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Measuring total evaporation in SAEON’s Cathedral Peak research catchments

By Michele Toucher, Byron Gray and Sue Janse Van Rensburg, SAEON Grasslands-Forests-Wetlands Node
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Given the growing pressures on South Africa’s water resources, the protection and good management of our key water source areas is becoming increasingly important.

The uKhahlamba Drakensberg in KwaZulu-Natal is one of those key water source areas which need to be protected and managed to ensure sustainable water supply to Gauteng and KwaZulu-Natal.

These high-altitude mountainous regions are facing several threats, one of which is vegetation change. How and why vegetation change is occurring in grasslands is one of the key research themes of the SAEON Grasslands Node. There are both local drivers of vegetation change, such as historical or current land use and fire management practices, and global drivers of vegetation change such as carbon fertilisation, nitrogen deposition and climatic shifts.

Understanding the impacts of vegetation change on water dynamics

The vegetation of an area and the hydrological response of that area are closely interlinked. When the vegetation changes, the way in which precipitation is divided into infiltration and runoff is altered, and therefore the streamflow generated is changed.

Understanding the impacts of vegetation change on localised water dynamics is thus vital for informing water resources planning. Vegetation can be managed to some extent at local scales, unlike the threat of climate change where little can be done locally about the temperature and rainfall.

With a better understanding of interaction between vegetation and water resources, under different land use or land management regimes, appropriate management opportunities can be identified that can be used to mitigate the potential negative impacts of changes in climate on our water resources.

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Byron Gray and Kent Lawrence install the Surface Renewal system in Catchment III, Cathedral Peak

The fully installed Surface Renewal system in the degraded Catchment III measuring over bracken fern (Pteridium aquilinum)

To date, research on the impacts of vegetation on water has focused on commercial forestry and agricultural crops. The SAEON Cathedral Peak research catchments, where different land use and land management treatments have been applied, provide a unique experimental opportunity to study the catchment scale impacts of woody encroachment and degraded grassland on water resources in comparison with the hydrological response under pristine grassland.

To be able to understand the impacts of vegetation on water, the components of the hydrological cycle that need to be measured are climate variables (radiation, wind speed and direction, air temperature, humidity and rainfall), total evaporation, soil water, interception and river flow. SAEON has monitored climate variables and river flows of the Cathedral Peak research catchments since 2012, as well as the total evaporation and soil water of the pristine grassland catchment.

However, until recently no total evaporation or soil water measurements were being made in the catchments showing vegetation change, namely Catchment III, which is degraded, and woody encroachment in Catchment IX. 

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Byron Gray installs the Automatic Weather Station to support the Surface Renewal measurements in Catchment IX, Cathedral Peak

The fully installed Surface Renewal system in the woody encroached Catchment IX

Measuring total evaporation

Measurement of total evaporation is very challenging, especially in mountainous, steep sites with varying vegetation structure. 

The total evaporation of the pristine grassland is being measured using an open path eddy covariance (EC) system. However, there were concerns about using these systems in the degraded and woody encroached catchment. Thus, after much discussion, the SAEON Grasslands Node team decided to test the Surface Renewal method in these catchments.

The Surface Renewal method estimates sensible heat in a simple and cost-effective way. The method is similar to the EC system in principle but uses high-frequency measurements of air temperature at two different heights above the canopy, measured by two exposed fine wire thermocouples as the basis for sensible heat flux measurements, as opposed to infrared gas analysis of water vapour in the EC system.

It was with great excitement that the Grasslands Node team installed the Surface Renewal systems in Catchments III and IX during November 2018. These systems will be monitored for the next two years to allow for total evaporation to be determined over different seasons.

The total evaporation data, together with the climate variables and streamflow, will allow us to better understand the impacts of these vegetation covers on the hydrological response. This study will form a component of SAEON student Byron Gray’s PhD.

Driest December on record for the Cathedral Peak research catchments

2018 ended on a low for the Cathedral Peak research catchments, with only 73 mm of rainfall recorded at the Mike’s Pass meteorological station for December 2018. This was well below the long-term historical (1949–1992) December mean rainfall of 202 mm, and below the previous December minimum rainfall recorded in 1970 of 83 mm.

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