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Depth | z | (cm)
0
20
40
calculated
water balance
60
July 25-August 2
August 2-9
July 11-18
July 18-25
80
02468 0
Root extraction rate S (10 -1 d -1 )
0
24
6802468
2468
Figure 4.33 Example of measured variations of root water uptake with depth and
time of red cabbage grown on a clay soil in the presence of a 90-110 cm groundwater
table obtained from water balance studies over 4 consecutive weeks (after Feddes,
1971 ).
provide a irst approximation of the water balance components. The hydraulic head
concept offers a more accurate and more general theoretical framework for soil water
low. The main components of the hydraulic head, both in the unsaturated and the
saturated zone, are the gravitational and pressure head. Darcy's law relates water lux
densities to hydraulic head differences, and applies also to both the unsaturated and
saturated zone. Richards' equation combines Darcy's equation and the water balance
and forms the basis of most physically based hydrological models. Semi-empirical
methods are discussed for iniltration (Horton and Green-Ampt) and capillary rise.
The soil hydraulic functions, which are required to solve the Richards' equation,
relate soil water pressure head, water content and hydraulic conductivity. Analytical
soil hydraulic functions based on Mualem-Van Genuchten, are described. Practical
measurement methods are treated for water contents, pressure heads, hydraulic con-
ductivities, soil water retention function and root water uptake.
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