Modeling the sink term under variable soil water osmotic and pressure heads

Abstract

Water extraction in vegetated areas plays a significant role in subsurface hydrology. The boundary between soil and roots is a major hydrological interface in that over 50 percent of the water that returns to the atmosphere from the soil crosses the soil root interface. In order to quantify such processes, the major difficulty in solving Richards' equation, either numerically or analytically stems from the lack of a sink term function that adequately accounts for the extraction term especially under variable soil water osmotic and pressure heads. The objective of this study was to verify the available models for such conditions and to investigate an alternative approach if unsatisfactory results obtained. Consequently, the existing macroscopic reduction functions for combined osmotic and pressure heads were used and tested against the experimental data. Further, some uncertainties involved in modeling are discussed. Alfalfa was grown in packed cylindrical loamy sand soil columns in the greenhouse and subjected to different levels of salinity and water stress simultaneously. Variations of soil water content, soil water pressure head, and osmotic head distributions in the root zone were obtained by varying the quantity of applied water, irrigation intervals, and irrigation water salinities. The collected experimental data under separate and combined stresses are compared and the available concepts are tested based on the mean soil solution osmotic and pressure head data. the experimentally obtained data from separate stress treatments were used to derive the parameter values needed for the calibration and simulation with hysteretic water and solute transport in the root zone model, HYSWASOR. When experimentally derived parameters were used in the simulation model for the separate stress treatments, calibration was still needed. No calibrations were made for simulations under joint stresses. Under combined variable osmotic and pressure heads, the additive and multiplicative reduction functions were first tested against the experimental data and then inserted in the simulation model. A newly developed combined reduction function that differs conceptually from the available classical functions was used in simulation model. Both experimental and simulated water contents, electrical conductivities and transpired water showed that the latter, which is neither additive nor multiplicative, fits the data range best. However, the small discrepancies can be related to the specified hydraulic functions of rootless soil and water compensation during the stress period.