The influence of Soil Surface Conditions and Rainfall Intensity on Runoff Generation Potential in Semiarid Botswana

The study analyzed an investigation of soil surface conditions and their runoff generation potential by means of field experiment and model predictions. The field experiment was carried out during the period January 2012 – September 2013 at the Botswana University of Agriculture & Natural Resources (BUAN) Farm, located at Sebele (24°33̍S, 25°54̍E) and 994m above mean sea level.

The specific objectives of the study were (1) to determine the proportion of rainwater that went off as runoff from common tillage systems, and (2) to determine the rainfall intensity for runoff initiation under the bare soil surface condition.

In order to achieve the above specific objectives a field experiment was carried out. For specific objective one, a Completely Randomized Block Design comprising of three blocks of runoff/catchment plots measuring 25m2, 50m2 and 100m2 were laid out. Within same plots/catchment, the following soil surface conditions were imposed: Mouldboard Ploughed Surface (PS), Poorly Managed Surface (PM), Harrowed Surface (HS) and Natural Vegetation (NV). At the bottom of the catchment area, a runoff collection system was laid out and runoff was measured after every storm event.

The rainfall intensity data collected from the recording float-and-syphon type rain gauge by the Botswana Dryland Farming Scheme (1985-1994) at Sebele was compiled and used to complement the rainfall intensity data recorded during the experimental period (2012/13).

The analysis of the field experimental results showed that a 5 x 5 m PM (un-weeded surface) yielded significantly more runoff than NV, PS and HS by 39%, 61% and 56% respectively. Thus, to build-up moisture conservation on farmland for crop growth, farmers should choose a tillage system that promotes good rainfall infiltration into the soil, such as ploughing and harrowing once.

The power regression curve analysis to test if measured rainfall intensity could initiate runoff on bare soil surface showed that a rainfall intensity of 1.0 mm/h initiated runoff of 1.3 mm at R2 = 60%, which was relatively lower. This could be attributed to the only 22 observations made due to the limited number of rainfall events during the period under review.