Infiltration (Week3)

Infiltration

water in ground surface enters soil

Infiltration Capacity

max rate at which the ground can absorb water

Field Capacity

volume of water the ground can hold

Rate of Infiltration

f = fp when i > or equal to fp

f = i when i > fp

fp - infiltration capacity

f - actual rate of infiltration

i - rainfall intensity

(all in cm/hr units)

Factors Affecting Infiltration Capacity

• Characteristics of Soil

• Surface of Entry

• Fluid Characteristics

Measurement of Infiltration

Flooding-type infiltrometers

• tube-type

• double-ring type

Tube-type

• 2-3hrs duration

• as infiltration proceeds, the volume is made up by adding water from a burette to keep water level at tip of pointer

Double-ring type

• water depths at both rings are kept the same

• measurement of water volume is done on inner ring only

Disadvantages of Infiltrometers

• raindrop effect us not simulated

• driving of the tube or rings disturb the soil structure

• results of infiltrometers depend to some extent on their size with larger meters giving less rates than smaller ones

Modelling Inflitration Capacity

focus on the curve as it is an important part on calculation and plotting the graph

Infiltration rate, fp(t)

rate at which water enters the soil at surface (in/hr or cm/hr)

Cumulative Infiltration, Fp(t)

accumulated depth of water infiltrating during given time period

Cumulative Infiltration Capacity, Fp

accumulation of infiltration over a time period since the start of process

Equations to Express Infiltration Capacities

• Horton’s (ONLY ONE DISCUSSED)

• Philip’s

• Kostiakov

• Green-Ampt

Horton’s Equation

fp - infiltration capacity at any time t from start of rainfall

fo - initial infiltration capacity at t = 0

fc - final steady state of infiltration capacity occurring at t = tc, aka constant rate of ultimate infiltration capacity

k - horton’s decay coefficient

Infiltration Indices

• used in hydrological calculations involving floods

• constant value of infiltration rate is assumed during the duration of storm

• Two types:

  • Phi Index (Only discussed)

  • W Index

Phi Index (Φ)

• average rainfall above which is the runoff

• derived from hyetograph and resulting runoff value

• Rainfall excess - amount of rainfall in excess of the index, also called effective rainfall

• expressed in length/time (mm/hr, in/hr, etc)

• Runoff = Total Rainfall - Infiltration

• Runoff = Total Rainfall - Φ(te)

  • where te = duratiom of rainfall excess values are in terms of depth

• Infiltration = Φ(te)