Wadleigh and Richards 417 



entirely gravity force, i.e., "one g," and corresponds to unit hydraulic 

 gradient. Permeability tests on the rate of movement of water in soil 

 cores or soil samples in the laboratory are often conducted at unit 

 hydraulic gradient. When a thin covering layer of water is maintained 

 on the top of the soil column and the water outflow at the bottom of 

 the soil column is at atmospheric pressure, there is no pressure gradient 

 in the soil column and gravity is the only force acting. The velocity of 

 water in soil at unit hydraulic gradient is designated as permeability 

 and is a measure of the readiness with which a soil transmits water. 



Darcy's law for the movement of water in soils simply states that 

 the velocity of water movement (v) is proportional to the hydraulic 

 gradient (/) as expressed by the equation v = Pi. The proportionality 

 constant, P, is the permeability. 



The hydraulic head at any point in a soil moisture system is equal 

 to the elevation at which water will stand in a riser or piezometer tube 

 connected to the point in question (101). If the soil is not saturated, 

 a porous cup can be used for establishing connection between water in 

 a manometer and water in the soil, as shown in Figure 2. In practice, 

 mercury manometers are used so as to allow measurements to be made 

 above ground, but actually, water manometers such as those illustrated 

 at the right in the figure could be used. When the porous cup is in 

 unsaturated soil, the free surface of the water in the manometer will 

 come to rest at a level below the porous cup. The difference in level 

 between the cup and the free water surface is a direct measure of the 

 soil moisture tension at the porous cup, whereas the elevation of the 

 free surface referred to any convenient datum is the hydraulic head of 

 the soil water at the cup. The average hydraulic gradient between two 

 points on a flow line, i.e., the soil moisture tension gradient plus the 

 component of gravity force, is equal to the difference in hydraulic head 

 between the points divided by the distance along the flow line. These 

 relations can be illustrated by the tensiometer system shown in Figure 2. 



Let it be assumed that the manometer readings represent moisture 

 conditions a week after a rain on a fallow soil and, for simplicity, let 

 it further be assumed that moisture conditions do not change in the 

 horizontal direction. The moisture tension at the depth of a porous 

 cup, when expressed in terms of a length of water column, is equal to 



