『Abstract
　In the present investigation, an effort has been 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, AVSWAT 2000. A total of 180 combinations of various management treatments including crops (rice, maize ground nut and soybean), tillage (zero, conservation, field cultivator, mold board plough and conventional practices) and fertilizer levels (existing half of recommended and recommended) have been evaluated. The investigation reveled that rice cannot be replaced by other crops such as groundnut, maize, mungbean, sorghum and soybean since comparatively these crops resulted in higher sediment yield. The tillage practices with disk 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, mold-board plough, and conservation tillage as compare to conventional tillage. Lowest NO3-N loss was observed in zero tillage in all the fertilizer treatments, whereas field cultivator, moldboard plough and disk plough resulted in increase of NO3-N loss. As compared to conventional tillage, the losses of soluble phosphorus were increased in moldboard plough. The losses of organic nitrogen were also increased as fertilizer does increased. After zero tillage the conservation tillage preformed 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 disk plough followed by moldboard 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 disk 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 in permissible limit.

Keywords: AVSWAT; GIS; NPS pollution; Remote sensing; Watershed management』

Introduction
　Theoretical considerations of the AVSWAT 2000 model
Materials and methods
　Study area and data collection
　Data availability
　Spatial database generation
　Model input parameter
　Sensitivity analysis
　Goodness-of-fit criteria for evaluation of SWAT model
　Model calibration and validation
　Identification and prioritization of the critical sub-watersheds
　Evaluation of BMPs for prioritized sub-watersheds
　Tillage implements and crops considered for effective management
　Fertility status of the soils of the watershed
Results and discussion
　Identification and prioritization of critical sub-watersheds
　Simulation of nutrient losses
　Effective management
　Fertilizer and tillage effect on crop yield, runoff, sediment yield and nutrient losses
　Effect of tillage on nutrient losses
　　Nitrate nitrogen (NO3-N) in runoff
　　Soluble phosphorus in runoff
　　Organic nitrogen and organic phosphorus in sediment
Conclusions
Acknowledgments
References