SIMULATION OF A CENTURY OF RUNOFF ACROSS THE TOMALES WATERSHED, MARINCOUNTY, CALIFORNIA

Tomales Bay has been the site of biogeochemical research on delivery o f dissolved and particulate materials from land to the bay and nutrien t cycling within the bay. Because that study seeks to infer controls o n inputs to the bay and historical changes in the land-bay system as w ell 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 Thornthwait e and his colleagues, to estimate both long-term runoff and spatial va riations of runoff as a function of rainfall, evapotranspiration, and field soil moisture capacity. The multi-cell water balance incorporate s temporal variation in runoff by using monthly rainfall values betwee n 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 km(2) watershed), c alculating the water balance for each cell, and summing the gridpoint results into runoff for 18 subwatersheds. Sensitivity analysis demonst rates that most of the spatial and temporal variation in annual runoff is due to rainfall, rather than either evapotranspiration or held soi l moisture capacity.