Geoscience Reference
In-Depth Information
The state of the soil moisture, as described by the level of saturation
relative to the soil field capacity, is regulated by rainfall and potential evap-
oration. Both of these atmospheric driving forces exert significant control
on the evolution of the soil moisture state and appear explicitly in the
soil water balance equation. On the other hand, the level of soil satura-
tion determines the availability of water as well as the hydraulic properties
of the soil; thus, soil saturation exerts significant control on the rates of
exfiltration and subsequent evaporation.
In the shiny days, the topsoil dried up quickly and formed the dry
surface layer (DSL). Liquid water transport from deeper soil layers stops at
the bottom boundary of the DSL, in turn vapor water transport is dominant
in the DSL. As a result, the effect of soil moisture deficit in the top layer
on the restriction of evaporation from soil surfaces of the study area kept
the soil water changing little at the soil below 30 cm in depth. Yamanaka
and Yonetani
6
showed a conceptual model to simulate the dynamics of the
evaporation zone and those related to the form of water content profile in
dry sandy soils, which can explain the difference in the surface resistance
and soil water content of surface soil relationship between interdiurnal and
diurnal time scales.
On the other hand, drying/wetting process creates cracks in the top soil
that forms the DSL and prevents the soil water loss at the underlaid layer
from evaporation in the study area. When the soil is comparatively dry, the
cracks are empty and do not contribute to the water flow. At moderate rate
of rainwater input to the ground surface, the cracks will rapidly be emptied
by flow through the walls in response to capillary potential gradients to
the surrounding soil matrix. But at high rates of rainwater inflow and/or
small saturated hydraulic conductivity of the soil matrix caused by sealing
during the rain event, saturated or near saturated zones built up around
the cracks, which then remain water-filled and made a major contribution
to the flow. When the rain continued, the cracks disappeared since the soil
particles filled full of the cracks. As a result, overland flow occurred and soil
matrix became the one factor to control infiltration process in the field.
3.3.
Variations of long-term variations of water table
Figure 4 shows the variations of water table and rainfall during 2001-2002
at study area. It was found that the water table depressed continually at the
early of rainy season. The water table began to rise on June 30, 2001 and
August 2, 2002, respectively. In spite of rain patterns were different for these
