Influence of cotton canopy on sprinkler irrigation uniformity

Irrigation system performance is often evaluated based on a uniformity coefficient calculated from water collected in a measuring apparatus or from changes in the soil water content. These measurements are generally made from one test and usually when no crop is present. However, a growing crop canopy has a significant potential to modify the distribution of water applied during an irrigation. This study was conducted to evaluate the influence of a cotton canopy on sprinkler irrigation uniformity. A seven-span lateral move sprinkler irrigation machine was used with sprinklers designed to create different levels of water application uniformity. Five uniformities and two depths of application were evaluated. Irrigation water was caught above and below the canopy by plastic troughs which spanned the distance between rows. Christiansen's Uniformity Coefficient (CUC) was used as the measure of uniformity and was calculated on both a daily and cumulative application basis. The cotton leaf canopy was found to be significantly influenced by the water application patterns and depths in the uniformity/application depth treatments. The row-to-row variations in plant height, leaf area, and leaf distribution had a significant effect on the distribution of water below the canopy. Water application data collected above and below the canopy showed that an increased percentage of the applied water was "lost" as the depth of application was reduced. Increasing the depth of applied water tended to improve the CUC both above and below the canopy. The CUC was higher below the canopy for any given application than above the canopy. In the high uniformity treatments, the depth of applied water had less effect on the difference in calculated CUC above and below the canopy than in the low uniformity treatments.