Field-Scale Modeling of Center Pivot Irrigated Cotton.

Regulatory ground water pumping restrictions continue to be debated in the Southern Ogallala Aquifer region and will eventually result in allocation of irrigation resources becoming more important. Models that address the temporal and spatial variability of water, energy, and nutrient balances at field-scale may provide the decision support information that producers will require to efficiently allocate resources within regulatory restrictions. The Precision Agricultural-Landscape Modeling System (PALMS) has unique abilities to address the magnified level of spatial and temporal variability in water, energy, and nutrient balances associated with pivot irrigation systems common in the Southern Ogallala Aquifer region. The model uses spatially and temporally representative irrigation data, now commonly available from pivot monitoring systems, and produce field-scale water balance output at resolutions suitable for use in variable rate irrigation prescriptions. The PALMS model was combined with the cotton growth model, COTTON2K, and the combined model is being evaluated against field data from several locations in the Southern Ogallala Aquifer region. Testing of the model, by comparing calculated and measured soil water content values, indicates that adjustment of soil physical parameters in the PALMS model is necessary to minimize mean squared deviation, increase correlation, and decrease squared bias. Adjustment of soil physical parameters was conducted following published methods for simultaneous scaling of saturated conductivity and soil water retention parameters. A scaling factor as low as 0.316, i.e., reducing saturated conductivity by an order of magnitude, has proven beneficial for modeling Pullman clay loam soils in our research.