HYDROLOGIC MODEL CONSTRAINTS ON LATE PLEISTOCENE CLIMATE IN CENTRAL NEW MEXICO

The climatic conditions leading to highstands of "pluvial" Lake Estancia in central New Mexico have been a matter of considerable debate, resulting in a range of estimates for Pleistocene precipitation and temperature in the southwestern United States. Using a hydrologic balance approach, Leopold (1951) determined that highstands were produced by precipitation values 50% greater than modern, given the assumption that summer temperatures were 9 °C colder while winter temperatures were unchanged. Antevs (1954) estimated that June-September temperatures were cooler by 5.5 °C and that precipitation was 60% greater. In contrast, Galloway (1970) called on temperature decreases of 10-11 °C throughout the year and a reduction in mean annual precipitation of 14% to raise Lake Estancia to highstand levels. In still another study, Brakenridge (1978) suggested that highstands could be achieved through no change in precipitation if all monthly temperatures were reduced by 7-8 °C.

We applied several models developed in recent decades to evaluate some of the above estimates of Last Glacial Maximum (LGM) climate (U.S. Department of Agriculture Soil and Water Assessment Tool for runoff, MODFLOW with LAK2 package for groundwater, and an energy balance lake model developed by Hostetler). Models were calibrated under modern climate and vegetation conditions using archival streamflow data and groundwater head values from wells, and were then forced with the climatic scenarios of Leopold and Brakenridge. Results show that simple year-round cooling of 7 °C is insufficient to raise Lake Estancia to its LGM highstand. Increased annual precipitation, in the range estimated by Leopold and Antevs, can raise water level to elevations near the LGM highstand, but only after a prolonged interval of time. Time constraints on lake level rise imposed by the Lake Estancia age model suggest that even greater increases in precipitation may have occurred.