Infiltration and Runoff
The process of water entering into the soil is called infiltration
Actually, the infiltration occurs on the ground surface plane. Below the surface, the penetration further is called percolation. Whatever rainfall occurs on surface of earth, some quantities infiltrate. The infiltrated quantities may further penetrate as percolation. If the voids in the soil are only partially filled by water during this percolation, it is called unsaturated flow. You have studied about unconfined and confined flows in aquifers. Those flows are saturated. -Saturated flows are those that have all voids occupied by water. The water table is the interface between the saturated and unsaturated flow.
- Horton’s Equation: He observed that the infiltration capacity reduced in an exponential fashion from an initial, maximum rate fcf to a final constant rate fco. Horton expressed the decay of infiltration capacity with time as an exponential decay given by
f = fc + (fo - fc ) e-kt
f = infiltration capacity at any time t from the start of the rainfall
fo = initial infiltration capacity at t = 0
fc = final steady state value
td = Duration of rainfall
kh = constant depending on soil.
In hydrological calculations involving floods, it is found convenient to use a constant value of filtration rate for the duration of the storm. The defined average infiltration rate is called infiltration index. Also this is the average infiltration rate during the time when the rainfall intensity exceeds the infiltration rate.
The W – index can be derived from the observed rainfall and runoff data. It differs from the φ - index in that it excludes surface storage and retention.
In an attempt to refine the φ-index the initial losses are separated from the total abstractions and an average value of infiltration rate, called W-index, is defined as
Where, P = Total storm precipitation (cm)
R = Total storm runoff (cm)
Ia = initial losses (cm)
te = Duration of rainfall excess
W-index = Avg. rate of infiltration (cm/hr)
The φ index is the average rainfall above which the rainfall volumes is equal to the runoff volume. The φ index is derived from the rainfall hyetograph with the edge of the resulting run- off volume.
R = Runoff in cm from a 24- h rainfall of intensity I cm/day
Runoff can be described as the part of the water cycle that flows over land as surface water instead of being absorbed into groundwater or evaporating. It thus represents the output from the catchment in a given unit of time.
There are a variety of factors that affect runoff. Some of those include:
Amount of Rainfall
The amount of rainfall directly affects the amount of runoff. As expected, if more rainfall hits the ground, more rainfall will turn into runoff. The same can be said about snowmelt. If a large amount of snow melts in a short time period, there will be a large amount of runoff.
The ability of the ground surface to absorb water will affect how much surface runoff occurs. If you have ever poured water onto sand, you may have noticed it sinks into the sand almost instantaneously. On the other hand, if you pour water on the street, the water will not sink but runoff to the gutter or a ditch. The less water the ground can absorb, the more runoff on the surface there will be. This is called permeability.
A surface with high absorption ability has high permeability, and a surface with low absorption ability has low permeability.
Vegetation needs water to survive, and a plant's root system is designed to absorb water from the soil. There is less runoff in highly vegetated areas because the water is used by the plants instead of flowing off the surface of the ground.
The slope of a surface is also important to the amount of runoff there will be. The steeper a surface is, the faster it will flow down the slope. A flat surface will allow the water time to absorb.
The part of runoff which enters the stream quickly after the rainfall or snow melting. To design soil conservation structure with proper capacity it is necessary to estimate peak runoff rate.
It includes surface runoff, prompt interflow and rainfall on the surface of the stream. In the case of snow-melt, the resulting flow entering the stream is also a direct runoff, sometimes terms such as direct storm runoff are used to designate direct runoff.
Baseflow (also called drought flow, groundwater recession flow, low flow, low-water flow, low-water discharge and sustained or fair-weather runoff) is the portion of streamflow that comes from "the sum of deep subsurface flow and delayed shallow subsurface flow. Also, it is the delayed flow that reaches a stream essentially as groundwater flow is called base flow.
(i) Direct runoff = surface runoff + Prompt interflow
(ii) Direct runoff = Total runoff- Base flow
(iii) Form Factor where, A = Area of the catchment l Axial length of basin.
(iv) Compactness coefficient
re = Radius of an equivalent circle whose Area is equal to area of the catchment (A)
(v) Elevation of the water shed, (z)
Where, A1, A2 … Area between successive contours.
Z1, z2 … mean elevation between two successive contours.
Methods to Compute Runoff
(i) By Runoff coefficient
Q = KP where, p = precipitation
K = Runoff coefficient
Q = Runoff
(ii) By infiltration Capacity Curve
(iii) By Rational Formula
Where, k = Runoff coefficient
PC = Critical design rainfall intensity in cm/hr
A = Area of catchment in the hectare
Qp = Peak discharge in m3/sec.
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