The contribution of fog to the water balance along the eastern escarpment of South Africa.

Abstract

Fog is a frequent phenomenon in South Africa, occurring mostly on the west coast and along the mountains forming the southern and eastern escarpments. Fog measurements are, however, neglected in water balance studies, resulting in an underestimate of the precipitation input to catchments that experience frequent fog occurrences. World-wide, tropical montane cloud forest (TMCF) studies have proven that fog deposition, facilitated via the interception of fog droplets by vegetation, can represent a significant fraction of the total hydrological input. In South Africa, limited literature exists on the contribution of fog to the country’s water yielding catchments. In particular, information on fog patterns and its contribution to the water balance is extremely scarce in the mountains forming South Africa’s eastern escarpment, where only one study has been previously conducted. Additionally, no forestry studies in the country have attempted to quantify fog. Thus, the aim of this study was to determine the contribution of fog to the water balance of two research catchments of different land use types and altitudes, situated along South Africa’s eastern escarpment. These sites included the Cathedral Peak research catchments and Two Streams; Cathedral Peak is a high altitude montane grassland catchment, whereas Two Streams is at a lower altitude and afforested by exotic plantations. At Two Streams, fog and the climatic conditions were monitored over a 16-month period (July 2015 to October 2016) and additional measurements of throughfall, stemflow and soil water content were carried out in an Acacia mearnsii plantation, to further determine the fog contribution in a forest plantation. At the Cathedral Peak research catchments, fog and the climatic conditions were monitored at three sites, including Mike’s Pass Meteorological Station, Catchment VI and a High Altitude site. Monitoring was conducted over a 14-month period (September 2015 to October 2016) at Mike’s Pass and over a two-month period (August 2015 to September 2015) at Catchment VI and the High Altitude site. Fog was found to be prevalent, occurring frequently and for long durations, potentially contributing fairly substantial amounts of water to the water balance. It occurred all year round, but was predominantly a summer phenomenon, however, it comprised a greater proportion of the total precipitation during the dry winter season. At Mike’s Pass, fog represented a contribution of almost 30 % during several drier months. At Two Streams, during the driest month of August 2015, fog represented a contribution of approximately 38 % of the total precipitation. Fog increased with altitude as a whole, but changes in other topographic features (i.e. hillslope orientation and slope) over short distances, meant that the delivery of fog was not uniform from one point to another at the same altitude. Fog occurrence and water yield increased with wind speed, although this was not found to be a very significant relationship. A stronger relationship between wind direction and fog was observed, particularly at Mike’s Pass, the higher altitude site, which was better exposed to fog-bearing winds. At Two Streams, fog did not facilitate throughfall of rainfall or contribute to soil water. The indirect effects of limiting wet canopy evaporation and transpiration rates were suggested to be a more relevant effect of fog on the water balance. These findings further the understanding of the contribution of fog to the water balance along the eastern escarpment of South Africa and will assist in future long-term climatological studies of fog and low cloud occurrence in the region.