Environmental Engineering Reference
In-Depth Information
remains after all water losses are satisfied. The most important rainfall losses along its
way of the rainfall to the water body could be defined as follows:
• Interception storage - This is the part of the rainfall, which adheres to the surface of
vegetation and other aboveground objects and is returned to the atmosphere by
evaporation. Its value depends on the type of vegetation, intensity and volume of the
rainfall and the growth stage of the vegetation. Grass and dense shrubbery could
intercept 1.2-1.8 mm of the rainfall (Novotny 2003).
• Depression storage - This is the part of the rainfall, which is detained in surface
depressions, which need to be filled before the runoff could be transported further.
This water evaporates or percolates to the soil. The amount of depression storage
depends on the moisture content of the ground cover and topography. In many cases,
the interception storage and depression storage are grouped together and evaluated as
one value, termed, “surface storage”.
• Evapotranspiration - it represents water loss into the atmosphere by the combined effect
of evaporation from soil and water surfaces and transpiration by plants. Transpiration
denotes the water abstracted by plants from soil moisture and released to the
atmosphere, as part of their life cycle.
• Infiltration - this is the process of percolation of the surface storage into the soil. It is a
function of soil permeability, moisture content, vegetation cover and other factors.
After reaching the aquifer, this portion of the rainfall is known as “groundwater
runoff” and is the main source for ground water recharge. It is the source of the “base
flow” in rivers and streams, maintained by springs and other forms of ground water
discharges to the natural water bodies. The infiltration of ground water to sewer
systems could be included in this category as well.
Considering the above-mentioned losses, we could differentiate between rainfall excess
and surface runoff in the following way:
• Rainfall excess or “net rain” is used to denote that part of the rainfall, which is left after
the subtraction of the above mentioned losses, and is expressed as the depth of water
in “mm” over a given surface area for a specified period of time.
• Surface runoff - it represents that part of rainfall, which has been generated by the
rainfall excess, and forms part of the surface flow in natural rivers and streams. It is
expressed as flow rate (volume per unit time).
4.2
Determining rainfall excess
Rainfall excess could be determined by the curve number method, known as the Natural
Resources Conservation Method (NRCM), developed first in the USA. It determines the
rainfall excess as a function of the rainfall volume, surface storage and infiltration
(Novotny 2003). Based on extensive rainfall/runoff data and variety of soil and cover
conditions, the method results in the development of a set of curves, with a specific
number, which links the 24-h rainfall with the corresponding rainfall excess. Each curve
number is dependent on the type of land use, level of imperviousness, hydrologic
conditions and type of soils. Different soil conditions are classified in four categories.
This method was adapted for the Southern African conditions and is known as the SA-
SCS method (Shulze et al. 1993). More detailed description of the procedure applied is
