Concerning the potential impacts of climate change on extreme events at the watershed scale, the nine extreme rainfall indices (Table 4), a subset of 27 extreme indices, recommended by the World Meteorological Organization-Commission for Climatology (WMO-CCI)/World Climate Research Programme (WRCP)/Climate Variability and Predictability (CLIVAR) project's Expert Team on Climate Change Detection and Indices (ETCCDI), was selected for comparative purposes with previous studies and other areas and based on available data that better describe the climatic behaviour of Thailand's climate. Moreover, they also can provide indicators for identifying risks of frequent extreme events in Thailand such as floods and droughts in terms of intensity, frequency, and severity. The selected indices were calculated from daily rainfall data using RClimDex software. RClimDexQC was used to control the data quality and homogeneity of historical data before calculating the extreme indices. The software can detect unreasonable values and then automatically replace their values with −99.9 or NA (not available).
Definitions of the selected extreme rainfall indices
| Index . | Index Name . | Definition . | Unit . |
|---|---|---|---|
| R95p | Very wet days | Annual total precipitation from days >95th percentile | mm |
| R99p | Extreme wet days | Annual total precipitation from days >99th percentile | mm |
| RX1day | Max 1-day precipitation amount | Annual maximum 1-day precipitation | mm |
| RX5day | Max 5-day precipitation amount | Annual maximum consecutive 5-day precipitation | mm |
| PRCPTOT | Annual total wet-day precipitation | Annual total precipitation from days ≥1 mm | mm |
| SDII | Simple daily intensity index | The ratio of annual total precipitation to the number of wet days (≥1 mm) | mm/day |
| CDD | Consecutive dry days | Maximum number of consecutive days when precipitation <1 mm | days |
| CWD | Consecutive wet days | Maximum number of consecutive days when precipitation ≥1 mm | days |
| R20 | Number of very heavy precipitation days | Annual count when precipitation ≥20 mm | days |
| Index . | Index Name . | Definition . | Unit . |
|---|---|---|---|
| R95p | Very wet days | Annual total precipitation from days >95th percentile | mm |
| R99p | Extreme wet days | Annual total precipitation from days >99th percentile | mm |
| RX1day | Max 1-day precipitation amount | Annual maximum 1-day precipitation | mm |
| RX5day | Max 5-day precipitation amount | Annual maximum consecutive 5-day precipitation | mm |
| PRCPTOT | Annual total wet-day precipitation | Annual total precipitation from days ≥1 mm | mm |
| SDII | Simple daily intensity index | The ratio of annual total precipitation to the number of wet days (≥1 mm) | mm/day |
| CDD | Consecutive dry days | Maximum number of consecutive days when precipitation <1 mm | days |
| CWD | Consecutive wet days | Maximum number of consecutive days when precipitation ≥1 mm | days |
| R20 | Number of very heavy precipitation days | Annual count when precipitation ≥20 mm | days |