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Streamflow Trends in Nepal Since the 1960s
Gautam, M.R. and Acharya, K. 2012. Streamflow trends in Nepal. Hydrological Sciences Journal 57: 344-357.

The authors write that "streamflows in unregulated catchments can carry signatures of climate forcing and changes," noting that identification of any trend in the streamflow of such a basin "can thus be important for understanding the impact of climatic variability and changes in the region." Unfortunately, they say "there are no studies that provide a comprehensive trend assessment of streamflows for the whole country of Nepal, which provides the southern pathway for most Himalayan snowpack melt and torrential seasonal monsoon rain."

What was done
In a study designed to fill this data void, the two researchers "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," which two data sets "comprised 23-33 hydrometric stations with 31 years and more than 20 years of published data, respectively."

What was learned
To start with, Gautam and Acharya state that "a majority (~66%) of the hydrometric stations showed an absence of trends," and they indicate that these stations were situated on "streams draining the larger basins." Thereafter, in regard to the remaining 34% of the stations, they report that tests on the 33-stations data set showed that "23% of the streamflow variables studied had statistically significant trends," but they say that these trends were "evenly divided between upward and downward trends." And in the last-remaining second and smaller data set, they found that a near-identical "24% of variables exhibited trends, of which 41% were downward and 59% were upward."

Although these results are indicative of essentially no significant mean result for the country as a whole, they say that "some spatial patterns were seen in the observed trend directions," with "a downward trend in the Karnali-Mahakali River basin and an upward trend in the West Rapti River basin," which trends again pretty much offset each other. And they additionally state - in the concluding sentence of the abstract of their paper - that there was "a nation-wide absence of trend in the post-monsoon season."

What it means
Apparently (and quite obviously so), however unnatural or unprecedented the global warming of the past few decades may (or may not) have been, it has had next to no impact on the annual amount of water coursing through the many rivers and streams of Nepal.

Reviewed 5 December 2012