Potential to assess nitrate leaching vulnerability of irrigated cropland

Irrigated cropland represents the largest source of groundwater nitrate pollution in the Central Valley (CV) of California. Mitigation, through the use of best management practices that maximize crop nitrogen use efficiency (NUE), will be most effective in reducing pollution if used where the risk of nitrate leaching loss is greatest. The University of California's Nitrate Groundwater Pollution Hazard Index (HI) tool was used to map the risk of nitrate leaching below the rootzone in irrigated fields in a portion of the CV. The HI is an expert system that calculates an index value based on soil properties, crop characteristics, and type of irrigation system in use. Depth to groundwater, aquifer recharge rate, and actual farm management practices (e.g., rate of N fertilizer applied) are not considered in the calculation. Application of the HI to 1,318,000 ha of irrigated cropland in the four southernmost counties of the CV revealed that 31% of the area is at high risk of nitrate leaching loss if not managed carefully. Adoption of drip or microsprinkler irrigation on all orchards, vineyards, and vegetable fields would decrease the area rated as most vulnerable from 31 to 20% of the area analyzed. Crop fields on permeable soils and/or irrigated by surface gravity methods contributed the most to the area at high risk. The HI can help the USDA, regulatory agencies, and cooperative extension target regulatory, research, and education efforts.