Page Banner

United States Department of Agriculture

Agricultural Research Service


Location: Cropping Systems and Water Quality Research

Title: Lessons Learned from the CEAP Watershed Assessment Studies

item Sadler, Edward

Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: August 5, 2009
Publication Date: August 5, 2009
Citation: Sadler, E.J. 2009. Lessons Learned from the CEAP Watershed Assessment Studies [abstract]. Science to Solutions Reducing Nutrient Export to the Gulf of Mexico, December 9-11, 2009, Des Moines, Iowa. p. 18.

Technical Abstract: The Conservation Effects Assessment Project (CEAP) has two major components: 1) a National Assessment and 2) a Watershed Assessment Study. The National Assessment is conducted using NRCS data and ARS watershed-scale models to provide estimates of conservation benefits at the national scale. The ARS Watershed Assessment Study (WAS) has since 2003 assessed effects and benefits of conservation practices at the watershed scale. The WAS goals are to provide detailed assessments of conservation programs in a few selected watersheds, a framework for improving the performance of the national assessment models, and support for coordinated research quantifying effects of conservation practices across a range of resource characteristics (e.g. climate, terrain, land use, and soils). The results have advanced our knowledge of how watershed scale assessments can capture impacts at multiple scales. They have also improved our understanding of what conservation practices can and cannot provide beyond the edge of a farm field. Fourteen ARS Benchmark Watersheds support watershed-scale assessment of environmental effects of USDA conservation program implementation. Thirteen of these represent primarily rainfed cropland and one is entirely irrigated. Many contain some areas of grazingland, wetlands, and confined animal feeding operations. Environmental effects and benefits have been estimated primarily for water and soil resources, with some assessment of wildlife habitat and air quality benefits in a few watersheds. The benchmark watersheds and their ARS units are located in Pennsylvania, Maryland, Ohio, Indiana, Georgia, Iowa, Missouri, Mississippi, Oklahoma, Texas, and Idaho, with modeling research being conducted in Colorado and Oregon. Of these, the Ohio, Iowa, Missouri, Mississippi, and Oklahoma watersheds drain into the Mississippi River and are thus directly relevant to the Gulf Hypoxia theme of this conference. The Indiana watershed drains to Lake Erie, but represents the conditions in southern Indiana that are within the Mississippi River Basin (MRB). Likewise, much of the research in other watersheds has also provided results that are applicable to the MRB. While it may not be the most earth-shattering finding, the CEAP results to date illustrate how critical it is to fully understand the system being considered for conservation practices. Furthermore, as there are a number of potential contaminants, BMP choices must be balanced among them and accommodate the complex, dynamic environmental interactions associated with each site. Often, we have observed that BMPs implemented for and successful in mitigating one environmental concern have caused an unintended increase in offsite effects of another contaminant. Changing public interests and knowledge have prompted re-interpretations of some research and asked questions that must be pursued using different experimental designs. To fully measure environmental effects of a BMP, we need to know information about flow, sediment, nutrients, and sometimes pesticides, pathogens, and emerging issues such as antibiotics and endocrine disruptors. It is also important to recognize what producers have already done and what limitations (e.g. time, labor, economic) they face. One example of the complexity of BMP interactions is provided by the Iowa watershed studies. In the South Fork of the Iowa River, they found that during the past 150 years, an average of 2.6 feet of sediment had been deposited in the floodplain affecting an area up to 260 feet wide. This deposition and concurrent loss of floodplain storage likely causes higher stages and exacerbated flooding. Furthermore, channel straightening increases flow velocity and exacerbates bank erosion. Another study showed distinguishable land use (watershed treatment) and climate trends (time) through simultaneous shifts in how energy (evaporative demand) and water (precipitation) were partitioned. In four Midwest watersheds, the observed trend toward increasing discharge was more attributable to climate change than land-use change. Their assessment methodology allows subtle cause-effect relationships to be teased out of an extremely dynamic hydrologic signal. At several CEAP watersheds, including MS, IA, and MO, it was demonstrated that channel sources of sediment are dominant at certain times and are substantially larger on a long-term basis than previously thought. Thus, upland erosion practices may have less impact on watershed outlet sediment load than stream bank stabilizing practices. Reducing the sediment load is relevant to hypoxia because of nutrients, particularly P, that bind to and are transported with sediment. Reducing sediment alone may not be sufficient to reduce environmental effects if the dissolved nutrient load is unchanged and significant. In this regard, several BMPs that substantially reduce erosion and sediment transport may actually exacerbate dissolved phase transport. The net effect depends on the balance between sediment-bound and dissolved nutrients, and there is no single answer that dominates. In several locations, tile drainage, pothole risers, and parallel terrace risers have been shown to allow dissolved nutrients to bypass parts of the landscape and deposit directly into streams. In an effort to maintain trafficability, productivity, and erosion control benefits of these practices, a number of BMPs have been studied to mitigate nutrient loads before discharge into the stream or drainage ditch. These include blind or French drains instead of risers in potholes (IN), C-rich materials placed between the tile and the ditch for tile drainage (IA), and wetlands or buffers intercepting underground outlets from parallel terraces (MO). Another approach in OH and elsewhere has been to manage wintertime flow by raising flashboard risers or other devices, thus reducing the flow of dissolved nutrients by decreasing off season flow. Modeling results from a number of watersheds suggests that local knowledge of the systems is required to satisfactorily calibrate and validate watershed-scale models of hydrology and contaminant transport. Knowledge of unusual soils or land use practices, which is beyond the local information on land use, soils, and weather, has usually been incorporated into the models and contributed to making them more applicable to other contexts. This illustrates that modeling, without benefit of the local information, may arrive at less than optimal results. Two critical lessons learned from WAS research are that a number of unanswered questions remain to be answered and .some may prove troublesome. For instance, catastrophic episodes of flooding and erosion have been observed and found important in the long- and broader timeframe. At present, this remains a critical need for research. Another question the CEAP watershed studies should be poised to address is the effect of climate change and the agricultural impacts that will be felt as a result. Substantial progress toward scaling results is expected based on preliminary indications. Continued improvement in performance of watershed models is expected as a result of research in these watersheds. From a national perspective, the benchmark watershed studies have provided and are expected to continue to provide rigorous science in support of conservation policy.

Last Modified: 4/18/2014
Footer Content Back to Top of Page