This is the first-of-its-kind study to quantify how legacy nitrogen in agricultural environments responds to changing hydrologic application rates and frequencies under AgMAR.
Scientists, engineers and practitioners seeking to apply AgMAR will find a description of water quality response under different stratigraphic configurations, antecedent moisture conditions and depth to water table.
AgMAR is a promising management strategy wherein surface waters are used to intentionally flood agricultural lands with the purposes of recharging underlying groundwater. However, it is not yet clear how legacy nitrogen that has been built up over the years from fertilizer use in these settings may respond to AgMAR practices, and more importantly, if flooding agricultural sites will enhance nitrate transport to the groundwater or attenuate it by supporting in situ denitrification. This study therefore uses a mechanistic model to evaluate the effects of different AgMAR management strategies (i.e., by varying the frequency, duration between flooding events, and amount of water) on nitrate leaching to groundwater under commonly-observed stratigraphic configurations in agricultural settings. Simulation results indicate that AgMAR is preferable where finer textured sediments exist that act as permanent sinks of nitrate via denitrification. Further, in comparing AgMAR strategies, our results indicate that applying same amount of recharge water all-at-once is desirable than smaller, incremental application but only under specific circumstances (e.g., lower antecedent soil moisture, lack of preferential flow paths). Our study concludes that ideal AgMAR recharge rates can be designed that honor groundwater quality with respect to nitrate, but need to account for underlying stratigraphy, antecedent moisture conditions and depth to water table.
Waterhouse, H., Arora, B., Spycher, N.F., Nico, P.S., Ulrich, C., Dahlke, H.E. and Horwath, W.R., 2021. Influence of Agricultural Managed Aquifer Recharge (AgMAR) and Stratigraphic Heterogeneities on Nitrate Reduction in the Deep Subsurface. Water Resources Research, DOI: 10.1029/2020WR029148.