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if drainage status is 1,2 or 3 and moisture supply 
capacity is 1 or 2 then the grazing suitability is 
1 
if drainage status is 4 and bearing capacity is 2 
and moisture supply capacity is 1 or 2 then the 
grazing suitability is 2 
if drainage status is 5 and bearing capacity is 2 
and moisture supply capacity is 3 then the grazing 
suitability is 3 
if moisture supply capacity is 4 or 5 then the 
grazing suitability is 3 
etc. 
4.2 Soil Drainage Status 
  
Drainage status is linked to the height of the 
water table, and more particularly its Mean 
Highest Vater Level (or GHG value), as follows: 
  
Drainage Status Level|GHG cm below land surface 
  
>80 
40-80 
25-40 
15-25 
<15 
Ui Æ OU NH 
  
  
  
  
Following field testing [MARSMAN and DE GRUIJTER, 
1986] it was found that the standard deviation of 
the GHG is 14cm. Using estimation by confidence 
intervals the probability of a land parcel vith a 
certain measured GHG value being in a specified 
Drainage Status Level can be calculated (see 
[DRUMMOND and RAMLAL, 1992]). For example vith a 
GHG value of 60cm, the probability of the parcel 
being in Drainage Status Level 2 is 85%. 
4.3 Soil Bearing Capacity 
  
Bearing capacity (3 classes) is related to 
Soiltype (5 classes) and GHG, as follows: 
  
  
Soiltype 1. 2 3 4 5 
GHG(cm) 
0-12 3 3 3 3 3 
13-24 3 3 3 3 2 
25-33 3.22 372 
34-40 2 1 3 2 1 
41-60 2 2° 2 2 |! 
61-80 1172 2-2 
80-140 1 11 2 1 
  
  
  
Thus, eg, Soiltype 3 with a water table 41-60 cm 
below the surface has a Bearing Capacity Class of 
2. 
Soiltype is related to Soiltexture (the organic 
and clay content of the soil) as follows: 
  
  
Soiltype Organic Clay 
content content 
1. Peat 15-100% 0-8% 
2. Clay with peat underlay | 22- 70% 8-100% 
3. Clay 0- 15% 25-100% 
4. Clayey sand 0- 2.5% 8-25% 
5. Sand 0- 2.5% 0-8% 
  
  
  
  
  
  
Uncertainty Sub-System 
Top-level DFD 
      
Spatial Processing 
  
  
  
  
   
  
  
  
Data Model 
Storing/Determining Storing 
quality of Data quality of the 
A model B 
  
  
  
  
  
  
      
   
suitable 
  
  
  
d 
Determining of Visualization of 
Final Information Quality 
Quality © A Information p 
m 
  
  
  
  
  
  
  
  
  
(7 
(Quali t] S 
Quality repo oot 
Figure 8 - An Overview of the ILWIS Uncertainty 
Subsystem 
  
As bearing capacity is determined from GHG, 
organic content, and clay content, the qualities 
of all three need to be known. Tests have shown 
that the probability of these particular organic 
content and clay content classes being correct is 
98% [MARSMAN and DE GRUIJTER, 1986]. The quality of 
GHG data was discussed in the previous section, 
and an example landparcel was shown to have a 
probability of 85% that it vas in its stated 
Drainage Status Level (or GHG level). Taking the 
same example landparcel, the probability of its 
Bearing Capacity Class (Pbc) being correct is: 
Pbc 20.85 « 0.98 x 0.98 = 0.82 = 82% 
4.4 Soil Moisture Supply Capacity 
  
Moisture supply capacity is recorded in 
millimeters and is calculated using a polynomial 
of twenty coefficients and three variables 
(rooting depth, mean lowest water-table depth, and 
mean spring water-table depth) [RAMLAL, 1991]. In 
this application it is reclassified into 5 
discrete classes: 
  
Moisture Supply Moisture Supply 
Capacity Class Capacity (mm) 
  
>200 
150-200 
100-150 
50-100 
<50 
Un £0) th = 
  
  
  
  
Following error propagations carried out by the 
Dutch Soil Research Institute  [MARSMAN and DE