Water Temperature Characteristics of Lakes Subjected to Climate Change

A deterministic, one dimensional, unsteady lake water temperature model was modified and validated to simulate the seasonal (spring to fall) temperature stratification structure over a wide range of lake morphometries, trophic and meteorological conditions. Model coefficients related to hypolimnetic eddy diffusivity, light attenuation, wind sheltering, and convective heat transfer were generalized using theoretical and empirical extensions. Propagation of uncertainty in the lake temperature model was studied using a vector state-space method. The output uncertainty was defined as the result of deviations of meteorological variables from their mean values. Surface water temperatures were affected by uncertain meteorological forcing. Air temperature and dew point temperature fluctuations had significant effects on lake temperature uncertainty. The method presents a useful alternative for studying long-term averages and variability of the water temperature structure in lakes due to variable meteorological forcing. The lake water temperature model was linked to a daily meteorological data base to simulate daily water temperature in several specific lakes as well as 27 lake classes characteristic for the north central US. Case studies of lake water temperature and stratification response to variable climate were made in a particularly warm year (1988) and a more normal one (1971). A regional analysis was conducted for 27 lake classes over a period of twenty-five years (1955-1979). Output from a global climate model (GISS) was used to modify the meteorological data base to account for a doubling of atmospheric C02' The simulations predict that after climate change: 1) epilimnetic water temperatures will be higher but will increa,se less than air temperature, 2) hypolimnetic temperatures in seasonally stratified dimictie lakes will be largely unchanged and in some cases lower than at present, 3) evaporative water loss will be increased by as much as 300 mm for the open water season, 4) onset of stratification will occur earlier and overturn will occur later in the season, and 5) overall lake stability will become greater in spring and summer.