Impact of El Nino and Southern oscillation (ENSO) on hydrometeorological characteristics and forest fires in 25 main river basins of Thailand: An investigation in Chao Phraya river basin

Results using historical data from 1950 to 1998 can be summarized as follows: 1) The ENSO events caused a reduction of rainfall amount in the Chao Phraya basin during the pre-monsoon period and at the end period of rainy season. Rainfall amount was below the average during the mentioned periods in the years that SOI was lower than long term average. The mean temperature of the mentioned periods showed the higher degree even in December. 2) ENSO showed small effect on annual rainfall in the Chao Phraya basin. The relatively larger effect was found in those area usually having small amount of rainfall while almost none in areas of heavy rainfall, especialy in those areas normally affected by the southwest monsoon. 3) Effect of Enso on Runoff of the Upper Chao Phraya basins: There was not strong effect of ENSO events on streamflow of head-water source area excepted in the 2 strong ENSO years when the runoff dropped considerably in both wet and dry periods. In general, however, the ENSO show less effect on streamflow in head-watershed areas. (R sup(2)=0.10-0.16) 4) Effect on ENSO on Hydrometeorological condition of Lower Chao Phraya basin. The upper air humidity and KI value indicating probability of rainfall in the ENSO years were considerably lower than those in the normal and La Nina Years. The was not sigificant relationship between the lower layer and upper lower air temperatures. The average surface air temperature in the ENSO years was about 0.7 deg C higher than that of the long-term average. 5) Effect of ENSO on Forest Fires in the Chao Phraya basin. ENSO index in terms of SOI and SST in general showed a rather low relationship with forest fire in Thailand. The SST of January (SSTj) in the years identified as ENSO year, however, indicates high influence of the ENSO event on forest area burnt (Ab) with the simple and polynomial regression equation. 6) Effect of ENSO on NDVI in the upper Ping basin. Investigation using LANDSAT imageries taken in February, which is a period before fire starting, during 1988-1999 indicates that NDVI in the ENSO years ranged between 0.1909-0.3734 in Mae Chaem Watershed and between 0.1433-0.4033 for Doi Suthep-Pui National Park. The average NDVI during the 1988-1999 ranges from 0.184-0.5026 and 0.1433-0.5060 for the respective areas.