Pond Morphology

       Morphological features of a pond include its area, depth and volume, the length of its shoreline, and hydraulic residence time. These features can have a large influence on pond trophic state. Pond area (As) can be determined directly from a topographic map or spatially indexed aerial photograph. Because it is so easily measured, area is often used as a convenient index of pond size. The range of pond areas in Chester County is shown in Figure 2. Determining pond volume (V) requires a depth profile. A bathymetric map of a pond looks much like a topographic map. Contour lines within the pond indicate points with the same depth, allowing quick interpretation of deep and shallow areas, and computer-assisted computation of pond volume. For example, the bathymetric map shown in Figure 11 indicates shallower areas on the west side of the pond, sloping uniformly toward a deeper hole (approximately 3.5 m, or 11.6 ft) near the east end.

Fig. 11. Bathymetric map of a pond in southern Chester County, with depth contours shown in meters. The deepest portion of the pond is near a standpipe at the south end, with greater sediment deposition and very shallow water near the inflow at the north end.

       The quotient of a pond’s volume/area (V/As) is termed its “mean depth”. Mean depth is especially important to primary producers in ponds. Deeper ponds have less light penetrating to the bottom (see Section H). Because light levels are too low to support adequate photosynthesis at the pond bottom, deeper ponds (and deeper areas of shallow ponds) often have fewer submersed aquatic plants. Pond area can provide a good estimate of pond volume in most instances, as shown in Figure 12.
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Fig. 12. Pond volume in relation to surface area in 13 Chester County ponds. Ponds above and to the left of the regression line had deeper mean depths while those at the lower right were shallower.

       The discharge, or the rate at which water volume leaves the pond at the outfall (standpipe or dam), is normally proportional to the combined inputs via stream inflows, surface runoff during rain events and groundwater inputs, subtracting water lost from the pond via evapotranspiration to the atmosphere (Fig. 13). Discharge often declines during summer, owing to reduced groundwater inflow and increased evapotranspiration.

Fig. 13. Sources and losses of pond water.

        “Hydraulic residence/retention time” (HRT) describes the amount of time that a water droplet remains within a pond before leaving via its outfall. HRT is usually expressed in days, and is computed as [Pond Volume, in m3]/[surface water discharge at the outfall, in m3/day]. For example, pond HW in Figure 14, with its large volume and low discharge, has a long retention time. Water entering ponds with longer retention times (e.g., many months) is often strongly transformed during its stay; nutrients are transformed, sediments are deposited and algae have time to grow. By contrast, water leaving a pond with a very short retention time (e.g., less than a week) differs little from the inflow.
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Fig. 14. Discharge and volume estimates based on measurements of 13 ponds. Large ponds with low discharges (lower right) had high residence times, while small ponds with high discharges had low residence times.