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Popular evapotranspiration (ET) partitioning methods make assumptions that might not be well‐suited to dryland ecosystems, such as high sensitivity of plant water‐use efficiency (WUE) to vapor pressure deficit (VPD). Our objectives were to (a) create an ET partitioning model that can produce fine‐scale estimates of transpiration (T) in drylands, and (b) use this approach to evaluate how climate controls T and WUE across ecosystem types and timescales along a dryland aridity gradient. We developed a novel, semi‐mechanistic ET partitioning method using a Bayesian approach that constrains abiotic evaporation using process‐based models, and loosely constrains time‐varying WUE within an autoregressive framework. We used this method to estimate daily T and weekly WUE across seven dryland ecosystem types and found that T dominates ET across the aridity gradient. Then, we applied cross‐wavelet coherence analysis to evaluate the temporal coherence between focal response variables (WUE and T/ET) and environmental variables. At yearly scales, we found that WUE at less arid, higher elevation sites was primarily limited by atmospheric moisture demand, and WUE at more arid, lower elevation sites was primarily limited by moisture supply. At sub‐yearly timescales, WUE and VPD were sporadically correlated. Hence, ecosystem‐scale dryland WUE is not always sensitive to changes in VPD at short timescales, despite this being a common assumption in many ET partitioning models. This new ET partitioning method can be used in dryland ecosystems to better understand how climate influences physically and biologically driven water fluxes. Plain Language Summary: We developed a new model to better understand how plants use and lose water in drylands and applied it to seven dryland sites. Our model partitions evapotranspiration—the total water lost to the atmosphere from the Earth's surface— into its components. Evapotranspiration consists of both evaporation from wet surfaces, such as wet soil, and the water lost from plants when they photosynthesize. Currently, models assume a strong relationship between the efficiency with which plants use water ("water‐use efficiency") and the dryness of the atmosphere, but this violates what we know about how plants function in drylands. For example, in drylands many plants are adapted to very dry conditions and their water use can be less sensitive to increasing atmospheric dryness compared to plants from wet environments. Using this new model, we found that plant water‐use efficiency is only correlated with atmospheric dryness some of the time and that evapotranspiration is primary controlled by water lost from plants. This model allows us to better understand the importance of timescale and ecosystem type in governing plant water‐use dynamics and more accurately assess the potential impact of changing climate conditions on dryland water fluxes and ecosystem processes. Key Points: A new evapotranspiration partitioning model (DEPART) was developed using eddy covariance flux tower measurements in a Bayesian frameworkThis method produces daily estimates of transpiration and weekly estimates of plant water‐use efficiency at the ecosystem scaleThis method reveals water‐use efficiency is limited by moisture supply in more arid climates and moisture demand in less arid climates [ABSTRACT FROM AUTHOR]

Abstract: Water relations in plant communities are influenced both by contrasting functional groups (grasses and shrubs) and by climate change via complex effects on interception, uptake, and transpiration. We modeled the effects of functional group replacement and biomass increase, both of which can be outcomes of invasion and vegetation management, and climate change on ecological drought (soil water potential below which photosynthesis stops) in 340 semiarid grassland sites over 30 year periods. Relative to control vegetation (climate and site‐determined mixes of functional groups), the frequency and duration of drought were increased by shrubs and decreased by annual grasses. The rankings of shrubs, control vegetation, and annual grasses in terms of drought effects were generally consistent in current and future climates, suggesting that current differences among functional groups on drought effects predict future differences. Climate change accompanied by experimentally increased biomass (i.e., the effects of invasions that increase community biomass or management that increases productivity through fertilization or respite from grazing) increased drought frequency and duration and advanced drought onset. Our results suggest that the replacement of perennial temperate semiarid grasslands by shrubs, or increased biomass, can increase ecological drought in both current and future climates. [ABSTRACT FROM AUTHOR]

The persistence and geographic expansion of dryland forests in the 21st century will be influenced by how climate change supports the demographic processes associated with tree regeneration. Yet, the way that climate change may alter regeneration is unclear. We developed a quantitative framework that estimates forest regeneration potential (RP) as a function of key environmental conditions for ponderosa pine, a key dryland forest species. We integrated meteorological data and climate projections for 47 ponderosa pine forest sites across the western United States, and evaluated RP using an ecosystem water balance model. Our primary goal was to contrast conditions supporting regeneration among historical, mid-21st century and late-21st century time frames. Future climatic conditions supported 50% higher RP in 2020-2059 relative to 1910-2014. As temperatures increased more substantially in 2060-2099, seedling survival decreased, RP declined by 50%, and the frequency of years with very low RP increased from 25% to 58%. Thus, climate change may initially support higher RP and increase the likelihood of successful regeneration events, yet will ultimately reduce average RP and the frequency of years with moderate climate support of regeneration. Our results suggest that climate change alone may begin to restrict the persistence and expansion of dryland forests by limiting seedling survival in the late 21st century. [ABSTRACT FROM AUTHOR]

The article reports on the study conducted on riverine floodplains in Tagliamento River in Northeast Italy. It is aimed at quantifying large-scale expansion and contraction dynamics of surface waters in the 41.5 kilometers braided section of the river. Researcher measured the discharge and vertical hydraulic gradients at multiple location to gauge surface-subsurface exchange patterns. Results of the research shows that there are two river sections delineated by distinct geomorphic knickpoints and the losing zone undergoes frequent and extensive drying and rewetting cycles. It also shows that frequent and irregular flow pulses brought the rapid expansion of the wetted channel.