Submitted to: Irrigation & Drainage Systems Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/20/2014
Publication Date: 6/28/2014
Citation: Zerihun, D., Sanchez, C.A., Lazarovitch, N., Warrick, A.W., Clemmens, A.J., Bautista, E. 2014. Modeling flow and solute transport in irrigation furrows. Irrigation & Drainage Systems Engineering. 3(2):1-16. doi: 10.4172/2168-9768.1000124. Interpretive Summary: The application of fertilizer using the irrigation water as the delivery mechanism, also known as fertigation, is a common practice in surface irrigated agriculture. This approach offers economic benefits to producers in comparison with the mechanical application of fertilizer, but it also represents a risk for nutrient losses to the environment through with the runoff stream and with deep percolation. In fertigation systems, the distribution of fertilizer along the field and down through the soil profile is linked to the distribution uniformity of the water application, and to the timing and rate of application of the nutrient during the course of the irrigation event. A solute transport model coupled to a surface irrigation hydraulic model can be used to identify optimal fertigation strategies for particular and soil conditions. This study describes a proposed fertigation model that couples a surface irrigation model, based on principles of conservation of mass and momentum, with a non-reactive solute transport model, based on the principles of advection and dispersion. The model is solved numerically using standard techniques used for solving the unsteady flow and advection-dispersion equations. Field data were collected to validate the model using a chemical tracer. Predictions compared adequately with tracer breakthrough curves measured on two irrigation furrows. Thus, for these tests, the model predicted with reasonably accuracy the surface tracer flow with distance and time. The tracer distribution in the soil was not investigated. This information should be of interest to researchers, extensionists, and agricultural producers.
Technical Abstract: This paper presents an internally coupled flow and solute transport model for free-draining irrigation furrows. Furrow hydraulics is simulated with a numerical zero-inertia model and solute transport is computed with a model based on a numerical solution of the cross-section averaged advection-dispersion equation. A procedure for integrating the furrow volumetric cumulative intake integral in the context of a hydraulic model is presented. Two hydraulic and solute transport data sets collected in sloping free-draining test furrows at the Desert Research and Extension Centre of the University of California-Davis (labeled as DREC-1 and DREC-2 data sets) were used in model evaluation. The soil intake and hydraulic parameters (the Kostiakov infiltration constants and Manning's roughness coefficient) were estimated with a simple approach that matches simulated and measured flow depth hydrographs. The field-scale Weighted Mean Relative Residual (WMRR) between measured and model predicted flow depth hydrographs (obtained based on estimates of the hydraulic parameter set) are 22.0% and 29.0% for DREC-1 and DREC-2 data set, respectively. However, the WMRR for DREC-2 data set falls sharply to 16.0%, when only the error associated with the downstream end computational node is excluded. This suggests that a large fraction of the error is associated with the form of the downstream boundary condition used and it also shows that the effect of the downstream boundary condition is localized (does not extend to a large segment of the flow upstream). The longitudinal dispersion coefficient, for the test furrows, is approximated with an explicit equation as a function of the hydraulic and geometric variables. Model evaluation is conducted in three steps: (1) cumulative intakes and intake rates computed with the numerical formulation presented here were compared with a subsurface flow model, HYDRUS-2D (the WMRR between the cumulative intake predictions of HYDRUS-2D and the method presented here is 2.2%); (2) solute breakthrough curves computed with the coupled flow and transport model were compared with those from exact analytical solutions for applicable conditions; and (3) model predicted solute breakthrough curves were compared with those obtained from field measurements. Overall the results suggest that the coupled flow and transport model is a useful irrigation and fertigation system management and evaluation tool.