THE USE OF CESIUM-137 MEASUREMENTS OF SOIL-EROSION AND FARMERS PERCEPTIONS TO INDICATE LAND DEGRADATION AMONGST SHIFTING CULTIVATORS IN NORTHERN THAILAND

Upland agriculture in northern Thailand has been claimed to cause decl ining soil fertility and to lead to sedimentation and water shortages in the lowlands. Research was undertaken in a Yao community of Chiang Rai province to identify the extent of soil degradation and to what ex tent physical changes in soil properties were perceived by farmers. GI S was used to classify land susceptibility and the distribution of cul tivation in historic times. Historic soil erosion was measured using t he Cesium-137 technique. Farmers were questioned to identify the perce ived importance of erosion. Results indicate that erosion of between 2 8 and 64 t ha-1 yr-1 occurred on steep slopes during the period 1963-1 991; these amounts are consistent with those measured on land underlai n by granite elsewhere in the world. Farmers perceive erosion as a haz ard and have generally avoided cultivating on steep slopes. In additio n, much erosion apparently results from naturally-occurring gullies. E vidence therefore suggests that the influence of upland agriculture on erosion is overstated. However, frequent cultivation is still resulti ng in declining soil fertility in the uplands.