Prediction of Runoff and Sediment Yield from Agricultural Watersheds Using Annagnps.


Sediments resulting from soil erosion are the major non-point source pollutant of  surface waters in agricultural watersheds.

Annualized Agricultural Non-Point Source Pollution Model (AnnAGNPS) is a computer-based watershed model that predicts non-point source pollutants and runoff loadings within agricultural watersheds.

AnnAGNPS v5.2 was used in conjunction with Arcview  3.2  GIS  to  predict streamflow and sediment discharges from a 250.89 ha agricultural watershed, Upper Ebonyi River watershed, located at Obollo-Etiti in Udenu Local Government Area in Enugu State, South-eastern Nigeria.

AnnAGNPS predictions were compared with two months (September and October) of streamflow and sediment discharge field measurements from the study watershed.

The September data were used  to  calibrate the model to a reasonable agreement with predicted data (R2 =  0.934  for streamflow and R2 = 0.707 for sediment yield).

Statistical performance  evaluation  of the model was carried out on the validation results. The model performed very well in following the trends, peaks and volumes of the measured hydrograph and  sediment  graph, with  R2 = 0.991 for hydrograph and R2 = 0.968 for sediment  graph.

The results show that  the model performed better in predicting runoff than predicting sediment yield.

Further, the model was applied to identify erosion hot spots  in  the study watershed. The model gave a sediment delivery ratio (SDR) of 0.419 for the watershed.


Background of Study

Agricultural non-point source pollution (NPSP) is the leading cause of surface water degradation due to intensification of agricultural production in last few decades (Ma et al., 2002).

The United States Environmental Protection  Agency  (USEPA,  1993) defines NPSP as pollution caused by diffuse sources not regulated as point sources and normally associated with runoff or percolation.

This type of pollution is often caused by poor management practices and includes soil erosion, nutrients and pesticides in agricultural runoff, pathogens from feedlots, urban runoff and sewage discharge (Tim and Jolly, 1994).

The major non-point source pollutants of interest for control and regulation include degradable organics, toxic compounds, sediment,  organic nitrogen (N) phosphorus (P) and inorganic N and  P  (USEPA,  1983).

Sediments resulting from soil erosion are by far the major pollutant  of surface waters  in rural and agricultural watersheds (Mbajiorgu, 2004).

The effects of different land uses (fallow, cropland, forest, pasture, rangeland and tillage) on runoff volume, soil erosion and sediment yield were modeled.

The results show that fallow land use gave the highest runoff volume, soil erosion and sediment yield followed by tillage land use while forest land use gave the least runoff volume, soil erosion and sediment yield.

Therefore, forest land use should be predominantly practiced to reduce sediment loss on the watershed. More conservation practices should be adopted at the landscape to reduce sediment loss for fallow and tillage land uses.


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