Lakes as surface storage

Lakes, wetlands, and reservoirs constitute essential components of the terrestrial branch of the hydrologic cycle due to their ability to retain water and influence streamflow levels and water quality. At the same time, these water bodies serve as giant evaporation features that support an upward moisture flux limited only by available energy (Coe, 2000). The net hydroclimatic result is not the same for all these water features, and their regional hydroclimatic significance is highly variable.

From the perspective of the global hydrologic cycle, natural lakes are wide places in rivers. While this is a nearly true characterization for most small lakes, large lakes are more accurately characterized as a body of water surrounded by land. Lakes occur where the physical setting favors accumulation of water and the hydroclimatic environment supports persistence of the water body. Crater Lake, Oregon, occupies a collapsed volcanic crater. The Great Lakes of Canada and the United States occupy huge basins formed by isostatic adjustment, glacial excavation, and moraine and outwash deposition. Lake Tanganyika in East Africa occupies a portion of an expansive Rift Valley. Lake Eyre in central Australia occupies a shallow depression that intermittently contains water following seasonally heavy rainfall. Lakes can be either freshwater or saline depending on the hydroclimate and geomorphology of the site. Many lakes in arid regions become salty due to evaporation exceeding precipitation, which concentrates salts arriving from tributary streams. This condition is exacerbated when the lake is endorheic or does not have a natural outlet.

Two commonly recognized hydroclimatic functions of lakes are that they provide storage that reduces the time variability of flow in the rivers that drain them, and they increase evaporation by providing large evaporating surfaces. Exorheic lakes which are drained by outflowing rivers serve as natural reservoirs that reduce high streamflow and augment low streamflow to the extent that they can adjust the water volume retained in the basin. By providing an unlimited moisture source, lakes support higher evaporation rates than would occur if the lake area was land instead of water. However, the net result is not the same for all lakes. Lakes in the temperate latitudes of Europe and North America contribute to an increase in river flow because the precipitation falling on their surfaces is greater than the evaporation from them, but the increase is relatively small and varies between 5% and 15% of the inflow. Rouse et al. (2005) estimate evaporation in western Canada is 32% greater due to the presence of lakes than would occur from uplands alone. In the Southern Hemisphere, most lakes with outlets have a negative influence on river flow, lowering it considerably because of intense evaporation from the water surface. The lake-related decrease in streamflow for African rivers is 30% to 90%. In South America, the lake influence produces a lowering of river flow by 4 to 5 times (Vikulina et al., 1978).

Lakes integrate the precipitation and runoff processes operating in the lake watershed and the hydroclimatic relationship for the lake surface. The depth and areal extent of water in lakes are indictors of water gains and losses that are functions of changes in hydroclimatic variations for both the lake and the surrounding watershed. The water balance for a lake is expressed as an equation based on the conservation of mass as

Date

Fig. 6.12. Daily lake level relative to an elevation datum of 1885 m for Lake Tahoe, California/Nevada (39° N), for 1 January 1958 to 1 August 2006. (Data courtesy of the U.S. Geological Survey from their website at http://waterdata.usgs.gov/nwis/.)

Date

Fig. 6.12. Daily lake level relative to an elevation datum of 1885 m for Lake Tahoe, California/Nevada (39° N), for 1 January 1958 to 1 August 2006. (Data courtesy of the U.S. Geological Survey from their website at http://waterdata.usgs.gov/nwis/.)

where V is the lace volume, Qi is the surface inflow due to the relationship between P and ETa for the watershed, Pw is the precipitation on the lake surface, Ew is evaporation from the lake surface, Q is the outflow from the lake, Qui is the underground inflow to the lake, and Quo is the underground outflow (Street-Perrott, 1995).

Solving Equation 6.14 for different time intervals permits assessment of lake volume changes on streamflow ranging from daily to annual periods. A highly simplified measure is to use the change in lake level, which is related to lake volume and water storage, to assess the lake influence because the hydrocli-matic role of lakes may change over the course of the year. For example, lakes in the western United States commonly reach high stages in the winter or spring while lakes in the eastern United States reach high stages in spring and early summer before beginning to decline. Seasonal changes in lake levels are superimposed on annual changes in lake levels responding to long-term hydrocli-matic relationships which may account for abnormally high or low lake levels during periods of wet or dry years.

Figure 6.12 displays water level fluctuations for Lake Tahoe, California/ Nevada (39° N). Lake Tahoe is the largest alpine lake in North America with a surface area of 487 km2, a volume of 156 km3, and a drainage area of 819 km2. Mountains surrounding the lake rise to 3297 m, and the lake surface elevation is 1886 m. Precipitation on the lake is greatest from November to March, but the dominant inflow to the lake is snowmelt runoff in May and June from the surrounding alpine watershed. The annual lake level time-series (Fig. 6.13) displays seasonal changes in the Lake Tahoe water level that are masked by the

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Fig. 6.13. Daily lake level relative to an elevation datum of 1885 m for Lake Tahoe, California/Nevada (39° N), for 1 January 1998 to 31 December 1998 showing the annual response to snowmelt inflow to the lake. (Data courtesy of the U.S. Geological Survey from their website at http://waterdata.usgs.gov/nwis/.)

Date

Fig. 6.13. Daily lake level relative to an elevation datum of 1885 m for Lake Tahoe, California/Nevada (39° N), for 1 January 1998 to 31 December 1998 showing the annual response to snowmelt inflow to the lake. (Data courtesy of the U.S. Geological Survey from their website at http://waterdata.usgs.gov/nwis/.)

long-term characteristics. Precipitation-related increases are evident in March, April, and December, but high sustained levels in late June, July, and August are products of snowmelt from high elevation areas of the watershed.

Figure 6.12 displays alternating periods of high and low levels of various durations for Lake Tahoe. The most prominent is the wet period from 1967 to 1975, and the dry period from 1986 to 1992. Not all years during these periods are wet or dry, but there is persistence toward wetness or dryness. The wet periods oscillate around a stationary mean, and the dry periods are characterized by abrupt declines and recoveries. Three of the four dry periods display similar minimum lake levels, but the event from 1986 to 1992 is longer in duration and reaches a lower minimum lake level than the other three periods.

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