Collection efficiency of fog events

Abstract

Fog is an important water resource, especially in arid and semi-arid zones. The usual method to collect fog water is placing a rectangular mesh perpendicular to the wind which traps fog droplets. The collector catches a fraction of the fog’s water droplets, allowing them to grow by coalescence until large enough to fall by gravity for collection. Fog events can last from minutes to hours, or even longer. This study aimed to evaluate the efficiency (η) of a standard fog collector (SFC) to gather fog water. Efficiency is defined as the ratio between the water reaching the collector’s gutter (CW) and the liquid water flux (LWF) normal to the collector’s mesh, integrated in a period of time. After a fog event begins, mesh collectors require some time to reach saturation and begin dripping; further, the system does not achieve a stationary state at field conditions. Therefore, we considered entire fog events when evaluating efficiency. The CW was measured using a rain gauge, and the LWF was calculated based on the liquid water content (LWC) obtained through a fog droplet spectrometer, and data of wind velocity registered by a meteorological station. The duration of the analyzed events ranged from 20 min to 11 h 40 min. A lag of up to one hour between the arrival of fog and the beginning of the measured water output was observed. Both processes started simultaneously when preceded by another fog event.For the analyzed events, η ranged between 0% and 36.8%. Isolated events of up to 30 minutes resulted in zero CW measurements because the mesh requires time to become saturated before dripping. Furthermore, η was zero if the mean dew point depression (DPD) was above 0.2 °C or the mean LWC was below 0.045 g m−3. We found that η decreases with increasing mean volume diameter (MVD) for MVD ≳ 10 μm. Finally, η increased with wind speed for events in which the MVD > 12 μm.