Minimizing Evaporation by Optimal Layering of Topsoil: Revisiting Ovsinsky's Smart Mulching‐Tillage Technology Via Gardner‐Warrick's Unsaturated Analytical Model and HYDRUS

Ovsinsky (1899, https://www.rulit.me/books/novaya‐sistema‐zemledeliya‐read‐193251‐1.html) suggested and tested a water conserving soil no‐till technology for rain‐snow‐fed field crops in a semiarid environment in southern Russia. We model Ovsynsky's unsaturated flow fragment, in which 1‐D steady evaporation and evapotranspiration through a two‐layered soil from a horizontal static water table to a dry soil surface takes place. Gardner's exponential and algebraic functions are used for the unsaturated hydraulic conductivity‐suction head relations. The vertical evaporation flux depends on the dyads and triads (correspondingly) of the parameters of these functions, for example, the saturated hydraulic conductivity and the sorptive number of the two layers. The flux, as a function of the relative thickness of the upper stratum, is analytically found from the solution of one or two nonlinear equations. This relation can be nonmonotonic and exhibits either a minimum or maximum depending on whether this stratum is coarser or finer than the subjacent stratum fed from a horizontal isobar. HYDRUS‐1D simulations confirm these extrema. This explains the experimental results from the literature on mulching/tillage/soil crusting‐sealing, which can increase, decrease, or have no impact on evaporation from a shallow water table. Alterations of the soil's homogeneity to reduce evaporation losses can improve the hydrological balance of soil profiles. Key Points For a given total thickness of a two‐layered soil the thickness of the upper layer minimizing or maximizing evaporation flux is found Gardner's, Philip's, and van Genuchten's soils are studied in 1‐D flows from a static water table to dry topsoil Analytical solutions of Richards ODEs are juxtaposed with HYDRUS finite element computations