Citation:

Johnson, Z., S. Leibowitz, AND R. Hill. Revising the index of watershed integrity national maps. SCIENCE OF THE TOTAL ENVIRONMENT. Elsevier BV, AMSTERDAM, Netherlands, 651:2615-2630, (2018). https://doi.org/10.1016/j.scitotenv.2018.10.112

Impact/Purpose:

The EPA’s Index of Watershed Integrity (IWI) is used to calculate and visualize the status of natural watershed infrastructure that supports ecological processes (e.g., nutrient cycling) and services provided to society (e.g., swimmable and fishable waters). The IWI is an accumulation of over 20 landscape stressors (e.g., road-stream crossings, percent urban land use) within 2.6 million watersheds in the conterminous US. Previous studies have shown significant correlations between the IWI and a number of water quality variables across the US, including stream nitrogen levels and benthic organic matter from lakes. In spite of this performance, the IWI was constructed based on an assumption of a negative linear relationship between stressors and watershed functions and equal weighting of stressors. However, the approach was designed to be improved over time. This study demonstrates how to iteratively update the IWI’s stressor-function relationships using random forest models and a nationwide response metric representative of one of the six watershed functions; specifically, the chemical regulation function (CHEM). Revision of CHEM values improved the prediction of several regional- to national-scale water quality indicators. The methods outlined in this study can be implemented iteratively as more and better data become available for all six of the watershed functions to elevate the accuracy and applicability of the IWI to various land management issues. This work contributes to SSWR 3.01B (Estimating and Predicting Water Resource Condition and Watershed Integrity) by incrementally improving the IWI and demonstrating how further improvements can be implemented.

Description:

Watersheds provide a range of services valued by society, incorporating biotic and abiotic functions within their boundaries. Watershed integrity has been defined as the capacity of a watershed to support and maintain the full range of ecological processes and functions essential to the sustainability of the watershed services provided to society. Recently, an operational definition of watershed integrity was applied and indices of watershed integrity (IWI) and catchment integrity (ICI) were developed and mapped for the conterminous United States. The IWI and ICI can be useful for many management efforts ranging from catchment to national scales, especially when combined with local site information. However, these indices were originally derived using equally weighted first-order approximations of relationships between anthropogenic stressors (obtained from the U.S. EPA’s StreamCat dataset) and six watershed functions. In addition, the original calculations of the IWI and ICI did not standardize metrics across these differing scales, resulting in IWI and ICI values that are not directly comparable. We provide an example of how to iteratively update the stressor-function relationships using random forest models and a nation-wide response metric representative of one of the six watershed functions. Specifically, we focused on the chemical regulation function (CHEM) of IWI and ICI by relating a composite metric of chemical water quality from 1914 samples to land use metrics explicit to CHEM to refine the nature of these relationships (e.g., non-linear versus linear). The rate of nitrogen fertilizer, agricultural land use, and urban land use were found to be the three most important stressors predicting the national water quality response metric. Revision of CHEM values improved the prediction of several regional- to national-scale water quality indicators. Opening up the revised random forest models to any stressor metrics within the StreamCat dataset did not significantly improve model performance, providing evidence for the continued use of the stressors chosen in the original calculation of IWI and ICI. In all cases, low-resiliency stressor-function relationships replaced the original negative linear relationship for CHEM. Therefore, the original IWI and ICI values are probably over-estimates (i.e., conservative estimates) of the actual integrity of the nation’s watersheds and catchments. With these revisions, we provide updated national maps of IWI and ICI; the data used to construct these maps are publicly available on the EPA’s StreamCat website (https://www.epa.gov/national-aquatic-resource-surveys/streamcat). The methods outlined here can be implemented iteratively as more and better data become available for all six of the watershed functions to elevate the accuracy and applicability of these indices to various land management issues.

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