Keeping tabs on the invisible movement of water in a hill
By Michele Hofmeyr
How water moves through the landscape affects both the vegetation and the animals living there. Water in the environment is very diverse and we often just think of flowing rivers and streams, but there is more to this dynamic than meets the eye.
The study of water, or hydrology, is a high priority in our arid country and of even greater importance in our protected areas. A detailed and high-tech research programme looking at how water moves down slopes through the soil within the herbivore and fire exclosures on the Sabie and Letaba Rivers is currently underway in the Kruger National Park.
The main purpose of these exclosures is to determine the effect of different combinations of fire, elephants and other herbivores on the vegetation. The exclosures will be maintained and managed as such for the next 20-25 years during which time close monitoring of changes will be done. The set-up also provides ideal opportunities for other research, such as soil-plant relationships, soil nutrient and population dynamic studies.
With these objectives in mind, this long-term monitoring programme being lead by Prof Simon Lorentz from the school of bioresources, engineering and environmental hydrology at the University of KwaZulu-Natal (UKZN) in conjunction with SANParks Scientific Services is well suited to provide information on changes in water movement patterns over time.
Using an interesting array of specialised equipment that monitors the moisture levels in the soil, the researchers hope to gain a better understanding of how water changes in abundance and availability from the top of the hill (the crest), right through to the bottom of the hill in the riparian or riverine zone.
The soil water monitoring is done within the exclosure sites to see if there are any effects in areas that have different numbers of herbivores impacting on the vegetation and different patterns of fire.
The monitoring of the soil water and how it changes over time and through the seasons from summer to winter is where the real science comes in. Specially designed probes have been placed at various depths in the soil at different points along the hillslope (or catena) from the crest area, dominated by broadleaved trees such as marula, through fine-leaved scrub vegetation characteristic of sodic areas and finally through to the taller trees along the river or drainage line.
At these monitoring stations the WatermarkTM sensors measure the resistance of the soil based on how much water is present and the TDR (time-domain reflectometry) meters measure the volume of water as a percentage of soil volume. These devices are able to detect subtle changes in the soil water and record all of these over time.
These data are stored and collected every two weeks. General weather data is also collected at the survey sites and this is done by specialist automated weather stations that take readings of rainfall, temperature, relative humidity, windspeed/ direction and solar radiation on a half hourly basis.
Eddie Riddell, an MSc graduate in ecology, presently studying for a PhD in environmental hydrology through UKZN, Pietermaritzburg, is the person on the ground collecting all the valuable data from the monitoring equipment stationed in the field.
Eddie is supported by the chief hydrology technician of the UKZN, Cobus Pretorius, and by staff from Scientific Services. He also co-ordinates specialist soil surveys required to learn about how soils conduct water vertically and laterally.
'The hillslopes in a landscape are what ultimately gives rivers their water' explained Eddie 'and if we have an understanding of how much water is moving through the system, even if it is not visible to us, we can have a better understanding of the dynamics involved in keeping the river flowing and the associated 'ecotypes' on the hillslopes well supplied with water'.
With careful analysis, the information gathered during this programme will help provide a greater understanding of the water patterns in the ecosystem. This will be invaluable to scientists and managers in the light of predicated climate changes and associated implications for broadscale water management of our protected areas.