SWAP offers eight options to prescribe the lower boundary condition. These conditions are given in this chapte

Dam, J.C., van

Kroes, J.G.

Wageningen Yield 

Alterra-report 773 85 
5 Soil water – groundwater interaction 
J.G. Kroes, J.C. van Dam 
5.1 Introduction 
In the unsaturated zone water flow and solute transport occur mainly in the vertical 
direction. Once in the saturated zone, water starts to move in a three dimensional pattern, 
following the prevailing pressure gradients. The bottom boundary of the one-dimensional 
SWAP is either in the unsaturated zone or in the upper part of the saturated zone where the 
transition takes place to three-dimensional groundwater flow.  
 
At the lower boundary we can define three types of conditions: 
- Dirichlet condition, the pressure head h is specified; 
- Neumann condition, the flux q is specified; 
- Cauchy condition, the flux depends on the groundwater level. 
 
The Dirichlet condition is a predescribed pressure head, often as a recorded phreatic surface 
of a present groundwater table.  
The Neumann condition is usually applied when a no-flow boundary (e.g. an impermeable 
layer) can be identified, or in case of a deep groundwater table, resulting in free drainage.  
The Cauchy condition is used when unsaturated flow models are combined with models for 
regional groundwater flow or when effects of surface water management are to be 
simulated. The relation between flux and groundwater level can be obtained from drainage 
formulae (see Par. 4.2.2) and/or from regional groundwater flow models (e.g. Van Bakel, 
1986). 
 
SWAP offers eight options to prescribe the lower boundary condition (Table 3), which each 
have their typical scale of application. 
 
Table 3. Eight options for the lower boundary condition 
Lower boundary 
condition (input 
switch SwBotb) 
Description Type of 
condition 
Typical scale of 
application 
1 Prescribe groundwater level Dirichlet field 
2 Prescribe bottom flux  ( botq ) Neumann region 
3 Calculate bottom flux from 
hydraulic head of deep aquifer 
Cauchy region 
4 Calculate bottom flux as function 
of groundwater level 
Cauchy region 
5 Prescribe soil water pressure 
head of bottom compartment 
Cauchy field 
6 Bottom flux equals zero Neumann field 
7 Free drainage of soil profile Neumann field 
8 Free outflow at soil-air interface Neumann field 
 
86 Alterra-report 773 
In case of options 1, 2, 3, 5 and 6, in addition to the bottom flux ( botq ), a drainage flux  
( drainq ) can be defined (Par. 4.2). In case of option 4 the lower boundary includes drainage 
to local ditches or drains so drainq  should not be defined separately. In case of options 7 and 
8, the simulated soil profile is unsaturated, so lateral drainage will not occur. 
5.2 Field scale 
When the model is applied at field scale with locally known/measured data, the following 5 
options are commonly applied: 
- Prescribe groundwater level      (SwBotB = 1) 
- Prescribe soil water pressure head of bottom compartment  (SwBotB = 5) 
- Bottom flux equals zero      (SwBotB = 6) 
- Free drainage of soil profile     (SwBotB = 7) 
- Free outflow at soil-air interface      (SwBotB = 8) 
 
Prescribed water levels ( avg ) are given as a function of time. This groundwater level 
respresents a field average groundwater level. For days with unknown values a linear 
interpolation occurs between the days with known values. The main advantage of this 
boundary condition is the easy recording of the phreatic surface in case of a present 
groundwater table. A drawback is that at shallow groundwater tables the simulated phreatic 
surface fluctuations are very sensitive to the soil hydraulic functions. This condition may 
result in strong fluctuations of the water fluxes across the lower boundary, which may not 
be desirable. Especially when the output of the Swap model is used as input in water quality 
calculations, it is is generally recommended to use another type of lower boundary 
condition.  
 
 
Prescribed soil water pressure heads of bottom compartment (hn) are input to the model 
and. The soil water pressure head is assigned to the lowest compartment. For days with 
unknown values a linear interpolation occurs between the days with known values. 
 
 
A bottom flux ( botq ) of zero may be applied when an impervious layer exists at the bottom 
of the profile. This option is implemented with a simple switch, which forces qbot to zero. 
 
 
Model input 
Variable Code Description Default 
avg  GWLEVEL Groundwater level as function of time (cm below soil surface) - 
Model input 
Variable Code Description Default 
hn HOBTS Soil water pressure head of bottom compartment as function of time (cm)- 
Alterra-report 773 87 
In case of free drainage of a soil profile, unit gradient is assumed at the bottom boundary 
and the bottom flux depends directly from the hydraulic conductivity of the lowest 
compartment: 
bot1 thus: n
H q K
z

  

 (5.1) 
In case of free outflow at soil-air interface, drainage will only occur if the pressure head in 
the bottom compartment (hn) increases until above zero. During drainage and after a 
drainage event, hn is set equal to zero and botq  is calculated by solving the Richards' 
equation. This option is commonly applied for lysimeters, where outflow only occurs when 
the lowest part of the lysimeter becomes saturated. 
5.3 Regional scale 
At regional scale the lower condition will generally be used describe the interaction with a 
regional groundwater system. In these cases 3 options are common: 
- Prescribe bottom flux       (SwBotB = 2) 
- Calculate bottom flux from hydraulic head of deep aquifer  (SwBotB = 3) 
- Calculate bottom flux as function of groundwater level   (SwBotB = 4) 
 
Prescribed bottom flux 
In this case the bottom flux ( botq ) is input to the model and should be given as a function of 
time. For days with unknown values a linear interpolation occurs between the days with 
known values. This option has a similar disadvantage as previously described option with 
the prescribed groundwater level. When a mismatch occurs between boundary conditions 
and soil physical properties the result may be a continuously declining or increasing 
groundwater level. Especially when the output of the Swap model is used as input in water 
quality calculations, it is is generally recommended to use another type of lower boundary 
condition. 
 
 
Calculate bottom flux from hydraulic head of deep aquifer 
This Par. discusses how a Cauchy condition may be applied to determine the bottom 
boundary flux botq , starting from a given hydraulic head of a deep aquifer. 
To illustrate this option Figure 26 shows a soil profile which is drained by ditches and 
which receives a seepage flux from a semi-confined aquifer. SWAP makes a distinction 
Model input 
Variable Code Description Default 
 SW2 Switch for kind of input: as sinus or as table 
 
When SW2=1: 
 SINAVE Average value of bottom flux  (cm d-1) - 
 SINAMP Amplitude of bottom flux  (cm d-1) - 
 SINMAX Time of the year with maximum value of bottom flux  (daynr from Jan 1)- 
When SW2=2 then enter a table: 
botq   QBOT2 Average value of bottom flux  (cm d
-1) - 
88 Alterra-report 773 
between the local drainage flux to ditches and drains qdrain, as calculated according to 
chapter 4, and the bottom flux due to regional groundwater flow, botq . 
 
Figure 26 Pseudo two-dimensional Cauchy lower boundary conditions, in case of drainage to 
ditches and seepage from a deep aquifer 
 
The bottom flux botq  depends on the average groundwater level, the hydraulic head in the 
semi-confined aquifer, and the resistance of the semi-confining layer. The bottom flux botq  
is calculated by: 
 
aquif avg
bot
conf
q
c
 

 (5.2) 
where aquif  is the hydraulic head in the semi-confined aquifer (cm), avg  is the average 
groundwater level, and confc  is the semi-confining layer resistance (d).  
The hydraulic head in the aquifer may be prescribed using a sinusoidal wave: 
 
 aquif aquif,m aquif,a max
aquif,p
2cos t t  

 
    
 	  (5.3) 
where aquif,m , aquif,a , and aquif,p  are the mean (cm), amplitude (cm) and period (d) of the 
hydraulic head sinus wave in the semi-confined aquifer, and maxt  is a time (d) at which aquif 
reaches its maximum. 
 
The average phreatic head, avg  (cm), is calculated as: 
 
 avg drain gwl gwl drain        (5.4) 
with drain  is the hydraulic head of the drain (cm) and gwl  the groundwater shape factor (-). 
Possible values for gwl  are 0.66 (parabolic), 0.64 (sinusoidal), 0.79 (elliptic) and 1.00 (no 
drains).  
 
Alterra-report 773 89 
 
Calculate bottom flux as function of groundwater level 
Calculate botq  from an exponential flux - average groundwater relationship, which is valid 
for deep sandy areas:  
 qbot avgbot qbot e
bq a   (5.5) 
where qbota  (cm d
-1) and qbotb  (cm
-1) are empirical coefficients. For additional data of botq  - 
avg  relationships, see Massop and De Wit (1994). 
 
 
 
 
 
Model input 
Variable Code Description Default 
gwl  SHAPE Shape factor to derive average groundwater level (-) 1.0 
drain  HDRAIN Mean drain base to correct for average groundwater level (cm) - 
confc  RIMLAY Vertical resistance of aquitard  (d) - 
 
- SW3 Switch for kind of input: as sinus or as table - 
When SW3=1 then enter a sinus wave: 
aquif,m  AQAVE Average value of hydraulic head in underlaying aquifer (cm) - 
aquif,a  AQAMP Amplitude of hydraulic head sinus wave (cm) - 
maxt  AQTMAX First time of the year with maximum hydraulic head (daynr from Jan 1) - 
aquif,p  AQPER Period hydraulic head sinus wave (d) - 
 
When SW3=2 then enter a table: 
aquif   HAQUIF Average value of hydraulic head in underlaying aquifer (cm) - 
Model input 
Variable Code Description Default 
qbota  COFQHA Coefficient A (cm d
-1) - 
qbotb  COFQHB Coefficient B (cm
-1) - 


Soil water - groundwater interaction

Abstract

Similar works

Full text

Available Versions

Wageningen Yield

Name not available