Results and discussion
3.1. Watershed and catena scale
Fig. 2 shows the distribution of Normalized Differenced Vegetation Index (NDVI) classes indicating different crop growing potential on the landscape scale. In order to reduce the randomness in prediction caused by varying management practices, the growing of different rice species and differing meteorological conditions throughout the years, the classification is based on a 4-year series (1999–2001, 2005) of LANDSAT ETM+-, SPOT 5- and ASTER-satellite images. Each of them was taken shortly before second rice harvest in fall. By using the ISODATA clustering method according to Tou and Gonzalez (1974) the NDVI was grouped into three classes, ranging from low (1) to high (3). As presented in Fig. 2, classes with high NDVI-values can be clearly located on the valley floors that are mainly covered with clay and silt enriched alluvial sediments, in depressional areas following the slope gradients and in concave shaped landforms. Regions with low NDVI values all over the years are usually related to the higher sections of the watershed, especially to the Red Clay forming the eastern and southern parts and to the small hilltops that are mainly developed from Red Sandstone.