Evaluation of Water Distribution Uniformity Equations in Sprinkler Irrigation and Feasibility of Using Them in Different Field Conditions

Distribution uniformity of water is one of the most important parameters for evaluation of irrigation systems. Numerous equations have been developed to calculate distribution uniformity coefficient in sprinkler irrigation systems. The provided equations do not necessarily yield the same results in calculation of uniformity coefficient for one specific farm in a particular condition. The aims of this study were the evaluation of various equations proposed by different researchers, investigation of the effects of different field conditions on the results, investigation of the existing relationships among outcomes of the equations, and feasibility of using these equations. For this purpose, distribution uniformity coefficients were calculated using the equations presented by Christiansen, Hawaiian Cane Society Specialists Hart and Reynolds, Wilcox and Swailes, Karmeli, Criddle et al, Benami and Hore, and Beale and Howell, for 10 sprinkler irrigation systems in Shahrekord fields. Data analysis was performed using Statistical Analysis System (SAS) Software in a randomized complete block design. The results indicated that there were significant differences (P< 0.05) between the aforesaid coefficients and some of these equations estimated a negative number or a number larger than 100% for coefficient of uniformity in some specific conditions of sprinkler irrigation. Furthermore, in all cases, beta distribution could estimate the water distribution uniformity coefficient (CU) better than the normal and uniform distributions. In all three beta, normal, and uniform distributions, minimum agreement between predicted and measured values was related to equations by Karmeli, Beall and Howell, and Benami and Hore; and the most agreement, particularly in the beta and normal distribution, was related to the Christiansen equation. The results conclusively indicated that a number of coefficients of uniformity such as Benami and Hore, Karmeli, and, to some extent, Beal and Howell coefficients are strongly dependent on specific field conditions and are not applicable under other field conditions.