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Soil Erosion Monitoring

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Detailed scientific research is carried out at the site to acquire better information about the conditions that favour or hinder soil erosion, a factor of major importance in mountain areas. Soil conservation on farmland and maintaining or improving its fertility are key research themes for ICIMOD. One of the main approaches used to control soil erosion is sloping agricultural land technology (SALT), or contour hedgerow intercropping agroforestry technology (CHIAT) (see sheet on Soil Conservation). In this method, double hedgerows of nitrogen fixing trees or shrubs are planted along contour lines at a spacing of 5-6 m. The hedgerows act as a barrier to water runoff and as a rich source of organic matter. Sediment washed down the slope by rain builds up behind the hedgerows, slowly transforming the slope into a series of natural less sloping terraces.

A series of plots were established in 1995 to measure the impact of nitrogen-fixing hedgerows on soil erosion and investigate the conditions that favour or hinder erosion. The surface runoff from each experimental plot is diverted through a gutter system into collection tanks and the soil erosion is evaluated from sediment concentration in runoff and total runoff.

The hedgerows are very effective in reducing soil erosion to a very low level, with a marked impact from the second year of planting and a reduction in soil loss by 80-99% from the fifth year on. Distribution of erosion over time is extremely inhomogeneous. Soil loss is associated with intense rainfall events, but only at certain times of year. Soil loss from control plots varied from around 3-8 t/ha in most years to a massive 131.6 t/ha in one year with a cloudburst event in the premonsoon period. This reflects the type of, often very localised, events that are devastating for mountain agriculture.

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