Rainfall Erosivity at Roma, Lesotho
To successfully predict soil erosion, an essential element is information on rainfall intensity. Lesotho has a reputation of being one of the most severely eroded countries in Africa. It has been estimated that in the lowlands of Lesotho annual soil loss is at 30 to 100 tons per hectare. Gully erosion is widespread and forms noticeably visible confirmation of water induced erosion. According to two experts, the amalgamation of surface wash and rill erosion is believed to be the most negative erosive method in this part of Africa.
Let us look at what rainfall erosivity is. Rainfall erosivity may partially be characterized as the possible power of rain to cause erosion through splash. “Precipitation induces soil erosion in a number of ways. Direct raindrop impact on a soil causes detachment and splashing of soil particles which are then redeposited in a denser arrangement. A downslope transport of particles by splash erosion as well as a surface sealing and crust formation occurs that causes a deterioration in infiltration capacity of the soil. The portion of precipitation that in-filtrates, percolates and may sometimes form concentrated interflow which can cause piping and enhance the risk of gullying”.
Lesotho is located on the east side of the African continent, with the 30° southern latitude and 28° eastern longitude crossing within its borders. From the arid west to the east, there is a significant distinction between the increasing rainfall patterns, 200 mm and 800 mm respectively. “Average annual rainfall in Lesotho ranges between 500-800 mm in the low-lands and over 1000 mm in the mountains. Lesotho receives about 80% of the total annual precipitation during the summer (October-April) with a pronounced dry season during winter”. In addition to this, Lesotho experiences different types of rainfalls, from drizzles to heavy downpours.
From the available intensity records we can see a correlation the rainfall recorded at Roma show a relationship between total sum of rainfall during individual storms and greatest concentration calculated during 15, 30 and 60 minutes. From this, we can identify that it is likely that rains causing soil erosion occur more frequently than storms themselves. “Standard criteria of rainfall erosivity indicate that Lesotho’s rains are relatively unerosive. This study shows that standard measures of erosivity are of limited value for the rainfall regime in Lesotho and would give misleading results if applied in erosion prediction equations”.
Studying the impact of Lesotho’s rainfall as an erosive force allows experts to help the country to find new ways to helping and developing more agricultural techniques for the industry.
Calles, Bengt & Kulander, Lena (1994) Rainfall Erosivity at Roma, Lesotho, Geografiska Annaler. Series A, Physical Geography, Blackwell Publishing on behalf of the Swedish Society for Anthropology and Geography.