ABSTRACT Sap flux density can be used as a surrogate of plants' transpiration rate; thus, measurements of sap flux density and the corresponding modelling efforts greatly improved our understanding of ecosystem transpiration. This study measured sap flux density every 10 min for three representative tree species (eight oak, Quercus variabilis ; four poplar, Populus adenopoda ; four pine, Pinus massoniana ) of a subtropical forest ecosystem in the Hanjiang River Basin from January 2021 to December 2023. The responses of nighttime sap flux density to major environmental factors were then analysed. Vapour pressure deficit (VPD) is the most important driving factor of nighttime sap flux density for all the three tree species, explaining 12%, 27% and 51% of the variation for pine, oak and poplar, respectively; temperature (Ta) and relative humidity (RH) can explain up to 29% of the variation; soil water content (SWC) showed relatively weaker control and explained up to 14% of the variation. Then three nighttime sap flux density models were compared in this study: regression, exponential and distribution models. In particular, the regression model was established using the stepwise multiple linear regression method based on the environmental responses of nighttime sap flux density; the exponential model was proposed using an exponential decay curve by assuming the continuity of nighttime sap flux density series; the distribution model was built on the positive relationship between accumulative nighttime sap flux and daytime sap flux. The regression model can explain 12%–70% of the variation in nighttime sap flux density, the distribution model can explain 22%–72%, and the exponential model can explain −82% to 62%. When evaluating the three models using root mean square error (RMSE) and the Akaike information criterion (AIC), the distribution model performed the best with the lowest RMSE and AIC. The findings advance our understanding of tree transpiration of this region and lay a foundation for modelling efforts of nighttime sap flux density.
ABSTRACT Changes in climate conditions and land‐use patterns exert significant influences on hydrological processes in agriculture‐dominated watersheds, with direct implications for regional water and food security. Although afforestation has been implemented in many agriculture‐dominated watersheds in the United States, the response of key hydrological processes to afforestation under future climate conditions remains poorly understood. In this study, we simulated the dynamics of three major hydrological variables–evapotranspiration (ET), groundwater recharge and water yield–in the Lower Mississippi Alluvial Valley (a humid river basin) using the SWAT+ model under 265 scenarios combining future climate conditions and afforestation levels. Our results show that, relative to the early 21st century (2000–2019), climate change alone is projected to decrease water yield by 1.4%–2.2% ( p > 0.05) and groundwater recharge by 15.1%–21.3% ( p < 0.001), while increasing ET by 7.2%–10.6% ( p < 0.001) by the late 21st century (2080–2099). Afforestation is also expected to have significant impacts on watershed hydrological processes. For instance, a 20% expansion in forest cover is projected to decrease water yield by 7.2%, while ET and groundwater recharge are projected to increase by 3.6% and 6.2% under SSP245, respectively. These findings indicate that large‐scale afforestation in humid regions can significantly reduce water yield while increasing groundwater recharge to the shallow aquifer water storage. Afforestation planning should therefore be spatially targeted and coordinated with irrigation and groundwater management to balance recharge gains against reductions in water yield.
Vegetation has been found effective in the mitigation of tsunamis and floods. Nature-based solutions employing vegetation are increasingly promoted for sustainable flood mitigation; however, the hydraulic role of vegetation structural attributes remains insufficiently quantified under flood-scale flow conditions. This study investigates the ecohydrological influence of vegetation crown height on inland flood dynamics through controlled laboratory experiments conducted over a range of Froude numbers representative of subcritical to near-critical flood flows. Physical experiments were performed to investigate parameters like water surface profile, backwater rise, flood energy reduction, fluid and moment force on trees and time-lapse delay under varying crown height and Froude number conditions. A real-world vegetation model (Eucalyptus tree) and flood conditions (Pakistan) were replicated using the concept of geometric and Froude similarity. Two vegetation densities (sparse: describes scattered with limited plant cover with significant portions of ground showing, and dense: means plants are tightly packed with little bare ground visible, creating a thick canopy), thicknesses and multiple crown heights (3d-7d (d: vegetation element diameter)) of vegetation were utilized in this study. The results demonstrated that by increasing crown height, the water retention capacity of the vegetation on the upstream side increased up to 87% due to vegetation arrangement. Further, it was observed that a maximum energy reduction of 45% was achieved by increasing vegetation thickness. It was also observed that by increasing crown height from WCS (without crown height for sparse) to 3d-S (S: sparse) and 3d-D (D: dense), the fluid force index value decreased up to 58% and 83% for sparse and dense vegetation respectively. A significant delay in floodwater arrival was observed (37%) in the case of a vegetation model with lower crown height compared with other conditions. The findings of the current research demonstrated a significant impact of crown height variation in flood risk management.
Determining environmental water requirements and ecohydrological thresholds is essential for sustaining wetland functionality in semi-arid regions subject to expanding water withdrawals and climatic variability. This study presents an integrated system-dynamics framework to identify thresholds and water requirements in the Ramsar-listed Alagol, Ulmagol and Ajigol (AUA) wetland complex in northeastern Iran. The framework couples daily water balance, wetland morphometry, vegetation dynamics, anthropogenic withdrawals and waterbird habitat responses within a STELLA modelling environment. Long-term seasonal and temporary inundation boundaries were delineated using a multidecadal satellite observation time series (1986-2025) and field evidence to define functional hydrological thresholds, whereas volume-area-elevation relationships converted simulated storage into habitat availability. The model was calibrated for the primary basin (Alagol Wetland) using daily hydrometric and remote-sensing observations from 2002 to 2025. Scenario simulations (2026-2040) indicate that anthropogenic withdrawals exert a stronger influence on long-term ecohydrological conditions than projected precipitation variability. Increased withdrawals substantially reduced seasonal inundation, shortened hydroperiods and accelerated drying, particularly in the shallower, morphologically non-linear Ajigol and Ulmagol basins. Conversely, reducing withdrawal pressures significantly improved threshold attainment and habitat availability, even under unfavourable climatic projections. The framework quantified critical, optimal and ideal storage states, providing quantitative benchmarks for environmental-flow planning. This reproducible approach links hydrological management to ecological outcomes, offering a transferable tool to support adaptive water allocation in data-scarce semi-arid wetlands globally.
ABSTRACT In recent years, extreme drought and extreme wetness events have occurred with increased frequency, resulting in legacy effects of varying intensity on ecosystem services (ESs) even after the events end, which poses a serious threat to global ecological security. This paper systematically reviews the research progress on the impacts of extreme drought and wetness legacy effects on ESs, focusing on their formation mechanisms, multidimensional impacts and the role of human management. The review shows the following: (1) Research in this field is growing overall, with hotspots regarding extreme drought, provisioning services and regulating services. (2) The impacts of these legacy effects on different ESs are complex and often asymmetric. Provisioning services focus primarily on ecosystem productivity, with forests and grasslands as key research subjects. Regulating services emphasize carbon sequestration and ecosystem resilience, whereas supporting services concentrate more on biodiversity and soil nutrients. Cultural services receive the least attention. (3) Human interventions, such as nutrient management and vegetation structure optimization, can mitigate negative legacy effects. (4) This study highlights future research priorities, including bridging individual physiological responses with aggregated scale analyses to clarify genuine legacy mechanisms, evaluating the overlapping legacy effects of compound extreme events and their cross‐seasonal compensatory mechanisms and determining ecological thresholds for human interventions while expanding mitigation strategies to address the largely unexplored extreme wetness legacies. These insights can support global climate change adaptation and sustainable ecosystem management.
