VALIDATION OF SURFACE IRRIGATION MODEL SIRMOD UNDER CLAY LOAM SOIL CONDITIONS IN EGYPT

Abstract

Surface irrigation (gravity) is the most dominant method currently accounts for 80-85% of irrigation water use in Egypt and surface application is by far the dominant irrigation method applied throughout the world. However, water use efficiencies with surface irrigation methods tend to be low. In recent years a number of surface irrigation simulation models for assessing surface irrigation system performance have been developed. One of the most commonly used models SIRMOD, developed by Utah State University, has seen wide use and evaluation throughout the world particularly by researchers and has been shown to offer potential for increasing surface irrigation water use efficiencies. The use of the SIRMOD model as a management tool for improving irrigation efficiencies was found to be a valuable aid. This study aims to validate SIRMOD model for using in Egypt under clay loam soil conditions. The SIRMOD model adequately describes advance and recession times and infiltrated depth under experimental site conditions for the furrow irrigation practice. In particular, for the experimental site the SIRMOD model provided acceptable predictions for 75 m and 50 m furrow lengths under 0.2% field slope, and for 100 m, 75 m and 50 m furrow lengths under 0.5% field slope at the 1st irrigation. For that, the good predicted values were for the later irrigations than the first one, due to the good relationship between the predicted and measured infiltration depths obtained from SIRMOD model which has high accuracy degree for furrow irrigation management decisions. Generally, predicted advance, recession times and infiltrated depth were highly correlating with measured one at 0.2% field slope more than 0.5% field slope for the two irrigations. INTRODUCTION urface irrigation methods within Egypt are currently responsible for greater than 85% of the total irrigated areas and hence make up the dominant method of irrigating both crops and trees. Although well designed and managed furrow-irrigated systems have the potential to operate at application efficiencies above 90% (Faulkner et al. 1998), many furrow systems operate at significantly lower efficiencies. One of the major constraints to the improvement of furrow irrigation performance has been the difficultly in assessing the many variables associated with furrow irrigation systems and their interactions, and to utilize these in irrigation management. One potential for improving the efficiency and performance of furrow irrigation systems lies in the use of simulation models to simulate and predict furrow irrigation performance and assess changes in management variables, which can lead to improvements in irrigation efficiency. A number of such models have been developed which aim to simulate surface irrigation systems. A few of these models have also been developed into user-friendly computer programs with the ultimate aim of being used by irrigation practitioners as a management tool such as SIRMOD model (Walker, 1998). The SIRMOD model (Walker, 1998) simulates the hydraulics of surface irrigation (border, basin and furrow) at the field level. The simulation routine used in SIRMOD is based on the numerical solution of the Saint-Venant equations for conservation of mass and momentum as described by Walker and Skogerboe (1987). Inputs required for the model to simulate an irrigation event include the infiltration characteristic, hydraulic resistance (Manning's n), furrow geometry, furrow slope, furrow length, inflow rate and advance cutoff time. Of these required inputs, the most difficult to determine adequately are the infiltration characteristics and the furrow inflows which often require either relatively expensive equipment or significant periods of time and skilled operators. These inputs have also been found to be the most sensitive in the SIRMOD model (McClymont et al. 1996). It should also be noted that a number of assumptions made in the SIRMOD model were not always present in the field investigations. These included a step inflow rate to the furrow, which was rarely found in the field data to due