Using soil hydromorphy degree for adjusting steady-state water table simulations along catenas in semiarid Russia

Abstract

Mapping soil properties has obvious utility for formulating agricultural management regimes. One that is particularly pertinent in areas with shallow water tables in semiarid regions, especially where catchments have both dry and moist parts, is hydromorphy degree. However, there is no convenient method for converting it to hydrological model state variables, which is a crucial first step for mapping (in the absence of extremely thorough and costly monitoring). Here we propose that the steady state continuity equation in kinematic wave form can be parameterized using expert knowledge of the typical water table depth (WTD) for soils with different hydromorphy degrees. To test the approach, six hillslope-based computational units (catenas) were obtained, for use in simulations, by automated of the Samovetc and Izberdey catchments in the Tambov region (Russia) using lumpR software. Five of the six catenas began at poorly drained flat upslope positions with soils with various degrees of saturation by shallow groundwater and one began at a well-drained upslope position. Each of the six hillslopes had a unique catenary sequence of soils, but each represented a typical unit. We genetically classified soils in the sequences according to hydromorphy degree based on redoximorphic features as diagnostic properties. For each resulting group, a historical database summarizing possible positions of the WTD was used. Application of expert knowledge in this manner alone yielded a broad range of possible WTD values (e.g. 1.5–5 m for a semi-hydromorphic soil) for each soil entity, but linking a catena by the fundamental physical constraint of flow continuity enabled narrowing of the range to 0.2–1 m thereby reducing it by ca. 80%. The results show that the approach could substantially improve crop and water management precision.