Estimating Nitrate Leaching to Groundwater from Orchards: Comparing Crop Nitrogen Excess, Deep Vadose Zone Data‐Driven Estimates, and HYDRUS Modeling

Core Ideas Leaching below the root zone is estimated based on eight sites of intensive vadose zone monitoring. Across methods N losses estimated at the annual orchard scale were in the same order of magnitude. Simple N mass balance provided a good proxy of the orchard scale annual N accumulation in the soil Under current BMP N load to groundwater is likely in the range of 60 to 100 kg N ha–1. Large spatial and temporal variability in water flow and N transport dynamics poses significant challenges to accurately estimating N losses form orchards. A 2‐yr study was conducted to explore nitrate (NO 3 − ) leaching below the root zone of an almond [ Prunus dulcis (Mill.) D. A. Webb] orchard. Temporal changes in water content, pore water NO 3 − concentrations and soil water potential were monitored within and below the root zone to a soil depth of 3 m at eight sites, which represented spatial variations in soil profiles within an almond orchard in California. Orchard monthly average NO 3 − concentrations below the root zone ranged from 225 to 710 mg L −1 with mean annual concentration of 468 and 333 mg L −1 for the 2014 and 2015 growing seasons, respectively. Despite the huge variability in pore water NO 3 − concentration between sites, the larger spatiotemporal scale N losses estimated at the annual orchard scale from surface N mass balance, vadose zone based water and N mass balance, flow calculations, and HYDRUS modeling were all on the same order of magnitude (80–240 kg N ha −1 yr −1 ). All methods indicated that most of the N losses occur early in the growing season (February–May) when fertilizer is applied to wet soil profiles. Simple mass balance (i.e., N load applied minus N load removed) provided a good proxy of the annual N accumulation in the soil profile at the orchard scale. Reduction of N losses at the orchard scale would require alternative fertigation and irrigation practices to decrease the difference between the N load removed and the N load applied to orchards.