Modeling Solute Transport in Sprinkler Irrigation Laterals

Abstract

The application of agricultural chemicals to crops in the form of aqueous solutions or emulsions blended with irrigation water (often referred to as chemigation) is widely practiced in modern farming systems. Optimal chemigation management can help reduce crop production costs and mitigate the environmental effects of agricultural chemicals. As a step toward the development of a field-scale sprinkler irrigation chemigation systems management model, a solute transport model that can simulate the time and distance evolution of the concentration of a nonreactive solute in an irrigation lateral is developed and evaluated in the study reported here. For modeling purposes, a lateral is conceptualized as a branched hydraulic network consisting of a series of connected pipes, each delimited by outlet nodes. At the pipe scale, solute transport is modeled as a one-dimensional advective process in which the applicable differential equation is solved with a quasi-Lagrangian integration scheme. Solutions to the transport problem, in a pair of consecutive pipes, are coupled through what is described here as the nodal condition, which can be stated as: the concentration computed at the downstream-end node of a pipe constitutes the upstream boundary condition for the advective transport problem in the pipe just downstream. The model was evaluated in two phases. First, the soundness of the formulation and programmatic implementation of the numerical solution to the pipe-scale advective transport problem was tested through comparison of model outputs with analytical solutions. Evaluation of the predictive capacity of the lateral-wide model, in the context of a real-world application, was then conducted by comparing computed breakthrough curves of a nonreactive tracer with data measured along a pair of laterals. The results suggest that model performance is satisfactory.