NITROGEN AND PHOSPHORUS REMOVAL WITH SURFACE RUNOFF FROM DIFFERENT LAND USES- A STUDY IN A SUB CATCHMENT OF UPPER MAHAWELI CATCHMENT AREA IN SRI LANKA

Catchment of the river Mahaweli is the largest watershed in Sri Lanka. There are four major reservoirs built across river Mahaweli to generate hydropower and to irrigate agricultural lands in dry zone of the country. However, siltation and water quality deterioration of reservoirs are major issues when considering their sustainability. Main agricultural land uses in the Upper Mahaweli Catchment Area (UMCA) are tea plantations and vegetable farming on terraces in hilly slopes. Most of these terraces are not properly managed hence highly vulnerable to soil erosion. Hence, this study was conducted to measure surface runoff and removal of sediments and plant nutrients from vegetable plots in well managed Terraces (WMT) and Poorly Managed Terraces (PMT), Tea Plantations (TP) and Forest Gardens (FG) in a sub catchment of UMCA. Runoff plots were established to measure surface runoff and sediment loads in different land uses. Water soluble and sediment attached nutrient fractions were quantified. Soil chemical parameters of different land uses were also assessed using standard methods. A linear relationship between rainfall and surface runoff were observed in all tested land uses. The sediment transport rate was higher in vegetable plots on poorly managed terraces followed by well managed vegetable plots, tea land and natural forest. Nutrient removal with surface runoff was higher in vegetable plots compared to other land uses. A greater fraction of NO3-N was removed as soluble form from farmlands but in the case of PO4-P and NH4-N removal of particulate fraction was prominent. Particulate losses were approximately 15 times higher than the dissolved losses for NH4-N and approximately 100 times higher than the dissolved losses for PO4-P. In the case of NO3-N, soluble fraction was approximately 1.5 times higher than the particulate losses. Results conclude that, greater amount of nutrients and sediments are transported from vegetable plots on poorly managed terraces. Hence, proper maintenance of vegetable growing terraces coupled with effective nutrient management is very important to reduce downstream siltation and nutrient pollution in surface waters.