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Soil water potential (SWP) is vital for controlling the various biological and non-biological processes occurring through and across the soil-plant-atmosphere continuum (SPAC). Although the dynamics and mechanisms of SWP have been investigated for several decades, they are not as widely explored in ecohydrology research as soil moisture, due at least partly to the limitation of field observation methods. This limitation restricts the understanding of the responses of plant physiology and ecological processes to the SWP gradient and the ecohydrological functions of SWP dynamics in different contexts. Hence, in this work, we first briefly revisit the origin and development of the concept of SWP and then analyze the comprehensive factors that influence SWP and the improvement of SWP observation techniques at field scales, as well as strategies for developing new sensors for soil water status. We also propose views of focusing on the response characteristics of plant lateral roots, rather than taproots, to SWP dynamics, and using hormone signaling research to evaluate plant response signals to water stress. We end by providing potential challenges and insights that remain in related research, such as the limitations of the SWP evaluation methods and the future development direction of SWP data collection, management, and analysis. We also emphasize directions for the application of SWP in controlling plant pathogens and promoting the efficiency of resource acquisition by plants. In short, these reflections revisit the unique role of SWP in eco-hydrological processes, provide an update on the development of SWP research, and support the assessment of plant drought vulnerability under current and future climatic conditions. [ABSTRACT FROM AUTHOR]

• Insolation differences among the aspects amplify the variability in soil moisture. • Soil moisture and vegetation variability are sensitive to geomorphic factors. • Aspect-related vegetation differences are more pronounced in smaller contributing area. • Landscapes generated under low uplift rates display less spatial variability. Landscapes evolve through nonlinear interactions between soil, vegetation, and climate. In semi-arid ecosystems, soil moisture variability (SMV) and vegetation variability (VV) can be strongly related to landscape organisation induced by differences in insolation on opposing north-facing slopes (NFS) and south-facing slopes (SFS). Due to its complex interactions with various processes and factors, soil moisture and vegetation exhibit significant variability in both space and time. In this study, the Channel-Hillslope Integrated Landscape Development (CHILD) landscape evolution model (LEM), coupled to a dynamic vegetation model (BGM) and equipped with a spatially distributed solar radiation component is used. The model is used to investigate the implications of various soil, climatic, and geomorphic factors on SMV and VV over landscapes with different characteristics. The analysis of model results indicates that SMV and VV are more sensitive to changes in geomorphic (hillslope diffusion and uplift rate) and climate (solar radiation, precipitation) factors than to soil hydrologic factors (anisotropy, porosity, infiltration capacity, root depth, and pore size distribution) considered in this study. Spatial variability increases with decreases in hillslope diffusion, and with increases in uplift rates and latitude, while temporal variability has the same response to those factors, and also increases with precipitation. All of these factors contribute to larger difference in condition on NFS and SFS, which ultimately is reflected in SMV and VV. Slope-area, soil moisture-area, and vegetation-area relationships revealed that the difference in SMV and VV between NFS and SFS is more pronounced for smaller contributing areas, where NFS are steeper than SFS. They also show that the temporal variability of soil moisture and vegetation is less in SFS than in NFS. [ABSTRACT FROM AUTHOR]

Previous studies on semi‐arid ecosystems have shown high values of soil moisture variability (SMV) primarily induced by the combined effects of non‐uniform precipitation, incoming solar radiation, and soil and vegetation properties. However, the relative impact of these various factors on SMV has been difficult to evaluate due to limited availability of field data. In addition, only a limited number of studies have analysed the role of landscape morphology on SMV. Here we use numerical simulations of a simple hydrological model, the Bucket Grassland Model, to systematically analyse the effect of each contributing factor on SMV on two different landscape morphologies. The two different landform morphologies represent landscapes dominated respectively by either diffusive erosion or fluvial erosion processes. We conducted various simulations driven by a stochastically generated 100‐year climate time series, which is long enough to capture climatic fluctuations, in order to understand the effect of various soil moisture controlling factors on the spatiotemporal SMV. Our modelling results show that the fluvial dominated landscapes promote higher spatial SMV than the diffusive dominated ones. Further, the role of landform morphology on SMV is more pronounced in regions where the spatial variability of incoming solar radiation and precipitation is high. [ABSTRACT FROM AUTHOR]

Water-limited ecosystems are characterized by considerable soil moisture variability (SMV) induced by the presence of a non-uniform vegetation distribution and climatic conditions, as well as the combined effects of aspect, topography, and incoming solar radiation. Partitioning of solar radiation and topographic influence is of utmost importance to know the exact source of variability in the soil moisture. To increase our understanding on this control, we used two different synthetic domains to conduct our numerical simulations that capture the joint effect of aspect and other key environmental factors on SMV. The topography of two different domains consisting of diffusion dominated (convex hillslopes) and fluvial dominated (concave hillslopes) landscapes were used. For each domain, we assume isotropic and homogeneous soil properties throughout the catchment. The sensitivity of different factors that were identified as relevant for SMV were investigated for spatially varied and uniform radiation. Simulations were performed by varying different factors within the prescribed range from the literature, and the coefficient of variation (CV) for mean soil moisture computed for analysis. We found that incoming solar radiation plays a vital role in determining SMV. In addition, SMV is exaggerated moving towards the higher latitude due to the latitudinal variation of incoming solar radiation for spatially varied radiation case. Further, fluvial dominated landscapes have higher coefficient of variation than diffusion dominated landscapes as heterogeneity in incoming solar radiation is more at steeper slopes. Heterogeneous precipitation over the landscape leads to significant variation in soil moisture for both spatially varied and uniform radiation cases. Consequently, heterogeneous precipitation with spatial varied solar radiation associated with the effect of topography could lead to the enhancement of spatio-temporal variability. These outcomes show: (1) the role of solar radiation is the major source of soil moisture heterogeneity; (2) the variability is modulated by topography under uniform soil properties and rainfall; (3) uniform changes in soil properties slightly affect SMV. [ABSTRACT FROM AUTHOR]

Contrasts in insolation lead to the development of aspect-controlled ecosystems characterized by heterogeneity in vegetation type and density in semi-arid ecosystems. Vegetation dynamics are strongly influenced by the availability of soil moisture on opposing hillslopes in semi-arid landscapes. Aspect controls on vegetation type and density have long fascinated ecologists as well as geomorphologists, and they mostly claim that polar-facing slopes (PFS) have denser vegetation than equator-facing slopes (EFS). While this observation characterizes the mean state of ecosystems, here we examine the seasonality of aspect-controlled ecosystems to gain insights about the vegetation protection for erosion. Here, we extracted NDVI values derived from Landsat 5, 7, and 8 (obtained from Google Earth Engine) for a 17 year period for 80 different catchments across the world having a size from 5 to 200 square km, where aspect-controlled vegetation has been reported. For each of the 80 catchments, we calculated the mean monthly NDVI values for PFS and EFS. Patterns of monthly mean NDVI reveal two broad categories for aspect dependent ecosystems; 1) higher NDVI on PFS all-year-round; (2) higher NDVI on EFS during the winter season. Higher NDVI on PFS is attributed to less insolation preserving valuable moisture in this semi-arid landscapes. One explanation for the observed winter reversal in NDVI may be attributed to preferential snow cover on PFS. The spatial NDVI analysis at such sites shows the seasonal reversal of NDVI on opposing hillslopes. Using the NDSI (normalized difference snow index) datasets for the last 17 years and air temperature records, we show that snow cover is absent from some sites still displaying a winter reversal in NDVI contrasts. Another explanation for this NDVI reversal in snow-free sites is favourable winter growing conditions for vegetation when the wet season in Mediterranean climate meets with relatively higher insolation on EFS. We suggest the winter reversal in NDVI in those sites can be ascribed to vegetation functionality and relatively higher insolation on EFS in winter with available soil moisture. These findings show that in aspect-controlled ecosystems, seasonal reversals in erosion protection provided by vegetation can alter erosion mechanisms on PFS and EFS. [ABSTRACT FROM AUTHOR]

• A soil moisture data-driven method for ET a was tested for the heterogeneous soils. • The soil hydraulic parameters were optimized using a genetic algorithm procedure. • ET a at sites of different soil textures and crop species were accurately estimated. • The inaccurate soil parameters and stratification may bias the estimates of ET a. • The lowest sensor should be placed away from the interface of stratified soils. A soil moisture data-driven method through inversely solving the Richards' equation was tested, and proved to be a reliable alternative estimator of actual evapotranspiration (ET a) for relatively homogeneous soils in an arid region. Few studies have examined the feasibility of the method using soil moisture for soils with more heterogeneities, which may lead to biased parameters and varied root depths, and thus inaccurate estimates of ET a. We used a genetic algorithm to optimize the parameters most sensitive to ET a estimation, then estimated daily and seasonal ET a using hourly soil moisture data, measured from four sites (from surface to 2-m depth at 20-cm intervals) with different soil textures and crop species in desert-oasis farmlands in northwest China. The seasonal ET a values were 487, 574, 436, and 377 mm at the sites cropped with field maize, spring wheat, seed maize, and seed maize, respectively, in 2018. The daily rates from one site were validated against the eddy-covariance measurements, and the daily rates from all the sites were compared against the estimations from a simple water balance method, and both showed good consistency (R2 = 0.72-0.93). The inverse method has great potential for relatively accurate estimates of ET a in the middle and late growing stages but may induce a slight underestimation in the initial growing season. To determine the minimum monitoring depths of sensors for relatively accurate estimates, ET a values were estimated using soil moisture data at various observation depths, and were compared with eddy-covariance measurements and hypothetical ET a -observed depth curves. The results showed that inaccurate stratification of soil types in the profiles may cause great uncertainty in the estimates, and if texture-contrasting interlayers exist in the profiles, the lowest soil moisture sensor should be placed sufficiently deep, with all the interlayers fully included. [ABSTRACT FROM AUTHOR]