An effort was made to identify the critical sub-watersheds for the development of best management plan for a small watershed of Eastern India using a hydrological model, namely, AVSWAT2000. A total of 180 combinations of various management treatments including crops (rice, maize, groundnut and soyabean), tillage (zero, conservation, field cultivator, mouldboard plough and conventional practices) and fertilizer levels (existing half of recommended and recommended) have been evaluated. The investigation revealed that rice cannot be replaced by other crops such as groundnut, maize, mung bean, sorghum and soyabean since comparatively these crops resulted in higher sediment yield. The tillage practices with disc plough have been found to have more impact on sediment yield and nutrient losses than conventional tillage practices for the existing level of fertilizer. Sediment yield decreased in the case of zero tillage, conservation tillage, field cultivator, mouldboard plough, and conservation tillage as compare to conventional tillage. Lowest NO 3-N loss was observed in zero tillage in all the fertilizer treatments, whereas field cultivator, mouldboard plough and disc plough resulted in increase of NO 3-N loss. As compared to conventional tillage, the losses of soluble phosphorus were increased in mouldboard plough. The losses of organic nitrogen were also increased as fertilizer dose increased. After zero tillage the conservation tillage performed better in all the fertilizer treatments as per loss of organic nitrogen and organic phosphorus is concerned. It can be concluded that the sediment yield was found to be the highest in the case of disc plough followed by mouldboard plough, field cultivator, conventional tillage, field cultivator and least in zero tillage practices. The nutrient losses were found to be in different order with tillage practices, resulted highest in disc plough tillage practices. In view of sediment yield and nutrient losses, the conservation tillage practice was found to be the best as the sediment yield is less than the average soil loss whereas nutrient loss is within the permissible limit.
