Evaluation of Soil Water Content Distribution in the Subsurface Drip Irrigation System: Field Measurement and Numerical Simulation.

In order to properly design and manage SDI systems to increase the efficiency of the water/fertilizer use and to reduce water losses due to evaporation and deep drainage, the precise distribution of water around emitters most be known. In this work, HYDRUS (2D/3D) was used to evaluate the distribution of soil water between two emitters when their wetting patterns overlap. In the first part of the study, two different modeling approaches were considered to predict the distribution of soil water content between two emitters and simulated results were compared with field data. Variably-saturated water flow was calculated using either a fully three-dimensional model or an axi-symmetric two-dimensional model. In the second part of the study, we compared results of numerical simulations of soil water content distributions for the SDI system with emitters that were represented (a) as a point source in an axi-symmetric two-dimensional domain, (b) as a line source in a planer two-dimensional domain, and (c) as a point source in a fully three-dimensional domain. When water movement from a lateral is considered as a two-dimensional process, it is assumed that water is discharged uniformly along an entire length of a lateral. However, water is in fact discharged from individual emitters located on a lateral, with no discharge between emitters. Therefore, the assumption of two-dimensional flow is only an approximation of a real three-dimensional flow process. Before the wetting patterns of two adjacent emitters start overlapping, this three-dimensional flow process could be approximated as a axi-symmetric two-dimensional process and solved using a two-dimensional version of HYDRUS (2D/3D) (or HYDRUS-2D). When two adjacent flow patterns start overlapping, the problem becomes a fully three-dimensional problem and needs to be solved using a three-dimensional model, such as a three-dimensional version of HYDRUS (2D/3D). Depending on the emitter spacing, irrigation duration, initial water contents, and soil properties, wetting patterns of two adjunct emitters will eventually overlap to a such extent that the water distribution along a lateral (between two emitters) appears to be relatively uniform. Only from this point forward, the flow can be approximated as a two-dimensional problem. The root-mean-square-error (RMSE) was used for statistical analysis. From results obtained in this study it can be concluded that the SDI system can be described using an axi-symmetric two-dimensional model only before wetting patterns start overlapping and a two-dimensional model only after full merging of neighboring wetting patterns. Only a fully three-dimensional model can describe the entire drip irrigation process in its entirety.