Streamflow, Fog, and Fog-Drip in the California Coast Range

The onshore movement of marine fog from coastal waters is a common occurrence during summer months along much of the contiguous U.S. Pacific Coast. Because the fog-season tends to occur during the precipitation-free dry-season, any additional input of moisture or reduction in loss of moisture through evapotranspiration provided by marine layer can be an important factor in localized hydrologic systems. In an effort to quantify some of the effects of fog on the regional dry-season hydrology, a study site within the Santa Cruz Mountains of central California was established. The fog-laden coastside and predominately fog-free San Francisco Bay-side of the study area provided an excellent opportunity to assess the impacts of the presence and absence of fog on ecohydrological processes. Streamflow, fog-drip, soil moisture, and weather conditions were measured from May-September. Bayside streams were found to be almost all intermittent, with much higher rates of baseflow recession compared to the predominately perennial coastside streams. Fog-drip was essentially nonexistent on the bayside, while highly variable amounts were recorded on the coastside. Maximum rates and seasonal totals of drip were found within stands of mature conifers (Sequoia sempervirens and Pseudotsuga menziesii) along exposed, often windy ridgelines. Rates of up to 19 in (48 cm)/month of fog-drip were recorded. Consequently, frequent infiltration events to depths of at least 9 in (23 cm) were also documented. Over the course of the study soil moisture levels at high fog-drip locations either increased, or were roughly equivalent to initial spring conditions from the onset of data collection. Increases of flow in coastside streams, under otherwise receding conditions, were found to coincide with fog and fog-drip events. These results indicate that the presence of fog can significantly affect dry-season hydrologic conditions of some coastal locations.