Evaporation from the Soil Surface

9.4 Evaporation from the Soil Surface

Water is lost from the root zone of the soil profile in three ways. It can percolate below the root zone, it can evaporate from the soil surface, and it

can be taken up by plants. The redistribution calculations just discussed can be used to

the percolation. Evaporation and transpiration still need to be discussed.

0 1 2 3 4 5 Time (days)

F I GURE 9.4. Water content vs. time at the 5 cm depth in Fig. 9.3.

Evaporation the Soil Surface

Time (days)

F I G U RE 9.5. Redistribution of soil water. Data from Fig. 9.4 plotted on a log-log scale and extended in time.

Figure 9.6 shows the course of evaporation rate over time for three soil drying experiments. Two stages of

can be identified, a steady constant rate stage and a falling rate stage. The transition between the two occurs when the soil surface becomes dry. The evaporation rate during the first stage is determined by the evaporative demand of the atmosphere. If the demand is high, this stage is short. The lower the evaporative demand, the longer this stage lasts. Coarse textured soils which store little water near the surface have short first stage drying periods. The sand in Fig. 9.6 stores so little water that first stage drying is almost absent.

At the onset of second stage drying, the soil limits the rate of supply to the soil surface. The rate of drying could be

by calculating the vapor conductance of the dry layer and the vapor pressure difference across it, but the rate is really determined by the ability of the soil to conduct water to the evaporating surface. The form of the solution is similar, again, to the heat flow equation. From the onset of second stage drying the evaporation rate decreases linearly with the inverse of the square root of time, so the cumulative soil surface evaporation during second stage drying (the integral of the rate over time) is proportional to the square root of time:

where is the time (days) that the first stage drying ends, and C is a constant that depends on soil type. Table 9.2 contains rough estimates of

C for several soil textures and also includes approximate values of total cumulative soil surface evaporation for both first and second stage drying.

Water Flow in Soil

9.6. Evaporation rate for loam at high and low evaporative demand and for sand at high evaporative demand.

F IGURE

Figure 9.7 shows cumulative evaporation versus time for the soils in Fig. 9.6. Interestingly, evaporative demand seems to have little effect on total water evaporated. Early on the high demand soil gets ahead, but the low demand soil stays in first stage evaporation longer and eventually almost catches up. The sand loses much less water than the loam. This is because the surface dries quickly and the coarse material has such a low

hydraulic conductivity that it is not able to conduct water to the surface. The ultimate in water conservation is attained with a fine gravel surface,

which has almost no storage and very low unsaturated conductivity, but transmits rain downward very readily. The pebble pavement sometimes

seen in deserts, where the fine material has been blown away by the wind

T A BLE 9.2. Approximate characteristics of soil surface evaporation for several soil textures.

Soil Texture Cumulative stage C Cumulative total evaporation

Clay loam 12 5.1 30 Loam

9 4 25 Clay

6 3.5 20 Sand

Transpiration and Plant Water Uptake 139

Time (hrs)

F IG U RE

9.7. Cumulative evaporation for the soils in Fig. 9.6.

leaving only coarse material, is a good natural example of a high efficiency storage system.