Integrated Models in Hydrology and Meteorology - Transport in the Atmosphere-Vegetation-Soil Continuum - page 312

When the groundwater level is relatively deep, we may assume a zero gradient of
the soil water pressure head at the bottom of the soil proile, so-called free drainage .
Application of Darcy's law gives for such a case:

∂
∂ +

h
z


 



qkh =− () 1

  =−

kh ()( 0 +=− 1 ) kh () (9.6)

In the case of lysimeter experiments where free outlow occurs at the lysimeter bot-
tom, SWAP will assume zero low as long as h ≤ 0 at the lysimeter bottom. As soon
as h tends to become larger than zero, SWAP will ix h at zero and calculate the bot-
tom lux.

Question 9.4: Imagine two columns of identical soils. Column 1 is taken to the lab-
oratory for leaching experiments, and free outlow applies at the bottom. Column 2 is
still undisturbed in the ield, where the groundwater level is so deep that free drainage
conditions apply at the bottom of the column. We irrigate both soil columns with the
same amount of water. At column 1, drainage starts 5 hours after water application.
Six hours after water application, which column contains more water, column 1 or
column 2?

9.1.5 Lateral Drainage
In the saturated part of the soil column, SWAP simulates lateral drainage and bot-
tom luxes separately. Drainage luxes refer to groundwater low to or from the local
drainage system. Bottom luxes refer to water luxes at the soil proile bottom, which
in general are governed by regional groundwater low and less by local water man-
agement ( Figure 9.6 ). In many soil water low models, bottom luxes include the
drainage luxes. SWAP can be used in the same way, by omitting the drainage compo-
nent. The feature of deining the lateral drainage lux separately allows the evaluation
of surface water management and drainage design alternatives.
SWAP offers three methods to calculate the drainage lux density q drain (m d -1 ):

1. A linear relation between groundwater level φ gwl (m) and q drain :

φφ
γ

gwl drain

(9.7)

q drain =

drain

where φ drain is the drain level (m) and γ drain is the drainage resistance (d). Simultaneous
drainage luxes to various drainage levels can be calculated, which are superimposed to
derive the total drainage lux. This is depicted in Figure 9.7 . Note that in the igure the
assumption is made that water level φ drain,1 is maintained in dry periods, resulting in inil-
tration. At the higher drainage levels, no water iniltration is assumed.