Enhancement of agricultural systems models for limited irrigated cropping systems research

Dwindling water supplies for irrigated crop production is the most limiting factor facing agriculture in the world today. In the evolving scenario, there is a need for making agricultural water use more efficient by bringing in up-to-date science based technologies in the irrigation field. In this context, crop water production functions (CWPFs, expressed as crop yield vs. consumptive water use or water applied) are helpful for optimal management of limited water resources for irrigation. However, they are site specific and vary from year to year, therefore for planning and managing limited irrigation, 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. The primary objective of this dissertation was to develop a methodology and use it to develop location (soil and climate) specific long-term averaged CWPFs for corn (Zea mays L) using available experimental data, long-term climate data, and a cropping system model, Root Zone Water Quality Model (RZWQM2), for various locations in the Great Plains of USA.;To begin with, several ways of quantifying water stresses (WS) based on soil water measurements and their relationship with grain yield, biomass and canopy cover of corn, winter wheat (Triticum aestivium L.) and dry (pinto) bean (Phaseolus vulgaris L. ) were investigated. There were six irrigation treatments for each crop, designed to meet 100 to 40% of potential crop ET (ETc) requirements during the growing season. Experiments were conducted from 2008 to 2011 near Greeley, Colorado in a sandy loam soil (LIRF, Limited irrigation Research Farm experiments). Water available for plant uptake (PAW, plant available water) and the maximum PAW (MAW) in the soil profile over the growing season were estimated from the soil water measurements.;Three additional WS factors (WSI1, WSI2 and WSI3) were developed as modifications of the default WS factors in the RZWQM2 using potential root water uptake (TRWUP) calculated by Nimah and Hanks (1973) approach and with accounting for stress due to heating of canopy from unused energy, when soil evaporation falls below the potential evaporation. These were incorporated in RZWQM2 and tested for simulation of corn in the LIRF experiments using CSM-CERES-Maize (v 4.0) module.;Simulations with the new stress factors showed that the simulations of crop responses to water can be substantially improved by incorporation of soil evaporation in both the supply and demand terms in the water stress quantifications in the model. Out of the three water stress factors tested, WSI2 was found to be better than others in simulations of corn grain yield, biomass and LAI. It was noteworthy, in the simulations with the WSI2 stress factor, that grain yield and biomass were improved simultaneously and the magnitudes of the errors were reasonable for model applications in water management.;Lastly, the RZWQM2 model was calibrated and validated for simulations of corn at two additional locations: Akron, Washington County and Rocky Ford, Otero County in Colorado, USA. Corn grain yield responses to different levels of irrigations at the three locations (Greeley, Akron and Rocky Ford) were simulated for multiple years, using available measured weather data (1992-2011) in RZWQM2. Mean CWPFs as functions of ET and applied water were developed for the soil types at the locations. A Cobb-Douglas type response function was used to explain the mean yield responses to applied irrigations and extend the CWPFs for drip, sprinkler and surface irrigations methods, assuming irrigation application efficiencies of 95, 85 and 55 %, respectively. (Abstract shortened by UMI.).