## 4 Discussion
Figure A1 shows a similar plot based on the respective 25 point-to-pixel grid locations of 4 IMERG datasets (Cal, Uncal, HQ, and IR) from 2001 to 2000. Upon characterizing the seasonal trend, pre-monsoon has been rising across more than half of the country's area, concentrated in the western region, which was also reported by [7; 79]. Likewise, winter precipitation is rising in the northwestern region [7; 78; 82; 100]. Moreover, the frequency of wet days is increasing at most locations, which is similar to findings reported by previous studies [77; 104; 105; 106]. However, most heavy precipitation-related indices show a sporadic rising tendency across the southern belt of the country [76; 79; 101; 107]. One of the possible reasons for this rising tendency is physiographic controls upon precipitation; topographic barriers induce rain shadows (leeward side) and precipitation hotspots (windward side); for instance, heavier rainfall occurs in the southern  
Figure 7: Spatial distribution of annual trends from 2001 to 2020 of the selected 28 precipitation indices across Nepal based on the IMERG Cal dataset. Refer to Table A1 in Appendix A for the details of the indices. The color code used for the precipitation stations (points) and background (grids) represents the same for each precipitation index. part as approaching monsoon winds in Nepal are first intercepted by the foothills of Churia range (southern region). Another explanation could be the behavior of the Indian summer monsoon, which interposes significantly to annual precipitation levels, and its influence decreases from the lower to Higher altitudes of Nepal [79; 107; 108]. Nevertheless, the harmonious trend is evident compared to GP observations, despite some noticeable contrasts at some locations. Importantly, as most precipitation indices reveal a strong inter-annual variation, it is challenging to compute a monotonic trend based on a short period of data (i.e., 20 years). Although the new generation of IMERG products is characterized by improved precipitation retrieval algorithms and more advanced satellite sensors, the accuracy of this product remains affected by various sources. Among them, the complex topography and atmospheric processes make it challenging to estimate precipitation. Monsoon and topographic variations may generate considerably. Furthermore, latitude, longitude, and elevation modulate seasonal and annual precipitation. Therefore, the location and topography characterize the climate of that location. However, most of the time, extreme precipitation events are unique and highly local in themselves. Nevertheless, topographic setting, monsoonal circulation, and climatic characteristics of location provide persistent information on the likelihood of severity of extreme precipitation. For instance, it is quite obvious that the foothills of Nepal are hardly hit by a strong monsoonal system every year. However, the severity depends on the location of the monsoon trough. In addition, some past cloud burst events have shown that when westerly and summer monsoon coincides, such episodes provide a favorable synoptic condition for heavy precipitation over a large area, affecting more than one major river basin. Depending on location and season, the impact of extreme precipitation events on humans, the economy, and nature vary a lot. Across Nepal, there are locations where one-day maximum precipitation towards the windward side of a mountain is almost equivalent to the total annual precipitation of the leeward side of the same mountain. Therefore, heavy precipitation in terms of amounts for one region could be a moderate event for many locations. Therefore, this study computed several indices based on amounts, frequencies, percentiles, and contribution percentages to annual precipitation. Precipitation intensity and duration on a sub-daily scale impact a lot. For instance, low-intensity but longer-duration precipitation can saturate the soil, causing landslides. Intense short-duration precipitation can bring local urban or flash floods.