Modeling shallow overland flow in surface irrigation

Based on a literature review, the suitability of the Manning equation to represent shallow flow in surface irrigation is questioned. This conclusion is supported by analysis of both experimental data of the present study on wheat and perennial pasture as well as other published and unpublished field data on wheat. The discharge-depth equation (Eq. 2) fits the data of most tests with R² values greater than 0.9. The values of the exponent a in the equation varies generally between 1.0 and 3.0 depending on the type of vegetation, stem density, height, sowing pattern, stage of growth, and flow condition, and confirms that assuming a constant value (i.e., 5/3) for the exponent in the Manning equation is not appropriate. It is proposed that the discharge-depth equation is a more suitable model than the Manning equation for shallow flow, and information is provided on the range of values of parameters for hydraulic resistance in surface irrigation flows.