Runoff variations in Lake Balkhash Basin, Central Asia, 1779-2015, inferred from tree rings

Long highly-resolved proxies for runoff are in high demand for hydrological forecasts and water management in arid Central Asia. An accurate (R2 = 0.53) reconstruction of October-September discharge of the Ili River in Kazakhstan, 1779–2015, is developed from moisture-sensitive tree rings of spruce sampled in the Tian Shan Mountains. The fivefold extension of the gauged discharge record represents the variability of runoff in the Lake Balkhash Basin for the last 235 years. The reconstruction shows a 40 year long interval of low discharge preceded a recent high peak in the first decade of the 2000s followed by a decline to more recent levels of discharge not seen since the start of the gauged record. Most reconstructed flow extremes (± 2σ) occur outside the instrumental record (1936–2015) and predate the start of large dam construction (1969). Decadal variability of the Ili discharge corresponds well with hydrological records of other Eurasian internal drainages modeled with tree rings. Spectral analysis identifies variance peaks (highest near 42 year) consistent with main hemispheric oscillations of the Eurasian climatic system. Seasonal comparison of the Ili discharge with sea-level-pressure and geopotential height data suggests periods of high flow likely result from the increased contribution of snow to runoff associated with the interaction of Arctic air circulation with the Siberian High-Pressure System and North Atlantic Oscillation

Projections of declining surface-water availability for the southwestern United States

Global warming driven by rising greenhouse-gas concentrations is expected to cause wet regions of the tropics and mid to high latitudes to get wetter and subtropical dry regions to get drier and expand polewards 1–4. Over southwest North America, models project a steady drop in precipitation minus evapotranspiration, P − E, the net flux of water at the land surface 5–7, leading to, for example, a decline in Colorado River flow 8–11. This would cause widespread and important social and ecological consequences 12–14. Here, using new simulations from the Coupled Model Intercomparison Project Five, to be assessed in Intergovernmental Panel on Climate Change Assessment Report Five, we extend previous work by examining changes in P, E, runoff and soil moisture by season and for three different water resource regions. Focusing on the near future, 2021–2040, the new simulations project declines in surface-water availability across the southwest that translate into reduced soil moisture and runoff in California and Nevada, the Colorado River headwaters and Texas. The global climate models used in this study include all simulations for all models that were continuous from 1950 to 2040 and that provided all of the data required. Historical simulations to December 2005 and future projections to 2040 using the Representative Concentration Pathway (RCP)85 scenario whereby anthropogenic radiative forcing equals 8.5 W m −2 by 2100 (refs. 15, 16) were analysed (see Methods). The RCP85 scenario involves stronger anthropogenic radiative forcing than the Special Repor