Page 409 - Fair, Geyer, and Okun's Water and wastewater engineering : water supply and wastewater removal

JWCL344_ch11_357-397.qxd 8/2/10 9:01 PM Page 369

11.5 Percolation 369
falling on or applied to the surface and (b) water draining from the bottom, corrections
being made for changes in soil moisture. Data of this kind are generally of limited use to
water supply engineers, but it is good to have some concept of the order of magnitudes in-
volved. For example, the transpiration ratio, or weight of water transpired by a plant per
unit weight of dry matter produced exclusive of roots, increases stepwise from about 350
for Indian corn through wheat and rice to almost 800 for flax; and from about 150 for firs
through oaks to almost 400 for birches.
Empirical formulations of consumptive use are generally based on temperature
records. A relationship proposed by Lowry and Johnson, for example, reads as follows:

U = 0.9 + 1.5 * 10 -4 g(T - 32) (U.S. Customary Units) (11.12)
m
where U is the consumptive use, in ft (acre-ft/acre); and T is the daily maximum temper-
m
ature, in F, for the growing season (last to earliest frost). Hence, (T 32) is the sum of
m
4
the degree days. When g(T - 32) = 4 * 10 , for example, U becomes 2.4 ft (0.73 m).
m
The following is an equivalent consumptive use equation using the SI Units:
U = 0.27 + 0.46 * 10 -4 g 1.8T (SI Units) (11.13)
m
where U is the consumptive use, in m (ha-m/ha); and T is the daily maximum tempera-
m
ture, in C, for the growing season (last to earliest frost).
Adding the proportion of daylight hours during specific months in the frost-free
period, Blaney and Criddle (1957) arrived at the equation:
U¿= K g pT¿ (U.S. Customary Units) (11.14a)
where the new factors are K, an empirical coefficient for specific crops varying from 0.6
for small vegetables to 1.2 for rice, and p, the monthly percentage of daylight hours in the
year. The consumptive use U
is measured in inches for a given period of time, and T
is
the mean monthly temperature, F
An equivalent SI equation using the SI units of U
(mm for a given period time), T
( C),
K (dimensionless) and p (dimensionless) is:

U¿= 25.4 K g p (1.8T + 32) (SI Units) (11.14b)

11.5 PERCOLATION

The term percolation is employed to describe the passage of water into, through, and out of
the ground. Figure 11.5 shows the conditions in which water occurs below the surface of the
ground. Only water in the saturated zone can be withdrawn from subsurface sources, the
development of groundwater supplies depending on the yields actually obtainable and their
cost. Unwanted entrance of groundwater into manholes and pipes is an important matter in
sewerage design.
Groundwater is derived directly or indirectly from precipitation: (a) directly as rain-
water and snowmelt that filter into the ground, seep through cracks or solution passages in
rock formations, and penetrate deep enough to reach the groundwater table; (b) indirectly
as surface water from streams, swamps, ponds, lakes, and reservoirs that filters into the
ground through permeable soils when the groundwater table is lower than free water sur-
faces. Streams that recharge the ground are known as influent streams; streams that draw
water from the ground as effluent streams.
The water table tops out the zone of saturation; the capillary fringe overrides it. The fringe
varies in thickness from a foot or so in sand to as much as 10 ft (3 m) in clay. It rises and falls
with the water table, lagging behind to become thicker above a falling table and thinner above
a rising table. Evaporation is increased when capillarity lifts water to, or close to, the ground