Abstract
For better water resources management, we proposed a method to estimate basin-scale seasonal rainfall over selected areas of the Chao Phraya River Basin, Thailand, from existing climate indices that represent variations in the Asian summer monsoon, the El Niño/Southern Oscillation, and sea surface temperatures (SST) in the Pacific Ocean. The basin-scale seasonal rainfall between 1965 and 2015 was calculated for the upper Ping River Basin (PRB) and the upper Nan River Basin (NRB) from a gridded rainfall dataset and rainfall data collected at several gauging stations. The corresponding climate indices, i.e., the Equatorial-Southern Oscillation Index (EQ-SOI), Indian Monsoon Index (IMI), and SST-related indices, were examined to quantify seasonal rainfall. Based on variations in the rainfall anomaly and each climate index, we found that IMI is the primary variable that can explain variations in seasonal rainfall when EQ-SOI is negative. Through a multiple regression analysis, we found that EQ-SOI and two SST-related indices, i.e., Pacific Decadal Oscillation Index (PDO) and SST anomalies in the tropical western Pacific (SSTNW), can quantify the seasonal rainfall for years with positive EQ-SOI. The seasonal rainfall calculated for 1975 to 2015 based on the proposed method was highly correlated with the observed rainfall, with correlation coefficients of 0.8 and 0.86 for PRB and NRB, respectively. These results suggest that the existing indices are useful for quantifying basin-scale seasonal rainfall, provided a proper classification and combination of the climate indices are introduced. The developed method could forecast seasonal rainfall over the target basins if well-forecasted climate indices are provided with sufficient leading time.
Highlights
Many regions have endured significant impacts from extreme floods and droughts, including Southeastern Asian countries [1,2,3,4]; disastrous floods and droughts are expected to occur more frequently due to the changing climate [2,5,6,7]
Areal rainfall data indicates that over 90% of the annual rainfall occurs during April and October almost every year, with the largest mean monthly rainfall occurring in September (PRB) or August (NRB) (Figure 2)
Upper Nan River Basin (NRB) in Thailand were analyzed to elucidate a quantitative relationship with existing climate indices
Summary
Many regions have endured significant impacts from extreme floods and droughts, including Southeastern Asian countries [1,2,3,4]; disastrous floods and droughts are expected to occur more frequently due to the changing climate [2,5,6,7]. To mitigate the impacts of extreme hydro-meteorological events, well-prepared water resource management practices are required, for which the quantification of rainfall and resulting runoff is key information. In Thailand, a devastating flood occurred in the Chao Phraya River Basin in 2011 [8,9], resulting in a change in policy to focus more on flood mitigation. Water 2018, 10, 800 during 2015 and 2016 due to the reduced water storage of the major reservoirs, i.e., the Bhumibol and Sirikit Dam reservoirs during the preceding years and the limited amount of rainfall in the wet season of 2015. Sufficient water storage in these reservoirs is required before the dry season while enough empty volume should be maintained for flood-control during the rainy season. Water release from the reservoirs needs to be well-managed by forecasting the areal rainfall and its resulting inflows to the reservoirs with sufficient leading time
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