Partitioning the relative contribution of dew and fog to total occult precipitation across a decreasing fog inundation gradient in the Pajaro Valley, CA

Abstract:

Traditionally, occult precipitation has been difficult to measure, especially given that it can take two predominant forms: fogs and dews. Recent studies have shown that estimates of fog deposition can be made using constructed fog collectors of various styles. Leaf-wetness sensors have also been deployed to try and estimate the relative wetness of leaves during fog events, but coastal systems can integrate both fog deposition and dew condensation, which cannot be differentiated with such sensors alone.

We measured fog interception with Decagon Leaf Wetness Sensors (LWSs) outfitted with a 1m² mesh passive fog collector in areas adjacent to the California Irrigation Management Information System (CIMIS) sites in the Pajaro Valley. The LWSs that were deployed were covered with a partial shield or left uncovered. Covering the LWS creates a microclimate that allows discrimination between fog events that horizontally advect droplets that impact individual leaves, and dew events, which condense once dew point temperature is reached. By measuring fog with a standard measurement device (mesh fog collector), and comparing it to LWSs that have either fog exposure or fog exclusion (covered LWSs), we can partition the relative contribution of dew to overall occult precipitation. This would be the first study quantifying the amount of water inputs (horizontally advecting fog) to the amount of recycled water (dew) to gain a greater understanding of the role of occult precipitation in catchment water balances across coastal California.