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a Dep. of Soil and Atmospheric Sciences, Univ. of Missouri-Columbia, 302 Anheuser-Busch Natural Resources Building, Columbia, MO 65211
b USDA-ARS, Univ. of Missouri, Columbia,Columbia, MO 65211
c Dep. of Biological Engineering, Univ. of Missouri, Columbia, 269 Agricultural Engineering Building, Columbia, MO 65211
* Corresponding author (hb91d{at}mizzou.edu)
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Abbreviations: EC, electrical conductivity • Keff, effective hydraulic conductivity • Ksat, saturated hydraulic conductivity • REV, representative elementary volume • SAR, Na adsorption ratio • WEPP, Water Erosion Prediction Project
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Reports have found that measurements on small samples (<100-mm diam.) tend to give higher Ksat values than do measurements on larger samples (Bagarello and Provenzano, 1996). The values of small samples are also questioned because samples are too small to embody a representative elementary volume (REV) of soil. The REV is a conceptual unit representing the smallest volume of a soil unit (Mallants et al., 1997). Its actual dimensions are ill defined. Bouma (1980) suggests three REV sizes for Ksat determinations: 100 cm3 for sand, 1000 cm3 for silt, and 10000 cm3 for clay soils. As a sample size increases, variability in Ksat values is expected to decrease.
The use of the REV is thought to reduce the sample-size dependence of Ksat, and thus facilitate better measurements (Mallants et al., 1997). Samples based on the REV often reflect the natural boundary conditions (Gupta et al., 1993), and diminish disturbance and compaction of soil during sampling (Vepraskas and Williams, 1995).
Soil texture is generally known to affect Ksat. Clay soils typically have low Ksat values (Bouma, 1980; Jamison and Peters, 1967). This is of interest in the midwest USA because about 4 million ha of claypan soils exist in this region (Jamison et al., 1968). These soils have an argillic horizon 130 to 460 mm deep, with clay contents >450 g kg-1 and are very slowly permeable although published data are limited (Jamison and Peters, 1967).
Because of the argillic horizon, claypan soils may perch water and create lateral flow. A study of claypan hydrology suggests that runoff rates may be equal to rainfall under saturated conditions (Saxton and Whitaker, 1970). Furthermore, studies of runoff and rainfall data from the McCredie rainfall-erosion plots near Kingdom City, MO, indicate that lateral flow known as interflow may be a significant component of the total runoff during 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, detailed in situ lateral Ksat studies have not been conducted for Missouri claypan soils because measurements are costly and time-consuming (Blevins et al., 1996). Lateral Ksat measurements are also limited elsewhere (Ahuja and Ross, 1983; Wallach and Zaslavsky, 1991). The need for in situ lateral Ksat determination for Missouri claypan soils has been recognized because of the probability of interflow (Jamison et al., 1968; Wilkinson and Blevins, 1999). Information on in situ lateral Ksat through the horizons above the claypan is important for determining their ability to conduct water laterally and assessing runoff and erosion.
Many have characterized the vertical Ksat for claypan soils (Doll, 1976; Zeng, 1994). However, most of the measurements were made only for the surface horizons (Jamison and Peters, 1967; McGinty, 1989), therefore, studies of Ksat variations with depth are few. Because of their hydrologic attributes, claypan soils probably have quite different effective Ksat values with depth from other Alfisols. The information on Ksat depth distribution would be valuable in explaining the claypan hydrology and for characterization of variability in horizons of low and high permeability required for accurate flow studies.
Because the Ksat values may vary by measurement method (Bouma, 1980; Bagarello and Provenzano, 1996; Mallants et al., 1997), the available Ksat data on these soils need to be studied to determine their consistency and uniformity by method. Data from such measurements should be statistically the same to be used for hydrologic prediction and modeling.
Since knowledge of Ksat is essential for the use of water flow models, it is useful to evaluate the influence of measured Ksat on modeled runoff. One modeling approach for erosion/runoff prediction is the WEPP. This model has been extensively used for runoff prediction since 1995 when it was publicly released by the USDA-ARS. Although Ksat is not the only factor that affects runoff, the WEPP model incorporates the estimated values of hydraulic conductivity as an important soil attribute to predict runoff (Flanagan and Nearing, 1995). The WEPP uses Keff values for surface layers, and internally computes Ksat values for subsurface soil layers. Studies indicate that runoff predictions are sensitive to the initial Keff values (Ghidey et al., 1999).
The objectives of this study are to: (i) measure lateral in situ Ksat of the 0- to 230-mm depth (above the claypan) using 250 mm wide by 500 mm long soil monoliths, (ii) measure the Ksat with and without bentonite of 76-mm diam. soil cores taken at 100-mm intervals to a depth of 2 m, (iii) compare the core Ksat vs. lateral in situ Ksat, (iv) compare previously measured Ksat data for Missouri claypan soils with results of this study, and (v) compare measured runoff vs. WEPP predicted runoff using measured Keff as a model input to illustrate the benefit of using measured Keff values.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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The monolith set-up had three compartments: (i) water supply pit, (ii) soil monolith, and (iii) water collection pit. Two 6.3-mm steel plates 500 by 700 mm were installed vertically along the two sides of the monolith, which allowed a 100-mm of additional length of steel plate at each end. The additional length allowed for construction of a water supply pit and a water collection pit. Two plates 250 by 500 mm were installed vertically at each end of the border to form a rectangular box. The steel plates extended from 150 mm above to 350 mm below the soil surface. Silicone caulking was used to waterproof the steel plate seams. A 20-mm discharge hole was made at the lower end of the collection pit. A divider screen was made from a metal screen with geotextile material, which separated the soil monolith from the collection and supply pits. A bentonite-slurry was used to seal the soil–steel plate interfaces. The excavated trench was backfilled with the original native soil.
Measurement of Lateral in situ Ksat
Monoliths were slowly wet for 48 h. The electrical conductivity (EC) of the in situ water used was 0.71 dS m-1, and the Na adsorption ratio (SAR) of the in situ water was 2.39. Once the monoliths were satiated, water was added to the supply pit using a Mariotte bottle for maintaining a constant head and measuring the inflow rate. When the water level rose to the soil surface in the collection pit, excess water flowed through the 20-mm discharge hole. Plastic tubing routed the outflow for measurement.
The monolith lateral Ksat was measured by applying water from the water supply pit and measuring outflow in the collection pit for 5 h. A difference in hydraulic head of 16 mm was measured along the in situ pedon. A polyethylene tent was used to cover the plot throughout the measurement to minimize water loss from evaporation. Time, inflow and outflow volumes, and hydraulic gradient were recorded to facilitate calculation of the lateral Ksat.
The time to steady flow conditions was 48 h. After 12 h of satiation, 18.9% of applied water was moving downward through the soil. Downward movement decreased to 4.8% of total inflow after 24 h. This continued to decrease to 1.5% when the plots were satiated for 48 h. Downward flow through the claypan was obtained by subtracting outflow from the inflow. Measurements were initiated when downward flow was 1.5%.
Laboratory Ksat
One hundred eighty soil cores were taken within 10 m of the in situ study sites, to determine the Ksat distribution with depth, and to facilitate comparison of lateral in situ Ksat with Ksat determined on small intact cores. Nine intact 76-mm diam. soil cores were collected every 100 mm with depth to 2 m using a core sampler (Blake and Hartge, 1986). A replicate area near each monolith was used to collect cores in a vertical orientation when the soil was slightly below field capacity. Samples were transported to the laboratory, and slowly wet from the bottom with tap water using a Mariotte bottle having a supply rate of about 3 mm h-1. The EC of the tap water used was 0.68 dS m-1, and the SAR of the tap water used was 2.34. Cores collected above the 200-mm depth were wet for 24 h. Cores collected at or below the claypan, were wet for 7 d. Measurements for samples with higher Ksat were determined with a constant head, and those with low Ksat were determined with a falling head (Klute and Dirksen, 1986).
Visible macropores (
1 mm) and interfacial voids located between the soil and the cylinder wall on a set of the cores were plugged using a bentonite-slurry. The reason for using this slurry was to eliminate the free flow of water through these macropores and voids. Elimination of bypass flow in small cores during Ksat determinations is a recommended methodology (Smith and Browning, 1946; Klute, 1965; Fadl, 1979). Blocking of macropores may seem at odds with the goal of estimating in situ Ksat, which measures flow through the naturally occurring macropores. However, a problem arises when small, 76-mm cores are used for Ksat measurements. Macropore continuity in field conditions is intact while this continuity is broken in small cores. These macropores are commonly finite in small cores and are often rapid pathways for bypass flow because of differences in the boundary conditions between in situ and core measurements. The dominant saturated flow in small cores is mainly via these macropores rather than through the soil matrix.
The Ksat values with bentonite were compared with those measured without bentonite to assess the effectiveness of the bentonite. A t-test was used to examine the hypotheses that the lateral in situ Ksat and laboratory Ksat determinations of the topsoil were not different by assuming anisotropic conditions (SAS Institute, 1985). This assumption is well supported by studies, which indicate that Ksat within the plow layer of silt loam soils is not appreciably influenced by core orientation (Dabney and Selim, 1987).
Comparison of Existing Ksat Data for Missouri Claypan Soils
The consistency of available Ksat data for the claypan soils was evaluated by comparing previously collected Ksat data with the results from this study. Data are based on studies of Jamison and Peters (1967), Doll (1976), McGinty (1989), Zeng (1994), and Baer and Anderson (1995). Soil characteristics and land use of the study sites are in Table 1. For data from the current study, a 95% confidence interval of the mean was calculated using the pooled variance of the Ksat with and without bentonite data of each depth separately.
Runoff Prediction Using Existing Ksat Data as Input for the WEPP Model
The study of Ksat influence on runoff prediction was conducted by using the WEPP Hillslope model (Version 98.4) on a single event basis using the input of Keff. The Keff input values for WEPP runoff prediction were determined using the Ksat measured on 76 by 76 mm diam. soil cores. The Keff was calculated as:
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The Keff was computed for the horizons within the upper 0 to 300 mm because this depth has soil that is much more permeable (Ksat 
71 mm h-1) than the underlying very slowly permeable argillic horizon (Ksat 1.83 µm h-1). Hence, the topsoil Ksat would largely control water flow in saturated conditions. The 0- to 300-mm depth reflects the inherent soil properties of this permeable soil. The best approach for Keff estimation would likely be to evaluate soil properties with depth on a case-by-case basis and allow the soil profile to direct the depth chosen for Keff estimation. However, this approach may be too costly and time-consuming for routine use.
The procedure used to compute Keff is different from that estimated internally by WEPP which predicts Keff based on approximate relationships with soil properties (Zhang et al., 1995). The predicted Keff determined by WEPP is optimized using measured runoff data from a database derived from multiple plots for various soil types. For instance, the Keff for the surface soil of the Mexico claypan soil is 0.34 mm h-1 (Nearing et al., 1996). The WEPP estimate of Keff is useful when measured Ksat data are not available. Because we had measured Ksat from five studies on Missouri claypan soils, we computed Keff for each study to evaluate the effect of Ksat variability on predicted runoff.
The computed Keff was used as an input parameter while using other WEPP input parameters as reported by Ghidey and Alberts (1996) for Missouri claypan soils. The only input parameter that was changed in this study was the Keff for the surface 300-mm depth. Below this depth, the WEPP internally predicted Keff values were used for runoff prediction (Zhang et al., 1995). The WEPP model requires four input files containing information on climate, slope, soil, and crop management to estimate runoff (Flanagan and Nearing, 1995). Ghidey and Alberts parameterized the required input files of WEPP Hillslope Model (Ver. 95.7) using measured runoff and soil data from long-term runoff-erosion plots at (McCredie) Kingdom City, MO.
The WEPP predicted runoff was compared with measured runoff data collected from the natural rainfall erosion plots located at the Midwest Research Claypan Farm (Ghidey and Alberts, 1996). The runoff-erosion plots were managed in no-till corn (Zea mays L.) for an 11-yr period (1983–1993). Only the 11 largest rainfall events were selected for study when runoff was likely to occur. Data from the no-till corn plots were used because these plots had the most protective crop residue (
95% residue cover), and thus Ksat would not be greatly reduced by surface seal from rainfall. Table 2 indicates that prior to the reported dates of largest rainfall event the soil was practically saturated in 1984, 1985, 1988, and 1993, and near saturation in 1983, 1986, 1987, and 1989 through 1992. Based on these data, the use of Keff using measured Ksat for runoff prediction was considered appropriate.
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RESULTS AND DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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9 µm h-1. Results suggest that the argillic horizons were a barrier directing the vertical flow horizontally above the claypan as lateral flow. A perched water table is thus likely as these soils are ponded for several hours. These results support earlier findings of Jamison and Peters (1967), and Saxton and Whitaker (1970), who reported the occurrence of lateral flow in these soils.
Comparison of Ksat Determined on in situ Monoliths and Intact Cores
The Ksat values for cores without bentonite were significantly larger than for the monoliths (P = 0.033). The mean value was four times more (312 ± 58 mm h-1) than for the monoliths' Ksat values (72 ± 0.7 mm h-1). The difference between the Ksat of the monoliths and the Ksat of intact cores with bentonite (71 ± 1.1 mm h-1) was not significant (P = 0.50). Inspection of the cores showed numerous vertically oriented macropores produced by flora and faunal biological activity (biochannels). Water flow through cores without bentonite was largely governed by flow through macropores extending throughout cores, and resulted in Ksat values that were unrealistically high. Under field conditions, such macropores would be expected to terminate in the subsoil because of the decrease in porosity with depth, soil swelling, and from clay hydration, and are thus much less conductive when satiated than continuous open-ended macropores in cores. Since the Ksat of cores with bentonite is similar to in situ values, the bentonite technique may be used to approximate laboratory Ksat to the Ksat of in situ soils. Small cores may not reflect the in situ Ksat values if continuous macropores that dominate flow in small cores are not eliminated.
The macropore effect on water flow is also a function of the pore orientation. A macropore extending vertically throughout a 76-mm core causes a higher Ksat value compared with values under field conditions. In contrast, a laterally oriented macropore in a core conducts less or no water because free water will not enter the pore, thus reducing the Ksat value (Hillel, 1998). This study found that cores having macropores visible at the exposed surface that were oriented vertically produced four times larger Ksat values compared with in situ measured Ksat, whereas Ksat values measured with bentonite injected to eliminate this effect were not statistically different from in situ measured Ksat values.
The Ksat Profiles with Depth of Intact Cores
Profile plots of Ksat are shown in Fig. 1
. Data show that Ksat with bentonite was significantly lower (P = 0.007) than Ksat without bentonite throughout the profile. Measurement of Ksat on cores without bentonite had higher conductivities even for samples within the claypan with high montmorillonitic clay content. This high clay content is commonly thought to increase swelling and thus close macropores reducing Ksat values. However, the measured data suggest this notion is not correct. The mean Ksat without bentonite (312 ± 58 mm h-1) is four times greater than Ksat with bentonite (71 ± 1.1 mm h-1) for the surface 100 mm of soil. The comparison of Ksat of cores with and without bentonite indicates that 90% of water flow through cores from the upper 100 mm of the soil can be conducted by the macropores. McGinty (1989) also measured the Ksat of 76-mm diam. cores without bentonite on claypan soils and found high Ksat values for the surface soil (333 mm h-1). This work was done on soil samples collected from no-till sites where some macropores were present and very likely were not closed, and thus conducted water very rapidly.
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Comparison of Ksat Determinations for Missouri Claypan Soils
Figure 1 shows the Ksat measured on selected Missouri claypan soils. The Ksat decreases with depth because of changes in soil density, texture, and structure. The Ksat values with bentonite were nearly 10 times greater than the Ksat values measured by Doll (1976), and Baer and Anderson (1995). The Ksat values were nearly five times less than those reported by McGinty (1989) for the upper 200-mm depth. The Ksat values reported by Zeng (1994) on 76-mm cores were 1.3 times greater than the Ksat with bentonite.
The variation in Ksat presented in Fig. 1 is mainly attributable to (i) the variations in depth to the claypan among the studies, (ii) the presence of conductive macropores, and (iii) the method of Ksat determination. First, the depth to claypan varies between 130 and 370 mm with an average of 250 mm (Jamison and Peters, 1967). Samples taken by previous investigators from different sites at the same depth may also have differed in clay content and bulk density, altering Ksat values (Table 1). The low Ksat values found by Baer and Anderson (1995) for example, may be explained because their samples were collected from severely eroded soil that had exposed the claypan. Secondly, claypan soils often have abundant macropores (Jordan et al., 1997). Small cores may overestimate Ksat values particularly for surface depths with abundant macropores. This is shown in Fig. 1 where the Ksat without bentonite is about four times higher than that the Ksat with bentonite. The Ksat values by McGinty (1989) are higher than the KSat values with bentonite because his measurements were made without bentonite, and had large macroporosity (about 3–5% porosity in the size range of 1- to 2-mm diam.). Thirdly, the Ksat variation may be due to different methods and different aspects of measurement (with vs. without macropores). For example, Jamison and Peters (1967) determined the Ksat with the double tube method, Doll (1976) used the crust method, and the core Ksat in this study was measured with and without bentonite.
Influence of Ksat on Modeled Runoff Prediction
Process-based hydrologic models require input of Ksat. However, model users often have limited access to measured data and thus use published or estimated values. Studies of claypan soils indicate that Ksat values may vary by 100 times due in part to spatial and temporal variability (Fig. 1). This variability in input Ksat has the undesirable effect of producing variable and inaccurate model predictions.
The impact of Ksat variability on runoff was evaluated by performing the WEPP runoff prediction using measured Ksat from selected studies for the Missouri claypan soils. The Keff values for the surface 300-mm depth are: Blanco (with bentonite) = 1.3 mm h-1, Doll = 2.7 mm h-1, Blanco (without bentonite) = 3.4 mm h-1, Baer and Anderson = 5.4 mm h-1, Zeng = 8.2 mm h-1, and McGinty = 183 mm h-1. Prediction results show runoff to vary greatly in response to changes in Keff input. The Keff of the other studies is significantly higher than the Keff with bentonite and underestimated the observed runoff. As expected, higher Keff values produce lower predicted runoff. Figure 2 compares the measured runoff with the predicted runoff using the Keff with and without bentonite (McCredie, MO), and the highest Keff (Novelty, MO) reported by McGinty. The effect on WEPP predicted runoff of using an Keff without bentonite measured at 40-mm runoff was 29 mm versus about 39 mm when using an Keff with bentonite. This indicates that the use of Keff without bentonite underestimated the runoff by 28% at a measured runoff of 40 mm. Use of Keff value calculated from cores with bentonite most closely correlated with the observed runoff (Fig. 2). This is attributed to the fact that Ksat measured with bentonite excluded macropore flow through continuous macropores in small cores and thus better reflected the in situ conditions where the water flow in macropores is reduced when the soil is saturated.
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The sensitivity index for the WEPP results reflects the change in runoff with respect to change in Keff. It was the greatest (0.25) for the highest Keff reported by McGinty indicating that for a 100% increase in Keff, runoff would be increased by 25%. The sensitivity values of other studies were: 0.10 for Zeng, 0.08 for Baer and Anderson, 0.07 for Blanco (without bentonite), 0.05 for Doll, and 0.04 for Blanco (with bentonite). This last sensitivity value was obtained using in situ Ksat values for comparison. Predicted runoff was sensitive to changes in Keff, indicating that Ksat is a critical parameter for obtaining accurate runoff estimates (Fig. 2). Indeed, Flanagan and Nearing (1991) stated that hydraulic conductivity is one of the most sensitive soil input parameters in predicting runoff. Consequently, model users should be cautious in using estimated Ksat without proper evaluation of its accuracy.
As suggested by Kutilec and Nielsen (1994), use of a pedotop-scale model will likely improve Ksat estimation for use as model input by accounting for some of the spatial variability in Ksat (the pedotop-scale consists of a surface area overlying similar soil REV units typically totaling from 100 to 1000 m2 in size). Laboratory Ksat of small cores should only be regarded as a rapid approximation of field conditions rather than a representative measure of the pedotop Ksat. Careful consideration of measurement method, presence of biochannels, natural variations in soil depth, and use of pedotop scaling should be pursued to improve the Ksat estimation.
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ACKNOWLEDGMENTS |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Received for publication July 25, 2001.
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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