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Interpolation of rainfall databases of Northeast Thailand of daily measurements at rain stations over 20 years were mapped with GIS tools. Various maps clearly visualized the characteristics of rainfall in this area. Annual and mid-rainy season rainfall showed a descending trend from the northeastern area to the southwestern area, probably due to the southwest monsoon and mountains in Laos and between Central and Northeast Thailand. Yearly variation in annual rainfall in this area was not particularly high, suggesting that its general image as a drought-prone area may be caused by the undulating topography and predominance of sandy soils in this area, which contribute to its vulnerability to flooding and drought. Both mean rainfall amount per rainy day and the number of rainy days in the rainy season showed large regional and yearly variation, but mean rainfall per rainy day was correlated to annual rainfall, whereas the number of rainy days was not strongly correlated to annual rainfall. Large regional variation in the number of rainy days in the rainy season suggests large regional variations in agricultural productivity in this area. The fact that areas near many of the provincial capitals have a high number of rainy days in the rainy season with small yearly variation may indicate relatively high and stable agricultural productivity in such areas that probably supported the establishment of those cities in early days. The duration, onset and end of the rainy season showed small yearly variation, suggesting that these parameters are not solely responsible for the unstable and erratic rainfall in Northeast Thailand.

The transport of three herbicides, pyriminobac-methyl, imazosulfuron and pyraclonil from a watershed that includes 40 ha of paddy fields to a drainage canal was monitored in the Lake Biwa Basin, Japan. Based on the intensive monitoring of all paddy plots and in the drainage canal conducted on 3 days during and after the application period, the passage of herbicide discharge from the paddy fields to the drainage canal was separated into surface runoff obtained from field observations and percolation flow calculated from the herbicide mass balance. Surface runoff from paddy plots immediately after herbicide application, discharging a large volume of paddy water, or both processes in conjunction had a significant effect on herbicide discharge. Without surface runoff, paddy fields discharged a consistently high amount of herbicide gradually by percolation flow. These results suggest that considerable amounts of herbicides were discharged into the drainage canal through percolation even if appropriate water management to prevent herbicide surface runoff was practiced.

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