Overview This major land resource area (MLRA 78) occupies about 13 million ha (32.11 million acres) in southern Kansas (0.3% of the total MLRA), western Oklahoma (32.7%), and the panhandle of Texas (67%). Much of the general information in this section was excerpted from USDA (1981).
Landuse About 60% of MLRA 78 is rangeland, and 35% is cropland. The rest of the area is urban land, woodland, or pasture. Nearly all the area is in farms or ranches. Most rangeland is in the west, but some is throughout the area. Ranges and pastures are grazed mainly by beef cattle. Winter wheat and grain sorghum are the major cash crops grown throughout the area. Cotton (Gossypium hirsutum L.) is an important crop south of the Cimarron River; alfalfa and peanuts are important locally. The crops grown under irrigation are also grown in nonirrigated areas. Soil erosion is a major concern of management if the soils are cultivated or if range and pasture are overgrazed.
Elevation and Topography Elevation in MLRA 78 ranges from 500 to 900 m, increasing gradually from east to west. On these dissected plains, the broad divides are nearly level to gently sloping, but slopes are short and steep in the valleys. In places the valleys are bordered by a rolling to steep irregular dune topography. Local relief is mainly in meters, but a few of the larger valleys are tens of meters or more below the general level of the plain. In southwestern Oklahoma, the Wichita Mountains are as much as 300 m above the surrounding plains.
Climate Average annual precipitation in MLRA 78 ranges from 500 to 750 mm, increasing from west to east (Fig. 1). Maximum precipitation is in spring, and the minimum is in winter. Snowfall ranges from 25 cm in the north to 10 cm in the south. The average annual temperature is from 4 to 18 oC, with an average of 185 to 230 freeze-free days. A typical distribution of annual precipitation for the MLRA is shown in Fig. 2.
Fig. 1. Spatial distribution of long-term average annual precipitation in southern region of United States. This precipitation map is based on data from W. H. William at Oak Ridge National Laboratory
Fig. 2. Typical temporal distribution of annual precipitation for MLRA 78. The long-term monthly precipitation data for the graph was obtained from a weather station at Erick, Oklahoma, which is located near the center of the MLRA.
Potential Natural Vegetation This area supports mid and tall grasses. Sand bluestem, little bluestem, and sand sagebrush are dominant on the coarse textured soils. Little bluestem, gramas, and associated grasses and forbs grow on the finer textured soils.
Water Resources The moderate but somewhat erratic precipitation supplies water for range and crops. Small ponds on individual farms provide water for livestock. Some larger ponds on individual farms are used for flood control, recreation, irrigation water, or water for livestock. A few large ponds and reservoirs are a source of municipal water and irrigation water. Rivers are a potential source of water for irrigation. Water in some of the larger rivers is highly mineralized. The deep sand and gravel in valleys yield some ground water. In sloping areas where the underlying sandstone and shale are near the surface, ground water is scarce.
Soils The major soils are Ustolls, Ustalfs, and Ochrepts. They have a thermic temperature regime, an ustic moisture regime, and mixed mineralogy. The nearly level to gently sloping, well drained and moderately well drained, deep Argiustolls (Abilene, Carey, and St. Paul series), Paleustolls (Hollister, Rotan, Sagerton, and Tillman series), and Natrustolls (Foard series) are on uplands. The Ustalfs mainly are deep and sandy or loamy and have a loamy subsoil. They have a thermic temperature regime and mixed mineralogy. The nearly level to undulating or rolling, well drained, deep Haplustalfs (Devol and Grandfield series) and Paleustalfs (Miles, Springer, Wichita, Winters, and Nobscot series) are on uplands. The gently sloping to moderately steep Ustochrepts (Dill, Enterprise, Hardeman, Obaro, Quinlan, Vernon, and Woodward series) are on uplands. Ustifluvents (Clairemont, Lincoln, Mangum, Yomont, and Yahola series) are minor soils on flood plains. The major soil series in MLRA 78 and selected physical properties of the major soils are listed in Tables 1 and 2. STATSGO soils are depicted in Fig. 3.
Fig. 3. STATSGO soils of the Central Rolling Red Plains MLRA 78.
Water and Chemical Transport Limited data on hydraulic properties for selected soils in MLRA 78 exist and well-documented data on chemical transport in the soils of this MLRA is scarce. Ouattara (1977) used a horizontal calibrated capillary tube to measure saturated hydraulic conductivity of Tillman-Hollister clay loam. An average vertical hydraulic conductivity of 0.15 cm day-1 was obtained with a standard deviation of 0.211 cm day-1 for the 130-cm deep soil profiles. The ratio of the hydraulic conductivity values from samples at different depths of the soil profile can be as large as 7 (Fig. 4).
Fig. 4. Saturated hydraulic conductivity of Tillman-Hollister clay loam at different depths.
Davidson (1977, unpublished), Nofziger et al. (1983), and Williams (1980) used modified Buchner funnels to determine soil water retention curves at relatively low suctions (up to 152 cm of water) and pressure plate extractors to determine water contents at higher suctions. They employed instantaneous profile method (Hillel et al., 1972) to measure unsaturated hydraulic conductivity in the field. Tensiometers were installed at 0.15, 0.3, 0.45, 0.6, 0.75, 0.9, 1.2, and 1.5 m depths to determine average hydraulic gradients across soil layers in the hydraulic conductivity measurement while water contents were measured by neutron scattering. Soil cores for retention curve measurements were taken at each tensiometer depth from the edge of the plots with tensiometer installations. Like the saturated hydraulic conductivity in Fig. 4, the retention curves and unsaturated hydraulic conductivity of different layers in a soil profile exhibited significant variations (Figs. 5 and 6).
Fig. 5. Water retention curves of soil samples of different textural classes from three soil profiles of Tipton sandy loam. The three sites are located within a 2-km region.
Fig. 6. Unsaturated hydraulic conductivity for layers of different texture classes from three soil pro-files of Tipton sandy loam. The three sites are located within a 2-km region.
Figures 5 and 6 also exhibit apparent grouping of retention curves and hydraulic conductivity by texture classes (i.e. data points of retention curves of soil samples from different depths and unsaturated hydraulic conductivity of different layers in a soil profile are clustered into separate groups by texture classes). Therefore, retention curves of soil samples of the same texture class from different depths in a soil profile are lumped together for an average retention curve of that texture class. Similar things are done for hydraulic conductivity from different layers of the same texture class in a soil profile. Tables 3 to 4 summarize the typical texture stratification and the hydraulic properties of selected soils in MLRA 78. The data for Cobb loamy sand, Cobb sandy loam, McLain silty clay loam, Port silty clay, Teller loam and Zaneis loam were provided by Davidson (unpublished) while the data on Tipton sandy loam were obtained in a southern region project (Nofziger et al., 1983). The h in Table 4 is soil water potential, q is soil water content, and K is unsaturated hydraulic conductivity. The value in the parentheses after a water content in Table 4 is the standard deviation associated with the water content corresponding to the soil water potential on the left. For a texture class occurring at more than one sampled depth, the standard deviation includes both cross-replicate and cross-depth variation. The measured hydraulic conductivity associated with a specific soil water potential in a soil texture class may vary by 1 order of magnitude in range. It’s worthwhile to notice that the measured hydraulic conductivity for layers of sandy loam texture in Fig. 6 form two separate groups while retention curves of the samples of the same texture class fall into one group in Fig. 5. The hydraulic conductivity of the sandy loam layer between 0- and 15-cm depths is lower than that between 30- and 90-cm depths by almost one order of magnitude. A plausible reason for this discrepancy may lie in the difference in bulk density in these two layers (Fig. 7).
Fig. 7. Bulk density of Tipton sandy loam at different depths.
Literature Cited Davidson, J.M. (unpublished). Measured hydraulic properties of Cobb loamy sand, Cobb sandy loam, McLain silty clay loam, Port silty clay, Teller loam, and Zaneis loam. University of Florida, Gainesville.
Hillel, D., V.D. Krentos, and Y. Stylianou. 1972. Procedure and test of an internal drainage method for measuring soil hydraulic characteristics in situ. Soil Sci. 114:395-400.
Nofziger, D.L., J.R. Williams, A.G. Hornsby, and A.L. Wood. 1983. Physical characteristics of soils of the southern region - Bethany, Konawa, and Tipton series. Southern Cooperative Series Bull. 265, Project No. S-124. Agricultural Experiment Station, Oklahoma State University, Stillwater.
Ouattara, M. 1977. Variation of saturated hydraulic conductivity with depth for selected profiles of Tillman-Hollister soil. Master degree thesis, Oklahoma State University, Stillwater.
USDA. 1981. Land Resource Regions and Major Land Resource Areas of the United States. United States Department of Agriculture, Soil Conservation Service Handbook 296.
USDA. 1994. State Soil Geographic (STATSGO) Data Base data use information. National Cartography and GIS Center, United States Department of Agriculture, National Resources Conservation Service, Fort Worth, Texas.
Williams, J.R. 1980. Spatial variability of unsaturated hydraulic conductivity of Tipton soil series. M.S. thesis, Oklahoma State University, Stillwater.