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Solute concentration time series reflect hydrological and biological drivers through various frequencies, phases, and amplitudes of change. Untangling these signals facilitates the understanding of dynamic ecosystem conditions and transient water quality issues. A case in point is the inference of biogeochemical processes from diel solute concentration variations. This analysis requires approaches capable of isolating subtle diel signals from background variability at other scales. Conventional time series analyses typically assume stationary or deterministic background variability; however, most rivers do not respect such niceties. We developed a time-series filtering method that uses empirical mode decomposition to decompose a measured solute concentration time series into intrinsic mode frequencies. Based on externally supplied mechanistic knowledge, we then filter these modes by periodicity, phase, and coherence with neighboring days. This method is tested on three synthetic series that incorporate environmental variability and sensor noise and on a year of 15 min sampled concentration time series from three hydrologically and ecologically distinct rivers in the eastern United States. The proposed method successfully isolated signals in the measured data sets that corresponded with variability in gross primary productivity. The strength the diel signal isolated through this method was smaller compared to the true signal in the synthetic series; however, uncertainty analysis showed that the process-model-based estimates derived from these signals were similar to other inference methods. This signal decomposition method retains information that can be used for further process modeling while making different assumptions about the data than Fourier and wavelet analyses.

The significance of riparian vegetation on river flow and material transport is not in dispute. Conveyance laws, sediment erosion and deposition, and element cycling must all be adjusted from their canonical rough-wall boundary layer to accommodate the presence of aquatic plants. In turn, the growth and colonization of riparian vegetation are affected by fluvial processes and river morphology on longer time scales. These interactions and feedbacks at multiple time scales are now drawing significant attention within the research community given their relevance to river restoration. For this reason, a review summarizing methods, general laws, qualitative cognition, and quantitative models regarding the interplay between aquatic plants, flow dynamics, and sediment transport in vegetated rivers is in order. Shortcomings, pitfalls, knowledge gaps, and daunting challenges to the current state of knowledge are also covered. As a multidisciplinary research topic, a future research agenda and opportunities pertinent to river management and enhancement of ecosystem services are also highlighted.

Transpiration, the dominant component of terrestrial evapotranspiration (ET), directly connects the water, energy and carbon cycles and is typically restricted by soil and atmospheric (for example, the vapour pressure deficit (VPD)) moisture stresses through plant hydraulic processes. These sources of stress are likely to diverge under climate change, with a globally enhanced VPD but more variable and uncertain changes in soil moisture. Here, using a model–data fusion approach, we demonstrate that the common empirical approach used in most Earth system models to evaluate the ET response to soil moisture and VPD, which neglects plant hydraulics, underestimates ET sensitivity to VPD and compensates by overestimating the sensitivity to soil moisture stress. A hydraulic model that describes water transport through the plant better captures ET under high VPD conditions for wide-ranging soil moisture states. These findings highlight the central role of plant hydraulics in regulating the increasing importance of atmospheric moisture stress on biosphere–atmosphere interactions under elevated temperatures.

How convective boundary‐layer (CBL) processes modify fluxes of sensible (SH) and latent (LH) heat and CO2(Fc) in the atmospheric surface layer (ASL) remains a recalcitrant problem. Here, large eddy simulations for the CBL show that while SHin the ASL decreases linearly with height regardless of soil moisture conditions, LHand Fcdecrease linearly with height over wet soils but increase with height over dry soils. This varying flux divergence/convergence is regulated by changes in asymmetric flux transport between top‐down and bottom‐up processes. Such flux divergence and convergence indicate that turbulent fluxes measured in the ASL underestimate and overestimate the “true” surface interfacial fluxes, respectively. While the non‐closure of the surface energy balance persists across all soil moisture states, it improves over drier soils due to overestimated LH. The non‐closure does not imply that Fcis always underestimated; Fccan be overestimated over dry soils despite the non‐closure issue. Large swirling motions, called large turbulent eddies, efficiently transport water vapor, carbon dioxide, and heat up and down throughout the convective boundary layer (CBL). To what extent scalar fluxes in the atmospheric surface layer (ASL) are modulated by large turbulent eddies from the top of the CBL (i.e., top‐down eddies) remains a recalcitrant problem in many fields spanning atmospheric sciences, hydrology, ecology, and climate change. Here, high‐resolution computational simulations of the CBL show that scalar fluxes in the ASL linearly change with height across soil wetness conditions largely due to changes in the interactions of top‐down processes and bottom‐up surface exchange. Such linear height‐dependence of the fluxes indicates that reported fluxes from direct turbulent measurements in the ASL are not identical to their sought surface values. As a result, the non‐closure of the surface energy balance occurs across all soil moisture conditions but improves as soil becomes dry. CO2measured fluxes are underestimated over wet soils and overestimated over dry soils, which has its implication when interpreting CO2exchanges from global flux measuring networks utilizing turbulence theories. Height dependence of fluxes, which confirms that the constant flux layer assumption is not routinely satisfied, is a fundamental reason for the non‐closure. Asymmetric flux transport by bottom‐up and top‐down processes leads to varying flux divergence/convergence (FDC) in the surface layerLatent heat and CO2fluxes are underestimated when soil is wet and overestimated when dry, but sensible heat flux is always underestimatedNon‐closure of the surface energy balance is regulated by varying FDC and improves for dry soils due to overestimated latent heat flux Asymmetric flux transport by bottom‐up and top‐down processes leads to varying flux divergence/convergence (FDC) in the surface layer Latent heat and CO2fluxes are underestimated when soil is wet and overestimated when dry, but sensible heat flux is always underestimated Non‐closure of the surface energy balance is regulated by varying FDC and improves for dry soils due to overestimated latent heat flux

Climate-induced forest mortality is being widely observed across the globe. Predicting forest mortality remains challenging because the physiological mechanisms causing mortality are not fully understood and empirical relations between climatology and mortality are subject to change. Here, we show that the temporal loss of resilience, a phenomenon often detected as a system approaches a tipping point, can be used as an early warning signal (EWS) to predict the likelihood of forest mortality directly from remotely sensed vegetation dynamics. We tested the proposed approach on data from Californian forests and found that the EWS can often be detected before reduced greenness, between 6 to 19 months before mortality. The EWS shows a species-specific relation with mortality, and is able to capture its spatio-temporal variations. These findings highlight the potential for such an EWS to predict forest mortality in the near-term.

Urban heat islands (UHIs) exacerbate the risk of heat-related mortality associated with global climate change. The intensity of UHIs varies with population size and mean annual precipitation, but a unifying explanation for this variation is lacking, and there are no geographically targeted guidelines for heat mitigation. Here we analyse summertime differences between urban and rural surface temperatures (ΔTs) worldwide and find a nonlinear increase in ΔTswith precipitation that is controlled by water or energy limitations on evapotranspiration and that modulates the scaling of ΔTswith city size. We introduce a coarse-grained model that links population, background climate, and UHI intensity, and show that urban–rural differences in evapotranspiration and convection efficiency are the main determinants of warming. The direct implication of these nonlinearities is that mitigation strategies aimed at increasing green cover and albedo are more efficient in dry regions, whereas the challenge of cooling tropical cities will require innovative solutions. The effect of cities on urban climate (often warmer but sometimes cooler than their surroundings) is largely explained by local hydroclimate and patterns of city development.

Floating treatment wetlands (FTWs) are efficient at wastewater treatment; however, data and physical models describing water flow through them remain limited. A two‐domain model is proposed dividing the flow region into an upper part characterizing the flow through suspended vegetation and an inner part describing the vegetation‐free zone. The suspended vegetation domain is represented as a porous medium characterized by constant permeability thereby allowing Biot's Law to be used to describe the mean velocity and stress profiles. The flow in the inner part is bounded by asymmetric stresses arising from interactions with the suspended vegetated (porous) base and solid channel bed. An asymmetric eddy viscosity model is employed to derive an integral expression for the shear stress and the mean velocity profiles in this inner layer. The solution features an asymmetric shear stress index that reflects two different roughness conditions over the vegetation‐induced auxiliary bed and the physical channel bed. A phenomenological model is then presented to explain this index. An expression for the penetration depth into the porous medium defined by 10% of the maximum shear stress is also derived. The predicted shear stress profile, local mean velocity profile, and bulk velocity agree with the limited experiments published in the literature. A two‐domain analytical model for flow inside floating treatment wetlands (FTWs) is proposedThe model accounts for the shear stress asymmetry bounding the vegetation‐free zone below the FTWThe model recovers the mean velocity and shear stress profiles reported in laboratory experiments configured to resemble FTW

Due to strong mean wind shear, the stable boundary layer (SBL) becomes vertically coupled. In a coupled SBL, large turbulent eddies enhance cross‐layer mixing and vertically mix turbulent kinetic energy. However, large gradients in momentum and heat fluxes are frequently observed, degrading the performance of Monin‐Obukhov similarity theory. It is shown that increased vertical gradients of mean variables (i.e., wind speed and potential temperature) can cause flux gradients. This process is operated upon by downward penetrating large eddies with altered phase difference, contributing unevenly to fluxes and thus causing flux gradients. In the coupled SBL with small gradients in mean variables, large eddies are vertically synchronized, contributing evenly to fluxes across layers and thus causing small flux gradients. As turbulent production becomes weak and the cross‐layer difference in turbulent flux transport increases, large eddies become less vertically synchronized, contributing unevenly to fluxes across layers and thus causing large flux gradients. Vertical gradients of turbulent fluxes and turbulent kinetic energy (TKE) in the stable boundary layer (SBL) are experimentally analyzedCross‐layer phase difference of large eddies is indicative of momentum and heat flux gradients even when TKE gradients are small in the SBLWeak turbulent production and different cross‐layer turbulent flux transport associated with large eddies lead to large flux gradients

The hydrodynamics of turbulent flow through submerged flexible vegetation is investigated in a flume using acoustic Doppler velocimetery (ADV) measurements. The flow characteristics such as the energetics and momentum transfer derived from conventional spectral and quadrant analyses are considered as the flow encounters a finite vegetation patch. Consistent with numerous canopy flow experiments, a shear layer and coherent vortex structures near the canopy top emerge caused by Kelvin–Helmholtz instabilities after the flow equilibrates with the vegetated layer. These instabilities are commonly attributed to velocity differences between non-vegetated and vegetated canopy layers in agreement with numerous experiments and simulations conducted on dense rigid canopies. The power-spectral density function for vertical velocity turbulent fluctuations at different downstream positions starting from the edge of the vegetation layer are also computed. For a preset water depth, the dominant dimensionless frequency is found to be surprisingly invariant around 0.027 despite large differences in vegetation densities. The ejection and sweep events significantly contribute to the Reynolds stresses near the top of the vegetation. The momentum flux carried by ejections is larger than its counterpart carried by the sweeps above the canopy top. However, the momentum flux carried by sweeps is larger below the top of the canopy.

The effect of hydraulic resistance on the downstream evolution of the water surface profile hin a sloping channel covered by a uniform dense rod canopy following the instantaneous collapse of a dam was examined using flume experiments. Near the head of the advancing wavefront, where hmeets the rods, the conventional picture of a turbulent boundary layer was contrasted to a distributed drag force representation. The details of the boundary layer around the rod and any interferences between rods were lumped into a drag coefficient Cd. The study demonstrated the following: In the absence of a canopy, the Ritter solution agreed well with the measurements. When the canopy was represented by an equivalent wall friction as common when employing Manning's formula with constant roughness, it was possible to match the measured wavefront speed but not the precise shape of the water surface profile. However, upon adopting a distributed drag force with a constant Cd, the agreement between measured and modeled hwas quite satisfactory at all positions and times. The measurements and model calculations suggested that the shape of hnear the wavefront was quasilinear with longitudinal distance for a constant Cd. The computed constant Cd(≈0.4) was surprisingly much smaller than the Cd(≈1) reported in uniform flow experiments with staggered cylinders for the same element Reynolds number. This finding suggested that drag reduction mechanisms associated with unsteadiness, nonuniformity, transient waves, and other flow disturbances were more likely to play a role when compared to conventional sheltering effects. Drag forces on unsteady shallow flow over cylindrical vegetation on sloping channel are investigated after a dam breakThe resulting vegetation drag appears much smaller than predictions from uniform canopy flow experimentsA proposed expression is experimentally tested for the dam break problem in a flume

The behavior of suspended particles in turbulent flows is a recalcitrant problem spanning wide‐ranging fields including geomorphology, hydrology, and dispersion of particulate matter in the atmosphere. One key mechanism underlying particle suspension is the difference between particle settling velocity (ws) in turbulence and its still water counterpart (wso). This difference is explored here for a range of particle‐to‐fluid densities (1–10) and particle diameter to Kolmogorov micro‐eddy sizes (0.1–10). Conventional models of particle fluxes that equate wsto wsoresult in eddy diffusivities and turbulent Schmidt numbers contradictory to laboratory experiments. Incorporating virtual mass and Basset history forces resolves these inconsistencies, providing clarity as to why ws/wsois sub‐unity for the aforementioned conditions. The proposed formulation can be imminently used to model particle settling in turbulence, especially when sediment distribution outcomes over extended time scales far surpassing turbulence time scales are sought. In rivers and streams, mixed‐up dirt called “suspended sediments” is known to influence water quality and its concomitant effects on many physical, chemical, and biological processes. Suspended sediments can be harmful to aquatic ecosystem productivity because they reduce light penetration. They can clog gills of fish and other aquatic organism, and they can impact reservoir operation and their capacity necessitating frequent dredging. There is debate about how these sediments interact with swirling motions (“turbulent flows”) in moving water. Traditional mathematical models overlook how individual grains are affected by these swirls (“turbulent eddies”) and how they displace water (“virtual mass”) when sediments move in a fluid. For large grains, these overlooked details can generate new terms in the force balance that are associated with complex eddying motion lagging behind the grain motion (“Basset history term”). Including the Basset history and virtual mass factors reconciles controversies between recent laboratory experiments and traditional theories about how grains settle in turbulent flows. Settling velocity of suspended sediments is reduced by virtual mass and Basset history for grains larger than turbulent micro‐scalesAn operational Rouse‐like budget that models these two effects is derived and tested against experimentsTurbulent Schmidt number exceeding unity in the experiments can be linked to these two effects Settling velocity of suspended sediments is reduced by virtual mass and Basset history for grains larger than turbulent micro‐scales An operational Rouse‐like budget that models these two effects is derived and tested against experiments Turbulent Schmidt number exceeding unity in the experiments can be linked to these two effects

Land-use and land-cover changes are accelerating. Such changes can homogenize the water cycle and undermine planetary resilience. Policymakers and practitioners must consider water–vegetation interactions in their land-management decisions.

Two ideas regarding the structure of turbulence near a clear air‐water interface are used to derive a waterside gas transfer velocity kLfor sparingly and slightly soluble gases. The first is that kLis proportional to the turnover velocity described by the vertical velocity structure function Dww(r), where ris separation distance between two points. The second is that the scalar exchange between the air‐water interface and the waterside turbulence can be suitably described by a length scale proportional to the Batchelor scale lB=ηSc−1/2, where Scis the molecular Schmidt number and ηis the Kolmogorov microscale defining the smallest scale of turbulent eddies impacted by fluid viscosity. Using an approximate solution to the von Kármán‐Howarth equation predicting Dww(r) in the inertial and viscous regimes, prior formulations for kLare recovered including (i) kL=2/15Sc−1/2vK, vKis the Kolmogorov velocity defined by the Reynolds number vKη/ν= 1 and νis the kinematic viscosity of water; (ii) surface divergence formulations; (iii) kL∝Sc−1/2u∗, where u∗is the waterside friction velocity; (iv) kL∝Sc−1/2gν/u∗for Keulegan numbers exceeding a threshold needed for long‐wave generation, where the proportionality constant varies with wave age, gis the gravitational acceleration; and (v) kL=2/15Sc−1/2(νgβoqo)1/4in free convection, where qois the surface heat flux and βois the thermal expansion of water. The work demonstrates that the aforementioned kLformulations can be recovered from a single structure function model derived for locally homogeneous and isotropic turbulence. The problem considered here is a theoretical prediction of mass transfer across and air‐water interface. This interfacial transfer phenomenon is featured prominently in global carbon balances, methane, nitrous oxides, dimethyl sulfide, and other gases. It is used to assess the metabolic health of aquatic ecosystems and to determine evasion rates of volatile organic compounds from lakes, estuaries, reservoirs, and large water treatment plants. The novelty of the approach is to link a bulk quantity reflecting the efficiency of swirling motions near the air‐water interface to the sizes of eddies and their energetic content responsible for the aforementioned swirling motion. The proposed approach is shown to recover a number of equations describing gas transport across interfaces that summarize a large corpus of experiments and simulations. Scaling laws empirically describing gas transfer velocity kLacross marine and coastal systems are theoretically explainedNew theory predicts kLusing Kolmogorov's universal structure function scaling laws for turbulence instead of surface renewal theoryWork shows how multiple mechanisms with different assumptions subject to eddy‐turnover time constraint lead to similar scaling laws for kL

Turbulence structures and exchange of momentum and heat in the nocturnal stable boundary layer (SBL) show distinct features under different stability conditions prompting interest in their connection. Here eddy covariance data collected at four different heights on a 62‐m meteorological tower over a large open flat terrain are used to characterize different SBL states and associated turbulence structures. In a SBL characterized by strong near‐surface winds, turbulent eddy sizes scale with their observational heights and the SBL experiences enhanced turbulent mixing of momentum and heat throughout (a state hereafter referred to as a “coupled” state). Conversely, in a decoupled SBL, weak winds occur near the surface and turbulent eddies are depressed and detached from the boundary leading to suppressed vertical mixing and layered SBL profiles. Because the transport of momentum and heat to the surface across SBL layers is determined by turbulent eddies, cross‐layer correlation and the aforementioned SBL coupling states can be delineated by distinct turbulence structures. Stable boundary layer (SBL) states are classified by near‐surface mean wind speedWeak and strong coupling across vertical layers form two distinct end‐member statesDistinct turbulence structures delineate these two end‐member SBL states

A number of studies already suggested that entrainment of warm/dry air from aloft degrades the similarity between air temperature (T) and specific humidity (q) in the atmospheric surface layer (ASL). Less is known about entrainment of cool/dry air on the ASL T‐qsimilarity. Using eddy covariance measurements over a large inland water surface, enhanced T‐qsimilarity with increased friction velocity (u*) was measured during daytime unstable conditions. Explaining this enhancement is the main goal of this work. Cool/dry air masses originating and advecting from land set the upper boundary condition on a quasi‐equilibrated internal boundary layer embedding the warm/moist ASL over water. With increased u*, the entrainment velocity (dhI/dt∝ u*) increases, thereby enhancing entrainment of cool/dry air originating from land, where tis time and hIis the internal boundary layer depth. By analyzing the scale‐dependent (spectral) correlation coefficients, the enhanced T‐qsimilarity with increased u*was shown to originate from spectral correlation coefficients in the middle‐ to low‐frequency ranges (large eddies ≫ z, the distance from the surface). With further increases in u*, similarity between Tand qwithin large eddies was further enhanced and phase differences between Tand qwas reduced. Quadrant analysis showed large increases in flux and time fractions of downward large cool/dry eddies with increasing u*. These results provide evidence that enhanced entrainment of synchronized, large cool/dry eddies was a primary cause for the increased T‐qsimilarity with increasing u*. Similarity between temperature and humidity improved with increasing wind shearTemperature and humidity of large eddies became more synchronized with increased wind shearEnhanced temperature‐humidity similarity was conjectured to be due to intensified entrainment of more synchronized, large cool/dry eddies at higher wind shear conditions

A widely used assumption in boundary layer meteorology is the zindependence of turbulent scalar fluxes Fsthroughout the atmospheric surface layer, where zis the distance from the boundary. This assumption is necessary for the usage of Monin‐Obukhov Similarity Theory and for the interpretation of eddy covariance measurements of Fswhen using them to represent emissions or uptake from the surface. It is demonstrated here that the constant flux assumption offers intrinsic constraints on the third‐order turbulent transport of Fsin the unstable atmospheric surface layer. When enforcing zindependence of Fson multilevel Fsmeasurements collected above different surface cover types, it is shown that increasing instability leads to a novel and universal description of (i) the imbalance between ejecting and sweeping eddy contributions to Fsand (ii) the ratio formed by a dimensionless turbulent transport of Fsand a dimensionless turbulent transport of scalar variance. When combined with structural models for the turbulent transport of Fs, these two findings offer a new perspective on “closing” triple moments beyond conventional gradient diffusion schemes. A practical outcome is a diagnostic of the constant flux assumption from single‐level Fsmeasurements. The constant flux layer assumption does not necessarily require turbulent transport to be symmetricThe constant flux layer assumption imposes intrinsic constraints on the eddy structure in the lower atmosphereThe structural model provides a new perspective on closing triple moments and diagnosing constant flux layer from single‐level measurements

In 1965, Brutsaert proposed a model that predicted mean evaporation rate E¯from rough surfaces to scale with the 3/4 power law of the friction velocity ( u∗) and the square‐root of molecular diffusivity (Dm) for water vapor. In arriving at these results, a number of assumptions were made regarding the surface renewal rate describing the contact durations between eddies and the evaporating surface, the diffusional mass process from the surface into eddies, and the cascade of turbulent kinetic energy sustaining the eddy renewal process itself. The working hypothesis explored here is that E¯∼Dmu∗3/4is a direct outcome of the Kolmogorov scaling for inertial subrange eddies modified to include viscous cutoff thereby bypassing the need for a surface renewal assumption. It is demonstrated that Brutsaert's model for E¯may be more general than its original derivation implied. The movement of water vapor molecules from rough surfaces such as soils by eddies into the atmosphere is of primary significance to a plethora of applications including hydrological and meteorological forecasting, irrigation planning, energy partitioning, and subsequent growth of the atmospheric boundary layer, to name a few. In 1965, W. Brutsaert proposed a general model that links the movement of water vapor molecules from the surface to the molecular diffusivity of water vapor in air and the wind‐induced shear stress at the surface. The derivation considered air packets sweeping down and coming in contact with the wet surface. When in contact with the surface, these packets become enriched with water vapor molecules during a finite contact duration, after which these packets are ejected from the surface. Brutsaert made key restrictive assumptions about the statistical properties of the contact duration of these packets with the surface to arrive at the final form of the evaporation equation. The work here demonstrates that the same result can be derived by assuming a turnover velocity of these air packets to follow a universal form based on a widely accepted theory of turbulent flows proposed by A. N. Kolmogorov. Mean evaporation rate was shown to scale with the 3/4 power law of the friction velocity and the square‐root of molecular diffusivityThe scaling was derived from surface renewal theory with ad hoc specification of contact probability between eddies and the surfaceA new approach that links these scaling results with the universal structure of the turbulent cascade is derived

A large corpus of field and laboratory experiments support the finding that the water side transfer velocity kLof sparingly soluble gases near air‐water interfaces scales as kL∼(νε)1/4, where νis the kinematic water viscosity and εis the mean turbulent kinetic energy dissipation rate. Originally predicted from surface renewal theory, this scaling appears to hold for marine and coastal systems and across many environmental conditions. It is shown that multiple approaches to representing the effects of turbulence on kLlead to this expression when the Kolmogorov microscale is assumed to be the most efficient transporting eddy near the interface. The approaches considered range from simplified surface renewal schemes with distinct models for renewal durations, scaling and dimensional considerations, and a new structure function approach derived using analogies between scalar and momentum transfer. The work offers a new perspective as to why the aforementioned 1/4 scaling is robust. Scaling laws empirically describing gas transfer velocity kLacross marine and coastal systems are theoretically explainedNew theory predicts kLusing Kolmogorov's universal structure function scaling laws for turbulence instead of surface renewal theoryWork shows how multiple mechanisms with different assumptions subject to eddy turnover time constraint lead to similar scaling laws for kL

The global air‐sea CO2flux (F) impacts and is impacted by a plethora of climate‐related processes operating at multiple time scales. In bulk mass transfer formulations, F is driven by physico‐ and bio‐chemical factors such as the air‐sea partial pressure difference (∆pCO2), gas transfer velocity, sea surface temperature, and salinity–all varying at multiple time scales. To de‐convolve the impact of these factors on variability in F at different time scales, time‐resolved estimates of F were computed using a global data set assembled between 1988 and 2015. The F anomalies were defined as temporal deviations from the 28‐year time‐averaged value. Spectral analysis revealed four dominant timescales of variability in F–subseasonal, seasonal, interannual, and decadal with relative amplitude differences varying across regions. A second‐order Taylor series expansion was then conducted along these four timescales to separate drivers across differing regions. The analysis showed that on subseasonal timescales, wind speed variability explains some 66% of the global F anomaly and is the dominant driver. On seasonal, interannual, and decadal timescales, the ∆pCO2effect controlled by the ∆pCO2anomaly, explained much of the F anomaly. On decadal timescales, the F anomaly was almost entirely governed by the ∆pCO2effect with large contributions from high latitudes. The main drivers across timescales also dominate the regional F anomaly, particularly in the mid‐high latitude regions. Finally, the driver of the ∆pCO2effect was closely connected with the relative strength of atmospheric pCO2and the nonthermal component of oceanic pCO2anomaly associated with dissolved inorganic carbon and alkalinity. The number of carbon dioxide (CO2) molecules per unit surface area per unit time that enter the ocean surface from the atmosphere is quantified by the air‐sea CO2flux (F). These CO2molecules impact many chemical and biological properties within the ocean. Yet, the direct controls on how many molecules can possibly be exchanged between the atmosphere and the ocean surface depend on several environmental factors such as wind speed at some reference height, the amount of CO2molecules in the atmosphere and in the water (or their imbalance ∆pCO2), the wave height, and sea surface temperature. These environmental factors vary on many time scales such as daily, monthly, seasonal, annual, inter‐annual, and decadal. The work demonstrates that the CO2gas exchange is dominated by the wind effect on subseasonal time scales, while on longer time scales, the ∆pCO2term, closely related to the variability of both atmospheric and oceanic CO2, emerges as a leading driver. Global air‐sea CO2flux is dominated by wind effect on subseasonal time scales, on longer time scales, ∆pCO2effect is the main driverThe decadal variability in global flux anomaly was almost entirely driven by ∆pCO2effect with highest contribution from high latitudesDrivers of global air‐sea CO2flux anomaly also dominate the regional variability, particularly in the mid‐high latitude oceans Global air‐sea CO2flux is dominated by wind effect on subseasonal time scales, on longer time scales, ∆pCO2effect is the main driver The decadal variability in global flux anomaly was almost entirely driven by ∆pCO2effect with highest contribution from high latitudes Drivers of global air‐sea CO2flux anomaly also dominate the regional variability, particularly in the mid‐high latitude oceans

The significance of phloem hydrodynamics to plant mortality and survival, which impacts ecosystem‐scale carbon and water cycling, is not in dispute. The phloem provides the conduits for products of photosynthesis to be transported to different parts of the plant for consumption or storage. The Mü$\ddot{u}$nch pressure flow hypothesis (PFH) is the leading framework to mathematically represent this transport. It assumes that osmosis provides the necessary pressure differences to drive water and sucrose within the phloem. Mathematical models utilizing the PFH approximate the phloem by a relatively rigid slender semi‐permeable tube. However, the phloem consists of living cells that contract and expand in response to pressure fluctuations. The effect of membrane elasticity on osmotically driven sucrose front speed has rarely been considered and frames the scope here. Laboratory experiments were conducted to elucidate the elastic‐to‐plastic pressure‐deformation relation in membranes and their effect on sucrose front speeds. It is demonstrated that membrane elasticity acts to retard the sucrose front speed. The retardation emerges because of two effects: (a) part of the osmotic pressure is diverted to perform mechanical work to expand the membrane instead of pressurizing water, and (b) expansion of the membrane reduces the sucrose concentration driving osmotic potential due to volume increases and concomitant dilution effects. These results offer a novel perspective about the much discussed presence of sieve plates throughout the phloem acting as structural expansion dampers. Plants utilize two interconnected hydraulic systems, the xylem and the phloem, to transport water and sucrose. The xylem provides the necessary pathway for water molecules, extracted from the soil and released back to the atmosphere during photosynthesis, to be transported to the leaf. The phloem moves photosynthates, mainly sucrose, from the leaf to different parts of the plant using the osmotic potential of sucrose along the pathway. The importance of phloem hydrodynamics to plant mortality and survival, which impacts the ecosystem‐scale carbon and water cycling, is not in dispute. While the xylem consists of dead cells and is rigid, the phloem consists of living cells and is elastic. The work presented here revisits osmotically driven flows in elastic tubes to focus on the influence of phloem elasticity on flow efficiency using laboratory experiments on idealized elastic semi‐permeable membranes. Experimental results are combined with mathematical modeling to show a decrease in transport efficiency because of losses in the osmotic potential necessary to drive the flow. These results offer a new perspective about phloem resilience and the role of sieve plates that connect the sieve elements of the phloem. The physics of sugar translocation in phloem cells is complicated by their elasticityMembrane elasticity leads to a slower front speed when compared to a theoretical rigid membraneThe role of sieve plates, porous media that exist within the phloem, is reconsidered to be structural instead of hydraulic The physics of sugar translocation in phloem cells is complicated by their elasticity Membrane elasticity leads to a slower front speed when compared to a theoretical rigid membrane The role of sieve plates, porous media that exist within the phloem, is reconsidered to be structural instead of hydraulic

While the claim that water-carbon interactions result in spatially coherent vegetation patterning is rarely disputed in many arid and semiarid regions, the significance of the detailed water pathways and other high frequency variability remain an open question. How the short temporal scale meteorological fluctuations form the long-term spatial variability of available soil water in complex terrains due to the various hydrological, land surface, and vegetation dynamic feedbacks frames the scope of the work here. Knowledge of the detailed mechanistic feedbacks among soil, plants, and the atmosphere will lead to advances in our understanding of plant water availability in arid and semiarid ecosystems and will provide insights for future model development concerning vegetation pattern formation. In this study, quantitative estimates of water fluxes and vegetation productivity are provided for a semiarid ecosystem with established vegetation bands on hillslopes using numerical simulations. A state-of-the-science process based ecohydrological model is used, which resolves hydrological and plant physiological processes at the relevant space and time scales, for relatively small periods (e.g., decades) of mature ecosystems (i.e., spatially static vegetation distribution). To unfold the mechanisms that shape the spatial distribution of soil moisture, plant productivity and the relevant surface/subsurface and atmospheric water fluxes, idealized hillslope numerical experiments are constructed, where the effects of soil type, slope steepness, and overland flow accumulation area are quantified. Those mechanisms are also simulated in the presence of complex topography features on landscapes. The main results are (a) short temporal scale meteorological variability and accurate representation of the scales at which each ecohydrological process operates are crucial for the estimation of the spatial variability of soil water availability to the plant root zone; (b) water fluxes such as evapotranspiration, infiltration, runoff-run-on, and subsurface soil water movement have a dynamic short temporal scale behavior that determines the long-term spatial organization of plant soil water availability in ecosystems with established vegetation patterns; and (c) hypotheses concerning the hydrological responses that can lead to vegetation pattern formation have to accommodate realistic and physically based representations of the fast dynamics of key ecohydrological fluxes. Correct scale representations are essential for quantifying water fluxes and vegetation productivityThe dynamic behavior of water fluxes determines the long-term spatial organization of soil moistureNew challenges for the hypotheses of the hydrological influence on vegetation pattern formation

Deviations in the Priestley-Taylor (PT) coefficient aPTfrom its accepted 1.26 value are analyzed over large lakes, reservoirs, and wetlands where stomatal or soil controls are minimal or absent. The data sets feature wide variations in water body sizes and climatic conditions. Neither surface temperature nor sensible heat flux variations alone, which proved successful in characterizing aPTvariations over some crops, explain measured deviations in aPTover water. It is shown that the relative transport efficiency of turbulent heat and water vapor is key to explaining variations in aPTover water surfaces, thereby offering a new perspective over the concept of minimal advection or entrainment introduced by PT. Methods that allow the determination of aPTbased on low-frequency sampling (i.e., 0.1 Hz) are then developed and tested, which are usable with standard meteorological sensors that filter some but not all turbulent fluctuations. Using approximations to the Gramdeterminant inequality, the relative transport efficiency is derived as a function of the correlation coefficient between temperature and water vapor concentration fluctuations (RTq). The proposed approach reasonably explains the measured deviations from the conventional aPT?=?1.26 value even when RTqis determined from air temperature and water vapor concentration time series that are Gaussian-filtered and subsampled to a cutoff frequency of 0.1 Hz. Because over water bodies, RTqdeviations from unity are often associated with advection and/or entrainment, linkages between aPTand RTqoffer both a diagnostic approach to assess their significance and a prognostic approach to correct the 1.26 value when using routine meteorological measurements of temperature and humidity. Variability of the Priesley-Taylor coefficient over water bodies is explainedA new method for estimating the Priesley-Taylor coefficient is proposedThe new method can be applied to standard meteorological sensors

In very stable boundary layers (VSBL), a “cocktail” of submeso motions routinely result in elevated mean wind speed maxima above the ground, acting as a new source of turbulence generation. This new source of turbulent kinetic energy enhances turbulent mixing and causes mean wind profile distortion (WPD). As a results, this transient distortion in the wind profile adjusts the classical log‐law. Addressing how WPD‐induced turbulence regulates flow structures, turbulent fluxes, and transitions in stability regimes across layers remains a challenge. Eddy covariance data measured at four levels on a 62‐m tower are employed to address these questions. It is shown that the WPD initiates large turbulent eddies that penetrate downward, leading to enhanced vertical mixing and comparable turbulent transport efficiencies across layers. As a consequence, turbulence intensity and fluxes are increased. As the WPD is intensified, turbulent fluxes and turbulent flux transport caused by large eddies are also enhanced, leading to a transition from very stable to weakly stable regimes. Due to the influence of WPD‐induced large eddies, the large‐eddy turbulent Prandtl number does not deviate appreciably from unity and the partitioning between turbulent kinetic and potential energies is linearly related to the gradient Richardson number. Frequently observed wind profile distortion (WPD) in the very stable boundary layer (VSBL) generates large turbulent eddiesWPD‐induced large eddies penetrate downward, leading to enhanced vertical mixing and increased downward turbulent fluxesThese large eddies alter turbulence structures and cause transitions from a VSBL to a weakly stable boundary layer Frequently observed wind profile distortion (WPD) in the very stable boundary layer (VSBL) generates large turbulent eddies WPD‐induced large eddies penetrate downward, leading to enhanced vertical mixing and increased downward turbulent fluxes These large eddies alter turbulence structures and cause transitions from a VSBL to a weakly stable boundary layer

Current literature provides large number of publications about ecohydrological processes and their effect on the biota in drylands. Given the limited laboratory and field experiments in such systems, many of these publications are based on mathematical models of varying complexity. The underlying implicit assumption is that the data set used to evaluate these models covers the parameter space of conditions that characterize drylands and that the models represent the actual processes with acceptable certainty. However, a question raised is to what extent these mathematical models are valid when confronted with observed ecosystem complexity? This Introduction reviews the 16 papers that comprise the Special Section on Eco‐hydrology of Semiarid Environments: Confronting Mathematical Models with Ecosystem Complexity. The subjects studied in these papers include rainfall regime, infiltration and preferential flow, evaporation and evapotranspiration, annual net primary production, dispersal and invasion, and vegetation greening. The findings in the papers published in this Special Section show that innovative mathematical modeling approaches can represent actual field measurements. Hence, there are strong grounds for suggesting that mathematical models can contribute to greater understanding of ecosystem complexity through characterization of space‐time dynamics of biomass and water storage as well as their multiscale interactions. However, the generality of the models and their low‐dimensional representation of many processes may also be a “curse” that results in failures when particulars of an ecosystem are required. It is envisaged that the search for a unifying “general” model, while seductive, may remain elusive in the foreseeable future. It is for this reason that improving the merger between experiments and models of various degrees of complexity continues to shape the future research agenda.

While the importance of ecosystem functioning is undisputed in the context of climate change and Earth system modeling, the role of short‐scale temporal variability of hydrometeorological forcing (~1 h) on the related ecosystem processes remains to be fully understood. Various impacts of meteorological forcing variability on water and carbon fluxes across a range of scales are explored here using numerical simulations. Synthetic meteorological drivers that highlight dynamic features of the short temporal scale in series of precipitation, temperature, and radiation are constructed. These drivers force a mechanistic ecohydrological model that propagates information content into the dynamics of water and carbon fluxes for an ensemble of representative ecosystems. The focus of the analysis is on a cross‐scale effect of the short‐scale forcing variability on the modeled evapotranspiration and ecosystem carbon assimilation. Interannual variability of water and carbon fluxes is emphasized in the analysis. The main study inferences are summarized as follows: (a) short‐scale variability of meteorological input does affect water and carbon fluxes across a wide range of time scales, spanning from the hourly to the annual and longer scales; (b) different ecosystems respond to the various characteristics of the short‐scale variability of the climate forcing in various ways, depending on dominant factors limiting system productivity; (c) whenever short‐scale variability of meteorological forcing influences primarily fast processes such as photosynthesis, its impact on the slow‐scale variability of water and carbon fluxes is small; and (d) whenever short‐scale variability of the meteorological forcing impacts slow processes such as movement and storage of water in the soil, the effects of the variability can propagate to annual and longer time scales. Simulations evaluate meteorologic variability on water/carbon fluxes and storesCross‐scale information transfer is ecosystem/resource‐limitation dependentFast meteorological drivers affect long‐term fluxes when stores are affected

With the “green economy” being promoted as a path to sustainable development and food security within the African continent, the influx of agricultural land is proliferating at a rapid pace often replacing natural savannah forests. Where agriculture is primarily rainfed, the possible adverse impacts of agricultural land influx on rainfall occurrences in water‐limited areas such as West Africa warrant attention. Using field observations complemented by model calculations in southeastern Burkina Faso, the main causes of a 10–30% suppressed daytime rainfall recorded over agricultural fields when referenced to natural savannah forests are examined. Measurements and model runs reveal that the crossing of the mixed layer height and lifting condensation levels, a necessary condition for cloud formation and subsequent rainfall occurrence, was 30% more frequent above the natural savannah forest. This increase in crossing statistics was primarily explained by increases in measured sensible heat flux above the savannah forest rather than differences in lifting condensation heights. Convective rainfall (CR) reduced by agricultural expansion in Burkina FasoReductions in CR explained by reduced sensible heat (H) over agricultural sitesReduced H reduced crossing between ABL height and lifting condensation level

An expression that separates biotic and abiotic controls on the temporal dynamics of the soil moisture spatial coefficient of variation Cv(θ) was explored via numerical simulations using a mechanistic ecohydrological model, Tethys‐Chloris. Continuous soil moisture spatiotemporal dynamics at an exemplary hillslope domain were computed for six case studies characterized by different climate and vegetation cover and for three configurations of soil properties. It was shown that abiotic controls largely exceed their biotic counterparts in wet climates. Biotic controls on Cv(θ) were found to be more pronounced in Mediterranean climates. The relation between Cv(θ) and spatial mean soil moisture θ¯was found to be unique in wet locations, regardless of the soil properties. For the case of homogeneous soil texture, hysteretic cycles between Cv(θ) and θ¯were observed in all Mediterranean climate locations considered here and to a lesser extent in a deciduous temperate forest. Heterogeneity in soil properties increased Cv(θ) to values commensurate with field observations and weakened signatures of hysteresis at all of the studied locations. This finding highlights the role of site‐specific heterogeneities in hiding or even eliminating the signature of climatic and biotic controls on Cv(θ), thereby offering a new perspective on causes of confounding results reported across field experiments. Abiotic controls are larger than biotic and are dominant in wet climatesHysteresis is stronger for Mediterranean than wet climatesHeterogeneity in soil properties weakens signatures of hysteresis

A mechanistic model for the soil‐plant system is coupled to a conventional slab representation of the atmospheric boundary layer (ABL) to explore the role of groundwater table (WT) variations and free atmospheric (FA) states on convective rainfall predisposition (CRP) at a Loblolly pine plantation site situated in the lower coastal plain of North Carolina. Predisposition is quantified using the crossing between modeled lifting condensation level (LCL) and convectively grown ABL depth. The LCL‐ABL depth crossing is necessary for air saturation but not sufficient for cloud formation and subsequent convective rainfall occurrence. However, such crossing forms the main template for which all subsequent dynamical processes regulating the formation (or suppression) of convective rainfall operate on. If the feedback between surface fluxes and FA conditions is neglected, a reduction in latent heat flux associated with reduced WT levels is shown to enhance the ABL‐LCL crossing probability. When the soil‐plant system is fully coupled with ABL dynamics thereby allowing feedback with ABL temperature and humidity, FA states remain the leading control on CRP. However, vegetation water stress plays a role in controlling ABL‐LCL crossing when the humidity supply by the FA is within an intermediate range of values. When FA humidity supply is low, cloud formation is suppressed independent of surface latent heat flux. Similarly, when FA moisture supply is high, cloud formation can occur independent of surface latent heat flux. In an intermediate regime of FA moisture supply, the surface latent heat flux controlled by soil water availability can supplement (or suppress) the necessary water vapor leading to reduced LCL and subsequent ABL‐LCL crossing. It is shown that this intermediate state corresponds to FA values around the mode in observed humidity lapse rates γw(between −2.5 × 10−6and −1.5 × 10−6kg kg−1m−1), suggesting that vegetation water uptake may be controlling CRP at the study site. Water Table affects Convective Rainfall (CR) via the plant‐atmosphere continuumFeedbacks from the Free Atmosphere (FA) can dominate CR for wet FA statesVegetation can control CR for relatively dry FA states

Estimation of evapotranspiration is a key requirement of hydrologic balance studies and climate analysis. The study reported involved collection of precise weighing lysimeter and meteorological data from three sites representing distinct climates. The combined data set for daily amounts of evapotranspiration and meteorological variables covers a total of 19 years on either an annual or growing season basis. The pan evaporation, PriestleyTaylor, original Penman, and PenmanMonteith evapotranspiration estimating methods are compared with lysimeter measurements using a moving average of 1–30 days. The results indicate the applicability of the various methods as a function of climate regime and the reduction in standard error of the estimate and increase in the coefficient of determination as a function of length of the moving average period. The results can be used both to determine which methods are most applicable for different climates and the expected magnitude of the error as a function of the estimating interval. This study indicates that a 5–10day moving average can reduce the standard error of the estimate and increase the coefficient of determination significantly between estimated and measured reference evapotranspiration for several estimating methods for various climates.

Temporal variability in root zone soil moisture content (w) exhibits a Lorentzian spectrum with memory dictated by a damping term when forced with white‐noise precipitation. In the context of regional dimming, radiation and precipitation variability are needed to reproduce wtrends prompting interest in how the wmemory is altered by radiative forcing. A hierarchy of models that sequentially introduce the spectrum of precipitation, net radiation, and the effect of won evaporative and drainage losses was used to analyze the spectrum of wat subtropical and temperate forested sites. Reproducing the wspectra at long time scales necessitated simultaneous precipitation and net radiation measurements depending on site conditions. The wmemory inferred from observed wspectra was 25–38 days, larger than that determined from maximum wet evapotranspiration and field capacity. The wmemory can be reasonably inferred from the Lorentzian spectrum when precipitation and evapotranspiration are in phase. Climate models predict Lorentzian temporal spectra for root zone soil moisture wLong‐term site measurements deviate from Lorentzian and report shorter wmemoryPrecipitation and net radiation impact wspectra and its memory at long scales

The vertical distribution of suspended sediment concentration remains a subject of active research given its relevance to a plethora of problems in hydraulics, hydrology, ecology, and water quality control. Many of the classical theories developed over the course of 90 years represent the effects of turbulence on suspended sediments (SS) using an effective mixing length or eddy diffusivity without explicitly accounting for the energetics of turbulent eddies across scales. To address this gap, the turbulent flux of sediments is derived using a co‐spectral budget (CSB) model that can be imminently used in SS and other fine particle transport models. The CSB closes the pressure‐redistribution effect using a spectral linear Rotta scheme modified to include isotropization of production and interactions between turbulent eddies and sediment grains through a modified scale‐dependent de‐correlation time. The result is a formulation similar in complexity to the widely used Rouse's equation but with all characteristic scales, Reynolds number, and Schmidt number effects derived from well‐established spectral shapes of the vertical velocity and accepted constants from turbulence models. Finally, the proposed CSB model can recover Prandtl's and Rouse's equations under restricted conditions. A suspended sediment concentration (SSC) equation for turbulent flows is proposed and testedThe equation is derived from a co‐spectral budget that accounts for energy distribution in all eddy sizesThe effects of Reynolds number and a scale‐dependent Schmidt number on SSC are explicitly described A suspended sediment concentration (SSC) equation for turbulent flows is proposed and tested The equation is derived from a co‐spectral budget that accounts for energy distribution in all eddy sizes The effects of Reynolds number and a scale‐dependent Schmidt number on SSC are explicitly described

Abstract: Resolving every detail of the three-dimensional canopy morphology and its underlying topography remains untenable when modeling high Reynolds number geophysical flows. How to represent the effects of such a complex morphological variability and any concomittant topographic variability into one-dimensional bulk flow representation remains a fundamental challenge to be confronted in canopy turbulence research. Theoretically, planar averaging to the scale of interest should be applied to the time-averaged mean momentum balance; however, such averaging gives rise to covariance or dispersive terms produced by spatial correlations of time-averaged quantities that remain ‘unclosed’ or require parameterization. When the averaging scale is commensurate with few canopy heights, these covariances can be labeled as ‘micro-dispersive’ stresses. When averaging is intended to eliminate low-wavenumber topographic variations, we refer to these covariances as ‘macro-dispersive’ terms. Two flume experiments were used to explore the magnitude and sign of both micro- and macro-dispersive fluxes relative to their conventional Reynolds stresses counterparts: a rod-canopy with variable roughness density and a dense rod canopy situated on gentle hilly terrain. When compared to the conventional momentum flux, the micro-dispersive fluxes in the lowest layers of sparse canopies can be significant (∼50%). For dense canopies, the dispersive terms remain negligible when compared to the conventional momentum fluxes throughout. For the macro-dispersive fluxes, model calculations suggest that these terms can be neglected relative to the Reynolds stresses for a deep canopy situated on a narrow hill. For the region in which topographic variations can interact with the pressure, both model calculations and flume experiments suggest that the macro-dispersive fluxes cannot be neglected, and their value can be 20% of the typical Reynolds stresses.

Estimation of evapotranspiration is a key requirement of hydrologic balance studies and climate analysis. The study reported involved collection of precise weighing lysimeter and meteorological data from three sites representing distinct climates. The combined data set for daily amounts of evapotranspiration and meteorological variables covers a total of 19 years on either an annual or growing season basis. The pan evaporation, Priestley‐Taylor, original Penman, and Penman‐Monteith evapotranspiration estimating methods are compared with lysimeter measurements using a moving average of 1–30 days. The results indicate the applicability of the various methods as a function of climate regime and the reduction in standard error of the estimate and increase in the coefficient of determination as a function of length of the moving average period. The results can be used both to determine which methods are most applicable for different climates and the expected magnitude of the error as a function of the estimating interval. This study indicates that a 5–10‐day moving average can reduce the standard error of the estimate and increase the coefficient of determination significantly between estimated and measured reference evapotranspiration for several estimating methods for various climates.

Longitudinal velocity measurements above a uniform dry lakebed were carried out to investigate the applicability of the random sweeping decorrelation hypothesis to thermally stratified turbulent flow. The higher order velocity structure functions of order mwere measured and modeled using the sweeping decorreltion hypothesis. In order to reduce the influence of Taylor's frozen hypothesis on the assessment of the sweeping decorrelation hypothesis, two dimensionless quantities, developed by Praskovsky et al. (1993), were used. Based on these dimensionless quantities, the sweeping decorrelation hypothesis predictions agreed well with the higher order structure function measurements. Assumptions inherent in the sweeping decorrelation hypothesis were also considered. It was found that strong interaction existed between the energy containing scales and the inertial subrange scales, indicating that the sweeping action alone does not fully describe the higher order structure function. Also, local temperature-velocity interactions were measured and found to be significant thus weakening the validity of the sweeping decorrelation hypothesis. However, these two interaction mechanisms appeared to be opposite in sign and counteracted each other.

A simple procedure is presented for the determination of field scale soil surface temperature using meteorological and surface energy measurements. An advectionaridity evaporation model, in conjunction with the surface energy budget, is used to solve for the sensible heat flux. The calculated sensible heat flux and Monin-Obukhov surface layer similarity theory for temperature are employed to obtain the field scale temperature. A theoretical model to define the scalar roughness length is applied. The model correlated well (r2= 0.91) with infrared surface temperature measurements. Error analysis indicated that the procedure conserved the higher statistical moments and the maximum daily observed surface temperature.

Abstract:  The use of stem sap flow data to estimate diurnal whole-tree transpiration and canopy stomatal conductance depends critically upon knowledge of the time lag between transpiration and water flux through the stem. In this study, the time constant for water movement in stems of 12-year-old Pinus taeda L. individuals was estimated from analysis of time series data of stem water flux and canopy transpiration computed from mean daytime canopy conductance, and diurnal vapor pressure deficit and solar radiation measurements. Water uptake through stems was measured using a constant-heat sapflow probe. Canopy transpiration was correlated to stem uptake using a resistance-capacitance equation that incorporates a time constant parameter. A least-squares auto-regression determined the parameters of the resistance-capacitance equation. The time constants for ten loblolly pine trees averaged 48.0 (SE = 2.0) min and the time lag for the diurnal frequency averaged 47.0 (SE = 2.0) min. A direct-cross correlation analysis between canopy transpiration and sap flow time series showed maximum correlation at an approximately 30 min lag. Residuals (model-predicted minus actual stem flow data) increased with increasing soil moisture depletion. While the time constants did not vary significantly within the range of tree sizes studied, hydraulic resistance and capacitance terms were individually dependent on stem cross-sectional area: capacitance increased and resistance decreased with stem volume. This result may indicate an inverse adjustment of resistance and capacitance to maintain a similar time constant over the range of tree sizes studied.

Channel networks increase in complexity as the importance of erosion grows compared to diffusion by soil creep, giving rise to a channelization cascade. Simulations, laboratory experiments, and data from a natural landscape are used to uncover the signature of such a cascade in the wavenumber spectrum of elevation fluctuations. Power spectra at intermediate distances from the boundaries are characterized by a peak wavenumber that is related to the quasi‐cyclic valleys superimposed on a power‐law scaling with exponent (α) at smaller scales. Dimensional analysis and self‐similarity arguments show that αis uniquely linked to the power‐law relation (with exponent m) between erosion potential and the specific drainage area via α= 2m− 3. This finding connects the spectral behavior of erosional surfaces to the exponent mthat distinguishes between the steep landscapes with debris‐flow‐dominated channels and relatively flat fluvial landscapes and directly controls the shape of the channel profile. Landscapes exhibit periodic valley spacing at the onset of channelization instability, a process by which elevation changes are no longer “smeared out” by random sediment redistribution. As the importance of erosion grows, different sequences of valleys and ridges emerge with a characteristic statistical behavior recognizable in the power spectrum. Dimensional analysis and self‐similarity arguments are used to show that the exponent of this power‐law scaling is uniquely linked to the relation between erosion potential and the specific drainage area. This finding is illustrated in numerical simulation and using data from a laboratory experiment and a natural landscape. The elevation spectrum presents a power‐law scaling for a wide range of scalesDimensional and self‐similarity arguments connect the power‐law scaling of the spectrum to the nonlinearity of the erosion termNumerical simulations and data from experimental and natural landscapes are used to test these findings The elevation spectrum presents a power‐law scaling for a wide range of scales Dimensional and self‐similarity arguments connect the power‐law scaling of the spectrum to the nonlinearity of the erosion term Numerical simulations and data from experimental and natural landscapes are used to test these findings

How large turbulent eddies influence non‐closure of the surface energy balance is an active research topic that cannot be uncovered by the mean continuity equation in isolation. It is demonstrated here that asymmetric turbulent flux transport of heat and water vapor by sweeps and ejections of large eddies under unstable atmospheric stability conditions reduce fluxes. Such asymmetry causes positive gradients in the third‐order moments in the turbulent flux budget equations, primarily attributed to substantially reduced flux contributions by sweeps and sustained large flux contributions by ejections. Small‐scale surface heterogeneity in heating generates ejecting eddies with larger air temperature variance than sweeping eddies, causing asymmetric flux transport in the atmospheric surface layer. Changes in asymmetry with increasing instability are congruent with observed increases in the surface energy balance non‐closure. To assess the contributions of asymmetric flux transport by large eddies to the non‐closure requires two eddy covariance systems on the tower to measure the gradients of the turbulent heat flux and other third‐order moments. A recalcitrant problem in land‐atmosphere interaction is the surface energy balance non‐closure. This problem arises when the measured available net energy at the surface is compared to the measured sum of sensible heat and latent heat fluxes sensed by the atmosphere. Several conjectures and hypotheses have been offered to explain this non‐closure ranging from experimental uncertainty to statistical uncertainty in averaging of processes associated with the conservation equations. Theories pertaining to the averaged conservation equations for heat and water vapor alone mask processes associated with the role of complex swirling motion (or eddies) in the atmosphere. Likewise, experimental uncertainty cannot explain why the closure becomes poorer during daytime when intense surface heating and weak winds coincide. Under such unstable conditions, large eddies transport heat and water vapor through ascending (ejections) and descending (sweeps) motions. Vertical transport of heat and water vapor carried by sweeps and ejections is unequal (i.e., asymmetric transport). The role of such asymmetric transport in the non‐closure is investigated here using heat and water vapor flux budget equations. Such asymmetry causes a reduction in heat and water vapor fluxes and it appears to follow patterns of non‐closure as the surface heating increases and wind speeds are reduced. Surface energy balance non‐closure under unstable conditions is linked to asymmetric turbulent flux transport of scalars by large eddiesAsymmetry primarily results from heterogeneous landscapes that lead to larger air temperature variances of ejecting than sweeping eddiesEnhanced asymmetry with increasing instability leads to reduced fluxes and increases non‐closure Surface energy balance non‐closure under unstable conditions is linked to asymmetric turbulent flux transport of scalars by large eddies Asymmetry primarily results from heterogeneous landscapes that lead to larger air temperature variances of ejecting than sweeping eddies Enhanced asymmetry with increasing instability leads to reduced fluxes and increases non‐closure

While it has been known that wave breaking and bubble generation at high wind speeds enhance air‐sea carbon dioxide (CO2) exchange rates (F), quantification of their contribution at the global scale remains a formidable challenge. There is urgency to make progress on this issue as a significant uptick in both magnitude and frequency of high wind events (HW) has been documented over the last 3 decades. Using a wind‐wave dependent expression for gas transfer velocity (k) that explicitly considers bubbles and a widely used wind‐only parameterization, the spatial pattern of kat high winds can be explained by sea surface temperature distribution. The HW, which represent some 3% of wind conditions, contribute disproportionally to the global F(18%) with an increasing trend. Approximately 50% of the global Fat high winds is attributed to bubble contribution. The findings are of significance to quantifying CO2transfer to the ocean interior. Studies on air‐sea carbon dioxide (CO2) exchange seek to determine how rapidly CO2molecules traverse the air‐water interface. This exchange impacts a plethora of processes related to ocean biogeochemistry, oceanic carbon cycling, and CO2buildup in the atmosphere. At high wind speeds, both conventional aerodynamic transfer processes and bubbles generated by wave breaking are expected to enhance air‐sea CO2gas exchange. Yet, the impact of bubbles in isolation on global and regional CO2exchange remains a subject of inquiry and speculation. There is some urgency to make progress on this topic because the magnitude and frequency of high wind events (HW) have been steadily increasing over the last 3 decades. The work here demonstrates that bubbles contribute as much as 50% of CO2gas exchange under high wind speeds conditions, which rarely occur over the ocean (less than 3% of the time). Yet, HW contribute disproportionally to the global air‐sea CO2gas exchange (about 18%). Global CO2fluxes at high wind speeds were estimated using a new wave‐wind dependent formulationHigh wind speed events, which represent some 3% of wind conditions, contribute 18% of the global air‐sea CO2fluxApproximately 50% of the global flux at high wind speeds is attributed to bubbles associated with wave breaking Global CO2fluxes at high wind speeds were estimated using a new wave‐wind dependent formulation High wind speed events, which represent some 3% of wind conditions, contribute 18% of the global air‐sea CO2flux Approximately 50% of the global flux at high wind speeds is attributed to bubbles associated with wave breaking

The sensitivity of soil carbon dynamics to climate change is a major uncertainty in carbon cycle models. Of particular interest is the response of soil biogeochemical cycles to variability in hydroclimatic states and the related quantification of soil memory. Toward this goal, the power spectra of soil hydrologic and biogeochemical states were analyzed using measurements of soil temperature, moisture, oxygen, and carbon dioxide at two sites. Power spectra indicated multiscale power law scaling across subhourly to annual timescales. Precipitation fluctuations were most strongly expressed in the soil biogeochemical signals at monthly to annual timescales. Soil moisture and temperature fluctuations were comparable in strength at one site, while temperature was dominant at the other. The effect of soil hydrologic, thermal, and biogeochemical processes on gas concentration variability was evidenced by low spectral entropy relative to the white noise character of precipitation. A full mass balance model was unable to capture high‐frequency soil temperature influence, indicating a gap in commonly used model assumptions. A linearized model was shown to capture the main features of the observed and modeled gas concentration spectra and demonstrated how the means and variances of soil moisture and temperature interact to produce the gas concentration spectra. Breakpoints in the spectra corresponded to the mean rate of gas efflux, providing a first‐order estimate of the soil biogeochemical integral timescale (∼1 min). These methods can be used to identify biogeochemical system dynamics to develop robust, process‐based soil biogeochemistry models that capture variability in addition to long‐term mean values. The ability to describe how climate change impacts soil carbon and nutrient cycles with models is a necessary tool for ecosystem management and sustainability. One difficulty in developing these predictive models is the so‐called “legacy effect”—for example, one wet summer may alter the ecosystem for many years afterward. Soil data and models are used here to quantify the relative strength of short‐ and long‐term variability of soil biogeochemical systems and how it responds to rainfall, soil moisture, and soil temperature. We found that variability in soil biogeochemistry is concentrated at longer timescales of several weeks to months and this is because the soil stores water and heat, retaining a “memory” of past rainfall and temperature. Further, this analysis offered a new perspective on the equations used in current models—models driven by soil moisture and temperature are able to capture the legacy in soil biogeochemical data. Soil gases retain variability of precipitation, soil moisture, and temperature at different frequenciesSoil biogeochemical memory is driven by diffusion at frequencies higher than hydrologic memoryLinear models driven by soil moisture and temperature capture the main features of the gas concentration power spectrum Soil gases retain variability of precipitation, soil moisture, and temperature at different frequencies Soil biogeochemical memory is driven by diffusion at frequencies higher than hydrologic memory Linear models driven by soil moisture and temperature capture the main features of the gas concentration power spectrum

Momentum and mass exchanges between the atmosphere and forests situated on complex terrain continue to draw significant research attention primarily because of their significance to a plethora of applications. In this paper, we investigated flows behavior on the leeward side of a two‐dimensional forested ridge under neutrally stratified conditions using large‐eddy simulations (LESs). The goal is to understand how variations in leaf area index (LAI), vertical canopy foliage distributions, and forest edge positions affect mean/turbulent flow statistics, momentum fluxes, and onset of recirculation patterns. Although pressure perturbations are dominated by the hill shape, it is demonstrated here that changes in canopy foliage distribution modulate intensities and patterns of the leeward adverse pressure gradients. Such changes in the adverse pressure gradients alter the mean velocity streamlines including the patterns and magnitudes of the leeward downward mean vertical velocity and the velocity variances and momentum flux in the wake region. While a downwind recirculation zone develops in all cases, the details regarding the incipient location and recirculation zone size vary including positions of the separation and reattachment points. Furthermore, changes in the strength and depth of the zone occur due to canopy‐induced changes in adverse pressure gradients, advection, and canopy drag. Because the recirculation zone impacts the local mean advective terms in momentum and scalar exchanges, the simulations here indicate that canopy morphology‐induced changes in the leeward flows have significant implications to both measurements and models of biosphere‐atmosphere exchange over complex terrain. Flows over the leeward side of a partially and fully forested hill are studied using a multiple‐layer canopy module in the WRF‐LESCanopy morphologies and partial forest cover lead to pressure perturbations that deviate from hydrostatic approximationThe leeward recirculation is altered by canopy morphological changes and partily modulated the pressure‐velocity interaction

The interpretation of tower‐based eddy‐covariance (EC) turbulent flux measurements above forests hinges on three key assumptions: (1) steadiness in the flow statistics, (2) planar homogeneity of scalar sources or sinks, and (3) planar homogeneity in the flow statistics. Large eddy simulations (LESs) were used to control the first two so as to explore the break‐down of the third for idealized and real gentle topography such as those encountered in Amazonia. The LES runs were conducted using uniformly distributed sources inside homogeneous forests covering complex terrain to link the spatial patterns of scalar turbulent fluxes to topographic features. Results showed strong modulation of the fluxes by flow features induced by topography, including large area with negative fluxes compensating “chimney” regions with fluxes almost an order of magnitude larger than the landscape flux. Significant spatial heterogeneity persisted up to at least two canopy heights, where most eddy‐covariance measurements are performed above tall forests. A heterogeneity index was introduced to characterize and contrast different scenarios, and a topography categorization was shown to have predictive capabilities in identifying regions of negative and enhanced fluxes. Topography‐induced flow structure causes strong heterogeneity in fluxes of gases emitted in forestsFlux heterogeneity is not reduced with increasing measurement heightImplications for tower‐based eddy‐covariance measurements are discussed