Soil properties - Fundamentals of the Physical Environment - page 577

Table 18.4 Hydraulic conductivity of soil

Hydraulic conductivity K (cm hr −1 )

Texture Structure

Coarse sand Single grain > 50

Sandy loam Blocky, fine crumb 6-12

Loam, silt loam Blocky 2-6

Clay, clay loam Blocky, prismatic 0·5-2

Clay, clay loam Blocky, prismatic, fine platy 0·25-0·5

Clay, heavy clay Massive, fine columnar < 0·25

infiltration rate . Its units are velocity, cm hr −1 . Initially in dry soils infiltration rates can
be high, especially in coarse-textured soils and in heavy-textured soils with surface
cracking. The infiltration rate then falls as pores fill with water, as cracks close up owing
to swelling clays, and as structure starts to collapse in the wet state. Infiltration rates can
vary from over 50 cm of water per hour in coarse permeable sands to as low as 0·02 cm
of water per hour in low-permeability clays.
Infiltration rates of soils can be measured in the field by means of a commercial
infiltrometer or a home-made device. The commercial infiltrometers are often double-
ring, with the ability to maintain standard moist conditions in the outer ring. In home-
made infiltrometers the vessel can be plastic piping or a tin. Three broad techniques are
available. The first is to note the time required for a volume of water, say 250 ml, to
infiltrate completely. A second technique is to construct a scale on the inside of the pipe
or tin, add 250 ml of water to the container, and note the time taken for unit amounts, say
50 ml, to infiltrate. The level is then topped up after each reading. A third set of methods
involves an inverted bottle, with a suitable air intake, so that the level of water in the pipe
or tin is maintained at a constant level. In this case the scale is on the bottle. The latter
two methods are designed to maintain a more or less constant head of water. The rate of
infiltration