Effects of different landuses on runoff and erosion characteristics of the Riam Kanan Catchment, Indonesia

The effect of four different landuses on runoff and soil loss were examined over a period of two cropping seasons in the Riam Kanan Catchment (RKC), located in the Indonesian province of South Kalimantan. The study was carried out in the context of sedimentation concerns for the Muhammed Nur Reservoir - an important source of drinking and irrigation water supply for the area. The landuses considered were cropland with minimum tillage, cropland with conventional tillage, grassland and areas reforested with rubber trees. Runoff and soil erosion data collected from plot experiments in the RKC, during two years of field experiments, were used to examine the relationship between rainfall, runoff and soil loss. Also, the field data were used to evaluate the effect of landuse on erosion and relationships between rainfall/runoff and erosion in the RKC. Field data collected from the RKC indicated that changes in landuse and vegetative cover from forest to cropland or grassland increased surface runoff during the wet season. The increase in the runoff was about four times for cropland and two times for grassland compared to that under forest. Runoff in the catchment was likely to occur when the rainfall intensity was >5 mm/h for all landuse types. The study also suggested that changes in landuse will result in decreased groundwater recharge in the catchment and reduced inflows to the Muhammad Nur Reservoir through groundwater flow from the catchment. Similarly, the field data indicated changes in landuse and vegetative cover from forest to cropland or grassland increased erosion during the wet season. The increase in erosion was about 20 times under cropland and five times under grassland compared to that under forest. Minimum tillage did not significantly assist in reducing erosion compared to conventional tillage. Additional soil conservation measures will still be required in cropping with minimum tillage for controlling soil erosion effectively. The linear regression model was used to describe the relationships between erosion and rainfall and runoff parameters in the RKC. The level of erosion was found to be better correlated to the average event rainfall intensity than to the total rainfall depth under the four landuses considered in the study. In general, the study concluded that the changes in landuse will result in alteration of erosion processes and may affect the capacity and useful life of the Muhammad Nur Reservoir.