A COMPARISON OF UNIFORMITY MEASURES FOR DRIP IRRIGATION SYSTEMS

Three drip irrigation systems were installed in 1984, two with lateral s on the soil surface and one with laterals about 0.30 m below the soi l surface. These systems were used to apply irrigation water and nutri ents to several experiments from 1985 to 1992. Emitter plugging, syste m uniformity, and overall performance were evaluated for both surface and subsurface systems using several methods, and the results were com pared To those obtained for unused tubing that had been saved from the original lot. Emitter uniformity values calculated for the unused lat erals by the traditional and ASAE EP458 methods were similar bur diffe rences between parameter values calculated by the Two methods were gre ater for the used laterals, especially in the subsurface system. All u niformity values were lower for the subsurface system, primarily becau se of plugged emitters. Uniformity values calculated by the EP458 meth od (randomly selected emitters) indicate lower emitter uniformity for the used laterals in the subsurface system, but these values were not as low as those calculated using all emitters on three laterals for th at system. These values indicate that the EP458 method may not always reflect true system uniformity, probably because of the small sample s ize and existence of completely plugged emitters. Correction of emitte r flow rates for temporal pressure variation among test times improved uniformity parameter values slightly. Uniformity values predicted by design/evaluation models were similar for both surface and subsurface systems, and generally indicate better system uniformity than values c alculated from emitter flow measurements. The models were unable to pr edict reliable uniformity values for systems because of their inabilit y to handle emitter plugging. Based on these results, it appears that both traditional and ASAE EP458 methods can be used to evaluate drip i rrigation systems, but the EP458 method generally indicates lower unif ormity and should be used carefully for systems where completely plugg ed emitters may exist. When emitter plugging occurs, the accuracy of p redictions by either method will depend primarily upon the number of e mitters measured and the extent of plugging. Entry of soil particles i nto this eight-year-old subsurface system during construction and/or r epair operations probably caused the observed emitter plugging, which emphasizes the need for exercising great care in installation and main tenance of subsurface systems ifa long system life (10-15 years) is ex pected.