Simulation of a Century of Runoff Across the Tomales Watershed, Marin County, California

Fischer, D; Smith, S; Churchhill, R

Tomales Bay has been the site of biogeochemical lese arch on delivery of dissolved and particulate materials from land to the bay and nutrient cycling within the bay. Because that study seeks to infer controls on inputs to the bay and historical changes in the land-bay system as well as contemporary processes, a long-term and spatially comprehensive hydrological record for the watershed was desired. A multi-cell water balance model has been developed based on the analysis by Thornthwaite and his colleagues, to estimate both long-term runoff and spatial variations of runoff as a function of rainfall, evapotranspiration, and field soil moisture capacity. The multi-cell water balance incorporates temporal variation in runoff by using monthly rainfall values between 1879 and 1992 and average monthly evapotranspiration values. Spatial variation is addressed by dividing the watershed into grid cells 60 m on a side (approximately 160 000 cells in the 561 km2 watershed), calculating the water balance for each cell, and summing the gridpoint results into runoff for 18 subwatersheds. Sensitivity analysis demonstrates that most of the spatial and temporal variation in annual runoff is due to rainfall, rather than either evapotranspiration or field soil moisture capacity.

Simulation of a Century of Runoff Across the Tomales Watershed, Marin County, California

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