Measurement of Water Infiltration Rates for Soil

Measurement of Water Infiltration Rates for Soil
Rationale

Some of the rain water (or irrigation water) runs off the ground and the rest soaks into the ground though its pores. Plants survive and grow on this infiltrated water. The infiltration rate is influenzed by the structure, compaction and organic matter of the soil. If the infiltration rate is low, more of the rain water will run off and be wasted.  Infiltration refers to the rate at which water enters the soil. Soil permeability refers to the rate at which water moves through the soil. (Extract from  Farmer Field Schools Facilitators' manual). At a more basic level, infiltration and permeability can be related to the hydraulic conductivity K (m/d  metre/day) of the soil.
 
While both infiltration and permeability are important for soil moisture, it is easier to measure infiltration, especially relative infiltration rates between soil of different structure, land use and vegetation. It is much more difficult to measure the rate at which water moves through the soil.  In laboratory methods of measurement, core samples of the soil with fixed boundaries are subject to hydraulic measurements to arrive at conductivity values. As opposed to this hydraulic small scale in­situ methods carry out simple tests like measuring the infiltration rate from a pipe driven into a
hole in the ground. In these methods, the outer boundary of the soil investigated is not known. But these methods give fast results in the field using very simple apparatus.  The method used here is classified as a small scale, above water table, infiltrometer or inversed auger hole method [ILRI162 Chap 12).
 
[ILRI162]  Drainage principles and applications, ILRI publication 16 Second edition, H.P Ritzema (Editor­in­Chief], The Netherlands 1994 ( http://www2.alterra.wur.nl/Internet/webdocs/ilri­-publicaties/publicaties/Pub162/download­162.html)
 
Hypothesis or Theory
 
The infiltration rate over a small soil surface is a constant (m/day).  The actual infiltration will be proportional to the head of water above the ground. If the head is kept constant, then the rate of infiltration has to be measured. It is easier to have a let the head fall with the infiltration and measure the time for the head to cross between  high and low points.
 
If a pipe is partially driven into the ground and water is continuosly poured into it, then over a period the soil below would be saturated with water to a considerable depth. Assume that the level of water above the soil surface is at h in the pipe. Consider the deep boundary layer at z metres below the ground. The average   hydraulic gradient in between the soil surface and the deep surface at z is approximately (z+h)/z. As z tends to numbers greater than 10 times h, the hydraulic gradient approaches value of 1. In this case, according to Darcy's laws, the mean flow velocity at z,
approaches K, the hydraulic conductivity.     
 
It is shown (ILRI162 equation 12.14) that
 
K = 1.15r(log(h0+0.5r)­log(ht + 0.5r))/(t)
 
t= time since start of measurement (s)
 
ht = height of water in pipe at time t
 
h0 = height of water in pipe at time 0.
 
In this experiment we have to pour the continuosly till the infiltration rate is almost constant. But it is easier to take pour a reasonable amount of water, say 5 litres, into a a pipe and assume that it has penetrated to sufficient depth.  The time taken for the water level to cross two marked points can be measured.
 
Methodology
 
The method is directly taken from:

Farmer Field Schools Facilitators' manual
Vol 1, Integrated soil, water and nutrient management in semi arid Zimbabwe
O. Hughes and J.H.Venema (eds), 131 pages,
Dept of Agricultural Research and Extension, and  FAO of the UN
Harare, Zimbabwe,  Feb 2005