Abstract
Abstract. Dryland regions are characterised by water scarcity and are facing major challenges under climate change. One difficulty is anticipating how rainfall will be partitioned into evaporative losses, groundwater, soil moisture, and runoff (the water balance) in the future, which has important implications for water resources and dryland ecosystems. However, in order to effectively estimate the water balance, hydrological models in drylands need to capture the key processes at the appropriate spatio-temporal scales. These include spatially restricted and temporally brief rainfall, high evaporation rates, transmission losses, and focused groundwater recharge. Lack of available input and evaluation data and the high computational costs of explicit representation of ephemeral surface–groundwater interactions restrict the usefulness of most hydrological models in these environments. Therefore, here we have developed a parsimonious distributed hydrological model for DRYland Partitioning (DRYP). The DRYP model incorporates the key processes of water partitioning in dryland regions with limited data requirements, and we tested it in the data-rich Walnut Gulch Experimental Watershed against measurements of streamflow, soil moisture, and evapotranspiration. Overall, DRYP showed skill in quantifying the main components of the dryland water balance including monthly observations of streamflow (Nash–Sutcliffe efficiency, NSE, ∼ 0.7), evapotranspiration (NSE > 0.6), and soil moisture (NSE ∼ 0.7). The model showed that evapotranspiration consumes > 90 % of the total precipitation input to the catchment and that < 1 % leaves the catchment as streamflow. Greater than 90 % of the overland flow generated in the catchment is lost through ephemeral channels as transmission losses. However, only ∼ 35 % of the total transmission losses percolate to the groundwater aquifer as focused groundwater recharge, whereas the rest is lost to the atmosphere as riparian evapotranspiration. Overall, DRYP is a modular, versatile, and parsimonious Python-based model which can be used to anticipate and plan for climatic and anthropogenic changes to water fluxes and storage in dryland regions.
Highlights
Drylands are regions where potential evapotranspiration far exceeds precipitation and where water is scarce
The modelling results show a good agreement between DRYland Partitioning (DRYP) and MODFLOW, and both models ran with negligible mass balance errors
More fluctuations are observed in the MODFLOW simulations, which can be attributed to the time step used for the simulation, which needs to be reduced in order to smoothly capture the variation in water table depth as the model switches boundary conditions (Eq 41)
Summary
Drylands are regions where potential evapotranspiration far exceeds precipitation and where water is scarce. Precipitation events are characterised by high-intensity and low-duration rainfall over restricted spatial areas (Pilgrim et al, 1988). This results in a highly dynamic hydrological system prone to flash flooding, and to water scarcity and food insecurity, societal risks that are exacerbated by climate change, population growth, and dryland expansion (Reynolds et al, 2007; Giordano, 2009; Siebert et al, 2010; Taylor et al, 2012; Huang et al, 2015, 2017; Wang et al, 2017; Cuthbert et al, 2019a)
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