Water Resources and Erosion Unit Source Watersheds
USDA, ARS, Grazinglands Research Laboratory El Reno, Oklahoma
The experimental facility consists of eight controlled watersheds located on the grounds of the Grazinglands Research Laboratory near El Reno, Oklahoma, about 30 miles west of Oklahoma City. The watersheds are representative of native grass and wheat management in the Reddish Prairies, Major Land Resources Areas in the Southern Plains region of Kansas, Oklahoma, and Texas.
Each of the eight watersheds is 80 [m] wide and 200 [m] long with a drainage area of 1.6 [ha]. The approximate longitudinal slope of the watersheds is between 3 and 4 [%] (Figure 1). The dominant soil is Bethany and Kirkland silt loam, with smaller areas of Milan loam, Aydelotte silt loam, and Renfrow silt loam. Watershed stratigraphy involves undifferentiated Permian Dog Creek Shale and Blaine Formation of the El Reno Group with a varied lithography consisting of reddish brown sand, silt and clay shale and discontinuous beds of gypsum. The watersheds were constructed and instrumented in 1976. Prior to construction all watersheds were in native grass. After construction four of the eight watersheds were cropped into winter wheat, and the other four remained in native grass. The cropped watersheds are surrounded by manmade berms and the grassed watersheds by ridges and natural boundaries. Land use of the watersheds has remained constant since 1976 with variations in land managements as dictated by research objectives.
The instrumentation of the watersheds measure precipitation, soil moisture, ground water, and surface runoff (Figure 2). Runoff from each watershed is measured with precalibrated flumes equipped with water-stage recorders. Sediment discharge is determined from suspended sediment concentration samples taken automatically before, during and after runoff peak flow for each runoff event (from five to 15 runoff samples). Precipitation is measured with four Belford weighing rain gages located around the watersheds. Rain gage charts are available from 1977 to present. New automated rain gages have been installed in February 1998. A nearby comprehensive weather monitoring station provides information on wind, air temperature, humidity and solar radiation. Finally, groundwater levels are measured at 10 groundwater wells surrounding the watersheds. Soil moisture measurements have been made regularly in the past. In early 1998 modern soil moisture measurement equipment has been installed in all the fields and regular measurements are being taken.
The WRE watersheds have been used to address a number of research objectives in the general area of surface runoff and erosion, water quality, spatial variability of soil properties, soil moisture distribution, groundwater levels, impact of land management alternatives, effects of land use and so forth. Current investigations address the impact of livestock grazing on runoff and erosion, and the impact of climate variations on the dynamics of soil moisture availability.