Streamflow trends in Nepal

This study presents the first comprehensive nationwide trend detection of streamflow in Nepal, a country that has been historically understudied despite its critical location as the southern pathway for most of the Himalayan snowpack melt and torrential seasonal monsoon rains. We applied Mann-Kendall and Sen's trend tests using trend-free pre-whitening and bootstrap approaches to two streamflow data sets to deal with serial and cross-correlation. The two data sets comprised 23-33 hydrometric stations with 31 years and more than 20 years of published data, respectively. The test on the 33 stations data set showed that 23% of the streamflow variables studied had statistically significant trends, evenly divided between upward and downward trends. Similarly, in the second, relatively smaller data set, 24% of variables exhibited trends, of which 41% were downward and 59% upward. The higher percentage of observed upward trends in pre-monsoon and winter seasonal average flow is noteworthy given the potential snowmelt contribution in many of the studied sites. Trends were mostly absent in stations draining the larger basins. However, some spatial patterns were seen in the observed trend directions, specifically, a downward trend in the Karnali-Mahakali River basin and an upward trend in the West Rapti River basin, as well as a nationwide absence of trend in the post-monsoon season.