New Evapotranspiration Estimates and Crop Coefficients for Optimizing Sub-Surface Drip Irrigation Scheduling.

Agriculture is a major user of ground and surface water in the United States. However, with the increasing demand for water due to population growth and environmental issues as well as uncertainty linked with climate change, water allocation to the agriculture sector may be declining in the future.  One approach to conserve water is to optimize irrigation scheduling through the development of new evapotranspiration (ET) estimates and crop coefficients (Kc) that better reflect the current agricultural and irrigation management practices.  Thus, the objectives of this study were to obtain new ET and Kc estimates for tomatoes grown under sub-surface drip irrigation.  The experiment was performed for two years in Central California using large weighing lysimeters.  Daily measurements of crop and reference ET were collected to derive Kc values. Weekly measurements of crop ground cover were also performed to develop relationships between Kc and fractional ground cover. Results indicated that coefficients obtained at peak season were relatively lower than those generally reported for tomatoes. Data also showed good correlation between fractional cover and Kc (r2 = 0.91). The Kc increased curve linearly until canopy reached about 75% of fractional cover. These results will then be integrated into a decision support system to guide irrigation scheduling and ultimately optimize crop water use efficiency.