The aim of this study is to investigate and identify the impacts of the large scale El
Niño-Southern Oscillation (ENSO) on rainfall, generalized monsoon index (GMI) and stream
flow in the Mae Rim watershed which is connected to the Upper Ping River Basin (UPRB), a
tributary of the Chao Phraya River Basin in Central Thailand, to help water management
decisions. The monthly serial data of rainfall, GMI and stream flow of the Mae Rim watershed
during 1982 to 2011 were used to study for a correlation with the ENSO index. South oscillation
index (SOI) is the selected ENSO index representing the optimized correlation with the serial
rainfall and stream flow of the study area. The two methods used to investigate impact on the
rainfall are the cross-lag monthly correlation and the ENSO phased classification within three
phases of El Niño, La Niña and Normal and sub-phases of strong, medium and weak. The
results indicated that the impacts of SOI on rainfall and stream flow are only found to be
significant during the strong and medium of El Niño and La Niña phases. Additionally,
significant impact mostly occurred in the early months of the southwest monsoon season. Thus,
the 3-mrm SOI is the available index used to forecast rainfall, GMI and stream flow up to 8
months in advance, especially for the hydro climates in April to July. The methods employed in
our study may be modified to gain a better understanding of the impact of the large scale ENSO
on rainfall, GMI and stream flow in other locations in Southeast Asia.