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Abstract Despite some progress, the importance of stemflow remains obscured partly due to computations emphasizing canopy interception loss. We advocate for two metrics—the stand-scale funneling ratio and the stand-scale infiltration funneling ratio—to more accurately portray stemflow inputs and increase comparability across ecosystems. These metrics yield per unit area stemflow inputs orders of magnitude greater than what would have been delivered by throughfall or precipitation alone. We recommend that future studies employ these stand-scale funnelling metrics to express stemflow commensurate with its ecohydrological importance and better conceptualize the role of stemflow in plant-soil interactions, permitting advances in critical zone science. [ABSTRACT FROM AUTHOR]

Tipping-bucket rain gauges are used widely to measure the amount and intensity of gross rainfall and throughfall in forests, despite the fact that their systematic underestimations are well known. To the knowledge of the authors, no dynamic calibrations for the budget-conscious Davis gauge (Rain Collector II, Davis Instruments, California) have been published. Thus, five Davis gauges were dynamically calibrated under different constant intensities of inflow and a correction equation was derived. The derived correction equation for the Davis rain gauge is V=-0.2005Q2+0.702Q+1 (R2=0.95, p<0.001), where V is the actual volume of a single tip scaled by the static volume of single tip c (cm3 cm-3), and Q is actual inflow scaled by c (s-1). The Davis rain gauge was then compared to the Onset rain gauge and the Ota rain gauge, and the corrections were applied to field observation data of canopy interception loss from a temperate forest in Japan. It is necessary to apply corrections to gross rainfall and throughfall data by tipping-bucket gauges because the results reveal that such corrections change the actual interception loss computed by values from 220% to 40%, depending on the combination of gauges employed. This difference is not trivial. The systematic bias of the Davis gauge is larger than the Onset and Ota gauges. Thus, it is recommended that researchers using Davis rain gauges apply the dynamically calibrated correction equation presented here to ensure more reliable estimates of gross rainfall and canopy interception loss. [ABSTRACT FROM AUTHOR]

Canopy interception of rainfall is an important process in the water balance of forests. The intrastorm dynamics of canopy interception is less well understood than event scale interception. Accordingly, armed with measurements of hourly interception intensity ( i ) from the field, this study is among the first to examine the differences in canopy interception dynamics between the first and second halves of rainfall events to quantify dynamic storage values for a coniferous forest in Japan. At this site, experimental results demonstrated that: (1) the relationship between interception loss ( I ) and gross rainfall ( GR ) at the event scale is better explained by a parabolic curve than a linear relationship, and there is a low correlation between rainfall intensity ( gr ) and i ; (2) the ratio of accumulated i during the first half ( I F ) to that of gr ( GR F ) was larger than the second half ( I S / GR S ), with no significant correlations between potential evaporation during first half ( PE F ) vs I F or the second half ( PE S ) vs I S ; and (3) water storage capacity was similar to the magnitude of maximum I . By emphasizing the comparison between I F and I S , this study concludes that the water storage on tree surface is more important than losses by wet canopy evaporation and splash during rain. This study also adds insights into intrastorm interception dynamics of coniferous forests which are necessary to better model and forecast interception losses. [ABSTRACT FROM AUTHOR]

Background: Dry deciduous forest (DDF) is distributed throughout the monsoon area of South-east Asia. Leaf morphological and photosynthetic traits are essential for adaptation to the environmental variability and usually change with tree height and season. Aims: To clarify the differences in leaf traits as influenced by tree height, light conditions and season for two dominant dipterocarps of DDF:Dipterocarpus tuberculatusandShorea obtusa. Methods: We measured changes in leaf traits with tree height, namely leaf mass per area (LMA), maximum photosynthetic rate (Amax), stomatal conductance (gs) and nitrogen concentration (N), in the rainy and early and mid-dry seasons. To identify the effects of height and light intensity on leaf traits, we conducted multiple regression analyses using leaf traits as the dependent variables and the height, light and season as independent variables. Results: Tree height was more important for gradients in most leaf traits, especially in LMA in both species, than light intensity. Tall canopy tree leaves had greater LMA,Amaxand N than shorter tree leaves in the rainy and early dry seasons, althoughgswas almost constant. The lowestAmaxandgswere recorded in immature leaves, sampled in the mid-dry season, at all tree heights. Conclusions: The two dipterocarp species achieved highAmaxby having highgs, LMA and N in a high-light canopy environment, whereas larger, thinner leaves with low respiratory costs may contribute to efficient light capture under low-light conditions in the understorey. [ABSTRACT FROM PUBLISHER]

Abstract: We measured evapotranspiration in an eastern Siberian boreal forest, in which the understory was cowberry and the overstory was larch, during the entire growing seasons of 2005 and 2006. We compared evapotranspiration from the understory vegetation above the forest floor E U with evapotranspiration from the whole ecosystem above the overstory canopy E O. The E U/E O ratio had a seasonal trend with a flat-bottomed U-shape during the growing season (4 May–30 September). High-E U/E O ratios at the beginning and end of the growing season were observed because larch, one of the two sources of E O, was a deciduous tree, while the understory was the evergreen cowberry. The mean daily E U values during the foliated period of larch (1 June–31 August) were 0.8 and 0.9mmday−1, or 51.4 and 51.8% of E O in 2005 and 2006, respectively. The understory vegetation was one of the most important components of the hydrologic cycle in this forest. A significant amount of E U was caused by plant physiological control, due to the aerodynamic conductance, which was much larger than the surface conductance, leading to a smaller decoupling coefficient. We found that 71% of E U was caused by the vapour pressure deficit above the forest floor. [Copyright &y& Elsevier]

Abstract: Extensive measurements of water balance components in a forest under the succession from Japanese red pine (Pinus densiflora) to evergreen oak (Quercus myrsinaefolia) were carried out. Significant decreases of transpiration from the red pines and forest interception loss were found based on the observational results in 1984/1985 and 2001/2002. The former was the pre-succession period and the latter was the middle succession period in the study forest. Although the forest had a main canopy of the red pines in 1984/1985, the multi-layered canopies were observed in 2001/2002, which were consisted of the upper canopy layer of the red pines and of the lower canopy layer of the evergreen oaks. On the other hand, no significant difference in the amount of total evapotranspiration between the two periods was observed, because the decrease of transpiration from the red pines was compensated by the increase of transpiration from the lower canopy layer, which was about three times larger than that of the red pines. Three factors were identified to produce the large amount of transpiration from the lower canopy layer: (i) increased net radiation over the lower canopy due to the increased gaps in the canopy of red pine, (ii) a relatively small difference of total sapwood area between the red pine and the lower canopies and (iii) larger sap flux densities of main species in the lower canopy which were around two times larger than that of the red pine canopy. [Copyright &y& Elsevier]

Abstract: Extensive measurements of rainfall, throughfall and stemflow in a forest during succession from Japanese red pine (Pinus densiflora Sieb. et Zucc.) to a combination of red pine and lower canopy trees—evergreen oak (Quercus myrsinaefolia Blume) and evergreen theaceous tree (Eurya japonica Thunb.) allowed the effect of this succession on the interception of rainfall to be evaluated. The measurements were conducted on two occasions: 1984/1985, and 2001/2002 when the lower canopy trees had become dominant. During this period, 75% of the red pines had been removed, and there was a substantial increase in stemflow (p<0.01), essentially no change in throughfall (p<0.01), and a substantial decrease in interception (p<0.01). The increase in stemflow was attributed to the increase in lower canopy trees; trees that have steeply angled branches, smooth bark surfaces and water repellent leaves; all of which enhance stemflow. The decrease in interception was due to the decrease in canopy water storage (2.6–1.1mm/event) and an increase in evaporation during rainfall event (0.7–1.1mm/event). The decrease in storage partly resulted from the removal of red pines, the bark of which is thick, flaky, and therefore, very absorptive. It was responsible for 88% of the actual rainfall storage at the beginning of the experiment. During the 17 year-period, the size of the lower canopy trees increased more rapidly than that of red pines. The increase in evaporation was due to the increase in canopy gaps by the removal of 75% of the red pines during the succession, and was a minor factor in affecting interception loss. [Copyright &y& Elsevier]

• ET and its components were measured separately in a tropical dry deciduous forest. • Understory ET is important, contributing 35% to whole-ecosystem ET. • Overstory LAI strongly affects the proportion of understory ET relative to whole ET. • Underestimates of stand transpiration can be corrected with calibrated coefficients. The water balance of tropical dry deciduous forests is less well understood than some other forest ecosystems. To help close this knowledge gap, we separately measured the evapotranspiration from the whole ecosystem (ET W), transpiration (TR) and interception loss (IL) from overstory trees, and evapotranspiration from the understory vegetation (ET U) in a tropical dry deciduous forest in Cambodia. It was found that ET W was equivalent to 73.7% of rainfall (P) at the annual scale. In the dry season, ET W corresponded to 120.1% of P , which indicates the utilization of soil water replenished during the wet season. The sum of transpiration estimated by the thermal dissipation (TD) method with the original coefficient (TR G), IL , and ET U was smaller than ET W, except for the middle of the dry season, due to an underestimation of TR G. Although recently reported calibration coefficients can reasonably correct TR G , future calibrations of the TD method are highly recommended for the precise evaluation of single-tree-scale transpiration in tropical dry forests. The annual contribution of the understory vegetation to ET W (ET U / ET W) was 34.6%, leading to the conclusion that the understory vegetation cannot be ignored when trying to gain a comprehensive understanding of the hydrologic cycle in tropical dry forests. The seasonal variations in ET U / ET W were mainly controlled by the leaf area index (LAI) of overstory trees, resulting from the overall stability of ET W and decreasing trend of ET U with increasing LAI in the wet season, with the opposite holding true in the dry season, i.e., decreasing ET W with the decline of LAI and less variations of ET U. Thus, LAI influenced both the seasonality and the annual contribution in ET U / ET W , exerting a notable influence on hydrological cycling in this forest. [ABSTRACT FROM AUTHOR]

Precipitation that has been intercepted by tree canopies and subsequently routed to the boles of trees is termed stemflow. Stemflow, because of its concentrated nature at the base of trees and the resultant depth equivalent inputs to near-stem soils, has been shown to contribute to overland flow and erosion, preferential and subsurface flow, perched water table development and groundwater recharge, and has been linked to by-pass flow promoting storm runoff in certain streams. Although the quantitative importance of stemflow has been captured at the tree-scale using the funneling ratio –a metric that compares the volume of stemflow generated by a tree to the volume of precipitation that would have fallen on an open area equivalent to the basal area of the tree-, representative expressions of the volumetric importance of stemflow at the stand-scale have been seldom used. As such, two metrics for expressing stemflow at the stand-scale are advocated for: the stand-scale funneling ratio and the stand-scale infiltration funneling ratio. The stand-scale funneling ratio is akin to the original funneling ratio, however, the stemflow volume from all trees within a stand is compared to the volume of water that would have fallen as rain in the open over an area equivalent to the basal area of that stand, while the stand-scale infiltration funneling ratio utilizes the total infiltration area of the stand in place of the stand basal area. Two case studies, one from a juvenile lodgepole pine stand in British Columbia, Canada, and one from a lowland tropical deciduous forest in Kratie, Cambodia, are used to illustrate the quantitative importance of stemflow using these two metrics even though stemflow is a relatively small percentage of rainfall in both forests. The stand-scale funneling ratio is also derived for eighteen mature forests from around the globe using the results from recent studies (January 2017 – June 2018, inclusive) and, when coupled with the stand-scale funneling ratios reported for 16 mature forests in the stemflow review paper of Levia and Germer (2015), it is shown that stemflow only needs to be as little as 0.5% of rainfall for stand-scale funneling ratios to exceed unity. As such, near-stem soils in mature forest environments typically receive inputs from stemflow that far exceed what they would have received from throughfall or precipitation alone. Typical values of the stand-scale infiltration funneling ratio that should be expected in mature forests given typical stemflow flow rates, stand basal areas, and surface soil saturated hydraulic conductivities in these environments is also discussed. The case for future studies to report these stand-scale funneling metrics is made so that this water input is expressed in a way that is commensurate with its ecohydrological importance and so that the role of stemflow in plant-soil interactions may be better conceptualized, permitting advances in critical zone science. [ABSTRACT FROM AUTHOR]

Large tipping-bucket flow meters (TBFs, one tip > 200 ml) have been employed by hydrologists to quantify various water fluxes in a variety of contexts. The over-arching goal of this study is to develop a generalized correction equation for various TBFs. Based on our testing, we recommend the following to minimize TBF error: (1) periodic checking of the static calibration volume ( c ) since c is a gauge-specific value which has been found to vary after field deployment; and (2) for dynamic calibration, the use of our newly derived generalized correction equation when the tipping rate is less than 0.2 Hz. In equation form, the generalized correction equation for common TBFs with flat triangular buckets is: V  = −0.75 Q 2  + 0.72 Q  + 1 ( R 2  = 0.843; p  <0.0001), where V  =  v/c and v is the water volume for one tip under dynamic conditions, and Q  =  q / c [s − 1 ] and q is the water flow rate into the TBF. From our field test in a Japanese cedar forest stand, using stemflow ( S F ) as an example, we found that use of the generalized correction equation was successful in eliminating the 2–3% error in S F amounts. Moreover, we found that the generalized correction equation performed nearly as well as gauge-specific derived correction equations. Thus, our generalized correction equation is applicable to correct flow estimates of TBFs when one does not have time and/or laboratory set-up for the laborious task of testing individual TBFs themselves. Although our correction procedure may not completely eliminate all error, we recommend use of the generalized correction equation for TBFs to improve the accuracy of water flux calculations in hydrologic studies. [ABSTRACT FROM AUTHOR]

Boreal forests play an important role in the global balance of energy and CO2. Our previous study of elaborate eddy covariance observations in a Siberian boreal larch forest, conducted both above the forest canopy and at the forest floor, revealed a significant contribution of latent heat flux (LE) from the cowberry understory to the whole ecosystem LE. Thus, in the present study, we examined what factors control the partitioning of whole ecosystem LE and CO2 flux into the understory and overstory vegetation, using detailed leaf-level physiology (for both understory and overstory vegetation) and soil respiration property measurements as well as a multilayer soil-vegetation-atmosphere transfer (SVAT) model. The modeling results showed that the larch overstory's leaf area index (LAI) and vertical profile of leaf photosynthetic capacity were major factors determining the flux partitioning in this boreal forest ecosystem. This is unlike other forest ecosystems that tend to have dense LAI. We concluded that control of the larch overstory's LAI had a relationship with both the coexistence of the larch with the cowberry understory and with the water resources available to the total forest ecosystem. [Copyright &y& Elsevier]

Irrigated agriculture contributes 40% of total global food production. In the US High Plains, which produces more than 50 million tons per year of grain, as much as 90% of irrigation originates from groundwater resources, including the Ogallala aquifer. In parts of the High Plains, groundwater resources are being depleted so rapidly that they are considered nonrenewable, compromising food security. When groundwater becomes scarce, groundwater withdrawals peak, causing a subsequent peak in crop production. Previous descriptions of finite natural resource depletion have utilized the Hubbert curve. By coupling the dynamics of groundwater pumping, recharge, and crop production, Hubbert-like curves emerge, responding to the linked variations in groundwater pumping and grain production. On a state level, this approach predicted when groundwater withdrawal and grain production peaked and the lag between them. The lags increased with the adoption of efficient irrigation practices and higher recharge rates. Results indicate that, in Texas, withdrawals peaked in 1966, followed by a peak in grain production 9 y later. After better irrigation technologies were adopted, the lag increased to 15 y from 1997 to 2012. In Kansas, where these technologies were employed concurrently with the rise of irrigated grain production, this lag was predicted to be 24 y starting in 1994. In Nebraska, grain production is projected to continue rising through 2050 because of high recharge rates. While Texas and Nebraska had equal irrigated output in 1975, by 2050, it is projected that Nebraska will have almost 10 times the groundwater-based production of Texas. [ABSTRACT FROM AUTHOR]

Two morphological variants of Terminalia alata ( Combretaceae) differed in leaf flushing phenology and spatial distribution in a Cambodian deciduous forest. The hairy-type trees displayed leaf exchange behavior in the middle of the dry season. The glabrous type flushed new leaves 3 months after the wet season started. The leafless period of the hairy type was estimated to be <1 month, whereas that of the glabrous type lasted more than 5 months. The landscape-scale leaf exchange behavior was similar to that of the hairy type. The two types showed clear spatial separation. The hairy type was limited to flat areas with deep soils. The dominance of the glabrous type in hilly areas with shallow soils suggests that it is adapted to water-limited environments. The abundance of the glabrous type in hilly areas and its unique leaf phenology probably influence the carbon, energy and water balance at the landscape level. [ABSTRACT FROM AUTHOR]

Summary Evapotranspiration (ET) was estimated from a planted coniferous forest in southwestern Japan by applying three methods: the eddy covariance method; the measurement of rainfall ( P ) and runoff ( Q ) in a small watershed; and a combination of rainfall interception loss ( I C ), upper canopy transpiration based on a sap-flux density measurement in Japanese cedar ( Cryptomeria Japonica D. Don) stands ( E UC ), and modeled sub-canopy ET ( E SC ). After inverse multiplication of the energy imbalance ratio, ET by the eddy covariance method (ET EC ) was 839.9 mm in 2007 and 811.8 mm in 2008. The yearly values of P – Q were partially affected by P in the previous autumn. After continuous data collection for more than 5 years, P – Q became stable. The 9-year (2000–2008) average P – Q , which was considered most reliable in this study, was 897.5 mm y −1 . The cumulative ET EC during the daylight hours from the right stream bank, covered mainly with large Japanese cedars, was 894.1 mm from April 2007 to March 2008. The value was almost the same as that calculated as the components sum (ET COMP = I C + E UC + E SC : 911.4 mm), and the comparison suggested that the annual totals of ET EC with an energy imbalance correction provide a reliable estimate of ET in a forest stand on a complex topography. Spatial variation in the watershed was likely caused by differences in soil water retention at each slope position. The slight difference in annual ET EC in 2007 compared with 2008 was attributed to differences in the radiative energy input. In the monthly–weekly analysis, ET COMP was frequently higher than ET EC after heavy rainfall, while ET EC was higher under dry conditions and during active ET. Even under dry canopy conditions, daily ET EC was often higher than E UC + E SC . The results suggested a time-lag in evaporation from the ecosystem and/or under-estimated ET EC after rainfall. [ABSTRACT FROM AUTHOR]

Abstract: The dependence of surface conductance G s on various variables in a forest environment is important for characterising the spatiotemporal variations of water, energy and CO2 exchange between the forest and the atmosphere. Using a Jarvis-type model, we examined the variation of G s in five different mature forests from three climate zones (boreal, cool- and warm-temperate) in the Far East. First, we applied the model using summertime G s data from each site separately (within-site analysis). We evaluated the maximum surface conductance G smax and parameters related to the response of G s to the environments. We found that these values differed among the locations. Second, we applied the model for pooled G s data from all the sites and calculated a common parameter set (pooled analysis). In the pooled model, the difference in the observed G smax among the sites was expressed as a function of soil water content and the leaf area index. In addition, the responses of G s to radiation, vapour pressure deficit and air temperature in different forests were also closely represented by common response functions. As a result, the pooled model could estimate the variation of G s at each site using one parameter set with similar precision to the within-site models. These results suggest that the surface conductance of the various mature forests had the same maximum value and response properties, although we were not able to verify this. Our new parameterisation concept for pooled G s data should be effective for simultaneous evaluations of the water, energy and CO2 exchanges of forests over wide regions. [Copyright &y& Elsevier]

Abstract: We measured sensible and latent heat fluxes (H and λE) in five forests located in boreal, cool-temperate, and warm-temperate zones of the Far East concurrently over several years to clarify their energy-consumption characteristics and the variation in and factors controlling evapotranspiration. The consumption of energy for evapotranspiration was larger at the southern sites than at the northern sites, and evapotranspiration in summer (July–August) was larger (average 2.9mmday−1) for temperate forests than for boreal forests (average 1.7mmday−1). Differences in energy-consumption characteristics between the forest types (e.g., deciduous vs. coniferous) were not as distinct as those by location. This inter-locational difference resulted from differing evapotranspiration restrictions caused by land-surface characteristics, rather than differing atmospheric evaporation demand. [Copyright &y& Elsevier]

We used an aerodynamic method to objectively determine a representative canopy height, using standard meteorological measurements. The canopy height may change if the tree height is used to represent the actual canopy, but little work to date has focused on creating a standard for determining the representative canopy height. Here we propose the ‘aerodynamic canopy height’ h a as the most effective means of resolving the representative canopy height for all forests. We determined h a by simple linear regression between zero-plane displacement d and roughness length z 0, without the need for stand inventory data. The applicability of h a was confirmed in five different forests, including a forest with a complex canopy structure. Comparison with stand inventory data showed that h a was almost equivalent to the representative height of trees composing the crown surface if the forest had a simple structure, or to the representative height of taller trees composing the upper canopy in forests with a complex canopy structure. The linear relationship between d and z 0 was explained by assuming that the logarithmic wind profile above the canopy and the exponential wind profile within the canopy were continuous and smooth at canopy height. This was supported by observations, which showed that h a was essentially the same as the height defined by the inflection point of the vertical profile of wind speed. The applicability of h a was also verified using data from several previous studies. [ABSTRACT FROM AUTHOR]