Infiltration rate

DETERMINATION OF INFILTRATION RATE IN SOIL

(Double ring infiltrometer)

Infiltration refers to the downward entry of water into the soil and the rate at which it occurs is called the infiltration rate or water intake rate. It is rapid in the beginning and decreases with time and attains almost a steady value called “infiltration capacity” or “basic infiltration rate”.

         The infiltration rate is usually measured under field conditions and involves ponding of water on the surface or sprinkling of water to stimulate the rain. There is no generally accepted method suitable for all situations.

Apparatus

         Two metal rings : Inner ring or 30 cm diameter and 20 cm in height and a guard ring of 50 cm diameter and 20 cm in length. One edge of the rings should be sharp so as to facilitate easy penetration into the soil.

         Dividing plate: Steel plate 1.25 cm thick and 60 cm in diameter with legs to keep the plate on the rings. Hammer, water, spade.

Procedure

         Drive the inner and guard rings in the soil by hammering on the driving plate placed on the rings until the top 10 cm of the rings remain above the soil surface. Place a gunny bag piece on the soil surface inside the inner ring and pour water both inside and outside the ring so that the water is ponded to a depth of 6 cm. There should not be any puddling of a soil during the addition of water. Both the rings should be pushed to the same depth in the soil and also the water inside and outside the inner ring should be approximately at the same level.

         Record the initial level and continue recording the receding water level after suitable intervals. Note the observations after 2, 5, 20, 25, 30, 60, 90, and 120 minutes after infiltration has commenced.  Note the depth of water infiltration (mm) and the rate of infiltration.

Classification of soil on the basis of infiltration rate

Very rapid                     >25.4 cm/ hour

Rapid                           12.7 - 25.4 cm/ hour

Moderately rapid          6.3 - 12.7 cm/ hour

Moderate                      2.0 - 6.3 cm/ hour

Moderately slow           0.5 - 2.0 cm/ hour

Slow                             0.1 - 0.5 cm/ hour

Very slow                     <0.1 cm/ hour

Calculation

                 Time (minutes)                          Water level (cm)            T (min)         H (cm) 

                  T 0            T 1                           H 0              H 1

Infiltration rate (IR)             =  H (average) / Time (average)                  

Infiltration rate per hour  = (IR / T) * 60

                                         =  X  cm/hr

Result

The infiltration rate of given soil =            cm/hr