In Rwanda more than 745 thousand hectares of potential agricultural land are eroded annually in Rwanda. Using the reference year 2021A, more than 3 million tons (six million tons annually) of crop produce are expected to be lost each season, with severe erosion affecting 22,000 tons of maize and 15,000 tons of beans. Each season, severe erosion results in a total economic loss of 37.9 billion Rwandan francs (RWf) in agricultural productivity. The research’s overall objective was to assess the impact of soil erosion on soil fertility on sebeya catchement. the specific objectives was to analyze to determine amount of soil loss in Sebeya catchment, determine the impact soil erosion on soil nutrients loss and estimate the value of replacement cost of soil nutrients lost in RWf for period of 2022, To achieve these objectives RUSLE model & Geographic Information System (GIS) techniques were combined to determine soil erosion .Soil mapdatabase was used to determine the quantinty of soil nutrients, whereas ministerial price guideline from MINAGRI was used to determine the value (RWf) of fertlizers (NPK) to replace the nutrients lost due to soil erosion. This study showed that amount of soil loss by combining all the five factors influencing soil erosion. After observation and analysis, it revealed that the big losses of soil in the sebeya watershed are only found on the surface covered by cropland and settlements with an estimated amount >300 t/ha/year, while other part of the catchment covered by forest and grasses loose a low quantity of soil with an estimation of 0 to 50 t/ha/year, which reconfirms that human activities are well managed and structured by rules and guidelines given by responsible institutions in charge such as MINAGRI, REMA, RWB, etc the sebeya catchment that is facing large amount of soil loss each year can be protected efficiently. The study has revealed that erosion is seriously taking place in Sebeya watershed, the results shows that, In general, the average soil composition of Nitrogen in the sebeya watershed is 0.378%, 9.051 mg/l of Phosphorous and 1,205 Cmol/kg of Potassium. Generally, Nitrogen is the soil nutrient that is highly lost with an estimated amount of 90 kg/ha/y. the highest amount of N lost is found in cropland about 173.4kg.ha/y while forestland loses the lowest amount of N estimated about 14.45kg/ha/y. the highest loss of nutrients amount of P and K are also observed in cropland with respective amount of 56.82kg/ha/y and 147.89kg/ha/y. These soil nutrients losses have a huge impact on soil fertility reduction, because the soil fertility is made at 80% of these three elements (N, P and K). This average value of soil loss in the catchment, which is estimated to be about 135t/ha/y, naturally indicates 90 kg/ha/y of N, 15kg/ha/y of P, and 74kg/ha/y of K. If three or five consecutive years of the same soil loss amount occur, soil fertility will be gone, and we will be left with a marginal soil unsuitable for cultivation. This study recommends the consideration of soil erosion control measures for all government plans to increase agricultural productivity through intensification and commercialization and suggest to conduct more research for other watershed to know exactly the amount for of soil and nutrient loss.

Keywords : Soil loss, Soil Fertility , GIS and Remote Sensing, RUSLE, Sebeya Catchement