Vegetation as indicator of flowpaths

Paul Meulenbeld DWS

Presentation based on PhD study and encouraged by Prof F Hodgson and the late Prof G van Tonder (UV)

Deep flowpaths

Typical vegetation as a response to changes in geology, weathering, groundwater availability and soil nutrient improvement (hence: differences in soil classification). Micro climate variations will also play a part.

The picture indicates Kirkia acuminata (white seringa) on a gabbro (nutrient rich, deeper weathering) ridge amongst a area characterised by nutrient deficient granite (shallow weathering).

The contact between granite and gabbro can be established through geophysical techniques. Soil classification not always appropriate due to rock exposures and a very small linear area.

Flowpaths linked to shallow subsurface flow and wetlands

The picture indicates Cyathea dregei (common tree fern) growing on the centre drainage line of an unchannelled valley bottom wetland. These trees require constant water access in order to survive.

Flowpaths linked to shallow subsurface flow and wetlands

Hillslope seepage wetlands are actually localised springs in the landscape resulting from changes in the geology. The water reaches the surface as the rock becomes impermeable, flows over this rock until it reaches a permeable surface again and disappears subsurface and flows into a lower elevated wetland.

The picture on top indicates this situation, the blue arrows indicating potential hillslope seepage wetlands and the green arrow the connection area with a lower elevated wetland.

The bottom picture illustrates Cyathea dregei nestling in a cliff. The presence of a permanent flowpath.

Vegetation with flowpaths: Increase in cation exchange capacity

(CEC). pH neutral to slightly alkaline.

Increase in groundwater availability (yield).

Figures: Distribution of CEC-values among geobotanical and non-geobotanical communities (n = 100 for each pie diagram).

The case studies (4) representing the Limpopo Granulite-Gneiss Belt indicate an important correlation:

Soil evidence

Non-geobotanical Community CEC-values (cmol(+)/kg)

45%

31%

10%

14%

<5.00

5.00-10.00

10.01-15.00

>15.00

Geobotanical Community CEC-values (cmol(+)/kg)

10%

23%

33%

34%

<5.00

5.00-10.00

10.01-15.00

>15.00

More examples exist.

A significant correlation of 81% exists between identified geobotanic species and groundwater, geology and the resulting geophysical anomaly.