LESSON 4.1-4.2 Flashcards
is the flow of water into the ground through the soil surface.
Infiltration
At the top, a thin layer of saturated zone is created.
Zone 1
Transition Zone
Zone 2
the downward
motion of the moisture takes place. The moisture content in this zone is above
field capacity but below saturation. Further, it is characterized by unsaturated
flow and fairly uniform moisture content.
Zone 3 -Transmission Zone
The soil moisture in this zone will be at or near field capacity and the moisture
content decreases with the depth.
Zone 4 - Wetting Zone
maximum rate at which the ground can absorb water,
Infiltration Capacity
volume of water that the ground can hold
Field Capacity
Infiltration depends upon this 7 factors namely:
- intensity and duration of rainfall,
- weather (temperature),
- soil characteristics,
- vegetal cover and land use,
- initial soil moisture content (initial wetness),
- entrapped air; and
- depth of the ground
water table.
The greatest factor controlling infiltration is the amount and
characteristics of precipitation that falls as rain or snow.
intensity and duration of rainfall
Some soils, such as clays, absorb less water at a slower rate than sandy soils. Soils
absorbing less water result in more runoff overland into streams.
soil characteristics
Some land covers have a great impact on infiltration and rainfall runoff.
Vegetal cover and land use
Like a wet sponge, soil already saturated from previous
rainfall can’t absorb much more … thus more rainfall will become surface runoff.
Initial soil moisture content (initial wetness)
The maximum rate at which a given soil at a given time can absorb water
Infiltration capacity
Factors Affecting Infiltration Capacity, fp
✓ Characteristics of the soil (Texture, porosity and hydraulic conductivity)
✓ Condition of the soil surface
✓ Current moisture content
✓ Vegetative cover and
✓ Soil temperature
A few important factors affecting fp are
Characteristics of Soil
Surface of Entry
Fluid Characteristics
The type of soil, viz. sand, silt or clay, its texture, structure, permeability and underdrainage are the
important characteristics under this category.
Characteristics of Soil
At the soil surface, the impact of raindrops causes the fines in the soil to be displaced and these in
turn can clog the pore spaces in the upper layers of the soil.
Surface Entry
Water infiltrating into the soil will have many impurities, both in solution and in suspension. The
turbidity of the water, especially the clay and colloid content, is an important factor and such
suspended particles block the fine pores in the soil and reduce its infiltration capacity.
Fluid Characteristics
Measurement of Infiltration
✓ Using flooding-type infiltrometers
✓ Measurement of subsidence of free water in a large basin or pond
✓ Rainfall simulator
✓ Hydrograph analysis
are experimental devices used to obtain data relating to variation of
infiltration capacity with time.
Flooding-Type Infiltrometer
Two types of flooding-type infiltrometers are in common use.
(a) Tube-type (or Simple) infiltrometer, and
(b) Double ring infiltrometer.
This is a simple instrument consisting essentially of a metal cylinder, 30 cm diameter and 60 cm long,
open at both ends. The cylinder is driven into the ground to a depth of 50 cm
Simple (tube-type) Infiltrometers
This most commonly used infiltrometer is designed to overcome the basic objection of the tube
infiltrometer, viz. the tube area is not representative of the infiltrating area.
Double-ring Infiltrometer
Some of the chief disadvantages of flooding-type
infiltrometers are:
✓ the raindrop impact effect is not simulated;
✓ the driving of the tube or rings disturbs the soil structure; and
✓ the results of the infiltrometers depend to some extent on their size with the larger meters giving
less rates than the smaller ones; this is due to the border effect
Experiments are conducted under controlled conditions with various combinations of intensities and
durations and the surface runoff rates and volumes are measured in each case. Using the water-budget
equation involving the volume of rainfall, infiltration, and runoff, the infiltration rate and its variation with
time are estimated. If the rainfall intensity is higher than the infiltration rate, infiltration capacity values
are obtained.
Rainfall Simulator
A reasonable estimation of the infiltration capacity of a small watershed can be obtained by analyzing
measured runoff hydrographs and corresponding rainfall records. If
Hyetograph Analysis
