Initial-Inflow-Variation Impacts on Furrow Irrigation Evaluation

One of the basic assumptions of the hydrological volume balance models in surface irrigation implies that furrow inflow instantaneously increases to a constant rate: the step-flow condition. Unfortunately this assumption is not always met in the field. Inflow variation is mainly due to the operations required to switch from one set to another, even with an automated furrow irrigation system such as gated pipe. In that regard, siphons are less sensitive to the operations and may be generally considered as a step-flow application system. Furthermore, the presence of a flume at the upstream end of the furrow can also perturb flow. The resulting gradual increase in flow reduces the stream advance rate at the beginning of the run. However, we demonstrate that later in the advance phase the velocity converges to the standard step-flow solution. Generally, advance-phase evaluation models incorrectly attribute the front slowdown to a very high infiltration rate. Two models were compared. The first, the Elliott and Walker model limited to the advance phase, identifies a Kostiakov-Lewis infiltration function with the constant term of infiltration set equal to zero. The second, the advance linear velocity (ALIVE) model, identifies a Horton function from the analysis of the advance rate. This study, based on analytical developments and on 72 furrow irrigation events, shows that the Elliott and Walker model is influenced by a non–step-inflow variation and underpredicts the infiltration amount. In contrast, the ALIVE model proposed by the writers is not as sensitive to the initial behavior of the flow, because it uses two-term rate functions for advance and infiltration.