Redistribution of Water in Soil

9.3 Redistribution of Water in Soil

When infiltration ceases, water continues to move down into the soil under the influence of matric and gravitational forces. Infiltration was stopped with the final profile shown in Fig. 9.2. The redistribution profiles at four

Water Flow in Soil

Water Content

F IG U RE

9.3. Redistribution of water in soil following infiltration.

times following are plotted in Fig. 9.3. The water content of the wetted zone decreases rapidly at first, but the rate of decrease becomes smaller with time, and eventually there is little change in water content, even over fairly long times. As the water content of the soil decreases, the hydraulic conductivity decreases, so the rate of movement of water upper to lower layers decreases. This decrease in hydraulic conductivity with drying allows the soil to store water and gives rise to field capacity.

Darcy's law and some of the characteristics of the redistribution pro- cess can be used to get a better understanding of field capacity. Note that the water content in the wetted part of the profile in Fig. 9.3 is almost constant with depth, implying that the matric potential is also almost

constant. To do a simple (but rough) analysis of redistribution, the induced flow can be neglected. Then just the gravitational part is left. Since neither nor K are ever zero (except when the soil is completely

dry), there will always be some flow out of the wetted zone. We choose

a value E for the flow out of the wetted portion of the profile, such that the flow can be considered negligible when compared to water inputs or

other water losses. Using Eqs. (9.1) and

the field capacity water potential corre- sponding to a drainage rate of in units of

where = 2 + and

Substituting the

Redistribution of Water in Soil

constant for the product and solving for gives:

One way to obtain a value for to set it equal to, say, ten percent of evapotranspiration (ET). If ET is 7

then would be 0.7 or 8.1 x

kg Campbell (1985) gives = Putting these values into Eq. (9.11) and assuming n =

2.5 gives a field capacity water potential of - 17 Field capacity determined more empirically is between - 10 and -33

If had been set to one percent of ET, then Eq. (9.1 1) would give -43

for

Example 9.1. Use the parameters in Table 9.1 for a silt loam soil to estimate the water content at 10 and -33

matric potential. Assume m 3 /m 3 .

Solution. Solve Eq. (9.2) for 6:

Using this equation, the water contents are

0.5 = ( 0.278- and

-2.1 J k g

m3

The water content at -33 J k g that is listed in Table 9.1 is 0.33 m 3 /m 3 . The difference between that value and the one computed here comes the way the values in Table 9.1 were obtained. The values in the table are averages of many samples for that texture class. Because of the nonlinear nature of Eq.

a calculation done using averages of the parameters normally would not be equal to the average of the measured water contents. Another source of uncertainty is the value of 6, for the soils in Table 9.1.

the water content

A number of factors influence field capacity in addition to the ones brought out in this simple analysis. It is actually the hydraulic conductiv- ity function of the soil profile that determines redistribution rates, not the conductivity of a particular location in the profile. If there is layering (which there usually is), field capacity will increase. In addition,

some texture and density effects appear to cancel in the simple analysis, but may influence field capacity of real profiles. Finally, the matric po-

tential gradient is almost never negligible compared to the gravitational gradient, so there are always matric effects on field capacity. In spite of these limitations, however, several key points are apparent from the

analysis. First, we see that field capacity is determined by the ability of the soil to transmit water. Some people suppose that the soil holds water

Water Flow in Soil

because of the attraction of the matrix for the water. We were able to de- termine a matric potential at which drainage was considered negligible, but this came from the dependence of conductivity on matric potential, not from the attraction of the matrix for the water. The second point is that

a sealed, soil column will continue to drain until it reaches zero water content. There is no point at which drainage ceases. We can therefore think of the soil as a leaky bucket. The size of the leak, however, decreases as the bucket empties.

Some of this is illustrated in Fig. 9.4. The figure shows the water content at a 5 cm depth for the soil in Fig. 9.3. Initially the water content decreases rapidly, but after two to three days the rate of decrease slows.

fikld capacity is defined as the water content initially wetted soil two to three days after a heavy rain or irrigation when there is no evaporation or transpiration. If Fig. 9.3 were extended to weeks, months, years, or even hundreds of years, water content would continue to decrease. Figure 9.5 extends the graph to about 3 years, and shows that

a log-log plot of water content versus time is a straight line.