## 2 Study Area and Data Preparation  
### Study Area  
Myanmar is the second largest country in Southeast Asia, sharing borders with China, Laos and Thailand in the east, and Bangladesh and India in the north. It has a 2832 km seaboard with the Indian Ocean to the west and south (Figure 1). There are four main rivers in Myanmar: Aveyarwaddy, Chindwin, Thanlwin and Sittoung. Among them, the Chindwin River is the third largest river in Myanmar and serves as one of the principal water resources of the country and the most important tributary of Aveyarwaddy, which is one of the major rivers in Asia. Located in northwestern Myanmar (Figure 1), the Chindwin River basin has a drainage area of 110,350 km\({}^{2}\) and a river length of 985 km. It is dominated by a mountainous forested terrain, except for the wide flood plain at its lowest southern part. The river flows into the Central Valley Region, which is the most economically developed area of the country. Several major biodiversity and conservation areas are located in the basin, such as the Hukaung Valley and the Htamanthi wildlife sanctuaries, as well as the Upper Chindwin Catchment Corridor and the Lower Chindwin Forest Corridor with important populations of critically endangered animals [(42)]. In general, the Chindwin River basin is economically and ecologically important for the development of the country [(42)]. Figure 1: Location of the Chindwin River basin. The Chindwin River basin has subtropical and tropical climates, with distinct humid and dry seasons. The southwest monsoon season starts from June and lasts until September, with a cloudy, rainy, hot, and humid summer. Meanwhile, the northeast monsoon season starts from December and ends in April, with fewer clouds, minimal rainfall, low humidity, and moderate temperature (Figure 2). Precipitation is generally uneven in space due to the effects of the southwest monsoon and the complex topography. According to the precipitation data statistics from five weather stations in the basin (Table 1), the mean annual precipitation exceeds 3700 mm in the northern mountainous source area (Hkamti station) but sharply drops to 750 mm in the southern part (Monywa station). The basin has an annual runoff of 1260-2630 mm, and its spatiotemporal pattern of runoff is similar to that of precipitation (Figure 2). The most dominant land is closed forest (occupying 50% of the basin), followed by degraded forest and shrubs (covering approximately 33%) and agricultural land (covering 15% of the basin area). Shifting cultivation and swamp account for 2%. During the southwest monsoon season, the Chindwin River basin is prone to severe floods every year at one place or another, due to the high rainfall intensities with significant spatial and temporal variations [42; 43; 44; 45]. According to the Department of Meteorology and Hydrology (DMH), Myanmar, flood occurs most frequently in July and August, accounting for 72% of the total number of floods in the basin [42]. The downstream area of the basin is located in the central dry zone of Myanmar, which is the country's major agricultural region. Owing to less rainfall and higher air temperature, the downstream region is facing frequent droughs, and severe and extreme droughs hit the region in 1997-1998, 2001, 2005, 2008-2009, and 2012-2014 [46]. Severe flood and drought disasters threaten property, human lives, food security and ecological biodiversity in the basin, making it important and urgent to assess flood/drought risks and adopt effective control measures. For flood/drought risk assessment, one fundamental job is to estimate streamflow regimes at any watershed locations through rainfall-runoff modeling. However, the rainfall-monitoring network in the Chindwin River basin is very sparse with a limited number of rain gauges, which likely affects the accuracy of streamflow simulation and forecasting.