Zones and Shapes in Lakes and Reservoirs - Hydro-Environmental Analysis: Freshwater Environments - page 320

between depths and lows. These relationships are used by agencies such as the U.S. Geological
Survey (USGS) to convert measured gage heights to lows, which are then reported.
However, in the backwater area of a reservoir, there is typically no such relationship. That is,
there is no unique relationship between water-surface elevations and lows. For stations located
within this backwater zone, agencies such as the USGS only report elevations. Since rating curves
are not applicable and velocities are small and variable, measuring the lows within a reservoir is
often problematic.
Upstream inlows to reservoirs are commonly measured, such as at the USGS gauging stations.
However, rarely are all of the tributaries into a lake or reservoir monitored. In addition to the tribu-
taries, there are often ungaged nonpoint source inlows, as well as precipitation falling directly onto
the lake or reservoir. Since large portions of the inlow are commonly unknown, a common practice
is to use the known outlow (discharge from the dam) and the known increase or decrease in the
amount of water stored in the reservoir (based on changes in pool elevation) to back compute the
inlow from the continuity equation (see Equation 11.1). Where possible, actual measurements of the
upstream lows are used to verify the validity of the calculated reservoir inlow.
Discharges from reservoirs may be monitored, providing a detailed record of outlows. For lakes,
the outlows are uncontrolled. However, for lakes modiied by structures, the low may be monitored
at the structure or through ungaged weirs or spillways. In large reservoirs, such as those used for
hydropower, there may be multiple outlet structures, such as multiple penstocks and turbines for
hydropower, multiple-gated spillways for lood releases as well as emergency spillways, which may
or may not be monitored. The discharges, such as releases through gates, may be based on measured
water-surface elevations in comparison with the operational guide or rule curve for the reservoir, as
discussed in Chapter 10.
11.3.3 b atHyMetry
The depth of a lake or a reservoir is not constant, but varies spatially and temporally. The depth
and its variations are important for navigation, isheries management, recreation, and other pur-
poses and are commonly incorporated into bathymetric maps. Bathymetric maps are analogous to
terrestrial topographic maps, which show contours of equal elevations. But, bathymetric maps are
typically based on contours of equal depths. As with topographic maps, the closer the lines are, the
more rapid the changes that occur (in depth or elevation). Bathymetric maps are typically estimated
from sedimentation or sounding (bathymetry or hydrographic) surveys. The U.S. Army Corps of
Engineers (the Corps) (USACE 1995, 2002) provides guidance on methods, accuracy standards,
and quality control criteria. These maps are available from a variety of agencies and organizations.
Bathymetric maps for many Florida lakes (e.g., Figure 11.3) are available from Florida Lake Watch
(University of Florida; http://lakewatch.ifas.ul.edu/).
11.3.4 S urface a rea and H ypSoGrapHIc c urVeS
Lake surface areas are used not only to indicate the size of a lake or a waterbody, but also to esti-
mate the potential impacts of wind, waves, evaporation, precipitation, gas exchange, solar heating,
and other impacts that vary as a function of the surface area. Changes in the surface area are also
important in lake management, such as to control littoral areas.
The surface area varies as a function of depth (or surface elevation) and the relationship is com-
monly expressed using a hypsographic curve or a plot of the elevation or depth versus the surface area.
Note that, depending on the purpose, the area may be a discrete surface area; that is, the plan area at a
particular depth, such as to estimate wind effects, etc. Alternatively, the area may represent the benthic
area. The two areas may be similar, but the differences are a function of the bottom slope. Particularly
for lakes, the curve may also be expressed as a percentage of the mean surface area (Wetzel 2001).