Modeling Large Water Infiltration Events in Small Plots Using the 1-D Finite Water-content Method and Numerical Solutions to the Richards' Equation.

Abstract:

The ability of soil to infiltrate large volumes of water is fundamental to managed aquifer recharge (MAR) when using infiltration basins or agricultural fields. In order to investigate the feasibility of using agricultural fields for MAR we conducted a field experiment designed to not only assess the resilience of alfalfa (Medicago sativa) to large (300 mm), short duration (1.5 hour), repeated irrigation events during the winter but also how crop resilience was influenced by soil water movement. We hypothesized that large irrigation amounts designed for groundwater recharge could cause prolonged saturated conditions in the root-zone and yield loss. Tensiometers were installed at two depths (60 and 150 cm) in a loam soil to monitor the changes in soil matric potential within and below the root-zone following irrigation events in each of five experimental plots (8 x 16 m²). To simulate the individual infiltration events we employed the HYDRUS-1D computational module (Simunek et al., 2005) and compared the finite-water content vadose zone flow method (Ogden et al. 2015) with numerical solutions to the Richards' equation. For both models we assumed a homogenous and isotropic root zone that is initially unsaturated with no water flow.  Here we assess the ability of these two models to account for the control volume applied to the plots and to capture sharp changes in matric potential that were observed in the early time after an irrigation pulse. The goodness-of-fit of the models was evaluated using the root mean square error (RMSE) for observed and predicted values of cumulative infiltration over time, wetting front depth over time and water content at observation nodes. For the finite-water content method, the RMSE values and output for observation nodes were similar to that from the HYDRUS-1D solution. This indicates that the finite-water content method may be useful for predicting the fate of large volumes of water applied for MAR. Moreover, both models suggest a relatively short residence time of that water in the soil profile which may explain the lack of observed deleterious effects on the crop.