## 2 Study Area and Data Processing
### Satellite Precipitation Data
#### 2.3.4 Spatial Distribution of Total Precipitation from SPPs and Gauge-Based Precipitation Data  
Figure 2a shows the spatial distribution of total precipitation amount, which was derived from the gauge-based gridded precipitation data on a 0.25\({}^{\circ}\) resolution during the period of 1 April 2014-31 December 2016. This figure indicates that precipitation has large spatial variability and tends to decrease sharply from north to south. The maximum total precipitation amount (9701.4 mm) was observed in the headwater region, whereas the lowest amount (2506.5 mm) appeared in the basin outlet region. Since part of the Arakan Mountain Range is located in the northern region of the watershed, the high mountains remarkably block the atmospheric moist from moving northward and considerably increase precipitation in the northern mountainous region. Figure 2b-i show that the eight SPPs demonstrate a spatial pattern similar to that of the gauge-based gridded precipitation data, with a noticeable decreasing trend from the upper to the lower reaches. However, 3B42 RT generally estimates a wetter condition throughout the watershed than the gauge-based data set, except in the western highland region, where remarkably less precipitation is derived (Figure 2b). 3B42V7 and  
IMERG-F generally provide slightly lower precipitation estimates in the northern part of the watershed than the gauge-based data set does (Figure 2c,f). IMERG-E, IMERG-L, GSMaP-NRT, GSMaP-MVK, and GSMaP-GAUGE derive an obviously dry situation throughout the basin, particularly in the northern and western parts of the watershed (Figure 2d,e,g-i), and GSMaP-GAUGE provides the driest situation among all SPPs (Figure 2i). Notably, nearly all SPPs (excluding GSMaP-GAUGE) estimate that precipitation in the west part of the river basin and the southern flood plain region is lesser than those in other regions. These regions, where less precipitation is estimated, are located in the eastern slopes of the Arakan Mountains. Meteorologically, the Arakan Mountains act as a barrier to the southwestern monsoon rains, thereby making the western slopes extraordinarily wet and the eastern slopes dry during the monsoon. However, the spatial precipitation pattern derived from the gauge-based data set does not show this topographical effect. The possible reason is that all seven weather stations are situated in the Chindwin Valley, while the Arakan highland region has no stations (Figure 1). As a result, the gauge-based gridded precipitation data might not sufficiently capture the actual precipitation situation in the Arakan mountainous region. IMERG-E, IMERG-L, IMERG-F, GSMaP-NRT, and GSMaP-MVK also represent more detailed spatial characteristics than 3B42 RT and 3B42V7 do because IMERG and GSMaP provide precipitation estimates on a finer spatial resolution (0.1\({}^{\circ}\)) than TMPA (0.25\({}^{\circ}\)). Nevertheless, the GSMaP-GAUGE precipitation map (Figure 2i) appears spatially coarser than other SPPs, because the GSMaP-GAUGE precipitation estimates are bias-adjusted using the CPC Unified Gauge-based Analysis of Global Daily Precipitation data set at a coarse spatial resolution (0.5\({}^{\circ}\)).  
Figure 2: Spatial distribution of total precipitation depth estimated from the gauge-based gridded precipitation data set and eight SPPs during the period 1 April 2014–31 December 2016.