Mapping the Risk of Nitrate Leaching From Irrigated Fields by Use of a Nitrate Hazard Index: Case Study in the San Joaquin Valley of California.

We used a Nitrate Hazard Index (HI) developed in 2005 by the University of California Water Resources Center to map the risk of nitrate leaching from crop rootzones in the southern San Joaquin Valley of California.  The total area analyzed was 1,318,000 ha of irrigated cropland, including grapes, tree fruits and nuts, cotton, forages, grains, and vegetables.  The composite HI value is calculated by multiplying together index values for three separate factors: crop species, type of irrigation system (surface gravity, sprinkler, drip, etc.), and soil properties -- and not on actual farming practices.  Crop species and irrigation types were identified using the most recent state Department of Water Resources land use maps of the four counties in the study area (Tulare 1999, Fresno 2000, Kings 2003, Kern 2006).  In the HI, crops are rated according to rooting depth, sensitivity to N deficiency, and other factors.  The HI soil component value for each soil series is based on several properties related to permeability and the potential for denitrification.  In our analysis, soil series were assumed to be the predominant series in USDA-NRCS SSURGO map units.  Complete conversion of fruit, nut, and vegetable crops to drip or microsprinkler irrigation from the earlier (1999-2006) adoption levels would decrease the percentage of the area estimated by the HI as vulnerable to significant nitrate leaching loss from 33% to 22%.  A large proportion of the cropped area remaining at risk of nitrate leaching loss after such a conversion is used to produce silage corn and other forages, which typically receive applications of dairy manure and are irrigated by furrow or border methods. The HI is useful for identifying the type of management practices that would be effective in limiting leaching of nitrate from the crop rootzone to low levels.