Phosphorus SPARROW Model for the Yakima River Completed

Development and Calibration of a Total Phosphorus SPARROW Model for the Yakima River Basin - Completed FY2013

Problem - A recent study by the USGS found nutrient concentrations in the lower Yakima River during the 2004-07 irrigation seasons were high enough at certain times and locations to support the abundant growth of periphytic algae and macrophytes which resulted in large daily fluctuations in dissolved oxygen concentrations and pH levels that exceeded the Washington State water-quality standards for these parameters. The nutrient concentrations in the lower reaches of the Yakima River during this time were almost always greater than the reference conditions suggested by the U.S. Environmental Protection Agency to protect water bodies from the negative effects of nutrient enrichment. To date, an efficient and effective way of determining where nutrient loads are elevated throughout the Yakima River Basin and what factors are responsible for these elevated loads does not exist.

Objectives - The USGS is proposing to develop a model to estimate mean annual total phosphorus (TP) loads throughout the Yakima River Basin, and to estimate the relative contribution from various sources of TP to each modeled stream reach. The proposed model will refine the USGS SPARROW model (SPAtially Referenced Regression On Watershed attributes) recently developed for the entire Pacific Northwest (PNW).

Relevance and Benefits - - The proposed study is consistent with the USGS science strategy to provide information and management tools to help manage and improve water quality and the health of aquatic ecosystems of the nation's waters. The Yakima River TP SPARROW model will allow managers of this resource to better assess the sources and transport of TP throughout the Yakima River. Model output will provide resource managers with the information necessary to efficiently and effectively manage TP in the Yakima Basin and to more effectively implement TMDLs. This study is appropriate for inclusion in the USGS Cooperative Program because it will provide information that advances our understanding of important hydrologic processes.

Approach - SPARROW is a watershed modeling technique for relating water-quality measurements made at a network of monitoring stations to attributes of the watersheds such as nutrient sources and environmental factors that affect rates of delivery of nutrients to streams and in-stream processing. The core of the model consists of a nonlinear regression equation describing the non-conservative transport of nutrients from point and non-point sources on land to rivers and through the stream and river network. SPARROW is designed to (a) utilize existing nutrient monitoring data and watershed information to explain the factors that affect water quality, (b) examine the statistical significance of contaminant sources, environmental factors, and transport processes in explaining predicted nutrient loads, and (c) provide a statistical basis for estimating stream nutrient loads in unmonitored locations. The model estimates nutrient fluxes and loads in streams, and evaluates the contributions of selected nutrient sources and watershed properties that control transport throughout large river networks.