1-kunkle models such as SFB requires calibration1-kunkle models such as SFB requires calibration

1-kunkle (1962) states that the
hydrograph separation of base and surface runoff, furthermore, it can separate
base runoff into discharge from bank and basin storage. The method is based on
several assumptions, for the separation of basin-storage discharge; first the
minimum-discharge values at the beginning and end of a groundwater year are
assumed to represent groundwater discharge from basin storage, because the bank
storage has been depleted and, provided that there has been no recent precipitation,
therefore, all the discharge is coming from basin storage, second the minimum
discharge values at the beginning and end of the groundwater year are very
nearly equal due to the close balance between the amount of discharge and the
amount of recharge.

2- Conceptual
rainfall-runoff models such as SFB requires calibration of a number of
parameter values prior to application to real catchments. Sensitivity analysis
usually suggests that model performance is much more sensitive to some
parameters than others (Calver, 1988).

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langbein and iseri (1960) states that  hydrograph is a graphical representation of
streamflow plotted with respect to time 
and can be used to analyze runoff characteristics associated with a
basin and storm.

4- The hydrograph shows the
integrated effects of the physical basin characteristics and storm
characteristics within the basin boundaries, and the separation of a hydrograph
in terms of time can be useful for hydrologic analysis of drainage structures.( (Chow, 1964; Freeze, 1974).

5-The single most important property of the hydrograph that is essential
to drainage structure design is the peak rate of runoff (Wigham 1970)

6- The hydrologic analysis
allows for estimates of runoff characteristics such as peak rate of runoff or
runoff volume, but exact solutions to drainage design problems should not be
expected (Virginia Department of Transportation, 2002).

7-Most scientists and transportation engineers recognize that runoff
occurs in response to complex interactions between surface flow and saturated
and unsaturated subsurface regimes (Freeze 1972).

8-Horton (1933) developed a
widely accepted theory where overland flow dominates runoff generation. When
precipitation falls to the earth, a portion of the moisture evaporates or is
intercepted by plants, litter, and soil. Initial surface detention storage must
be satisfied before infiltration into the soil column occurs. Infiltration rate
is greatest initially, and is reduced as precipitation continues. If the
precipitation rate exceeds the infiltration rate after satisfying interception
requirements, the excess moisture initially forms small puddles, creating
depression storage. As surface depressions are filled and depth of surface
detention increases, surface runoff begins. This runoff is referred to as
overland flow or surface runoff (Horton, 1933) and the theory is most
appropriately applied to hill slopes with low infiltration capacity and little
soil depth. Horton’s theory suggests that most precipitation events exceed the
infiltration capacity of the soil and that overland flow is common (Freeze

9- Schwab (1971) states that two general classes of elements influence
runoff: precipitation characteristics and basin or watershed qualities. Precipitation
attributes include type, duration, amount, intensity, frequency, and
distribution. basin attributes are size, shape, topography, soils, geology, and
land utilize.

10- Virginia Department of Transportation( 2002) found that Precipitation
qualities describe the supply of water to a basin, a portion of which achieves
the basin outlet as surface runoff. Sum and term of the precipitation are the
most essential qualities of a tempest for hydrologic examination and can be
joined to describe intensiy and recurrence of the precipitation. Circulation of
precipitation in time and space is to some degree lessened in significance by
dissecting basins with little contributing seepage territories: the smaller the
basin size, the less the normal changeability of precipitation circulation over
the basin. One assumption made during the hydrologic examination and plan of
hydraulic structures for small basins is that the precipitation sum is uniform
over the basin in time and space. There is no single acknowledged basin measure
restrict for which the uniform precipitation supposition remains constant.
Different offices and examiners utilize most extreme measure limits from under
20 acres  of land to a few square miles
for their meaning of little bowls. VDOT’s meaning of a little basin is 200
acres of land or less.


Dougherty, D. E. &
Marryott, R. A. (1991) Optimal groundwater management.  Simulated annealing water  resources. p2493-2508.

Freeze, R.A., 1972a, Role
of subsurface flow in generating surface runoff 1, Base flow contributions to
channel flow: Water Resources Research, v. 8, no. 3, p. 609–623.

Langbein, W.B., and Iseri,
K.T., 1960, General introduction and hydrologic definitions: U.S. Geological
Survey WaterSupply Paper 1541-A, 29 p.

Wigham, J.M., 1970, Peak
flow–rainfall events, in Gray, D.M., ed., Handbook on the principles of
hydrology: Port Washington, New York, Water Information Center, Inc. 102 p.

Schwab, G.O., Frevert,
R.K., Barnes, K.K., and Edminster, T.W., 1971, Elementary soil and water
engineering: New York, John Wiley & Sons, Inc., 316 p

Horton, R.E., 1933, The
role of infiltration in the hydrologic cycle: Transactions of the American
Geophysical Union, no. 14, p. 446–460.

Freeze, R.A., 1972b, Role
of subsurface flow in generating surface runoff 2, Upstream source areas: Water
Resources Research, v. 8, no. 5, p. 1272–1283.

Hayes,C,D , Young,L,R.(2006) Comparison
of Peak Discharge and Runoff Characteristic Estimates from the Rational Method
to Field Observations for Small Basins in Central Virginia, Scientific
Investigations Report 2005-5254.