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…

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.

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

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

 Properties that lead to low erodibility High organic matter content Nonexpansive clays Strong granular structure

Some important principles in erosion control…

 Keeping disturbed soil covered  Controlling runoff  Trapping sediment  Altering soil properties (more difficult)

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

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.

Stats from the UWC…  Annual soil loss in South Africa is estimated at 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.

SOIL DEGRADATION IN THE KAROO Milton and Dean, 1996; Cowling, Roux, Pieterse, 1986, Roux, 1981  semi arid environment  low and highly variable rainfall

 grassy vegetation, woody shrubs and succulents  thin soils (ancient)  long slopes

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

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

 variable grazing capacity  : good rains, high stocking rates  : low rains overstocked degradation  Economics = keep numbers high

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

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

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