CLIMATE CHANGE IMPACTS: BASIN HYDROLOGY RESPONSES .1 Basin hydrology impacts: GCM steady-state / transient approach

Steady-state and transient changes in lake and basin hydrology using GCM 2xCO2 climate scenarios have been conducted on the Great Lakes (e.g., Croley, 1990; Hartmann, 1990; Croley

1992b). Simulations included 30 years of “present” Great Lakes hydrology using historical daily data with present diversions and channel conditions. Monthly adjustments of “present”

to 2xCO2 conditions for each meteorological variable was applied to the historical daily datasets to estimate 33-year sequences of atmospheric conditions associated with the 2xCO2 scenarios. Differences between the 2xCO2 scenario and the base case scenario are assumed to indicate possible changed climate conditions. The three scenarios (GISS, GFDL, OSU and CCC) changed precipitation little but snow-melt and runoff were greatly decreased, evapotranspiration and lake evaporation were greatly increased, and net basin supplies to the lakes and lake levels were decreased. Table 2 summaries some of the key basin hydrology changes for GCM steady-state scenario. Table 3, representing the period 1981-2060 with historical data from 1951-80 (Croley, 1995), shows changes in key lake hydrological variables for each of the Great Lakes as a result of GISS GCM transient climate scenarios.

4.2 Basin hydrology impacts: Climate transposition scenarios

An alternate to GCM derived climate scenarios is represented by climate transposition scenarios.

Transposed scenarios were selected to represent analogues of “future” climatic conditions (Croley et al., 1996; Schertzer & Croley, 1999) under the assumption that future changes in the basin climate may approximate latitudinal and/or longitudinal climatic shifts. Four scenarios were examined (a) Scenario 1 (warm and dry) corresponds to warmer temperatures and mixed precipitation changes, (b) Scenario 2 (warm and wet) corresponds to warmer temperatures and increases in precipitation amounts over the entire basin, (c) Scenario 3 (very warm and dry) corresponds to very high temperatures and mixed precipitation changes, and, (d) Scenario 4 (very warm and wet) corresponds to very high temperatures and large increases in precipitation over the entire basin.

Table 2. Average annual steady-state Great Lakes basin hydrology summary depicting current (base) climate conditions and potential changes using GCM climate scenarios (based on Croley et al., 1996;

Schertzer & Croley, 1999).

Scenario Overland Evapo- Basin Over-lake Over-lake Net Basin Precip. transpir. Runoff Precip. Evap. SUPPlY Basea (m3 s-l) 13855 7814 6206 6554 4958 7803

GISSb 2% 21% -24 % 4 % 26 % -37 %

GFDL’ 1% 19% -23 % 0% 44% -51 %

OSUd 6% 19% -11% 6% 26 % -23 %

CCC’ -2 % 22 % -32% 0% 32 % -46 %

‘Base Climate (present conditions) from Transposition Study (Croley et al., 1996).

bGoddard Institute for Space Studies GCM (Croley, 1990).

‘Geophysical Fluid Dynamics Laboratory GCM (Croley, 1990).

dOregon State University GCM (Croley, 1990).

‘Canadian Climate Centre (Croley, 1993a).

Table 3. GISS transient climate changes impacts summary (based on Croley, 1995; Schertzer & Croley, 1999).

“Expressed as a depth over the basin., DExpressed as a depth over the lake.

‘Computed over first 7 decades since Ontario regulation plan fails in eighth.

Potential changes in the Great Lakes Basin hydrology, resulting from application of climate transposition scenarios (Croley et al., 1996; Schertzer & Croley, 1999) are shown in Table 4 which can be compared to GCM steady-state scenarios (Table 2). The climate transposition coordinates for each scenario are included in Table 4. All transposition scenarios produced significant increases in lake evaporation influenced by decreased cloud cover and increased solar radiation receipt. Other factors influencing evaporation included increased longwave atmospheric radiation, changes in the partitioning of energy between sensible and latent heat flux, and a decrease in lake ice cover. Since evaporation is highly event orientated, (e.g., Arctic cold air outbreaks), accurate future estimates of lake evaporation will require estimates of the number and severity of cold air outbreaks.

Many scenarios result in lower soil moisture and reduced runoff despite higher precipitation. Total annual evapotranspiration from the ground and the vegetation increases in

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all scenarios. Also, higher temperatures significantly reduced total snowfall. In the four scenarios, the snow-melt season is shorter and less significant. The above results are likely to be a feature of any warmer climate. This means that in a warmer climate, greater precipitation is required to maintain runoff at present levels.

Warmer climates result in large negative pressures on net basin water supply. Net basin supply (NBS) is comprised of the sum of over-lake precipitation and surface runoff into the lake, minus lake evaporation. Analyses suggest that for temperature changes of 5 to 6”C, precipitation increases of 20 to 30% may be required to maintain Lake Superior NBS at today’s levels. If annual mean temperatures were to increase with no compensating increases in precipitation, it is highly likely that NBS levels would fall significantly.

Table 4. Average annual steady-state Great Lakes basin hydrology summary depicting current (base) climate conditions and potential changes using transposition climate scenarios (based on Croley et al., 1996;

Schertzer & Croley, 1999).

Scenario Overland Evapo- Basin Over-lake Over-lake Precip. transpir. Runoff Precip. Evap.

‘Base Climate (present conditions) from Transposition Study (Croley et al., 1996).

‘Transposed Climates from the Southwestern US (Croley et al., 1996). Note that number l-4 represents scenario discussed in the text.

The climate transposition scenarios suggest the occurrence of higher interannual variability in NBS ranging from 140 to 300 mm. Average NBS variability increases about 60% in warm scenarios 1 and 2 and about 90% in very warm scenarios 3 and 4, due primarily to increases in precipitation variability. Accurate estimates of precipitation variability expected in future climates will require an accurate simulation of the frequency and magnitude of infrequent large precipitation events.

Lake effects on regional climate have negligible hydrological effects. GLERL tested lake effects on basin hydrology by calculating outcomes with and without lake effects present and the analysis suggested that huge changes in lake effects would be required to level variability, both seasonal and inter-annual, than exist in the present regime.

5 CLIMATE CHANGE IMPACTS : LARGE-LAKE PHYSICAL / HYDROLOGICAL