Optimization of fertilization under furrow irrigation

Two mathematical models were developed and programmed separately to simulate and optimize the application efficiency and uniformity of water-soluble fertilizers through furrow irrigation systems. The models simulate solute mass balance and transport in a one-dimensional, unsteady open-channel hydraulic simulation of the advance and post-advance phases of an irrigation event (wetting, cutoff, and recession). The first model is based on a method-of-characteristics solution to the velocity-depth form of the Saint-Venant equations and a solute mass balance equation. The second model uses the area-discharge form of the same governing equations and a four-point implicit solution approach, allowing for much larger simulation time steps than the method-of-characteristics model. The Kostiakov-Lewis infiltration equation was used to account for the vertical movement of water into the soil.; The four-point implicit model was programmed with an optimization algorithm to determine the best timing and duration of solute (fertilizer) injection for the product of two performance indices: solute application efficiency and solute application uniformity. The results from the four-point implicit model were compared with three sets of field data from this research and were found to be in close agreement in terms of advance trajectories and surface water solute concentrations. Several dozen water samples from the field experiment were analyzed in a soils laboratory. Over 50,000 simulations were performed with the model and the results were analyzed in terms of optimal injection start and stop times for solute efficiency and uniformity. Management guidelines are given for the timing of solute injection in furrow irrigation. The four-point implicit model can be used in the formulation of irrigation and fertigation management plans, leading to savings in both water and fertilizer, as well as reductions in non-point source pollution from irrigated agriculture.