Citation:

Lin, J., J. Compton, AND S. Leibowitz. Modeling and assessing nitrogen delivery in the Calapooia River Watershed, and the impact of small streams delivery on downstream watershed. Soil and Water Conservation Society meetings, Louisville, KY, July 24 - 27, 2016.

Impact/Purpose:

Nutrient delivery from headwater streams to downstream navigable waters is related to nutrient inputs, hydrologic flows and nutrient retention mechanisms within the watershed. A team of EPA scientists and an NRC post-doc have used data from EPA, USGS and USDA-ARS to estimate nitrogen inputs and hydrologic export from 73 stations within the Calapooia Basin of Oregon. They examine the influence of N inputs from agriculture and other sources, stream size and duration, and timing of precipitation as important factors influencing N retention. The results of this study should provide insights into establishing more effective nutrient management and reduction plans within Willamette River Basin. This is a product under SSWR 4.03C.

Description:

The Calapooia River is a major tributary to the Willamette River in western Oregon, which is characterized by a mountainous forested upland and a flat agricultural lowland. Here we report on a modeling study of watershed’s N budget, and quantify the influence of different streams on the delivery of water and N to downstream waters. Surface water was sampled between 2003 and 2011 at 73 sites on the Calapooia River and its tributaries. Daily discharge simulations from a hydrologic model are integrated with concentration measurements to calculate TN loads using LOADEST (Load Estimator). ArcGIS data of landscape N inputs were used to calculate contributions of various N sources. Nearly 40% of the total area has a net TN input of 100-200 kg/ha/yr, dominated by agricultural land, and 48% has an input of <100 kg/ha/yr. About 2.4% of the area has a TN input of >200 kg/ha/yr, where animal feeding operation and farmland are the dominant land use. Mountainous area has a TN input of <30 kg/ha. Based on 2008 data, dry summer yield is as low as <1 kg/ha, while nearly 50% of the annual yield occurs during the wet winter, and reaches 50 kg/ha over the season. The fraction exported ranges from 2 to >60% at the subbasin level with the highest values found at the mountainous areas. Annual TN export is 25% of the inputs for the Calapooia watershed and is relatively high for western watersheds. Subbasins with the greatest TN input do not correspond with areas with the highest yield. Differences in crops, riparian buffers or soil could explain the discrepancies. Future modeling work with SWAT (Soil and Water Assessment Tool) will be used to understand stream N delivery under various land use, soil, and management scenarios, and to calculate the seasonal contribution of N from different stream types (e.g., intermittent vs. perennial). The results of this study should provide insights into establishing more effective nutrient management and reduction plans for the Calapooia watershed.

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