Applying phosphorus indices at a small agricultural watershed in Southern Brazil

Best management practices at watershed scale are essential to mitigate water pollution. The objectives of this study were: (1) to estimate the P-index in a small watershed with intensive agricultural use applying five P-index versions at three scales (watershed, sub-basin and agricultural field); (2) to assess the effect of the connectivity factors (distance between the agricultural field and the stream and width of riparian native vegetation) in estimating the risk of P loss. The five P-index versions resulted in a similar risk of P loss, 75 to 83 % of the whole watershed scale (agricultural plus forest areas) was classified as low or very low risk for P loss. At the agricultural area scale, 79 to 100 % of this area was classed as high and very high risk for P loss. The low risk of P loss at watershed scale is explained by the high occurrence of forest vegetation. The reduced distance between agricultural land and streams and/or the reduced width of riparian native vegetation increased the risk of P loss. Estimated P-index values at a sub-basin scale indicated lower risk of P loss compared to agricultural field scale. In order to better estimate the risk of P loss at an agricultural field scale, we advise using a P-index which considers also connectivity factors.