The analysis of groundwater recharge in Mongolia using vadose zone modeling

Knowledge of groundwater recharge (GR) is vital for optimal water resources management under an arid continental climate. However, in vast territories such as Mongolia, direct measurements of GR are unfeasible because they mandate excessive costs, stemming from time-consuming and labor-demanding efforts. A valid alternative to direct measurements is numerical models based on the monitoring of precipitation (P) and evapotranspiration (ET) for simulating GR. While direct measurements of ET are logistically problematic and unpractical for large-scale applications, a reliable prediction may be derived from crop reference evapotranspiration (ET0) which is calculable from limited data and will feed numerical models to evaluate a (pseudo) realistic GR as output. The crop reference evapotranspiration (ET0) was calculated employing the Hargreaves (Har) temperature-based ET0 method that closely simulated the internationally recognized standard FAO Penman-Monteith (FAO-56 PM) method (calculated with available data at limited locations). The set of weather data required for FAO-56 PM is still mostly unavailable or not easily accessible in data-limited countries such as Mongolia. The Har temperature-based method showed good potential to replace FAO-56 PM in the region according to our analysis. A time-variable and spatially-variable crop coefficient (Kc) was used to convert Har ET0 into a biome-specific potential evapotranspiration (ETp) for 41 study locations. However, there were no readily available estimates of Kc in natural vegetation specific to Mongolia. A dynamic (time-variable) radiation-dependent (in Gobi Desert) or LAI-dependent (in steppe) Kc was adopted from the literature and used for the first time in Mongolia. The LAI dependent Kc was also adjusted due to the climate features of the region. The developed Kc values are important to convert ET0 to ETp with consideration of region’s climate and any factors affecting the vegetation. The mean annual ET0 ranged from 685 mm to 1129 mm, while the ETp ranged from 147 mm to 695 mm. The GR rates were calculated using the estimated ETp as input in the HYDRUS-1D numerical vadose zone model for 41 study locations across Mongolia. The mean annual GR rates were smaller than 12 mm in study locations and the GR tends to decrease when vegetation cover increases. Advisors: Vitaly A.Zlotnik and Erin Haacke