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 erodibilityHigh organic matter contentNonexpansive claysStrong granular structure

Some important principles in erosion control…

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 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.

SOIL DEGRADATION IN THE KAROOMilton 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 1930 - 48 million small stock! Capacity estimated at - 7-10 million Current: 10 million

variable grazing capacity 1868-1902: good rains, high stocking

rates 1903-1957: 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 (1850-1925) 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