SAEON takes up the challenge of investigating fog precipitation

A view up the Jonkershoek valley during a mid-summer gale-force South-Easter resulting in large orographic clouds (Picture: Abri de Buys)

Dwarsberg fog collector shrouded in fog (Picture: Abri de Buys)

Water was a key reason for colonial utilisation in the 17^th century by the Dutch East India Company and associated European settlement of the area that eventually became known as Cape Town (Picture courtesy of The Roundhouse)

By Abri de Buys, Field Technician, SAEON Fynbos Node

Water was a key reason for colonial utilisation by the Dutch East India Company (Vereenigde Oostindische Compagnie, VOC) and associated European settlement in the 17^th century of the area that eventually became known as Cape Town.

This is confirmed by references to, and remnants of, the network of Dutch grachts and other waterways that persist in Cape Town until today. This agricultural settlement relied heavily on perennial streams from Table Mountain, as indigenous people had undoubtedly done for a long time prior to the arrival of the VOC.

Early observations

Observations reflecting a remarkable consistency of water availability were documented in the context of the suitability of these streams for early hydroelectric power in 1895. The first scientific publication on fog precipitation in South Africa was that of Dr Rudolf Marloth (1903) - on the famous “Tablecloth” fog formation that occurs on Cape Town’s iconic Table Mountain. This study confirmed suspicions that fog precipitation can contribute a significant amount of water to ecological systems.

The contribution of fog to the various mountain streams or groundwater that feed seeps and springs on lower mountain slopes has not been scientifically established or quantified. However, anecdotal evidence in the literature of stream levels increasing after heavy fog events certainly seems plausible. Fog precipitation is important for both the ecology and water utilisation in the area, because it affects the amount of water available in the system. Perhaps more importantly, orographic clouds like the “Tablecloth” are formed by south-east winds which dominate in summer, suggesting that fog inputs may reduce the severity of summer droughts, relieving plants from drought stress and contributing to summer stream flow.

Changes in fog precipitation

Since Marloth’s time, most published fog research in South Africa has focused on the potential of fog-water harvesting for human consumption in various parts of the country. Some work has been done on the mechanisms by which fog becomes part of the fynbos ecosystem.

One striking aspect of the literature on fog precipitation in South Africa is a general shortage of information on the role of fog precipitation in catchment hydrology and ecology. Some other fog-prone ecosystems, such as tropical montane cloud forests of central and South America and Californian redwood forests, do have a wealth of literature relating fog to ecological and hydrological processes.

In some of these systems, research has suggested that factors such as regional rises in sea surface temperature as well as local land use/land cover change can have an influence on the occurrence and amount of fog precipitation. These changes potentially have implications for ecosystems supported by this moisture, and fynbos mountain catchments may be affected as well.

Fynbos Node fog monitoring network

SAEON has taken up the challenge of monitoring and facilitating the investigation of fog precipitation as an integral part of its catchment-based monitoring platforms. Appropriately so, since research has clearly shown it to be an important environmental driver that has received little attention locally, and is likely to be subject to long-term change from anthropogenic causes.

The first step was to equip the high-elevation weather station at Dwarsberg, Jonkershoek with a fog collector in March 2013. The first year of baseline data already allows us some insight into seasonal precipitation patterns and relationships between fog precipitation and trade winds near the headwaters of a number of rivers that arise in these mountains.

Perhaps the most significant finding thus far is that while a normal rain gauge at the site measured 3556.7 mm of rain over a year, the adjacent fog gauge measured 1233.8 mm over the same period while no rain was falling. How much of this fog blowing over the site becomes part of the catchment hydrology is currently unknown.

A second fog monitoring station in the Table Mountain range has been fully operational since October 2013, after several months of testing. Most recently the Fynbos Node expanded the fog monitoring network by adding a transect of fog collectors paired with rain gauges across an elevation gradient at Jonkershoek, within the Langrivier catchment where we have been continuously monitoring stream flow.

SAEON will be quantifying the fog input along this gradient and investigating the influence of this form of precipitation on stream runoff measured at the weir. The establishment of this infrastructure also allows SAEON to investigate the most appropriate methods for continuous fog monitoring into the future.

The "tablecloth" is a regular sight in Cape Town during the dry season (Picture courtesy of UCT)

Opportunities for future research

SAEON’s new fog monitoring infrastructure presents many opportunities for future research. Fynbos botanists, ecologists and hydrologists have been flagging the role of fog precipitation as insufficiently understood for a long time.

Exciting questions can be asked of this monitoring platform and a lot of potential exists to use it as a base for future expansion and linkages to other projects. Some of the questions that require answering include: At what elevation does fog precipitation start playing an important role? Does fog precipitation influence the arrangement of vegetation in the landscape? Do plants utilise this wind-borne water source and if so, how? How seasonal is precipitation in different forms at different elevations and what does this mean for the runoff on which downstream processes depend? Can we predict what impacts a changing fog precipitation regime will have on runoff and biodiversity?

Much of the intact natural vegetation in the Fynbos biome occurs in mountain catchments and many of these are subjected to fog. These very catchments contribute to the water resources on which society relies. Understanding their functioning in the context of this understudied phenomenon is not only academically fascinating, but socially relevant.

Are there differences between fog, cloud, mist and occult precipitation?

The meteorological definition of "fog" is any cloud that is in contact with the surface of the earth and reduces visibility to less than 1 000 metres.

From this definition it is clear that visibility at the point of contact is important. A stratus cloud in contact with a mountain peak can meet the criteria for "fog" at the point of contact even though an observer may clearly view the cloud from two kilometres away.

There are many different sources and mechanisms by which a cloud of water droplets can occur at ground level. A mist developing in a valley on a cold damp night, an orographic "Tablecloth" cloud blowing over a mountain, a dense advection fog moving inland from the ocean, these all result in the definition criteria being met.

When these airborne water droplets come into contact with surfaces such as plants, rocks or buildings, they are collected and can drip or flow down or be absorbed through leaves and become part of the ecosystem. The fact that standard rainfall measurements often completely miss this form of water input has led people to also refer to it as "occult" precipitation.

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