Aspect-controlled spatial and temporal soil moisture patterns across three different latitudes

Soil moisture in semi-arid areas plays a critical role as it regulates numerous ecohydrologic processes in land surface hydrology, subsurface hydrology, and vegetation dynamics. Studies on soil moisture distribution and dynamics currently rely on data obtained using three types of approaches: In situ (generally point-scale) measurements, remotely sensed observations, and modelling approaches. The spatial variability of soil moisture plays a vital role in the estimation of land surface fluxes (evapotranspiration (ET) and runoff) due to the non-linear relationship between soil moisture and the associated physical processes. Understanding this variability is essential for the optimal management of water resources and ecosystem sustainability. Although a considerable amount of work has been done on the subject, the ability to understand and characterize the mechanisms that determine the distribution patterns of soil water content still remains a challenge at the centre of hydrological research, especially for ungauged catchments. It is necessary to understand the spatial variability of soil moisture and its influencing factors, which will provide a basis to improve our understanding of hydrological, biogeochemical processes, and lateral and subsurface flow processes. The effects of several factors that control soil moisture variability (SMV) in semi-arid landscapes (microclimate, vegetation, topography, soil depth, soil texture, etc.) have been documented in previous work. However, the control of latitude on SMV under different environmental conditions still remains poorly understood. Latitude significantly affects the availability of water and energy as the global distribution of solar radiation varies from the equator to higher latitudes. Latitude has a dominant control on the availability of water because of the varying amount of solar radiation on north-facing slopes (NFS) and south-facing slopes (SFS), which influences soil moisture variations. This study focusses on evaluating and comparing the effect of latitude on SMV, and its control on soil moisture patterns. To this end, we use a modelling framework to capture the joint effects of aspect and latitude on SMV. We used the Bucket Grassland Model (BGM), equipped with a vegetation dynamics component, to analyse soil moisture patterns and variability at various latitudes (45°N, 34°N, and 15°N). The main objective of this study is to investigate changes in soil moisture patterns at various latitudes and differences in SMV on the different aspects for a synthetic domain. We conducted different simulations as a sensitivity analysis (at various latitudes) using BGM to study the effect of aspect-related soil moisture variations in a semi-arid landscape. The latitudinal patterns of modeled soil moisture are analysed, and distinct variations are identified in the SMV. The results show that water stress varies with aspect and are affected by latitude, which in turn affect the SMV. Further, they show that SMV increases moving towards higher latitudes. Also, aspect-related soil moisture differences are enhanced at higher latitudes. Therefore, it is not possible to characterize soil moisture variations or model surface hydrological processes at the catchment scale, without explicitly accounting for aspect, particularly in ecosystems where the aspect has a dominant effect.