Optimizing Use of Green Water in Irrigated Agriculture: A Soil Survey Decision Support Tool.

Globally, irrigated agriculture occurs on 20% of cultivated land but is responsible for 40% of food production.  This productive capacity relies on a 2200 million-acre foot ‘blue' water input, that is, water diverted from streams or groundwater pumped from aquifers, threatening freshwater availability and quality.  In Mediterranean climates like California, the reliance on ‘blue' water is even more pronounced: 80% of California's diverted stream flows and groundwater is for agriculture, averaging 40-50% of annual runoff from California's watersheds.  ‘Green' water, the soil stored water from natural rainfall that is potentially available to plants, is a soil ecosystem service that can reduce reliance on ‘blue' water in irrigated agriculture and limit environmental impacts of food production.  Towards this end, we are developing a place-based decision support tool to help farmers schedule time to first irrigation of the growing season and highlight opportunities to improve water use in irrigated agriculture.  While the NRCS soil survey has published estimates of plant available water (PAW) for the soils of the United States, PAW's lower limit, the wilting point, is below the soil moisture threshold for profitable irrigation management.  To overcome this limitation of soil survey, Python software is used to process NRCS soil survey data, estimate soil moisture retention curves for each horizon using a pedotransfer function, and calculate crop-specific estimates of maximum ‘green' water availability for each soil based on crop-specific rooting depth and soil moisture tension thresholds.  We will then overlay precipitation data and place-based crop evapotranspiration estimates across a spectrum of water years.  It is hypothesized that these maps will show significant temporal and spatial variability of green water availability estimates, highlighting opportunities for farmers to optimize their water and soil resources through delayed irrigation at the beginning of the growing season and management practices that increase soil stored ‘green' water.