Interception Flashcards
What is interception?
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.
Depression storage
the fraction of rainfall excess that becomes entrapped in numerous small depressions from which the only escape is via evaporation or/and infiltration.
Evaporation
the process by which water
is transferred from land and water
masses of the earth to the atmosphere.
What is evaporation a function of?
Function of: • Solar radiation • Differences in vapour pressure between a water surface and the overlying air • Temperature • Wind • Atmospheric pressure • Quality of evaporating water
Importance of evaporation?
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.
Estimating Evaporation
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
What is transpiration?
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
Transpiration Process
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
Estimating Evapotranspiration
For estimating evapotranspiration – Theoretical, based on physics of the process – Analytical, based on energy or water budgets – Empirical
What is Infiltration?
the flow of water through the soil surface 1. Surface entry 2. Transmission through soil 3. Depletion of storage capacity
What are Factors affecting infiltration rate?
- The characteristics of infiltrating water
- Type and extent of vegetal cover
- Condition of the surface crust
- Biological activity and organic matter
- Soil texture and structure
- Antecedent water content of the soil
How does Characteristics of the fluid affect infiltration rate?
– 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
How does . Type and extent of vegetal cover affect the infiltration rate?
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.
How does Condition of the surface crust affect the infiltration rate?
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.
How does Biological activity and organic matter affect the infiltration rate?
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.
How does soil texture and structure affect infiltration rate?
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
How does Antecedent water content of soil affect infiltration rate?
Storage available depends on porosity,
thickness of the horizon, and the amount
of moisture already present.
What is Field Capacity ?
defined as the maximum amount of water which can be retained against the force of gravity and represents the storage capacity of capillary water.
Horton’s Equation
If rainfall supply exceeds the infiltration capacity,
Infiltration tens to decrease exponentially given by:
Infiltration Index
• 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.
Index
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:
