The 137Cs radioactivity of soils was used as a tracer of soil erosion in a catchment in the Netherlands: 143 samples were analysed to map the I3'Cs redistribution using geostatistical interpolation methods. Caesium-137 activities on grassland are significantly higher than on arable land. Also, 13'Cs activities on waning slopes are higher and activities on steep slopes are lower. The soil erosion estimates, derived from the 137Cs data, are used to validate the USLE erosion model. The recent Chernobyl nuclear accident also contributed to the lJ7Cs activity. However, the Chernobyl input of I3'Cs, with a constant ratio of 1.765:l to IJ4Cs, cannot be used as a tracer of soil erosion. Because of the rapid decay of 134Cs, we will not be possible to separate the sources of IJ7Cs in the near future in areas significantly influenced by Chernobyl fallout and in these areas I3'Cs can no longer be used as a soil erosion tracer.
KEY WORDS I3'Cs Soil erosion Chernobyl Geostatistics Digital Elevation Models Soil erosion models
SOIL EROSION IN SOUTH LIMBURG
Soil erosion and flooding have increased in the hill country of South Limburg (The Netherlands) during the last decades. Schouten et al. (1985) stated that the expansion of urban areas and the increased number of sealed roads, as well as the intensification of agriculture are the major causes. Due to recent reallotment, fields have become larger, which causes surface runoff to be more erosive. Changes in land use also contribute to increasing erosion. The area of grassland has decreased in favour of urban areas and, after 1975, in favour of arable land (Van der Helm et al., 1989). Moreover there has been a change in land use and in the kinds of crops grown in South Limburg. Between 1960 and 1986, the growing of crops causing a higher erosion risk, such as maize and sugarbeet, has increased in South Limburg, replacing cereals such as winter wheat.
MAPPING SOIL EROSION
There are several methods for mapping soil erosion. One method consists of monitoring height changes in the landscape due to soil erosion, through the use of erosion pins (Haigh, 1977). However, several problems with this method have been noted, such as soil disturbance on insertion, and the influence of flow in their vicinity, causing enhanced turbulence in runoff (Rogowski et al., 1985). Because erosion rates are generally of the order of a few millimetres per year, a long period of measurement is needed to obtain a reliable estimate. This difficulty is confirmed by field experience in the Netherlands. Furthermore, farmers may move or disturb the pins on arable land.
Another group of methods consists of the use of sediment traps to measure erosion. A box dug into the slope with its top flush with the ground surface, catches surface runoff and sediment from the slope above it. A major problem with this method is defining the contributing area to the troughs . Therefore, the plots are often bounded. Consequently, the influence of slope length is not well accounted for. These sediment traps are used to measure both short-term and long-term erosion rates.