It is essential to standardize the definitions and approaches to quantifying various irrigation per formance measures. The ASCE Task Committee on Defining Irrigation Efficiency and Uniformity provides a comprehensive examination of various performance indices such as irrigation efficiency, application efficiency, irrigation sagacity, distribution uniformity, and others. Consistency is provided among different irrigation meth ods and different scales. Clarification of common points of confusion is provided, and methods are proposed whereby the accuracy of numerical values of the performance indicators can be assessed. This issue has two companion papers that provide more detailed information on statistical distribution uniformity and the accuracy of irrigation efficiency estimates.

A set of drop size distribution data is presented covering a wide range of sprinkler types including single nozzle impact sprinklers with straight bore and square nozzles, and sprayheads with various types of deflector plates. Drop sizes were measured by the laser-optical method and comparisons with other types of drop size measurement techniques are presented. Distributions are parameterized with an exponential function, and a method is provided to estimate the parameters given the sprinkler type, nozzle size, and pressure head. Keywords. Irrigation, Sprinklers, Drop size. S prinkler irrigation can be defined as any irrigation system which distributes water as discrete droplets through the air. The variety of sprinkler devices available has increased dramatically in recent years, from the conventional single or double nozzle impact sprinkler with many types of nozzles to various types of deflection-plate sprinklers which influence the drop sizes and water distribution patterns over a wide range of flow rates and pressures. Accurate knowledge of drop size distributions for sprinklers is important because evaporation and drift losses are controlled by the extreme small size ranges and drop impact energy on the soil is determined primarily by the largest size ranges. Selection of a specific sprinkler package for a sprinkler system operating on particular soil, slope, crop, and climate conditions will be aided by knowledge of the drop sizes. Several articles have been published describing the drop size distributions of specific types of sprinklers (Kohl, 1974; Kohl and DeBoer, 1984; Solomon et al., 1985; Kohl and DeBoer, 1990). The reported data were collected using pellet, stain, and photographic methods. This article presents additional data using a laser-optical method compared with some distributions determined by some of the previously used methods, so that the entire body of published data is more comparable. The main objective is to parameterize the data and present a method to predict the parameters as a function of nozzle size and pressure.

Abstract:The Imperial Irrigation District is a large irrigation project in the western United States having a unique hydrogeologic structure such that only small amounts of deep percolation leave the project directly as subsurface flows. This structure is conducive to relatively accurate application of a surface water balance to the district, enabling the determination of crop evapotranspiration (ETJ as a residual of inflows and outflows. The ability to calculate ETc from discharge measurements provides the opportunity to assess the accuracy and consistency of an independently applied crop coefficient-reference evapotranspiration (K ET o ) procedure integrated over c the project. The accuracy of the annual crop evapotranspiration via water balance estimates was ±6% at the 95% confidence level. Calculations using K and ET o were based on the FAa-56 dual crop coefficient approach and included separate calculation of evaporation c from precipitation and irrigation events. Grass reference ET o was computed using the CIMIS Penman equation and ETc was computed for over 30 crop types. On average, Kc-based ET computations exceeded ETc determined by water balance (referred to as ETc WB) by 8% on an annual basis over a 7 year period. The 8% overprediction was concluded to stem primarily from use of K c that represents potential and ideal growing conditions, whereas crops in the study area were not always in full pristine condition due to various water and agronomic stresses. A 6% reduction to calculated Kc-based ET was applied to all crops, and a further 2% reduction was applied to lower value crops to bring the project-wide ET predicted by Kc-based ET into agreement with ETc WB' The standard error of estimate (SEE) for annual ETc for the entire project based on K c , following the reduction adjustment, was 3.4% of total annual ETc, which is considered to be quite good. The SEE for the average monthly ETc was 15% of average monthly ETc. A sensitivity analysis of the computational procedure for K c showed that relaxation from using the FAa-56 dual K method to the more simple mean (i.e., single) K curve and relaxation of specificity c c of planting and harvest dates did not substantially increase the projectwide prediction error The use of the mean K curves, where effects c of evaporation from wet soil are included as general averages, predicted 5% lower than the dual method for monthly estimates and 8% lower on an annual basis, so that no adjustment was required to match annual ET derived from water balance. About one half of the reduction in estimates when applying the single (or mean) K c method rather than the dual K c method was caused by the lack of accounting for evaporation from special irrigations during the off season (i.e., in between crops).

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