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 incr eases 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 tha t regard, siphons are less sensitive to the operations and may be gene rally considered as a step-how application system. Furthermore, the pr esence of a flume at the upstream end of the furrow can also perturb f low. 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 attri bute the front slowdown to a very high infiltration rate. Two models w ere compared. The first, the Elliott and Walker model limited to the a dvance phase, identifies a Kostiakov-Lewis infiltration function with the constant term of infiltration set equal to zero. The second, the a dvance linear velocity (ALIVE) model, identifies a Horton function fro m the analysis of the advance rate. This study, based on analytical de velopments and on 72 furrow irrigation events, shows that the Elliott and Walker model is influenced by a non-step-inflow variation and unde rpredicts the infiltration amount. In contrast, the ALIVE model propos ed by the writers is not as sensitive to the initial behavior of the f low, because it uses two-term rate functions for advance and infiltrat ion.