The two terms, Infiltration and Percolation, can be easily defined as 'the process of water penetrate surface layer into subsurface soil' and 'water percolates downwards under the influence of gravity until the zone of saturation is reached after infiltrating water has passed through the surface layers' respectively.
As different types of soil have different allowance of water to infiltrate, different infiltration capacities, f, are determined for each soil type.
Factors influencing fc
In the above interactive animation, the greater the number, the greater the quantity of designated factor..
For any soil under constant rainfall, infiltration rate decreases can be expressed as an equation:
where f is the infiltration rate at any time t (mm/h)
fc is the infiltration capacity at large value of t (mm/h)
μ = fo - fc
fo is the initial infiltration capacity at t = 0 (mm/h)
t is the time from beginning of rainfall (min)
K is the constant for a particular soil and surface (min-1)
From the above functions and parameters, we can conclude the factors affecting fc are:
1. Presence of vegetation
2. Type of soil
3. Initial moisture content of soil
4. Rainfall intensity
5. Slope of ground
Methods to find fc
Infiltrometers
An infiltrometer is a tube with wide-diameter and consists of two short concentric rings. The rings are flooded to a depth of 5mm over surface and continually refilled to maintain the depth and to measure the inflow to central tube. The outer ring is proposed to eliminate edge effects. For large areas, simulation of real conditions is needed and hence usually replaced by sprinkler tests. The sprinkler simulates rainfall and runoff measured, the difference is assumed to be the infiltrated amount.
Φ - index method
Φ- index is the average rainfall above which the rainfall volume is equal to the runoff volume. Φ- index is derived from the rainfall hyetograph with rainfall intensity (mm/h) to time (h). The shaded area in the hytrograph is the measured runoff in mm, where the un-shaded are is equal to the losses due to evaporation, surface detention and infiltration (= rainfall - runoff). Many determinations should be made and averaged before the index is used.
Soil moisture
Antecedent precipitation index
The antecedent precipitation index is based on the assumption that soil moisture is depleted proportional to the amount in storage in the soil.
where Io = initial value of index (mm)
It = index value t days later
k = a recession constant varying between 0.85 and 0.98
If precipitation occurs, the index would be increased by an amount. This amount is indeterminate as some rain may have left the catchment as surface runoff. k value varies seasonally due to reduction of the index by evapotranspiration.
Estimated soil-moisture deficit
Since evapotranspiration is coninually removeing soil moisture and precipitation replacing it, continuous measurements of these two processes provide an estimation of soil moisture deficit for any location at any time. This avoids the use of recession constant. The determination of soil-moisture deficit is proposed to predict proportion of runoff from a storm.
Measurement of soil moisture
In order to proceed in-situ measurement of soil moisture, an instrument called The Wallingford soil-moisture probe is developed by the United Kingdom Institute of Hydrology in conjunction with the Atomic Energy Authority. This instrument can be used for all terrain and weather. It consists of a radioactive source that can emit fast neutrons. Those neutrons are scattered and slowed by collisions with water particles. For wetter soil, the number of collisions would be larger and hence more slow neutrons would be detected.
Final Year Project - Web-based interactive computer-aided learning package on engineering hydrology
