UC Davis

Abstract Flood managed aquifer recharge (Flood‐MAR) is an emerging practice to enhance groundwater sustainability through recharge of aquifers. However, in agricultural systems Flood‐MAR may harm groundwater quality, given residual soil nitrate (NO 3 − ). This research evaluated Flood‐MAR NO 3 − leaching risk across a precipitation and soil textural gradient in a globally important agricultural region, California's Central Valley, and whether Flood‐MAR timing strategies could mitigate the risk. Using multi‐decadal root zone water quality model simulations of irrigated and fertilized maize (250 kg N ha −1 year −1 ) on well‐drained soils as a representative case‐study, results suggest Flood‐MAR can be used with near‐negligible additional NO 3 − leaching in locations with median annual precipitation >400 mm year −1 . In those locations, wet‐year precipitation leached most residual NO 3 − without practicing Flood‐MAR. At drier locations, Flood‐MAR NO 3 − leaching risk increased most clearly in loamy soils. Additional NO 3 − leaching risk increased in drier climates because minimal precipitation‐driven deep percolation maintained residual NO 3 − accumulation across growing seasons. In fine‐texture soils, NO 3 − leaching risk was mitigated by denitrification, preventing residual NO 3 − accumulation. Flood‐MAR practices diminished denitrification, leaching NO 3 − rapidly when soils were colder and biogeochemically inactive, and thereby decreased growing season denitrification when soils were warmer and denitrification rates higher. Effects of Flood‐MAR timing strategies (January Flood‐MAR vs. March Flood‐MAR), combined with variable pauses among applications (3‐ vs. 7‐ vs. 21‐day intervals) were negligible. Infrequent Flood‐MAR should be practiced with care in arid climates and especially after prolonged droughts coinciding with more limited irrigation water supplies that constrain salt‐leaching practices all favoring residual NO 3 − accumulation. Core Ideas Modeling suggests NO 3 − leaching risk during flood managed aquifer recharge (Flood‐MAR) varies by texture and climate. NO 3 − leaching risk diminished from Flood‐MAR in locations with annual precipitation >400 mm year −1 . Flood‐MAR should be practiced with care in arid climates and especially after prolonged droughts. Flood‐MAR timing strategies (early vs. late season) and durations (3‐ vs. 7‐ vs. 21‐day intervals) were negligible. Plain Language Summary Agricultural management of floodwater (Flood‐MAR) is a new practice where floodwaters are applied to agricultural fields to recharge groundwater. Flood‐MAR may harm groundwater quality by leaching soil nitrate (NO 3 − ) into groundwater. This modeling study evaluated the Flood‐MAR NO 3 − leaching risk in different climates and soil textures in California. It evaluated whether Flood‐MAR timing strategies (early‐ vs. late‐season irrigation application strategies) influenced risk. Flood‐MAR had near‐negligible NO 3 − leaching risk in locations with rainfall >400 mm year −1 . At drier locations, Flood‐MAR NO 3 − leaching risk was highest (especially in loamy soils) because low rainfall allowed NO 3 − to build up in soils from year to year. Different Flood‐MAR timing strategies (early season vs. late season), combined with variable pauses in water applications (3‐ vs. 7‐ vs. 21‐day intervals), showed no difference in NO 3 − leaching. Flood‐MAR should be practiced with care in arid climates.