UC Davis

Agriculture is responsible for 90% of nitrate leaching to groundwater in the Central Valley. This nitrate comes from excess fertilizer in the soil which is not taken up by crops. One method to reduce excess fertilizer, and therefore nitrate leaching, is by matching the crop’s nitrogen demand through high frequency, low concentration (HFLC) fertilizer applications. This method has been implemented at the study site, a 56-ha commercial almond orchard in Modesto, CA, since 2017. Over five years of data have been collected from a vadose zone and groundwater monitoring well network at the orchard to study how this nutrient management practice impacts nitrate leaching rates as well as the nitrate concentrations in shallow groundwater. This project develops numerical models of water flow and nitrate transport used to 1) quantify the resulting reduction in nitrate loading rates from the orchard, and 2) predict when improvements in shallow groundwater quality may be measured. The models integrate flow and transport in the unsaturated zone and shallow groundwater at the field-scale. A key challenge investigated is the role that spatial variability observed in both the geologic media and in nitrate concentrations plays in the vadose zone and in groundwater. The models demonstrate that spatial variability observed in groundwater nitrate concentrations at the sub-field scale primarily results from sub-field scale spatial variability in nitrate leaching from the vadose zone. HFLC is predicted to reduce nitrate leaching from an already well-managed orchard by 40%. Low recharge rates (7 cm/year on average) caused by dry climate and efficient irrigation make for long response times (up to 50 years or more) in field-scale improvements in shallow groundwater quality. Additionally, nitrate concentrations in groundwater recharge under HFLC are predicted to remain above the public health criteria of 10 mg/L NO3-N. These findings imply that under dry current climate conditions, reliance on nutrient efficiency goals alone may not resolve groundwater nitrate issues in the Central Valley.