Impact of Saturated Hydraulic Conductivity on the Prediction of Tile Flow

Preferential flow through macropores and other structural voids in field soils most often occurs at or near saturation. Our earlier research revealed significant differences in the value of the saturated hydraulic conductivity (A's) of a glacial till soil in central Iowa when obtained with five different measurement techniques. The five techniques included one laboratory constant-head penneameter method and four in situ methods: disc penneameter, Guelph penneameter, velocity permeameter, and double-tube permeameter. Differences in measured K, values were attributed to differences in sample size, the existence or absence of open-ended macropores, and measurement principles. In this study, we used the different A", estimates in a twodimensional numerical model, CHAIN_2D, to predict water flow into a subsurface tile drain in the same field. Comparisons between predicted and observed tile flows were made during four crop growing seasons. Preferential flow observed in the tile drain during large storm events was predicted best by the model when using A", values measured with the disc permeameter method, which least disturbed the boundary conditions of the flow field and better accounted for the macropore structures of the field soil. Quantitative and qualitative findings suggest that the disc permeameter was best suited for the field site. T SATURATED hydraulic conductivity (Ks) is a key parameter needed for analyzing or modeling water flow and chemical transport in the subsurface soil. Several laboratory and in situ techniques have been developed during the past several decades to measure this parameter. The different techniques often show significant differences in ,KS that reflect inherent experimental or mathematical limitations (Lee et al., 1985; Kanwar et al., 1989; Logsdon et al., 1990; Mohanty et al., 1991, 1994; Paige and Hillel, 1993; Gupta et al., 1993). One logical question that arises is how the different Ks measurements may impact predictions of flow and transport when used in a computer model. This question seems especially important when water flow or solute transport at or near saturation is considered in a macroporous field soil. In other words, Ks probably plays an especially important role in the vadose zone (between the soil surface and shallow groundwater table) during periods following heavy rainfall or irrigation. The objective of our study was to determine the impact of Ks values measured with four different techniques on the ability of the two-dimensional variably-saturated flow and transport numerical model (CHAIN_2D) to predict tile drain outflow from a macroporous, no-till, tile-drained agricultural field. For this study, the Ks measurement techniques included: (i) an in situ Guelph permeameter, (ii) an in situ velocity permeameter, (iii) an in situ disc permeameter, and (iv) a constant-head permeameter in the laboratory using detached soil cores. USDA ARS, U.S. Salinity Lab., 450 W. Big Springs Road, Riverside, CA 92507. Received 14 Nov. 1997. *Corresponding author (bmohanty® ussl.ars.usda.gov). Published in Soil Sci. Soc. Am. J. 62:1522-1529 (1998). MATERIALS AND METHODS