Saturated Hydraulic Conductivity and Its Impact on Simulated Runoff for Claypan Soils Humberto Blanco-Canqui,* Clark J. Gantzer, Stephen H. Anderson, E. E. Alberts, and F. Ghidey ABSTRACT surements (Mallants et al., 1997). Samples based on the REV often reflect the natural boundary conditions Saturated hydraulic conductivity (K sat ) is an essential parameter (Gupta et al., 1993), and diminish disturbance and com- for understanding soil hydrology. This study evaluated the K sat of in situ monoliths and intact cores and compared the results with other paction of soil during sampling (Vepraskas and Wil- studies for Missouri claypan soils. These K sat values were used as liams, 1995). runoff-model inputs to assess the impact of K sat variation on simulated Soil texture is generally known to affect K sat . Clay runoff. Lateral in situ K sat of the topsoil was determined on 250 by soils typically have low K sat values (Bouma, 1980; Jami- 500 by 230 mm deep monoliths. These values were compared with son and Peters, 1967). This is of interest in the midwest the K sat of 76 by 76 mm diam. intact cores with and without bentonite USA because about 4 million ha of claypan soils exist to seal macropores. Mean ( SD) lateral in situ K sat was 72  0.7 in this region (Jamison et al., 1968). These soils have mm h -1 and mean intact core K sat without bentonite was 312  58 an argillic horizon 130 to 460 mm deep, with clay con- mm h -1 . The mean intact core K sat without bentonite was significantly tents 450 g kg -1 and are very slowly permeable al- larger than the lateral in situ K sat (P = 0.03). The lateral in situ K sat though published data are limited (Jamison and Pe- was not different from core K sat with bentonite (71  1.1 mm h -1 ). The intact core K sat with bentonite differed from previous studies by ters, 1967). 10 times. This was attributed to the variations in soil depth to claypan, Because of the argillic horizon, claypan soils may macropore presence, and methodology. The impact of using an effec- perch water and create lateral flow. A study of claypan tive hydraulic conductivity (K eff ) computed from measured K sat on hydrology suggests that runoff rates may be equal to intact cores without bentonite underestimated the Water Erosion rainfall under saturated conditions (Saxton and Whitaker, Prediction Project (WEPP) simulated runoff by 28% for a measured 1970). Furthermore, studies of runoff and rainfall data runoff event of 40 mm. The core K sat with bentonite was correlated with from the McCredie rainfall-erosion plots near Kingdom measured runoff from long-term erosion-runoff plots. A quadratic City, MO, indicate that lateral flow known as interflow regression explained 95% of the variability between measured and may be a significant component of the total runoff dur- simulated runoff. ing springtime when precipitation is usually the most intense and the erosion rates are the highest (Minshall and Jamison, 1965; Ghidey and Alberts, 1998). To date, S aturated hydraulic conductivity is an essential detailed in situ lateral K sat studies have not been con- parameter for understanding soil water movement. ducted for Missouri claypan soils because measurements It is a fundamental input for modeling runoff, drainage, are costly and time-consuming (Blevins et al., 1996). and movement of solutes in soils (Mallants et al., 1997). Lateral K sat measurements are also limited elsewhere While K sat is widely studied, questions remain about (Ahuja and Ross, 1983; Wallach and Zaslavsky, 1991). how sample size and boundary conditions influence The need for in situ lateral K sat determination for Mis- its determination. souri claypan soils has been recognized because of the Reports have found that measurements on small sam- probability of interflow (Jamison et al., 1968; Wilkinson ples (100-mm diam.) tend to give higher K sat values and Blevins, 1999). Information on in situ lateral K sat than do measurements on larger samples (Bagarello and through the horizons above the claypan is important for Provenzano, 1996). The values of small samples are also determining their ability to conduct water laterally and questioned because samples are too small to embody a assessing runoff and erosion. representative elementary volume (REV) of soil. The Many have characterized the vertical K sat for claypan REV is a conceptual unit representing the smallest vol- soils (Doll, 1976; Zeng, 1994). However, most of the ume of a soil unit (Mallants et al., 1997). Its actual measurements were made only for the surface horizons dimensions are ill defined. Bouma (1980) suggests three (Jamison and Peters, 1967; McGinty, 1989), therefore, REV sizes for K sat determinations: 100 cm 3 for sand, studies of K sat variations with depth are few. Because 1000 cm 3 for silt, and 10 000 cm 3 for clay soils. As a of their hydrologic attributes, claypan soils probably sample size increases, variability in K sat values is ex- have quite different effective K sat values with depth from pected to decrease. other Alfisols. The information on K sat depth distribu- The use of the REV is thought to reduce the sample- tion would be valuable in explaining the claypan hydrol- size dependence of K sat , and thus facilitate better mea- ogy and for characterization of variability in horizons of low and high permeability required for accurate H. Blanco-Canqui, C.J. Gantzer, and S.H. Anderson, Dep. of Soil and flow studies. Atmospheric Sciences, Univ. of Missouri-Columbia, 302 Anheuser- Busch Natural Resources Building, Columbia, MO 65211; E.E. Al- Because the K sat values may vary by measurement berts, USDA-ARS, and F. Ghidey, Dep. of Biological Engineering, method (Bouma, 1980; Bagarello and Provenzano, 1996; Univ. of Missouri, Columbia, 269 Agricultural Engineering Building, Columbia, MO 65211. Contribution of the Missouri Agric. Exp. Stn. Abbreviations: EC, electrical conductivity; K eff , effective hydraulic Journal. No. 13159 Received 25 July 2001. *Corresponding author conductivity; K sat , saturated hydraulic conductivity; REV, representa- (hb91d@mizzou.edu). tive elementary volume; SAR, Na adsorption ratio; WEPP, Water Erosion Prediction Project. Published in Soil Sci. Soc. Am. J. 66:1596–1602 (2002). 1596