Types of infiltrometer - Single & Double ring infiltrometer

Types of infiltrometer - Single & Double ring infiltrometer

Measurement of Infiltration

• Infiltration characteristics of a soil at a given location can be estimated by Using flooding-type infiltrometers
• Measurement of subsidence of free water in a large basin or pond
• Rainfall simulator
• Hydrograph analysis

Types of Infiltrometer / Flooding-Type Infiltrometer

Flooding-type infiltrometers are experimental devices used to obtain data relating to variation of infiltration capacity with time. Two types of flooding-type infiltrometers are in common use. They are 

1. Tube-type (or Simple) infiltrometer, and 
2. Double- ring infiltrometer.

1. Simple (Tube Type) Infiltrometer 

Simple Infiltrometer(Tube Type Infiltrometer)

This is a simple instrument consisting essentially of a metal cylinder, 30 cm diameter and 60 cm long, open at both ends. The cylinder is driven into the ground to a depth of 50 cm . Water is poured into the top part to a depth of 5 cm and a pointer is set to mark the water level. As infiltration proceeds, the volume is made up by adding water from a burette to keep the water level at the tip of the pointer. Knowing the volume of water added during different time intervals, the plot of the infiltration capacity vs time is obtained. The experiments are continued till a uniform rate of infiltration is obtained and this may take 2-3 hours. The surface of the soil is usually protected by a perforated disc to prevent formation of turbidity and its settling on the soil surface.

A major objection to the simple infiltrometer as above is that the infiltered water spreads at the outlet from the tube  and as such the tube area is not representative of the area in which infiltration is taking place.

2. Double-ring Infiltrometer

This most commonly used infiltrometer is designed to overcome the basic objection of the tube infiltrometer, viz. the tube area is not representative of the infiltrating area. In this, two sets of concentrating rings with diameters of 30 cm and 60 cm and of a minimum length of 25 cm, as shown in , are used. The two rings are inserted into the ground and water is applied into both the rings to maintain a constant depth of about 5.0 cm. The outer ring provides water jacket to the infiltering water from the inner ring and hence prevents the spreading out of the infiltering water of the inner ring. The water depths in the inner and outer rings are kept the same during the observation period. The measurement of the water volume is done on the inner ring only. The experiment is carried out till a constant infiltration rate is obtained. A perforated disc to prevent formation of turbidity and settling of fines on the soil surface is provided on the surface of the soil in the inner ring as well as in the annular space. 

double-ring infiltrometer

As the flooding-type infiltrometer measures the infiltration characteristics at a spot only, a large number of pre-planned experiments are necessary to obtain representative infiltration characteristics for an entire watershed. Some of the chief disadvantages of flooding-type infiltrometers are

1. the raindrop impact effect is not simulated;
2. the driving of the tube or rings disturbs the soil structure; and 
3. the results of the infiltrometers depend to some extent on their size with the larger meters giving less rates than the smaller ones; this is due to the border effect.

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Rainfall Simulator

In this, a small plot of land, of about 2 mx4 m size, is provided with a series of nozzles on the longer side with arrangements to collect and measure the surface runoff rate. The specially designed nozzles produce raindrops falling from a height of 2 m and are capable of producing various intensities of rainfall. Experiments are conducted under controlled conditions with various combinations of intensities and durations and the surface runoff rates and volumes are measured in each case. Using the water-budget equation involving the volume of rainfall, infiltration and runoff, the infiltration rate and its variation with time are estimated. If the rainfall intensity is higher than the infiltration rate, infiltration capacity values are obtained.

Rainfall-simulator-type infiltrometers give lower values than flooding-type infiltrometers. This is due to effect of the rainfall impact and turbidity of the surface water present in the former.

Hydrograph Analysis

Reasonable estimation of the infiltration capacity of a small watershed can be obtained by analyzing measured runoff hydrographs and corresponding rainfall records. If sufficiently good rainfall records and runoff hydrographs corresponding to isolated storms in a small watershed with fairly homogeneous soils are available, the water-budget equation can be applied to estimate the abstraction by infiltration. In this, the evapotranspiration losses are estimated by knowing the land cover/use of the watershed.