Fluxes through the bottom boundary of the root zone in silty soils: Parametric approaches to estimate groundwater contribution and percolation

The paper revises empirical and semi-empirical approaches used with water balance models to estimate the capillary rise from a water table and the deep percolation through the bottom of the root zone and describes simple parametric functions to be used with the water balance simulation model ISAREG. The relationships between upward flux, water table depth, soil water storage and crop evapotranspiration were developed from analysing results of simulations with the deterministic flux model WAVE. Field experiments with wheat and maize were used to calibrate this model and to validate the new approaches with the ISAREG water balance model. The application refers to a silty soil in the North China Plain, which hydraulic properties were studied through in situ and laboratory measurements. Observations included the soil water content, the soil water potential and the water table dynamics. The validation of the approach was performed by comparing ISAREG simulations with observed soil water contents during periods when upward or downward fluxes were observed; the resulting regression coefficient is close to 1.0 and the average relative error of estimate, ARE, is 1.38%. The cumulated upward fluxes computed with ISAREG and WAVE for a 3-month period are similar, respectively, 27 and 31 mm. Simulations with ISAREG were performed for other soils to analyse the impacts on upward fluxes when the model parameters are changed to be applied to different soils. Results for the validation of the percolation approach when comparing simulated and observed soil water content data for a maize crop season produced a coefficient of regression also close to 1.0 and an ARE of 2.18%. It could be concluded that the proposed simulation approaches for the groundwater contribution and percolation produce accurate predictions of the daily soil water content in presence of a water table for irrigation scheduling purposes.