Comparison of Four Infiltration Models in Characterizing Infiltration Through Surface Mine Profiles

A infiltrometer system was used to evaluate the infiltration process through reconstructed surface mine profiles. Six different profiles were subjected to constant simulated rainfall intensities for selected test conditions. Surface runoff rates were monitored and transient soil moisture contents in the profiles were measured with a gamma density gauge. Unsaturated hydraulic conductivity relationships were determined using Campbelfs method and the "zero-flux" procedure. The infiltration process was modeled by the SCS curve number method, a form of Holtan's equation, the Green-Ampt model and Richards' equation. SCS curve numbers were determined by fitting the method to the observed results. Richards' equation gave very good estimates of the infiltration process through the spoil profiles, but was only slightly better than the Green-Ampt model. None of the models worked well for the profiles where macropore flow occurred through a two layer, topsoil-over-spoil system. INTRODUCTION One of the major components of the hydrologic cycle is infiltration. The ability of water to move through a soil profile has a direct effect on many of the components of the hydrologic cycle. In a previous paper (Ward et al., 1983a), results from an extensive series of infiltration experiments involving reconstructed profiles of surface mine spoil and topsoil material were presented. The purpose of this study was to evaluate four commonly used infiltration models in describing infiltration through reconstructed surface mine overburden and to broaden the limited data base of soil physical parameters which are essential in predicting infiltration in these media. Article was submitted for publication in June, 1985; reviewed and approved for publication by the Soil and Water Div. of ASAE in April, 1986. The work reported in this paper was supported in part by funds provided by the Office of Water Research and Technology, United States Department of Interior, as authorized under the Water Resources Act of 1964, and in part by the Kentucky Agricultural Experiment Station and is published with the approval of the Director of the Experiment Station as Journal Article No. 82-2-3-265. Mention of trade names is for informational purposes and does not necessarily imply endorsement by the Kentucky Agricultural Experiment Station. The authors are: L. G. WELLS, Professor, Agricultural Engineering Dept., University of Kentucky, Lexington; A. D. WARD, Associate Professor, Ohio State University, Columbus; I. D. MOORE, Senior Scientist, Division of Land and Water Resources, C.S.I.R.O., Canaberra City, Australia; and R. E. PHILLIPS, Professor, Agronomy Dept., University of Kentucky, Lexington. EXPERIMENTAL METHODS Profile Construction and Characteristics Surface mine spoils and soils were obtained from Peabody Coal Company's Alston Mine in Ohio County, KY. The spoil material was a mixture of grey and dark shale and sandstone. The topsoil material was a mixture of Belknap silt loam and Sadler silt loam. Physical and chemical properties of the spoil and soil materials are presented by Ward et al. (1983a). The infiltrometer system incorporates two soil/spoil bins which have dimensions of 0.91 x 1.83 x 1.07 m deep. These bins were packed with soil and spoil materials to form profiles similar to those found in Western Kentucky. After initially filling the empty bins, new profiles were constructed by replacing the top 15 to 20 cm of material. Six profiles were constructed, three consisted entirely of spoil material, and three consisted of 15 cm of topsoil over spoil. The physical characteristics of the profiles are in agreement to those documented from field measurements in similar reconstructed media as discussed in Ward et al. (1983a). Profile bulk densities and changes in soil moisture content were measured with a Troxler two-probe gamma density gauge. Soil suction was measured with a series of tensiometers which were inserted horizontally into the sides of the bins at depth increments of 15 cm. A complete description of the instrumentation and the infiltrometer system is presented by Ward et al. (1981). Infiltration Tests The six soil/spoil profiles were constructed to evaluate the influence of rainfall intensity, initial moisture content, and bulk density on infiltration through the profiles. Initial soil/spoil moisture contents ranged from air dried to field capacity, and tests were conducted at rainfall intensities of 1, 2 or 3 cm/h. At the beginning of each infiltration test, the gamma probe was used to determine initial moisture conditions in a profile. Scans were made every 2.54 cm down the profile in conjunction with the advances of the wetting front. Between movements, readings were taken at the same location every few minutes. Soil moisture contents behind the wetting front were determined by monitoring the grid locations above the wetting front every 30 to 60 min. Accumulated infiltration was determined by taking the difference between the initial and final moisture contents for a profile as determined by the gamma probe. The infiltration rate during a test was determined by measuring runoff rates from the soil surface. This approach assumes that the rainfall rate is constant, that all the rainfall is applied to the soil surface, and that the surface storage is small. This approach also provided Vol. 29(3):May-June, 1986 © 1986 American Society of Agricultural Engineers 0001-2351/86/2902-785$02.00 785 another measurement of the accumulated infiltration volume. The duration of each event was controlled so that the wetting front would pass beyond either the first or second level of tensiometers. The test durations ranged from 75 to 600 min depending on soil/spoil type and the density of the profile. INFILTRATION MODELS The SCS curve number method, a modified form of Holtan's equation, the Green-Ampt model, and Richards' equation were selected for evaluation because they are widely used, and have each been included in surface mine hydrology models. The Green-Ampt model (Green and Ampt, 1911) and Richards' equation (see, for instance. Smith and Woolhiser, 1971) are based on the physics of soil water movement, while the SCS curve number procedure (SCS, 1973) and Holtan's equation (Holtan, 1961) are empirical models which have parameters with no physical significance. SCS Curve Number Method This procedure was developed to predict surface runoff from small watersheds and was intended for use only where watershed data and daily rainfall records were available. The data used to develop the method were obtained from experimental plots for agricultural soils and agricultural land treatment measures (SCS, 1964). The expression for accumulated surface runoff, Q, is: