Geoscience Reference
In-Depth Information
rain, soil volumetric water content at 10 cm in depth decreased continually,
and returned to the value before the rain event about 112 h. However, soil
volumetric water content at other depths changed little during the event,
which indicates no rainwater recharge for the groundwater in this case.
Also, there was no overland flow observed during the event.
Comparison of Figs. 2 and 3, it was found that the soil volumetric water
content at 10 cm in depth was always low in the shiny day, which means that
soil water at top layer loses easily because the strong potential evaporation
in the study area. At the same time, soil volumetric water content at the top
layer responded rainfall well during the rain event. It took about 40-50 min
for rainfall infiltrated from the surface to the soil layer 10 cm in depth.
3.2.
Soil water movement effected by drying/wetting at the
top layers
Table 1 shows the changes soil volumetric water contents with depth dur-
ing the study period. Water in the soil resides within soil pores in close
association with soil particles. The largest pores transport water to fill
small pores and soil particles. After a complete wetting and time is allowed
for the soil to dewater the large pores, red soil in the study area has an half
of the pore space for water and the rest for air. Generally, this condition is
called field capacity ranging from 34% to 41% in the study area (Table 1).
Evapotranspiration, or evaporation if on a bare soil, is mainly driven
during the first stage by atmospheric conditions (radiation, wind speed,
etc.); during this stage the soil is wet and conductive enough to supply
water at a rate commensurate with the evaporative demand. During the
second stage (usually longer than the first one)
3
-
5
the evaporation rate falls
progressively below the potential rate; at this stage the evaporation rate is
limited by the rate at which the drying soil profile can deliver moisture
toward the surface (i.e. by the hydraulic soil characteristics). Therefore,
soil moisture is a crucial link of the air-soil interaction.
Table 1. Changes of soil water contents with depth during the
study period.
Depth (cm)
10
30
60
100
200
Minimum of soil water content (%)
14
24
34
34
34
Maximum of soil water content (%)
73
72
70
68
65
Field capacity (%)
34
35
39
41
40
