Estimating Ag-MAR flood duration based on crop tolerance, root depth, and soil texture data

Abstract:

Agricultural managed aquifer recharge (Ag-MAR) is a recharge technique for groundwater replenishment, in which farmland is flooded during the winter using excess surface water in order to recharge the underlying aquifer. Ag-MAR is currently being implemented in California as part of the efforts to mitigate California’s chronic groundwater overdraft. Root zone residence time, is defined as the duration that the root zone can remain saturated during Ag-MAR without crop damage. It is a key factor in Ag-MAR, as prolonged saturated conditions in the rhizosphere can damage perennial crops due to oxygen deficiency – hypoxia – a well-known situation in agricultural soils under intensive irrigation.

Usually, when Ag-MAR water application starts, aeration of the root-zone will be quickly suppressed by a water-layer covering the soil surface, as it prevents oxygen transport to the root-zone through diffusion in the gas phase. When water application ceases, re-aeration of the root-zone will depend on the soil’s drainage rate that controls the formation of connected air pores between the root-zone and atmosphere. Hence, proper estimation of the planned flood duration during Ag-MAR requires prior knowledge of both crop characteristics and soil texture.

Here we present a simple model to estimate the planned flood duration during Ag-MAR based on 4 main parameters: (1) soil texture; (2) crop-tolerance to waterlogging period; (3) effective root-zone depth; and (4) critical air content. First, soil texture is used to estimate the soil’s hydraulic parameters and the hydraulic conductivity function. Next, hydraulic parameters are used to calculate the wetting period, which is the time to achieve saturation at the effective root-zone depth using the Green and Ampt method (1911); and the drainage period, which is the time to achieve re-aeration of the root-zone using the Sisson’s method (1980, 1991). The estimated water application period equals to: wetting period + crop tolerance period – drainage period.

We will present the model results and a field validation for a few soils, and discuss the model sensitivity to measured and estimated parameters. The implementation of this model in a web-based app is currently under development. Finally, the app is intended to serve as a planning tool for Ag-MAR projects.