Observed daily rainfall data were collected from the Thai Meteorological Department and Royal Irrigation Department. The ten stations were selected according to Table 1 after comparing the completeness and continuity of the recorded data among the rainfall stations distributed across the watershed. The baseline period of 1985–2014 was also selected based on the aforementioned results, and missing data was estimated using the normal ratio method. This method is commonly used in estimating missing rainfall values due to its simplicity and efficiency and has been used in Thailand (Okwala et al. 2020). Rainfall data in the study area fell within the criteria of the method where the surrounding gauges have the normal annual rainfall exceeding 10% of the gauge considered. Moreover, this method is considered to be suitable for tropical climates (Silva et al. 2007). The stations were grouped into two groups: high (EL. >100 m.MSL) and low (EL. <100 m.MSL) altitude stations, according to their elevation range.