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locations for sampling were drawn a priori on a map, and the coordinates of the 278sampling locations were identified in the field using a global positioning system andthe nearest rice field sampled. Soil samples were collected from 0 to 15-cm (soilmonolith 0.2 m × 0.2 m × 0.15 m by spade) and 15 to 40-cm (dutch auger) depths.Five samples were bulked, one from the center of the field and four samples within a6-m radius around the center of the field. The samples were air-dried and ground topass through a 2-mm sieve. Soil particle size distribution was determined using thehydrometer method.From the collected point-data, maps of subsoil textures were generated for eachof the 250 × 250-m cells of the 40,000-ha area using indicator geostatistics with andwithout prior means (Goovaerts 1997), as described in Oberthür et al (1999). TheGIS-based map would allow us to classify and map out cells according to their subsoilclay contents.Results and discussionRainfall and water table depthFigure 1 displays the extent of rainfall variability during the wet season across yearsfor the Det Udom area. Seasonal rainfall distributions and groundwater table fluctuationsduring the 1993 and 1994 growing seasons contrasted markedly (Fig. 2). Totalrainfall received by the rainfed lowland rice crops was 1,035 mm for direct-seededrice in 1993 and 751 mm for the transplanted rice crop during the 1994 wet season. Inthe 1994 WS, 650 mm of rainfall were received from May to late July, when the greenmanure plants were growing before the rice crop. In 1993, significant rainshowersRainfall (mm)1,6001,4001,2001,0008006004002000AF80AF50AF2019931994Jun Jul Aug Sep Oct NovMonthFig. 1. Cumulative rainfall in 1993 and 1994 and at probability ofexceedence of 20% (AF20), 50% (AF50), and 80% (AF80) at DetUdom, Ubon Ratchathani, lower northeast Thailand. The frequencyanalysis was based on 1963-97 rainfall record.Identifying soil suitability for subsoil compaction . . . 101