Soil Water Flow - Transport in the Atmosphere-Vegetation-Soil Continuum - page 145

∂ H ∂ h

No flow: = 0, = -1

∂ z ∂ z

Soil surface

0

Infiltration

Capillary rise

Groundwater level

Pressure head h 0

Figure 4.11 Pressure head proiles in case of no low (hydrostatic equilibrium), inil-
tration and capillary rise.

4.3.3 Hydraulic Head of Water Vapour
Water is present in the soil not only in the liquid phase, but also in the gas phase as
water vapour. At static equilibrium, the total heads in both phases are equal. Water
vapour consists of pure water, and thus π = 0. Also it is not inluenced by matrix
forces, and thus h = 0. Therefore its total head is determined by its vapour pressure
e , and by its position in the gravitational ield. We may derive the following relation
between relative vapour pressure e / e sat (also called relative humidity) and the pres-
sure and osmotic head of soil water with which it is in equilibrium (Koorevaar et al.,
1983 ):


 


  =×

e
e

+ ( )

− 7 − 1

ln 75 . 10 cm h π

(4.10)

sat

Question 4.7: In the soil plant roots may extract water to about h = -16 000 cm, also
denoted as wilting point. Which relative humidity in the air-illed pores is in equilib-
rium with this pressure head? Which soil water pressure head corresponds to a relative
air humidity of 80%?

4.4 The Soil Water Characteristic
A soil water characteristic or retention curve relates volumetric water content to soil
water pressure head. Figure 4.12 shows a retention curve with some typical derived
data. Under unsaturated ield conditions the soil water pressure head may range over
six orders of magnitude: -10 6 < h < 0 cm. Because of this large range the pressure
head is often depicted on a logarithmic scale, for instance as pF (= log - h cm). Impor-
tant water contents correspond to ield capacity θ fc (1.7 < pF < 2.3) and wilting point
θ wp ( pF = 4.2). If at the start of a growing season a soil is at ield capacity, the water