Monitoring and Modeling of Nitrate Leaching in Micro Irrigation Across a Wide Range of California Crops and Soils.

There is a growing concern for agricultural lands input to groundwater pollution seeking sustainable agricultural practices. This requires the optimization of fertigation/irrigation practices in order to maximize their application efficiency and minimize fertilizer losses through leaching to groundwater. In an optimum irrigation/fertigation practice, irrigation water and fertilizers are applied at the rate, place and time that ensure maximum yield and fruit quality while leaching of water and nitrate to groundwater is minimized, and build-up of salts in the root zone of trees is avoided. Nitrate contamination of groundwater across the Central Valley has been related to its diverse and intensive agricultural practices. However, there has been no study to separately quantify and compare leaching of nitrate in each individual agricultural land within the complex and diversely managed studied area. We have developed a combined field monitoring and modeling approach to calculate leaching of water and nitrate at the scale of single tree, plot, and orchard. By calculating the total head gradients across a soil layer below the root zone, with known hydraulic properties, fluxes of water and nitrate (measured concentrations) can be calculated using Darcy equation. The relative small uncertainty introduced by this method offers a reliable and attractive tool for quantifying leaching of water and nitrate below the root zone under any management practices. This method is being tested in various crops in California including tomato, citrus, almond, pistachio, and walnut. We will present experimental approaches that provide the necessary data on soil moisture, water potential, and nitrate concentration and multi-dimensional modeling of unsaturated water flow and solute transport to calculate fluxes of water and nitrate under various agricultural practices. These results will be used to evaluate and optimize irrigation and fertigation management practices for multiple locations, crop types, and irrigation systems.