DETERMINING THE MINIMUM MANAGEMENT SCALE OF A COMMERCIAL VARIABLE-RATE IRRIGATION SYSTEM

Variable-rate irrigation (VRI) is a technology that can improve the precision of water distribution, allowing application to be matched to the variability of field conditions. Ideally, VRI can irrigate small (>= 100 m(2)) management areas as required by soil water content, crop ET, and infiltration rate. Under field conditions, achieving the ideal prescription for water application is hampered by confounding effects of mechanical limitations, surface redistribution, and wind drift. This study employed data from a two-dimensional array of 440 catch-cans measuring application from a multi-scale irrigation prescription. To evaluate the effectiveness of the VRI equipment, a new performance coefficient was developed based on the traditional uniformity coefficient. Initial analysis showed that there was an average under-application of water by similar to 7%, and the most significant disparity between the prescribed and measured water depths occurred near areas of transition in the prescription. Areas of transition were analyzed in more detail, and a characteristic length scale (sigma) of the transition zones was identified through a dispersion analogy. Analysis of the scale showed that sigma was independent of application depth, position along the pivot, or orientation of the pivot. This indicated that a traditional, single line catch-can test can provide sufficient information for determination of the length scale. The length scale was used to determine the size of the smallest independent management zone; the minimum management zone size was determined to be 5.6 sigma (23 m for the pivot under investigation).