SUITABILITY OF SELECTED FLOW EQUATIONS AND VARIATION OF MANNING N IN FURROW IRRIGATION

Simulation models are becoming valuable tools for design and managemen t of furrow irrigation systems. The Manning equation is used often for describing overland flow in these models. The past studies suggest th at the Manning equation is not suitable for describing the flow in bor der 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 o ther four similar flow equations for flow in furrow irrigation and eva luate how Manning's n varies spatially, temporally, and with flow rate in the furrow. Field tests were monitored for a range of flow rates a nd 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 constan t with changes in flow rate at high flow range. The Manning equation i s considered the best for modeling the overland flow in furrow irrigat ion among the equations examined in the study because the Manning equa tion requires only one parameter to be estimated, whereas in the other equations up to four parameters should be estimated.