Still Waters - Mathematics and Statistics for the Environment - page 576

Calculate water retention time:

Storage capacity = 2274 acre-ft

Annualrunoff = 38.8 in./yr × 10,220 acres

= 38.8 in./yr × 1ft/ 12 in. × 10,220 acres = 33,045 acre-ft/year

Storage capacity
Annualrunoff

2274
33 045

Ret entiontime = = = 0.069 years

,

Now calculate the ratio of drainage area to lake capacity:

Drainage area
Storage capacity

10 220
2274

, 4449

1

.

R = = =

20.3 STILL WATER SURFACE EVAPORATION
In lake, reservoir, and pond management, knowledge of evaporative processes is important
to the environmental professional in understanding how water losses through evaporation are
determined. Evaporation increases the storage requirement and decreases the yield of lakes and
reservoirs. Several models and empirical methods are used for calculating lake and reservoir
evaporative processes. The following sections discuss applications of the water budget and energy
budget models, along with four empirical methods: Priestly-Taylor, Penman, DeBruin-Keijman,
and Papadakis.

20.3.1 W ater b udget m odel
The water budget model for lake evaporation is used to estimate lake evaporation in some areas. It
depends on accurate measurement of the inflow and outflow of the lake and is expressed as

∆ S = P + R + G I - G O - E - T - O (20.9)

where
∆ S = Change in lake storage (mm).
P = Precipitation (mm).
R = Surface runoff or inflow (mm).
G I = Groundwater inflow (mm).
G O = Groundwater outflow (mm).
E = Evaporation (mm).
T = Transpiration (mm).
O = Surface water release (mm).

If a lake has little vegetation and negligible groundwater inflow and outflow, lake evaporation
( E ) can be estimated by

E = P + R - O ± ∆ S (20.10)