Crop Water Production Functions for Optimizing Use of Limited Water and N.

Crop water production functions (CWPFs) are expressed as crop yield vs. consumptive water use or water applied. The CWPFs are helpful for optimal management of limited water resources.  The CWPFs are site specific and vary from year to year, especially when yield is expressed as a function of irrigation water applied. Therefore, for planning and managing limited irrigation for enhanced water use efficiency (WUE), the CWPFs should be based on long-term field experiments to take into account the variations in precipitation and other climatic variables at the location. Our objectives here were to develop (1) a methodology for developing soil and climate specific long-term averaged CWPFs and (2) optimum N application level, for development of a decision support system ‘The Colorado Deficit Irrigation Tool (CDIT)’ that is designed to maximize profit and minimize water use for farmers in limited water environments.  The experimental data for this study were from the corn deficit irrigation trials conducted at the Limited Irrigation Research Farm (LIRF) (40° 26’ N, 104° 38’ W, and 1428 m MSL) of the USDA-ARS near Greeley, Colorado.  The RZWQM2 was calibrated and tested for simulation of corn with data at the location, and then used to simulate the crop yield responses and N requirements to different levels of irrigations, for multiple years, using available long-term measured weather data (1992-2011).  Mean CWPFs as functions of ET and applied water, and optimum N levels under deficit irrigations were developed for the soil type at the location. A Cobb-Douglas type response function was fitted to the mean yield responses to model-applied irrigation and then used to extend the CWPFs for drip, sprinkler and surface irrigations methods, assuming irrigation application efficiencies of 95, 85 and 55 %, respectively.