Infiltration - 2 | Engineering Hydrology - Civil Engineering (CE) PDF Download

Measurements of Infiltration

Infiltration is a highly complex process that can vary over time and across different locations. When selecting measurement and data analysis techniques, it's important to account for these variations and their spatial dimensions. Infiltration measurement techniques can be categorized based on these spatial aspects. Below is a brief introduction to various infiltration measurement techniques.

Areal Measurement
Areal infiltration estimation involves analyzing rainfall-runoff data from a watershed. For a storm with a single runoff peak, this process is similar to calculating a  index (refer to section 4.3.2). The total rainfall volume is calculated by integrating the rainfall hyetograph, and the runoff volume is determined by integrating the runoff hydrograph. The infiltration volume is found by subtracting the runoff volume from the rainfall volume. The average infiltration rate is then calculated by dividing the infiltration volume by the rainfall duration.

Point Measurement
Point infiltration measurements are conducted by applying water to a specific site over a finite area and measuring the soil's water intake. There are four types of infiltrometers used for this purpose:
Ponded-Water Ring or Cylinder Type

• Sprinkler Type
• Tension Type
• Furrow Type

The choice of infiltrometer should match the system being studied. For instance, ring infiltrometers are suitable for inundated soils like those in flood irrigation or pond seepage. Sprinkler infiltrometers are used where rainfall impacts surface conditions and, consequently, the infiltration rate. Tension infiltrometers help determine the infiltration rates of soil matrices in the presence of macropores. Furrow infiltrometers are ideal for scenarios where the effect of flowing water is significant, such as in furrow irrigation.

Ring or Cylinder Infiltrometers
These infiltrometers typically consist of metal rings with diameters ranging from 30 to 100 cm and heights of 20 cm. The ring is inserted about 5 cm into the ground, water is applied inside the ring using a constant-head device, and measurements are taken until a steady infiltration rate is achieved. To reduce the effect of lateral water spreading, a double-ring infiltrometer, which includes an additional larger outer ring, can be used.

Sprinkler Infiltrometer - Rain Simulator
With the use of a rain simulator, water is uniformly sprinkled over a specific experimental area at a rate that exceeds the soil's infiltration capacity. The resultant runoff (R) is measured, and the infiltration rate (f) is calculated using the formula f = (P - R) / t, where P represents the total rain sprinkled, R is the runoff collected, and t is the duration of rainfall.

Infiltration rate and cumulative infiltration variation with time

Estimating Infiltration Rate

In this section, four methods for estimating infiltration are discussed: Horton Infiltration, the -index, Philip infiltration, and Green-Ampt infiltration equations.

Horton Infiltration
Generally, for a constant storm, infiltration rates decrease over time. The initial infiltration rate is highest at the beginning of the storm and gradually decreases until it reaches a constant value.
Horton observed that infiltration starts at a rate fof_ofo and decreases exponentially until it reaches a constant rate fcf_cfc. He proposed the following infiltration equation, assuming rainfall intensity iii is greater than fpf_pfp at all times

Where:

f_p = infiltration capacity in at any time
f_o = initial infiltration capacity in
f_c = final constant infiltration capacity in  at saturation, dependent on soil  type and vegetation
t= time in hours from the beginning of rainfall
k = an exponential decay constant dependent on soil type and vegetation

Note that infiltration occurs at capacity rates only when rainfall intensity  equals or exceeds

The cumulative infiltration equation  for the Horton method is derived from the relationship

and is given by:

The φ -index method is the simplest technique for estimating infiltration. It calculates infiltration as the difference between gross rainfall and observed surface runoff. This method assumes that the loss is uniformly distributed across the rainfall pattern.

The Philip Method
Philip proposed an equation to estimate cumulative infiltration F(t):
Where:

S = sorpitivity which is a function of the soil suction potential (representing soil suction head
K = the hydraulic conductivity of the soil (representing gravity head)
t = time from the beginning of the rainfall.

Noting that f(t) = dF(t)/dt, the Phillip equation for infiltration rate is