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Title: Development of Duration-Curve Based Methods for Quantifying Variability and Change in Watershed Hydrology and Water Quality

Author
item MORRISON, M - EPA
item Bonta, James - Jim

Submitted to: Environmental Protection Agency
Publication Type: Government Publication
Publication Acceptance Date: 12/11/2007
Publication Date: 5/1/2008
Citation: Morrison, M.A., Bonta, J.V. 2008. Development of Duration-Curve Based Methods for Quantifying Variability and Change in Watershed Hydrology and Water Quality [abstract]. Environmental Protection Agency. Available: http://www.epa.gov/nrmrl/pubs/600r08065/600r08065.pdf.

Interpretive Summary: During the past decades, the U.S. Environmental Protection Agency (EPA), the U.S. Department of Agriculture (USDA), and other federal program administrative and regulatory agencies spent considerable amounts of time and money to manage risks to surface waters associated with agricultural activities, urbanization, and other avenues of nonpoint source pollution. A variety of best management practices (BMPs) exist for this purpose and have been installed throughout the country, yet very little is known about their overall effectiveness in reducing stressors at the watershed scale. The objective of this research is to explore and develop uniform methods for simple quantification of hydrology and water quality data, focusing on watersheds containing agricultural BMPs. A significant motivation for the research is to provide tools that can be used to identify and quantify the major factors that connect watershed hydrology and water quality (such as climate, soil type, slope, and land use). These connecting factors are important for evaluating the effectiveness of agricultural and other BMPs because they often determine stream and stressor management decisions. Research methods must take into account natural variability and uncertainty in watershed response to BMP installation and precipitation events. The research project documented in this report is a collaborative effort, funded through an Interagency Agreement between EPA’s National Risk Management Research Laboratory and USDA’s North Appalachian Experimental Watershed (NAEW) in Coshocton, OH. Project objectives were achieved through an examination of historical data collected at the NAEW, with examinations of other related databases. As a result of this research, methods were developed to quantify BMP effectiveness and to understand how natural systems respond to watershed changes over time. The research will benefit states and other stakeholders faced with assessing the performance and effectiveness of BMPs within a watershed management framework.

Technical Abstract: Little is known about effectiveness of land activities to control water quality. The objective was to explore the duration curve (DC) concept for comparing hydrology and water quality data from watersheds. DCs are plots of the percent of time that a given value of a variable (e.g., flow rate) is exceeded. DCs include the flow duration curve (FDC), concentration DCs, and load-rate DCs. DCs take into account natural variability and uncertainty in streamflow watershed response to land activities and precipitation events. Minimum number of samples, averaged streamflow data, and seasonal variation of concentrations were examined. It was found the smallest number of samples needed for relationships between streamflow (Q) and SO4 concentrations (C) (“C-Q relations”) was 25-35. Little is gained with sample numbers larger than about 150-400 samples from the C-Q relations. However, differences between DCs developed from the C-Q data and flow DCs for different sample sizes suggest that 30-100 samples are adequate. Based on the combined approaches, 50 samples is the suggested minimum number of samples for this study. There is little benefit from using more than about 2% of the 2500 samples collected, a savings in money. An investigation of the use of readily available average daily flow data suggests that midrange and small instantaneous flow rates may be represented by average daily flows which are more readily available data, but errors are more pronounced at larger or smaller flows. An investigation of seasonal variation in NO3-N concentrations and loads showed C-Q equation form changed from a 2-part relation to a linear relation to no relation. An illustration was given on how the DC methodology can be used to quantify the decrease in load rates and the reduction in the percent of time a given load is exceeded (or reduced risk) due to a land-management change. This research project is a collaborative effort between the USEPA and ARS-Coshocton, funded by the USEPA. The research will benefit states and other stakeholders faced with assessing the performance and effectiveness of BMPs within a watershed management framework.