Assessing whole-field uniformity of stationary sprinkler irrigation systems

Abstract

The procedure established in the literature for the evaluation of stationary sprinkler irrigation systems is limited in space and time since it is based on a sample of precipitation taken around one sprinkler during a given period of the whole irrigation event. This procedure also ignores what happens in the soil after water infiltrates. A model of the drop trajectory and of the water distribution pattern is formulated here for simulating precipitation from single sprinklers. The operating pressure determines sprinkler flow and maximum throw. Wind and evaporation distort the distribution patterns. The water distribution of individual sprinklers is overlapped to generate precipitation over the whole field and to calculate a coefficient of uniformity. Field effective uniformity is then calculated by averaging precipitation over the extension of plant roots or water redistribution within the soil profile. Application of the model has shown the impact of system management and design, field topography and wind on irrigation uniformity. Management factors such as lateral operation time or riser inclination may account for a large part of the field precipitation variations. A rough topography may also reduce uniformity significantly. Wind speed is important when it exceeds 1.8–2 m s^–1. The allowable maximum pressure loss of 20% fixed as a design criterion seems an overly strict limit when other factors may overcome pressure loss as sources of non-uniformity. The sources of non-uniformity have different scales of variation. Large-scale sources, such as lateral operation time or pressure loss, are not dampened by the crop or soil. Sources of smaller-scale variation, such as wind or inclination of the sprinkler riser, are better compensated by the crop and soil. The application of this kind of model to the design and management of sprinkler irrigation systems is discussed.