Development of an Approach for Calculating Temperature TMDLs in Northern California

Developing TMDLs for pollutants such as temperature is complicated by the difficulty in understanding the diffuse nature of potential sources, such as the condition of riparian vegetation or reduced base flows associated with hydromodification. In the Pacific Northwest, numerous stream systems have been assessed and determined to be impaired by elevated temperatures associated with land and water management practices. In the Eel River watershed in northern California, most of the stream reaches are impaired for temperature and require the calculation of TMDLs. To support the development of TMDLs in these and other northern California watersheds, a QUAL2E-SHADE modeling system was developed to quasi-dynamically simulate temperatures within selected stream reaches. This modeling system is comprised of a Geographical Information System (GIS)-based SHADE model linked to a modified QUAL2E receiving water model. To simulate stream temperatures on a watershed scale, shading dynamics due to riparian vegetation and variation in the topographic relief are computed to estimate the amount of solar radiation that is absorbed by each selected stream reach. This is completed using the underlying algorithms of the SHADE model in addition to a GIS-based preprocessor that calculates stream orientation, topographic shading, and vegetation buffers. Vegetation height is calculated using an asymptotic diameter at breast height to tree height relationship or constant values. The simulated shade attenuated solar radiation is routed through the stream using the QUAL2E-SHADE model, which was modified to predict the temperature in each selected stream reach. The modeling system has been modularized such that the user can run the SHADE model alone or in conjunction with QUAL2E-SHADE. Independently, the SHADE model can provide a screening level view of the influence of shade on in-stream temperatures by taking topography and riparian vegetation into account. Coupled with the QUAL2E-SHADE model, it provides the ability to simulate temperature in all or selected reaches within a particular watershed using heat flux and transport calculations. This allows more flexibility during modeling and supports the exclusion of reaches that are not considered hydrologically important (i. e., no flow during the summer). The modeling system can evaluate the effects of riparian-zone vegetation management strategies on stream temperatures during low-flow/critical conditions. Algorithms for calculating the height and density have been included and the system can simulate what if scenarios based on these parameters for baseline/existing conditions and alternative historical and future scenarios. Postprocessors for QUAL2E-SHADE for computing the percent shade and maximum weekly average stream temperature (MWAT) have been developed to provide a direct link between model results and GIS, thus allowing the user to immediately view model results in a spatial context. This paper describes the various components of the QUAL2E-SHADE modeling system, which has been tested and applied to the North Fork Eel River, Middle Fork Eel River, Upper Eel River, Middle Main Eel River, and Lower Eel River watersheds in northern California for temperature TMDLs. The paper will also provide comparisons of modeling results to thermal infrared remote sensing data collected in the Middle Main and Lower Eel Rivers. This comparison provides some insight into possible future uses of remote sensing data for assessing temperature impaired streams.