Development of a Runoff Coefficient Equation Using Spatial Variables for Feasibility Study of Small-scale Irrigation Projects

Abstract

The preliminary feasibility study of small-scale irrigation projects for water resources development under the Royal Irrigation Department (RID) guidelines requires a monthly runoff assessment at the headworks to determine storage capacity. Runoff is calculated using catchment area, average monthly rainfall, and a runoff coefficient (R.O.C.). Currently, the determination of R.O.C. relies on qualitative topographic assessments without clear numerical criteria, resulting in variations based on individual project engineer's judgment. This research aimed to develop an equation for estimating R.O.C. with explicit numerical criteria using spatial variables, including slope data derived from MERIT DEM and percent tree cover (PTC) grid data from ISCGM. Additionally, the consistency of the developed equation was evaluated by comparing calculated runoff volumes with those obtained from the traditional assessment method for 20 previously designed projects in the western region of Thailand. The study reveals that the developed equation can estimate R.O.C. values consistent with the traditional assessment method used in previous project designs, with an Index of Agreement of 0.995, effectively linking spatial variables to R.O.C. values and reducing dependence on the project engineer's judgment., and can be incorporated into a working manual for engineers conducting preliminary feasibility studies of small-scale irrigation projects nationwide, enhancing accuracy and standardization.