WATER-QUALITY MODELING OF THE CLEAR LAKE, CALIFORNIA WATERSHED: GEOLOGICAL AND ANTHROPOGENIC INFLUENCES ON NUTRIENT AND SEDIMENT TRANSPORT

Clear Lake, California, is the oldest lake in the State and is hyper-eutrophic because of natural features of the watershed and anthropogenic activities. Nutrients that stimulate primary productivity of algae are nitrogen and phosphorus that have accumulated in the lake sediments over time. The shallow nature of the lake contributes to wind-driven, vertical mixing multiple times per year. Anoxic bottom water conditions cause mobilization of bio-available phosphorus from lakebed sediments, leading to frequent algal blooms. The surrounding watershed has both agricultural and urban land uses that may contribute nutrients to the lake in addition to natural sources from forest and chapparal land. To address potential management options, it is necessary to understand the relative contribution of nutrient sources from the watershed and internal cycling so that effective decisions can be made. Natural sources of phosphorus are likely driven primarily by watershed geology. Rock types of the Clear Lake watershed are dominated by Franciscan Complex sedimentary and metamorphic rocks and volcanic rocks with variable phosphorus concentrations. Chemical weathering increases the clay content in soils and leads to enhanced amounts of rainfall-induced erosion and phosphorus transport. To understand nutrient and sediment sources, a dynamic version of the SPARROW (Spatially-Referenced Regression on Watershed attributes) model is being developed. SPARROW models can predict loads and sources of nutrients at the sub-watershed scale based on climatic and landscape features and distinguish between natural and anthropogenic factors. Geological sources of phosphorus are modeled as both a source and a delivery variable, based on concentrations in surrounding rocks, soils, and streambed sediments. Clay content of soils also drives phosphorus transport in streams because of erosion, whereas sand-dominated portions are less likely to transport nutrients because of recharge of water to the aquifers. Nitrogen inputs are likely dominated by atmospheric deposition and fertilizer use. This SPARROW model will be calibrated using three years of stream water-quality monitoring along with landscape features including rainfall, runoff, soil types, groundwater recharge, fertilizer use, cattle grazing, and urban density.