Response and contribution of shallow groundwater to soil water/salt budget and crop growth in layered soils

Shallow groundwater affects crop growth and yield by altering the root zone water and salt budgets, with the process being more complicated in layered soils. To gain insights into the role of shallow groundwater in layered soils, LAWSTAC, a physically-based agro-hydrological model, was employed to simulate soil water, salt, and groundwater fluxes and crop yield in three soil profiles: loam (L), loam with a sand interlayer (LSL), and loam with a clay interlayer (LCL). The model was verified using data collected in 2007 and 2008 from a spring wheat field with shallow groundwater tables. The model predictions matched well with the measured soil water content, salt concentration, groundwater table depth (GWD), leaf area index, biomass, and crop yield. Further simulations were conducted for the L, LSL, and LCL profiles with varying GWDs in the crop growth seasons and initial values specified at 130, 150, 170, 190, 210, 230, and 250 cm. The groundwater table showed greater fluctuations in LCL, with smaller fluctuations in LSL in comparison to L. When starting with smaller initial GWDs (of 130, 150, 170, and 190 cm), the coarse-textured soil under the root zone facilitated water and salt fluxes at the bottom of the root zone, resulting in larger averages for root zone water storage and more salt content during the crop growth seasons and lower crop yield due to salt stress. Meanwhile, in cases with an initial GWD larger than 210 cm, the coarse-textured soil inhibited the fluxes and improved crop yield due to the reduced salt stress. Fine-textured soil under the root zone inhibited water and salt fluxes, reducing root zone salt content and favoring crop yield; however, the effect decreased with larger initial GWDs. Thus, the groundwater table should be lowered to a level where coarse-textured soil acts as a capillary barrier for salt control and crop growth in soil profiles similar to LSL. Keeping a high groundwater table to sustain water for crop growth is acceptable in soil profiles similar to LCL, while keeping a lower groundwater table could be an effective practice to control salt and improve crop production in salinized croplands.