Prediction and modelling of soil erosion…

1. Prediction and modelling of soil erosion… • Why would we want to predict soil erosion? • Optimal resource management • Evaluation of consequences of different land use • Compliance with environmental requirements • Development of sediment control plans (particularly for construction projects) • Prediction of dam infiltration rates…

2. What do we need to understand before we can predict erosion? • What factors affect soils’ susceptibility to erosion? • Erosivity of erosion agents. • Erodibility of soils. • Length of slope. • Gradient of slope. • Land cover and management. • NB vegetation, plant residues, soil tillage • Erosion control practices.

3. Erosivity… • Total rainfall • Intensity and seasonal distribution of the rain • Why is intensity important? • Intense rains have large drop size • Higher rate of rainfall = more runoff

4. Erodibility… • Indicates a soil’s inherent susceptibility to erosion • Infiltration capacity • Structural stability • Properties that tend to result in high erodibility • High fine sand and silt content • Expansive clay minerals • Impervious soil layers • Blocky, platy or massive soil structure

5. Properties that lead to low erodibility • High organic matter content • Nonexpansive clays • Strong granular structure

6. Some important principles in erosion control…

7. Some important principles in erosion control… • Keeping disturbed soil covered • Controlling runoff • Trapping sediment • Altering soil properties (more difficult)

8. Soil Degradation… • Has many causes and various effects of which erosion is one. • See overhead.

9. South Africa…(work done at UWC) • In South Africa apartheid policies ensured that 42% of the people lived on 13 % of the land (the "homelands"). • This overcrowding has resulted in severe erosion. • Soil erosion can be seen as both a symptom of underdevelopment (i.e. poverty, inequality and exploitation), and as a cause of underdevelopment.

10. Stats from the UWC… • Annual soil loss in South Africa is estimated at 300 - 400 million tonnes, nearly three tonnes for each hectare of land. • Replacing the soil nutrients carried out to sea by our rivers each year, with fertilizer, would cost R1000 million. • For every tonne of maize, wheat, sugar or other agricultural crop produced, South Africa loses an average of 20 tonnes of soil. • The FAO (Food and Agriculture Organisation, a branch of United Nations) estimates that the global loss of productive land through erosion is 5-7 million ha/year.

11. SOIL DEGRADATION IN THE KAROOMilton and Dean, 1996; Cowling, Roux, Pieterse, 1986, Roux, 1981 • semi arid environment • low and highly variable rainfall

13. grassy vegetation, woody shrubs and succulents • thin soils (ancient) • long slopes

14. Silvery grasses: protect soil and moisture • bush ineffective ‘interceptor’ • Surface sealing and crusting = sheet erosion

16. HISTORY • early C20 - wool boom • Wall St crash in 1929: prices dropped • more stock to make a profit • 1930 - 48 million small stock! • Capacity estimated at - 7-10 million • Current: 10 million

17. variable grazing capacity • 1868-1902: good rains, high stocking rates • 1903-1957: low rains overstocked degradation • Economics = keep numbers high

19. DESTRUCTION OF VEGETATION COVER Summary of vegetation changes according to Roux et al, 1981 Phase 1: degradation (1850-1925) • destruction of pristine vegetation • thinning of palatable and soil protecting species

20. Phase 2: denudation (1930's ) • denudation and thinning out • exacerbated by drought • elimination of palatable species • High runoff and high erosion.

21. Phase 3: re-vegetation • Karoo bushes and less palatable species • Grasses appear after rains. Phase 4: stabilisation • Stable cover of scrub and bush, few grasses • low grazing capacity, high erosion Phase 5: desertification • vegetation cover at a minimum • soil exposed