Runoff and soil loss relationships for the Texas Blackland Prairies ecoregion

Summary

Hydrological and soil loss data have been collected since 1937 at the USDA-ARS Grassland Soil and Water Research Laboratory near Riesel, TX. Data from the site, originally named the Blacklands Experimental Watershed, have played a vital role in the evaluation of conservation management practices to limit soil erosion and offsite herbicide transport and in the development of several watershed models used worldwide. The entire record of precipitation, runoff, sediment loss, management practices, and limited meteorological information is publicly available (http://www.ars.usda.gov/spa/hydro-data). The data represent a valuable regional resource for use in water supply modeling, rural land development, and agricultural land management. Results of the present analyses confirmed the importance of soil–water phases to temporal runoff patterns in the Texas Blackland Prairies ecoregion. Little runoff occurs in the “dry” soil–water phase (avg. = 2–9 mm/month), but substantial surface runoff and lateral subsurface return flow occurs in the “saturated” phase (avg. = 19–28 mm/month). Strong linear relationships (P < 0.0001) were determined between watershed size and annual peak flow rates for return intervals from 2 to 100 yr. Long-term data indicate a drastic reduction in soil loss from small grain production compared to row crop production due to the presence of soil cover in both the spring and fall high precipitation periods. Thus, utilization of a winter cover crop in row crop production or conversion from row crop to small grain production can be effective in reducing offsite transport of sediment and associated contaminants, which may be important in watersheds with substantial agricultural contribution to water quality impairment.

Introduction

Historical hydrologic and sediment transport data are valuable in many applications including agricultural conservation practice design, urban development guidance, water resource planning, flood control, and ecological management. In the absence of regional data, relationships often are used that were established for areas with different hydrologic and climatic regimes. When extrapolated data are used in such applications, substantial uncertainty is added to already uncertain situations. Therefore, when long-term records are available they should be relied upon to guide water resource management and planning.

Baseline studies on local runoff processes are needed to guide development design and to adequately address land conversion issues and urban sprawl (Marsh and Marsh, 1995). Flow data are necessary for calibration and validation of water supply and water quality models, which are used to explore water resource scenarios. These data are also needed for the optimal design of hydraulic structures (e.g. dams, culverts, and detention basins). The lack of adequate data has been recognized for some time as a cause for failure of hydrologic structures but more commonly as contributing to unnecessarily conservative safety factors in structure design (USDA-SCS, 1942). Additionally, soil erosion and sediment transport data are valuable in estimating detention basin and water supply reservoir design factors. Historical region-specific soil erosion and sediment data in conjunction with modeled rates under future scenarios can provide the most accurate design life estimates. Simon et al. (2004) highlighted the need for ecoregion-specific sediment transport data because of the effect of sediment on stream channel stability and aquatic ecosystem health.

In many areas in Texas and other states, the diverse demands of agricultural water requirements and rapidly increasing urban populations stress available water supplies. One such area is Texas Blackland Prairies ecoregion (Omernik, 1987). This region contains Austin-San Marcos, one of the top ten fastest growing metropolitan areas in the US, and Dallas-Fort Worth, the fastest growing metropolitan area containing over 5 million people (US Census Bureau, 2001). Expensive and litigious water quality projects involving total maximum daily loads (TMDLs) are also underway. In this and other regions with intense water resource conflicts, measured flow and sediment transport data are especially valuable because cities, industry, legal interests, and regulatory agencies are hesitant to make decisions with estimates or information extrapolated from other regions.

It was in this Texas Blackland Prairies ecoregion that the Blacklands Experimental Watershed was established in 1937 (Fig. 1). The experimental watershed facility near Riesel, TX, is now part of the USDA-ARS Grassland Soil and Water Research Laboratory with headquarters in Temple, TX. In the mid-1930s, the United States Department of Agriculture Soil Conservation Service (USDA-SCS), now the Natural Resources Conservation Service (NRCS), realized a need to analyze and understand hydrologic processes on agricultural fields and watersheds because of their impact on soil erosion, flood events, water resources, and the agricultural economy. The research program of the Hydrologic Division of the SCS established a number of experimental watersheds across the US. Riesel was one of the three original watersheds, which were designed to collect hydrologic data (precipitation, percolation, evaporation, runoff) and to evaluate the hydrologic and soil loss response as influenced by various agricultural land management practices (USDA-SCS, 1942).

Hydrologic data collection at the Riesel experimental watersheds began in 1937 and continues to the present. It is one of the most intensively monitored small watershed research sites in the US. The continuous hydrologic records are particularly valuable for studies designed to identify trends or changes caused by climate shift or other factors and are necessary to determine the influence of extreme, rare events (Edwards and Owens, 1991). These data have been used for numerous purposes such as water quality studies (Kissel et al., 1976, Sharpley, 1995, Harmel et al., 2004), farming practice evaluations (Baird et al., 1970, Baird and Knisel, 1971, Chichester and Richardson, 1992), and natural resource model application and development (Williams et al., 1971, Arnold and Williams, 1987, Richardson and King, 1995, King et al., 1996, Ramanarayanan et al., 1998, Harmel et al., 2000).

The USDA-ARS experimental watershed databases are particularly valuable for field- to farm-scale research and design because of their long-term, detailed, continuous record on multiple watersheds. These small watershed data are vital to properly evaluate runoff and sediment transport processes from single land use, relatively homogeneous watersheds and to differentiate mechanisms for various land use conditions. Sediment transport and flow data collected at larger scales are often influenced by dams, channel processes, and variable land management, which alter sediment and discharge routing and generally confounds interpretation of smaller-scale land management. However, small watershed data are available only for limited regions of the US because of the resource commitment necessary to implement and maintain such data collection networks. In contrast, the United States Geologic Survey (USGS) operates a nationwide hydrologic monitoring network that has historically provided larger-scale data, which are needed to evaluate integrated effects and downstream routing processes. For years, the USGS hydrology database has provided valuable information such as peak flows, return intervals, and low flow durations, which have led to knowledgeable decisions that made without hydrologic data would have resulted in high costs to the US.

In a review of the importance of hydrometeorological data, Slaughter (2000) points to the need for the collection and preservation of long-term, spatially diverse data. These data are vital for research and planning related to water resources, climate change, ecological preservation, and the global food supply. The value of these data should not be disregarded in decisions made concerning budgets for monitoring programs. The USDA-ARS in cooperation with other federal agencies and programs (such as: USGS, USDA Forest Service, and National Science Foundation) has a unique opportunity and responsibility to provide continuous, watershed-based information to state and local governments, universities, and private organizations that will continue to need these data (Slaughter and Richardson, 2000). With these needs in mind, personnel at several USDA-ARS watershed networks have recently published long-term data and analyses on precipitation (Hanson, 2001, Nichols et al., 2002, Harmel et al., 2003) and discharge and sediment transport (Pierson et al., 2001, Van Liew and Garbrecht, 2003).

The specific objective of this study is to provide a summary and analysis of hydrologic and sediment loss data for the Texas Blackland Prairies ecoregion. The selected analyses, along with the publicly available data, should provide valuable information on water resource and erosion control management in the Texas Blackland Prairies ecoregion and in other Vertisol-dominated areas. When used in conjunction with regional USGS relationships for larger watersheds (e.g. Lanning-Rush, 2000, Raines, 1998), the analyses should support policy decisions in this region with rapidly growing population, increasing urban development, and intense debates over urban and agricultural contribution to water quality concerns.