Suitability of selected flow equations and variation of Manning's n in furrow irrigation

Simulation models are becoming valuable tools for design and management of furrow irrigation systems. The Manning equation is used often for describing overland flow in these models. The past studies suggest that the Manning equation is not suitable for describing the flow in border irrigation because the value of Manning's n varies with the depth of flow. In this study we examine the suitability of the Manning and other four similar flow equations for flow in furrow irrigation and evaluate how Manning's n varies spatially, temporally, and with flow rate in the furrow. Field tests were monitored for a range of flow rates and two furrow slopes. The Manning and the other equations fitted data satisfactorily. The value of the Manning's n varied slightly with the variation in flow rate at low flow range, but it became fairly constant with changes in flow rate at high flow range. The Manning equation is considered the best for modeling the overland flow in furrow irrigation among the equations examined in the study because the Manning equation requires only one parameter to be estimated, whereas in the other equations up to four parameters should be estimated.