THE LAuRENTIAN GREAT LAkEs TO CONsTRAIN EvAPORATIvE LOssEs
s. JasecHKo a,b, J.J. gIBson b,c, a. pIetronIro d, t.W.d. edWards a
a department of earth and environmental sciences, university of Waterloo, Waterloo,
ontario, canada
b alberta Innovates – technology futures, Victoria, British columbia, canada
c department of geography, university of Victoria, Victoria, British columbia, canada
d national Water research Institute, environment canada, saskatoon, saskatchewan, canada
Abstract
evaporation is an important yet poorly constrained component of the water budget of the Laurentian Great Lakes, but is known historically to have a significant impact on regional climate, including enhanced humidity and downwind lake effect precipitation. sparse over lake climate monitoring continues to limit ability to quantify bulk lake evaporation and pre-cipitation rates by physical measurements, impeded by logistical difficulties and costs of in -strumenting large areas of open water (103–105 km2). Measurements of stable isotopes of oxy-gen and hydrooxy-gen in water samples of precipitation and surface waters within the great lakes basin are used to better understand the controls on the region’s water cycle. a stable isotope mass balance approach to calculate long term evaporation as a proportion of input to each lake is discussed. the approach capitalizes on the well understood systematic isotopic separation of an evaporating water body, but includes added considerations for internal recycling of evapo-rated moisture in the overlying atmosphere that should be incorpoevapo-rated for surface waters suf-ficiently large to significantly influence surrounding climate.
1. IntroductIon
evaporation is an important component of the water budget for the laurentian great lakes of north america; it is the least understood control on lake levels, and
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has a significant influence on lake and local climates. The Great Lakes contain rough-ly 20% of the global reservoir of unfrozen fresh surface water, and support a robust binational regional economy. evaporation estimates by the great lakes environmen-tal research laboratory (glerl) lumped heat storage model estimate the evapora-tion flux to be similar in magnitude to catchment runoff or direct precipitaevapora-tion into each great lake. this model utilizes lakeshore climate station data and satellite measurements, but does not incorporate temperature and humidity data from over lake monitoring buoys [1]. further, lake superior’s residence time is greater than a century, and necessary data for the GLERL model does not extend sufficiently far back in time to produce a long term evaporation estimate. evaporation for the great lakes ranges from roughly 600 mm to 900 mm. evaporation is highly seasonal, weighted to fall and early winter months.
2. consIderatIon of laKe effects on tHe atMospHere and tHe use of an Isotope Mass Balance to calculate eVaporatIon
Use of stable isotopes of oxygen and hydrogen in water is a cost efficient meth-od of determining evaporation as a proportion of inflow (E/I) for small lake systems, particularly for regions in which it is difficult and expensive to set up instrumenta-tion [2]. also, this approach is shown to rapidly assess small scale spatial variabil-ity in evaporation as a proportion of input to lake systems [3, 4]. a synchronized approach using two conservative tracers, namely δ18O and δ2H, is used to calculate E/I and allows for a calculation check to ensure that the model realistically captures the physical characteristics of the lake evaporation process. evaporation calculations have been described for large systems such as the caspian sea, the eastern Mediter-ranean sea and lake titicaca [5–7]; however, a unifying approach to calculating E/I for large surface water bodies that accounts for effects on the overlying atmosphere, and matches outputs for both δ18O and δ2H has yet to be described.
the evaporation as a proportion of hydrologic inputs to a surface water body (E/I) can be calculated for δ18O and δ2H by equation 1.
where a δ symbol represents a value for either δ18O or δ2H, and the subscripts represent the isotope composition of inputs, outgoing evaporate, and the lake (δI, δE, δL, respectively). the application of an isotope mass balance model to calculate evaporation is reliant upon an accurate determination of δe, as all other values can be well approximated and readily measured. δe is best estimated as a function of air temperature, relative humidity, the isotope composition of the lake surface (δl) and of moisture in the overlying atmospheric (δa) following a linear resistance model [8].
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2.1. Appropriate weighting of calculation inputs
Hydrologic inputs to the great lakes are direct precipitation, catchment runoff, and input from another Great Lake by connecting channel inflow, or interlake mixing in the special case of great lakes Michigan and Huron that share a common lake lev-el. In order to produce a representative estimate for the isotope composition to a lake, the isotope composition of each lake input must be weighted to water amount. In the case of precipitation (P), runoff (R) and inflows (C), this is completed at a month-ly time step following equation 2.
1 Q represents the monthly mean precipitation flux, runoff flux, or connecting chan -nel discharge. results for the three possible inputs to each great lake are weight-ed against one another to produce a single value for δ18O and δ2H for the input to
fluxes of precipitation, runoff, and interlake input correspond to P, R and C, respectively. Subscripts attached to δ symbols represent the flux weighted δ18O or δ2H value for each flux. The Great Lakes are a chain lake system, and the contribution of connecting channel inflow is the dominating input to the lower Great Lakes, Erie and ontario, but is non-existent for lake superior.
finally, since evaporation is a seasonal occurrence, atmospheric and lake con-dition inputs to the calculation of E/I must be weighted to months when evapora-tion dominates as established by [9]. this is completed at a monthly time step using glerl monthly evaporation estimates. as a result, winter and fall conditions domi-nate the calculation inputs for the great lakes. Weighting by evaporation to winter weighting conditions opposes isotope mass balance studies for smaller lake systems in seasonal climates. for these lake systems, weighting by evaporation commonly results in a greater influence of warmer temperatures and generally higher δa values of summer months than the annual mean values.
2.2. Calculating δE : considerations for the Great Lakes
the immense areas of open water covered by the great lakes and the known effects the lakes have on the atmosphere encourage modifications to the formulation of the isotope value for evaporating water. since evaporated moisture in an upwind
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area of a Great Lake modifies the downwind over lake atmosphere, it is reasonable to include a certain proportion of evaporated moisture in the isotope value for the over-lying atmosphere (δa).
other considerations include the choice of a kinetic fractionation factor for ei-ther smooth or rough conditions. In many studies, fractionation factors developed in wind tunnel experiments are applied. However, in the case of the oceans smooth con-ditions best represent the kinetic fractionation process. due to the large areas covered by the north american laurentian great lakes, smooth kinetic fractionation factors are worth considering for use in calculating value for the isotope value of outgoing evaporate (δe).
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