The role of fog in the water balance of coastal mediterranean forest in Chile

In mediterranean-type climate conditions, fog contributes significantly to the water balance of the ecosystem, since precipitation rates are lower than evapotranspiration rates. However, it remains unclear whether fog contribution is a direct input of water to the system through canopy dripping or if it limits evapotranspiration by limiting radiation and vapor pressure deficit. Focusing on the former, in this study we aim to understand the fog water as a input to the water balance in mediterranean coastal forests. Using the water balance principle (ΔS = ET - (P + F)), we characterize the key elements of water storage (ΔS), evapotranspiration (ET), precipitation (P), and fog (F) over different vegetation units that compose the mediterranean forest. The data used for this characterization is gathered from in-situ measurements (meteorological stations and standard fog collectors) and remote sensing sources (GOES and MODIS products). To quantify fog interception by vegetation unit canopies, a numerical model (AMARU; Lobos-Roco et al., 2025) is used, which estimates the fog inflow from routine meteorological data. By solving the water balance equation for the forest areas, we are able to determine the fog collection efficiency, which allows us to estimate the amount of water collected by the forest. Finally, to evaluate the modeling outputs, we conduct an in-situ experiment to measure water collection from the forest canopy using analog rain gauges. Our preliminary results show that fog contributes as water input in forests located in south and west facing slopes, reaching ET requirements. Moreover, our estimates of forest fog collection efficiency round 10%, meaning that only 1/10 of fog inflow is captured by forest canopy. We expect that this study contributes to advance our understanding of forest dynamics in coastal mediterranean ecosystems and their responses to fog water input.