Water Towers of the Pamirs: II. Cryosphere dynamics and implications for runoff and livelihoods

<p>Cryosphere components in the Pamirs play an important role in the release of water to the Vakhsh and Panj river systems where both mountain and downstream communities rely on sustainable water supplies for their agriculture, potable water, and hydropower. Of the three primary cryosphere sources of water (glacial, snow, and permafrost melt), glacial melt is the most predictable and constitutes and intermediate supply of runoff to streams, while almost no estimates of permafrost contributions are available. Snowmelt is highly variable from year to year and because it is the largest water supply to these rivers, understanding the potential amount and timing of snowmelt is critical for local communities.  </p><p>Based on our remote sensing investigations during the past 20 years, we water found wide interannual variations in snow water, snowline elevation, and snow persistence throughout the Vakhsh and Paji basins, but no clear evidence of basin-wide climate change trends. Specific locations of the central Pamirs appear to be shifting from snow to rain due to climate warming, approximately offsetting each other, but likely producing more runoff in late spring to early summer and less in mid to late summer. In the high, glaciated Vakhsh basin, temperature increases have been offset by higher snowfall, resulting in little glacial ice change. By overlaying maps of glaciers on a digital elevation model (Alos Palsar 12.5 m) containing stream networks, we estimated that about 75% of the glaciers were closely connected to first-order or larger channels; however, this may be a liberal estimate because some first-order streams are not connected to major river systems. Based on the TTOP model nearly 24,000 km<sup>2</sup> of continuous permafrost terrain exists throughout the Panj and Vakhsh basins, the majority of which is located at elevations > 3577 m. Streamflow contributions from permafrost thaw during the summer were estimated as subsurface flux from streambanks; ≈ 638 x 10<sup>6</sup> m<sup>3</sup> each summer, which represents about 1.5% of the average annual river discharge for both basins.</p><p>The climate variability and localized changes we observed pose challenges for predicting runoff from high elevation cold regions due to the altered patterns of the timing of snow, glacier, and permafrost accumulation and melt, including temporal changes, interannual variability, and hydrological connectivity of sources. The various water sources will respond differently in a changing climate, generating complex runoff scenarios and socioeconomic consequences downstream.</p>