Water Resources Research Center, University of Minnesota
Newsletter or Bulletin
Investigation of a simple analytic model of an interfluvial water table
demonstrates that a shift in groundwater recharge N changes the water table elevation
the most near the middle of the interfluve. Consequently, lakes lying farthest from
rivers are most vulnerable to lake-level change. The partial derivative of groundwater
head with respect to N, the "positional sensitivity," is quantified for the simple model as a function of position across the interfluve. Despite its simplicity, the positional sensitivity of the model has some predictive value for water-table and lake-level changes in a sandplain in west-central Minnesota.
Lake levels are also a function of surficial hydrology. "Lake pumping" is
symbolized by y and defined as the net removal of water from a lake by hydrologic
processes acting at the lake surface, namely evaporation minus direct precipitation and
minus any input from overland runoff that reaches the lake. Investigation of a simple
analytic groundwater model of a circular Jake next to an infinitely long river shows that the sensitivity of the lake level to a change in Y is proportional to the radius of the lake
and its distance from the river. The analysis also indicates that lakes lying in highly permeable substrates are not very sensitive to changes in 1. The response of a lake level to a shift in climate depends on characteristics of surficial and groundwater hydrology that are unique to that lake. Determination of the past levels of several lakes, rather than just one, should help provide a more nearly
unique reconstruction of past hydrology and climate. Analysis of the sediments of
several closed-basin lakes lying in the Parkers Prairie sandplain in west-central
Minnesota indicates that lake levels were lowest about 8.5 to 8 ka. I manipulate the N
and y of a steady-stare analytic-element groundwater model such that the modeled water
table coincides with the paleo-lake levels for a given past time. Model results indicate
that lake levels at 8.5 to 8 ka can be explained primarily by reducing N to 4O% of the
modern value, coupled with a y of about 20 to 30 cm yrl. By 6 ka N had increased to
50 to 80% of the modern value allowing most lakes to rise in level, but y may also have
increased forcing at least one lake to remain nearly dry.
Almendinger, James E..
Lake and Groundwater Paleohydrology: Use of Groundwater Flow Theory to Explain Past Lake Levels in West-Central Minnesota.
Water Resources Research Center, University of Minnesota.
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