The Impacts of ENSO Phases on the Variation of Rainfall and Stream Flow in the Upper Ping River Basin, Northern Thailand

Abstract

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.