The use of HYDRUS-2D to simulate intermittent Agricultural Managed Aquifer Recharge (Ag-MAR) in Alfalfa in the San Joaquin Valley

Agricultural Managed Aquifer Recharge (Ag-MAR) is a potential and sustainable practice where agricultural fields can be used to recharge depleted aquifers using excess precipitation during winter. However, there is little information on the amount of Ag-MAR that can be applied to crops such as alfalfa. HYDRUS-2D was used to estimate the net recharge in an alfalfa field grown on a sandy loam soil in a Mediterranean climate at Parlier, California, USA in 2020-2022. The alfalfa field had four irrigation treatments: full irrigation during summer growing season (March through November), mid-summer deficit irrigation treatment (March to August and complete irrigation cutoff after August cutting), winter flooding treatment, and no winter flooding. Recharge, evapotranspiration (ETa), soil moisture dynamics, and root water uptake were simulated during the recharge period in winter. Previously fully irrigated treatments in summer, followed by winter recharge led to cumulative groundwater recharge of 1459, 1687, and 1415 mm for 2020, 2021, and 2022, respectively. These applications resulted in a net recharge of 85%, 89%, and 84% of the applied irrigation water during the winter period, a significant contribution to groundwater aquifers. Mid-summer deficit irrigation treatments, followed by winter recharge, resulted in net groundwater recharge of 1337, 1498, and 1272 mm for 2020, 2021, and 2022, respectively, amounting to 78%, 79%, and 76% of the applied irrigation water during winter flooding periods. HYDRUS simulation model predicted groundwater recharge potential in these experiments successfully with a coefficient of determination, R2 values of 0.91, and 0.89 for the groundwater recharge during winter flooding after the full irrigation in summer, and the mid-summer deficit irrigation, respectively. These results confirm the potential utilization of HYDRUS simulations in predicting groundwater recharge potential under similar sandy -soil conditions in California's San Joaquin Valley.