Soil Water Flow - Transport in the Atmosphere-Vegetation-Soil Continuum - page 134

Rain
Irrigation

Tr anspiration

Tr ansport of:
soil water
solutes
soil heat

Evaporation

Interception

Runoff

Snow

Top soil
Sub soil
Groundwater level

Local drainage flux

Saturated zone

Aquitard

Second aquifer

Figure 4.1 Overview of the soil hydrological domain and processes as considered
in this chapter.

low mechanisms is required, but also a precise quantitative knowledge of these pro-
cesses.
Figure 4.1 shows the domain and processes that we consider in this chapter. The
top boundary is located just above the vegetation. The depth of the lower boundary
depends on the drainage condition. In the depicted situation with a shallow ground-
water level, the lower boundary is situated below the groundwater level in order to
include local lateral drainage. The three-dimensional groundwater low patterns at
larger depth belong to the science of hydrogeology and are outside the scope of this
topic. In case of deep groundwater levels, the domain lower boundary may be located
at a few metres below soil surface. The moisture conditions at depths larger than 3
m below the root zone do not affect the soil water low near the soil surface. The
advantage of deining the top and lower boundary of the atmosphere-vegetation-soil
continuum in this way is that in this domain the main water low direction is vertical,
which simpliies computation and analysis considerably.
In the region between soil surface and groundwater level, the so-called vadose
zone, the volumetric water content shows large rapid luctuations. This variable water
content has a strong impact on other vadose zone processes such as root water and
nutrient uptake, biochemical transformations and soil temperatures. Therefore we will
consider how we can quantify the water content luctuations. An important aspect to
consider is the soil proile distinct layering with different soil hydraulic properties.
Investigations of soil physical behaviour can be conducted at molecular, pore and
macroscopic scales of observation. At the molecular scale, the molecules are the sys-
tem, and atomic particles like electrons and protons are the system elements. At the