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DETECTING TEMPORAL CHANGE IN WATERSHED NUTRIENT YIELDS
Citation:
WICKHAM, J. D., T. G. WADE, AND K. Riitters. DETECTING TEMPORAL CHANGE IN WATERSHED NUTRIENT YIELDS. ENVIRONMENTAL MANAGEMENT. Springer-Verlag, New York, NY, 42(2):223-231, (2008).
Impact/Purpose:
The two objectives of this paper are: 1) to develop a dataset of nutrient yields that incorporates intrasite, inter-annual variability using watersheds dominated by a single land-cover class (e.g., forest, urban), and; 2) use the dataset to show the effect of land-cover change on the distribution of TN and TP yields. The two objectives are related to water-quality management prescribed under the Clean Water Act (P.L. 92-500), which maintains water quality through the use of standards (www.epa.gov/waterscience/standards). Water-quality standards have three components: designated use, criteria, and anti-degradation. Individual states establish designated uses (e.g., fishing, swimming, drinking) for its waterbodies, numerical criteria serve as benchmarks to determine if designated uses are being met, and anti-degradation policies are put in place to maintain and protect the established designated uses. Quantifying the affect of land-cover change on N and P distributions can be used to guide development and interpretation of N and P criteria (USEPA1998) and anti-degradation policies.
Description:
Meta-analyses reveal that nutrient yields tend to be higher for watersheds dominated by anthropogenic uses (e.g., urban, agriculture) and lower for watersheds dominated by natural vegetation. One implication of this pattern is that loss of natural vegetation will produce increases in watershed nutrient yields. Yet, the same meta-analyses also reveal that, absent land-cover change, watershed nutrient yields vary from one year to the next due to many exogenous factors. The interacting effects of land cover and exogenous factors suggest nutrient yields should be treated as distributions, and the effect of land-cover change should be examined by looking for significant changes in the distributions. We compiled nutrient yield distributions from published data. The published data included watersheds with homogeneous land cover that typically reported two or more years of annual nutrient yields for the same watershed. These data were used to construct statistical models, and the models were used to estimate changes in the nutrient yield distributions as a result of land-cover change. Land-cover changes were derived from the National Land Cover Database (NLCD). Total nitrogen (TN) yield distributions increased significantly for 35 of 1550 watersheds and decreased significantly for 51. Total phosphorus (TP) yield distributions increased significantly for 142 watersheds and decreased significantly for 17. The amount of land-cover change required to produce significant shifts in nutrient yield distributions was not constant. Small land-cover changes led to significant shifts in nutrient yield distributions when watersheds were dominated by natural vegetation, whereas much larger land-cover changes were needed to produce significant shifts when watersheds were dominated by urban or agriculture. We discuss our results in the context of the Clean Water Act.