Increasing extreme events in the central Himalaya revealed from a tree-ring based multi-century streamflow reconstruction of Karnali River Basin

Gaire, Narayan Prasad

JOURNAL OF HYDROLOGY

2022

610

Fan, ZX

The Karnali River is one of the major transboundary rivers of the Nepalese central Himalaya and a major tributary of the Ganges River. In order to investigate regional historical hydroclimate variability, we compiled eleven tree ring-width chronologies based on 354 exactly dated tree cores from five coniferous species for the Karnali River catchment. The chronologies were moisture-sensitive and the composite chronology was significantly correlated with spring-to-early summer (March-July) (R = 0.67, P < 0.001) discharge at the Chisapani gauging station. The robust calibration (R2 = 0.45, 1962-2010) enabled us to reconstruct March-July streamflow variability of Karnali River Basin (KRB) over the past four centuries. The streamflow reconstruction showed prolonged pluvial periods during 1650-1661, 1711-1718, 1723-1731, 1824-1831, 1877-1889, 1924-1931, and 1971-1981, while prolonged low-flow periods occurred during 1639-1649, 1689-1710, 1753-1769, 1777-1784, 1803-1823, 1867-1876, 1942-1959, and 1963-1970. Our reconstructed March-July streamflow series correlated positively with the seasonally averaged drought index and precipitation in the central-west Himalayan region and vicinities. The streamflow reconstruction revealed an increasing frequency of extreme low-flow and pluvial events in the central-west Himalayan region, especially during the past century. We found short (2.1-3.3, 6.2-6.3, and 8.5-8.7 years) to medium (12.2-12.5 years) periodicities in the reconstructed streamflow series, implying teleconnections with broad-scale climate modes like the El Nin similar to o-Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO). Our Karnali River streamflow reconstruction represents an update of regional hydroclimate variability, which may serve as a useful reference for water resource management and appropriate water-use policies in the context of current and future climate change.