The water balance is a mass balance of the flow and storage of water
in surface soil on a per unit area basis, using the hydrologic equation:
Inflow - Outflow = Change in Storage. The inflow is precipitation
less overland runoff, the outflows are actual evapotranspiration (AET)
and deep-drainage percolation, and water is stored as soil moisture.
The wet season is that portion of the year where there is sufficient precipitation
to satisfy potential evapotranspiration (PET), while in the dry season
PET exceeds precipitation. The specific calculations in the monthly
water balance are summarized in the following 9 steps:
Adjusted Average Temperature - The temperature obtained from the LCD represents
the average monthly temperature at the LCD station. Using adiabatic
lapse rates, this temperature is adjusted for the elevation difference
that may exist between the site and the LCD station. The elevation
of the LCD station is listed in the LCD
Potential Evapotranspiration (PET) - The modified Blaney-Criddle method
(Doorenbos and Pruitt 1977), the Penman method (Penman 1948) and the Penman
method with correction factor (Doorenbos and Pruitt 1977) are used to estimate
PET rate. All three methods are based on average air temperature,
minimum relative humidity, ratio of actual to maximum possible sunshine
hours, and average wind speed. The Penman method and Penman method
with correction factor methods are also based on the maximum relative humidity
and latitude. These parameters are contained in the LCD and are different
than those used by Thornthwaite and Mather (1955, 1957), Fenn et al. (1975),
and Dass et al. (1977). Because the soil moisture storage calculations
are based on PET rate, the computation of PET rate has a significant influence
on the amount of water that percolates into the waste site
Adjusted Precipitation - Monthly precipitation is obtained from the LCD.
Precipitation occurring in months with an adjusted temperature below freezing
is assumed to be in the form of snowfall. Snowfall is assumed to
occur before any considerable ground-surface freezing has taken place.
This assumption is important because when snowmelt occurs, percolation
can also occur. It is also assumed that the snow is stored on the
ground during the months when the adjusted average monthly temperature
is below freezing. During the spring melt, a portion of the snowmelt
is combined with the precipitation. This adjusted precipitation is
used in the overland runoff and percolation computations. Snowmelt
computations consider melt from rainfall, vapor condensation, convection,
and radiation. Typically required parameters include average temperature,
average wind speed, site elevation, mean sky cover (i.e., degree of cloudiness),
and monthly precipitation as rainfall, all of which are contained in the
LCD.
Overland Runoff - The Soil Conservation Service (SCS) curve number technique
forms the basis of estimating the net monthly overland runoff. This
technique is described in a separate document (Whelan et al. 1987) and
will not be discussed in detail here.
Maximum and Potential Percolation - Maximum percolation represents the
difference between adjusted precipitation and monthly overland runoff.
Potential percolation represents the difference between the maximum percolation
and the PET. For months with an adjusted temperature below freezing,
both the maximum and potential percolation are zero.
Accumulated Potential Water Loss - This step represents the potential soil
moisture water loss during a year. It is computed using the potential
percolation and the soil moisture retention tables provided by Thornthwaite
and Mather (1957).
Soil Moisture Storage - This step computes the moisture contained in the
surface soil at the end of each month. In dry months (i.e., months
where PET is greater than adjusted precipitation), soil moisture storage
is computed using the accumulated potential water loss and the soil moisture
retention tables provided by Thornthwaite and Mather (1957). In wet
months (i.e., months where adjusted precipitation is greater than PET),
the soil moisture storage is computed using the potential percolation and
the previous month's soil moisture.
Actual Evapotranspiration (AET) - Actual evapotranspiration equals PET,
if the maximum percolation (i.e., adjusted precipitation less overland
runoff) is greater than or equal to PET. If maximum percolation is
less than PET, then there is not enough new moisture to entirely satisfy
PET. Thus, AET will equal the sum of maximum percolation and the
amount by which soil moisture storage is reduced (i.e., maximum percolation
minus the change in soil moisture storage between the current and previous
months).
Deep-Drainage Percolation - The leachate generated from a soil column is
zero if the adjusted temperature is below zero. If the temperature
is above zero, the leachate generated equals the maximum percolation minus
AET and change in soil moisture storage.
A complete description of the steps highlighted above is provided below
in sections 5.1.2 through 5.1.9.(a)
(a)
Many of the original equations in this chapter
were developed using English units. For consistency, all parameters and
equations are expressed in metric units, unless otherwise noted.