Soil erosion rates on sloping cultivated land on the Loess Plateau near Ansai, Shaanxi Province, China: An investigation using 137Cs and rill measurements

Sediment yields from the rolling hills area of the Loess Plateau in northern China (10 000-25 000 t km(-2) yr(-1)) are amongst the highest in the world. The sediment is believed to derive from both the deep gullies that dissect the rolling plateau and the steep cultivated fields on the slopes of the mounds between the gullies. However, there are few reliable data for erosion rates on the cultivated fields and it is suspected that current estimates (10 000-16 000 t km(-2) yr(-1)) based on empirical relationships (derived from erosion plot studies) exceed the true values. This study sought to address the need for more information concerning erosion of the cultivated fields through derivation of erosion rates from measurements of rill volume and caesium-137 (Cs-137) inventories for typical fields near the village of Ansai, Shaanxi Province. The derived erosion rates are discussed and compared with estimates based on empirical relationships derived from erosion plot data. Where erosion rate estimates based on both rill volume data and Cs-137 inventories are available, they show good agreement in the pattern of downslope variation. Both show a sharp decline in erosion rates at a slope length of c. 50 m. This is tentatively attributed to a change from transport-limited to detachment-limited conditions, where rill incision reaches the undisturbed loess at the base of the plough layer. No such decline is visible in the predictions based on empirical relationships derived from erosion plot data, Further evidence is presented that supports the suggestion that these empirical relationships overestimate erosion rates at slope lengths in excess of c. 50 m. It is tentatively suggested that the rates of soil erosion from sloping cultivated fields in the rolling hills area are more likely to lie in the range 8000-10 000 t km(-2) yr(-1) than in the higher range suggested by the empirical relationships. (C) 1998 John Wiley Sons, Ltd.

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