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Abstract
The rising cost of pumping energy has created the stimulus for reducing the operating pressure of center pivot irrigation machines. The reduction in operating pressures on these pivots has brought with it some problems which lead to an increase in the potential for runoff under them. Lowering operating pressures can lead to increased drop sizes and reduced distance of throw. Increasing drop sizes causes the drops to impart more energy to the soil and a reduction in the rate at which water can be infiltered. Reducing the distance of throw causes the application rate to be increased. The potential for runoff exists whenever the application rate exceeds the infiltration rate, so both factors lead to increased potential runoff.
A physicaly based infiltration model modified for reduction in the intake rate due to drop effects was merged with an application rate model. Several application packages, system flow rates, depths applied, and soils were simulated with the model, and potential runoff was estimated for each. Potential runoff was highest on soils with a high percentage of silt and for the 180(DEGREES) spray nozzle package. Results are presented in tabular and graphical form for each situation.
Using the results of the potential runoff estimations a method was developed to estimate the maximum allowable depth which could be applied with a selected application package as a function of soil, system flow, and surface storage. This maximum allowable depth method is intended for use in system design by providing a way to compare system application packages and allowable depths.
An economic comparison of application packages was developed incorporating both operating pressures and losses due to runoff. A chart for use in that comparison was presented.
