Observations on effects of ground cover and soil surface configurations on soil erosion in a mini-watershed

Undulating mini-watershed of approximately 14 ha (88 rai) was selected in a village of Khon Kaen province, Northeast Thailand. Seasonal changes of ground cover and soil surface configuration were monitored bi-monthly to monthly with special emphasis on the uplands from 1999-2001. Cropping patterns with different planting and harvesting dates of cassava and sugarcane interspersed by vacant plots brought about uneven distribution of percentage ground cover in the landscape at any time of monitoring. As a result, a mosaic pattern of ground cover was observed at any time of the year. From erosion plot study, there was tendency for the plot with higher ground cover percentage to lose less sediment from the plot under lower ground cover percentage. Sediment traps inside a plot consisted of the rough furrow-floor with small depressions, living weed plants and crop residues. They did not seem to have strong roles in trapping sediment at early growth stage of crop or immediately after soil tilling. However, living weed plants and crop residues could play a major role as sediment traps when crop plant became more mature. More sediment traps were found along the way from any field plots to paddy fields down the slope. Outside the field plots, sediment traps found were plough-blade depression, patches of grass and ridges present at the outer lower slope edge of a plot along the plot border, vacant land adjacent to a plot and drainage ditches or waterways, occasionally with dyke along the border between 2 adjacent plots. All sediment traps outside a field plot could trap sediment moved with run-off water to a certain extent. Some important traps were vacant land with grass, ridge with grass and dyke across a ditch. Major farmer management decisions that could influence soil erosion were selection of planting date and setting up of row direction. Crops planted at the end of the rainy season developed more crop canopy and protect the soil underneath from erosion than those planted at the other time of the rainy season. Farmers observed that planting cassava on ridges along the slope helped reduce erosion as water was freely drained in numerous furrows between ridges. Other observed management that might help reducing erosion included ploughing under the weed or residues from previous crop and applying filter cake from the sugarcane factory to the plot.