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Long-term hydrological monitoring at Jonkershoek aids climate change studies
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- By R Arthur Chapman, CSIR, Stellenbosch
From the 1850s onwards European settlers were over-exploiting South Africa’s indigenous forest resources for timber.
Concerns by the public and government officials led to a programme of afforestation (usually a grassland or fynbos conversion) using exotic species, particularly pines (P. pinaster initially), eucalypts and some acacias (A. mearnsii).
By the early 1900s it was clear from the numerous complaints by farmers that these plantations were affecting streamflow. A planned programme of research into “the influences of forests on water conservation and allied problems” was endorsed by the Fourth Empire Forestry Conference held in 1935 at Cape Town.
Construction of measuring weirs began in 1936 in the Jonkershoek State Forest and Dr CL Wicht was appointed as Founder and Research Director of the Jonkershoek Forestry Research Centre.
Experimental design
The experimental design was based on the classic paired-catchment principle used at Emmental in Switzerland and Wagon Wheel Gap, Colorado, USA. The principle of this approach is that the streamflow from two untreated catchments are compared, so as to establish their natural relationship. One is then treated, for example planted to trees. The change in the relationship between the two catchments after afforestation could then be ascribed to the treatment or influences of afforestation.
The instrumentation established at the time consisted of 29 rain gauges, of which 12 were continuously-recording and measured what water was going into the catchments. Eight continuously-recording weirs (six remain operational) measured what was coming out.
Eight catchments ranging from 27 – 246 ha were established. They have relatively steep slopes, with strong rainfall gradients caused by orographic forcing of incoming north-west frontal systems during the winter months (the Western Cape having a Mediterranean climate). Mean annual rainfalls of about 1 200 mm on the lower slopes can go as high as 3 000 mm/a (and 3 600 mm on the Dwarsberg at the top of the valley). The percentages of catchment afforested ranged from 36 – 98% with pinus species, mostly P. radiata.
Research outputs
The onset of streamflow reductions is evident in the data at ~5 years of plantation age, with a peak reduction occurring at ~15 years, followed by a gentle decline in water use. A rule of thumb is 30-40 mm streamflow reduction per 10% of catchment planted, at peak water use. A host of other effects and research questions subsequently arose from such findings.
South African forest hydrology experiments are world renowned and key papers from these research efforts are still cited in the international literature (e.g. Bosch and Hewlett, 1982). The Stream-flow Reduction Activities (SFRA) policy and Working for Water programme that emanated from these experiments are also used worldwide as examples of research supporting both policy and catchment management activities. More than 400 papers and conference proceedings have been produced using data collected from these research catchments.
Other national impacts include:
- Regulation of the R22 billion/yr forest industry. The commercial forest industry today derives benefit in export markets from the Sustainable Forest Certification (FSC) that the industry is able to achieve, in part because of previous catchment research and implementation of research findings imposed on the industry.
- Legislation related to SFRAs, riparian zone management and forest rotation regimes to conserve streamflow.
- Outputs include the Gush Tables, CSIR Flow Reduction Curves and the Handy Reference Manual, all of which concern streamflow reduction, and have been used for allocating permissible plantation areas to catchments,
- Validation of hydrological models, and
- Initiation of the Working for Water programme (~ R1.5 billion+ to date).
The down-side of these developments is that the forest industry mostly dislikes the impacts this research has had because of the constraints and water charges imposed on it that other crop types do not bear. However, we believe that the policies that have been implemented based on these results have led to greater sustainability of resource supply.
Where to now?
Is there still value to be obtained from these unparalleled data sets?
Firstly, questions in forest hydrology remain, particularly with respect to the recovery of streamflow after felling, as well as the hydrological effect of older plantations and the impacts of rehabilitation using forestry.
Secondly, daily hydrological models have not been shown to perform well against observed data. Thirdly, the amount of water apportioned to forestry is destined to become even more tightly defined with the increasing demand for water as the economy and population grow (including timber growers). It is imperative to keep an accurate track of how streamflow responds over time to tree growth.
Even more importantly now, the data sets give us a great opportunity for studying the evolution of climate change in South Africa. Apparent long-term trends in rainfall in Jonkershoek - of about a 14% decline since records began - indicate a decline of about 20% in runoff from the pristine catchments.
The mountain ranges in the Western Cape are crucial to water production for the economy of the Western Cape. Water managers need to understand how security of supply may be affected by changes in rainfall in these areas. Anecdotally, tree growth rates in the Jonkershoek plantations are known to have also declined over the same period, indicating the direct link between rainfall and wood production, and the economic implications of climate change.
Further, the nature of rainfall is also changing. As the atmosphere warms, more moisture can be held in the air. Rainfalls become more intense, but the duration of dry periods becomes longer. These effects have been found elsewhere in the World and research has tentatively identified these trends in South Africa.
The much finer time resolution of the Jonkershoek data will be a boon to studies of changes in rainfall intensity with time. We are now planning research on trends in quantity, intensity and timing of rainfalls and the impact this has on runoff. We hope to include studies on extreme value distributions as well – these also have economic implications (for design floods).
Strong downward trends have been tentatively identified in the KwaZulu-Natal Drakensberg, a water catchment region crucial to the KwaZulu-Natal and Gauteng economies. Unfortunately, monitoring in these catchments stopped in the 1990s and it seems unlikely that monitoring will start again, now that the record has been broken, indicating the importance of maintaining the continuity of observations.
Internationally, there is an understanding that trends in the direction of change require the continual commitment to rainfall and streamflow gauging programmes.
From this point of view, we gratefully acknowledge SAEON’s support.
