Interception

Question 1

What is interception?

Answer 1

The fraction of
gross rainfall input that wets and
adheres to above ground objects until it
is returned to the atmosphere through
evaporation.

May be appreciable depending on:
– the storm size; and
– the characteristics of the forest—species
composition, age, density of stands.

Question 2

Depression storage

Answer 2

the fraction
of rainfall excess that becomes
entrapped in numerous small
depressions from which the only escape
is via evaporation or/and infiltration.

Question 3

Evaporation

Answer 3

the process by which water
is transferred from land and water
masses of the earth to the atmosphere.

Question 4

What is evaporation a function of?

Answer 4

Function of:
• Solar radiation
• Differences in vapour pressure between a water
surface and the overlying air
• Temperature
• Wind
• Atmospheric pressure
• Quality of evaporating water

Question 5

Importance of evaporation?

Answer 5

A significant fraction (~60%) of water
falling as rainfall is returned to the
atmosphere
• Evaporation is important in water
resources planning and management
programmes and for evaluating the
potential for water resources
development and water supply studies.
• Evaporation is not significant during
storm periods and can be disregarded
in flow estimations.

Question 6

Estimating Evaporation

Answer 6

For estimating lake (reservoir)
evaporation
– Water Budget Method
– Energy Budget Method (pg 150 Viessman and
Lewis, 2003)
– Mass Transfer Techniques
– Estimation from Pan Evaporation
Measurements

Question 7

What is transpiration?

Answer 7

Controlled by factors dominating evaporation:
– Solar radiation
– Temperature
– Wind velocity
– Vapour pressure gradients
– Atmospheric pressure
– Quality of evaporating water
• But also affected by the character of plant and
plant density

Question 8

Transpiration Process

Answer 8

Blown up picture of a single
plant showing uptake of
water through its root system
and transport to the leaves.
• Large surface area of leaves
and water available through
many microscopic pores in
the leaves.
• Increased temperature of the
leaves increases the
saturation vapour pressure,
thereby allowing increased
loss of water to evap

Question 9

Estimating Evapotranspiration

Answer 9

For estimating evapotranspiration
– Theoretical, based on physics of the
process
– Analytical, based on energy or water
budgets
– Empirical

Question 10

What is Infiltration?

Answer 10

the flow of water through the soil surface
1. Surface entry
2. Transmission
through soil
3. Depletion of
storage capacity

Question 11

What are Factors affecting infiltration rate?

Answer 11

1. The characteristics of infiltrating water
2. Type and extent of vegetal cover
3. Condition of the surface crust
4. Biological activity and organic matter
5. Soil texture and structure
6. Antecedent water content of the soil

Question 12

How does Characteristics of the fluid affect infiltration rate?

Answer 12

– Turbidity—“pollution” of the rainwater by the
fine clays and colloids in soils
– “Contamination” by salts (especially in alkali
soils)
• Affects fluid viscosity
– Temperature
• Affects fluid viscosity

Question 13

How does . Type and extent of vegetal cover affect the infiltration rate?

Answer 13

Vegetation is one of the most significant
factors affecting surface entry of water
– Vegetation and mulches protect the soil
surface from rainfall impact.
– Root system perforates the soil, keeping
it unconsolidated and porous.
– Forest litter reduces washout of colloidal
material which tend to plug soil pores.

Question 14

How does Condition of the surface crust affect the infiltration rate?

Answer 14

Prevention or retardation of entry of water
into the soil due to surface sealing by
the inwash of fines or other
arrangements of particles.
Note that even if soils may have excellent
underdrainage, the effect of sealing
causes a net low infiltration rate.

Question 15

How does Biological activity and organic matter affect the infiltration rate?

Answer 15

Organic matter from crops promotes a
crumb structure and promotes permeability
– High biotic activity in and beneath the soil
surface opens up the soil, resulting in high
entrance capacities.

Question 16

How does soil texture and structure affect infiltration rate?

Answer 16

Concerns pore size and pore-size distribution,
that is the proportion of different sizes present,
and their relative stability during storms
– Eg, sands, pores are relatively stable, since the
sand particles that form them are not readily
disintegrated, nor do they swell.
– Soils with silt or clay are subject during a storm to
the disintegration of the crumbs or aggregates,
which in their dry state may provide relatively large
pores

Question 17

How does Antecedent water content of soil affect infiltration rate?

Answer 17

Storage available depends on porosity,
thickness of the horizon, and the amount
of moisture already present.

Question 18

What is Field Capacity ?

Answer 18

defined as the
maximum amount of water which can be
retained against the force of gravity and
represents the storage capacity of
capillary water.

Question 19

Horton’s Equation

Answer 19

If rainfall supply exceeds the infiltration capacity,

Infiltration tens to decrease exponentially given by:

Question 20

Infiltration Index

Answer 20

• Generally assumes that infiltration
occurs at some constant or average
rate throughout a storm.
• Initial rates are usually underestimated,
and final rates are overstated.
• Suited only for large storms, or for
storms where infiltration rates may be
assumed relatively uniform.

Question 21

 Index

Answer 21

is the average rainfall rate
above which the volume of runoff
equals the volume of rainfall. If the
mass infiltration is called Basin
Recharge, the  Index can be
computed from: