QUANTIFYING MAGNITUDES AND RATES OF CLIMATE-INDUCED HYDROLOGIC CHANGE

Some of the best-documented proxies of the hydrologic effects of climate change are lake-surface fluctuations in closed basins because they accurately record changes in the absolute flux of water moving across the landscape. Due to limitations in most age models, however, the actual rates of change are typically not well constrained.

Even for the most precise radiocarbon age models for late Pleistocene pluvial lakes, only broad constraints can be placed on rates of change. For example, the surface of Lake Lahontan was at about 1335 m at 13,070 +/- 60 ¹⁴C yr B.P. (15,200 – 16,410 cal yr B.P.; 2s) (Adams and Wesnousky, 1998) but had fallen to an elevation below 1230 m by 12,070 +/- 210 ¹⁴C yr B.P. (13,440 – 14,870 cal yr B.P.) (Thompson et al., 1986). Based on the bounds of these age ranges, this change may have occurred in as little as 330 years or in as long as 2970 years. It is unlikely that this range could be significantly reduced through additional radiocarbon, luminescence, or surface exposure dating because of inherent uncertainties associated with each technique.

The increased availability of moisture-sensitive tree ring chronologies provides a means to place much tighter constraints on the timing and durations of wet and dry periods over the last several thousand years. Correlating periods of increased wetness, recorded by tree rings, with radiocarbon-dated late Holocene highstands in closed basin lakes allows both the rates and magnitude of hydrologic changes to be documented. As an example, the Medieval climate anomaly was expressed in tree-ring records in the Great Basin as two prolonged droughts separated by a 46 year period of above average wetness (Cook et al., 2010). During this Medieval pluvial (AD 1075 to 1121), a lake grew to cover ~ 3000 km², attained a depth of ~ 25 m, and contained ~ 47 km³ of water in the normally dry Carson Sink (Adams, 2003). Simple water balance modeling indicates that discharge would have to increase by a factor of four or five to account for a lake of this size. Enhanced runoff caused by an increase in the runoff:rainfall ratio may have been responsible for the abrupt increase in lake size during this extended wet period. Further developing the approach of constraining the timing of lake-level changes with tree rings should allow more precise estimates of the rates and magnitudes of past hydrologic changes.