UNSTEADY FURROW INFILTRATION IN THE LIGHT OF THE RICHARDS EQUATION

Authors: PEREA, H - U OF AZ, TUCSON, AZ; Strelkoff, Theodor; SIMUNEK, J - USDA, RIVERSIDE, CA; Bautista, Eduardo; Clemmens, Albert

Submitted to: United States Committee of Irrigation and Drainage Engineering Conference
Publication Type: Proceedings
Publication Acceptance Date: 3/1/2003
Publication Date: 5/15/2003
Citation: Perea, H., Strelkoff, T., Simunek, J., Bautista, E., Clemmens, A.J. 2003. Unsteady furrow infiltration in the light of the richards equation. United States Committee of Irrigation and Drainage Engineering Conference. p.625-636.

Interpretive Summary: In the competition among the agricultural, municipal, and environmental sectors for water, agriculture, as a major user, must use irrigation water prudently. Furthermore, with the potential for runoff and deep seepage to carry agricultural chemicals off site into the environment, the issues of irrigation design and management take on a critical importance. About half of the irrigated acreage in the U.S. is watered by surface means. In many developing countries, over 90% of agricultural lands are supplied by surface irrigation systems. Often, these systems do not live up to their potential for efficient water application, which, under appropriate field conditions, can rival that of pressurized systems. Computer models simulating the surface-irrigation process have been constructed to facilitate the search for conditions that provide optimum performance. A key calculation in this software describes how water infiltrates into the soil. Physically based methods exist for modeling soil-moisture movement, but they are cumbersome and would greatly increase the size and running time of the models. In addition, their field-data requirements are often not readily met. Hence, surface-irrigation models typically rely on a more empirical approach. This research describes a pilot study comparing furrow infiltration as computed by an existing stand-alone model of unsaturated soil-moisture flow with a much simpler procedure incorporated into a simulation model of surface irrigation. This procedure utilizes empirical data on infiltration in border strips to calculate the greater infiltration exhibited in furrows -- stemming from lateral suction into the soil through the furrow walls. The aim is to test, and if possible, improve the simple empirical procedures of the surface-irrigation models. The work forms part of a charge to the Environmental and Water Resources Institute (ASCE) Task Committee on Soil and Crop Hydraulic Properties to review existing field-parameter estimation methods and make recommendations. The results of improved methods for simulating furrow irrigation are intended for application by research, consulting, and extension personnel for the benefit of both growers and the population at large.

Technical Abstract: This paper addresses the treatment of infiltration in the context of simulating irrigation-stream flow in surface irrigation. It bridges the gap between traditional empirical formulations of infiltration and more recent treatments of infiltration as a process, to be mathematically modeled on the basis of physical principles as much as the flow in the surface stream. Solutions of the Richards equation for unsaturated flow have become ever more practical since the advent of user-friendly software for numerical modeling. But these do not provide the definitive solution to infiltration during surface irrigation, because of important but as yet not predictable changes in soil structure that take place at the stream-soil interface. Still, much can be learned by comparing empirical approaches with the results of Richards-equation solutions in homogeneous isotropic soils. Particular emphasis is accorded to furrow infiltration, a phenomenon more complex than infiltration from border strips or basins because of lateral and upward suction into the soil surrounding the furrow.