de Bie, Kees 
  
  
  
Soil texture and the use of NPKs are noticeably missing in the model. Their 
absence is due to the already high amount of variability explained, the relatively 
low number of orchards with yields, and their possible correlation with included 
variables, e.g. the relation between texture, terrain and WHC. 
8. Yield gap and yield constraints 
Before evaluation of the model, interacting variables were pooled to establish their 
combined effect on yields. The combined effects are labeled as ‘location’ and 
‘pruning + use of a motor sprayer’ (Table 4). 
Quantification of effects by variable on yields is based on comparing the ‘best’ 
value that occurred amongst the 45 sites surveyed with the ‘average’ value. For 
instance, if 9 farmers used Azodrin, the ‘average’ value would become 9/45 or 0.2 
while the ‘best’ value remains 1. The constraint specific yield gap is the difference 
between the two values. 
  
  
  
  
Table 4. Quantified break-down of the mango yield gap by yield 
constraint (‘000 Bath/ha; 1993 season) 
Ln(Yield+1) Yield 
ffi measured | m.valuesx | . al ield 
Independents coer'^| values coeff. yle % 17° 
cient gap c gap 
avg. best | avg. best 
Constant|-1.109] 1.000 1 1.11  -1.11 
  
If spraying by motor sprayer AND 
pruning done 
If pruning done AND not sprayed by 
motor sprayer) 
1.139] 0.178 1 020 1.14 
0.523} 0.356 1 0.19. 0.32 
    
    
ombined effect of 'pruning * use 0.39 1.14] 0.75 | 13% 45 
of a motor sprayer’ 
If poor water holding capacity 0.870] 0.289 1 0.25 0.87 
If on footslopes -0.398] 0.444 1 -0.18  -0.40 
If not in hills AND if poor water 
holding capacity -1.845] 0.156 0 -0.29 0.00 
Combined effect of 'location' -0.21 0.87] 1.08 | 18% 65 
Year effect (1=good, 0=avg., -1=bad) 1.165] -0.067 1 -0.08 1.17] 1.24 | 21% 74 
If sprayed with Azodrin 1.322% 0.200-..1 0.26 1.32] 1.06 | 18% 63 
If weeded by tractor MULTIPLIED BY 
canopy cover (%) 
If ability to apply supplementary 
irrigation water 
0.008] 38.44 95 0.31 0.76] 0.45 | 8% 27 
0.777} 0.267 1 0.21 0.78] 0.57 | 10% 34 
  
  
  
  
  
  
  
pH of the top-soil 0.354] 6.000 8 2.12 283] 0.71 | 12% 42 
Ln(Yield+1):] 1.89 5.86 
Estimated yield '000 Bath/ha: 6 351 
Actual yield '000 Bath/ha: 23 250) 227 
Sum: 100% 351 
  
Environmental factors (location and pH) in the model explain 30% of the yield gap, 
management factors 49% and the year effect (species attribute) 21%. The total 
estimated yield gap (best-average) follows from an Ln(Yield+1) value of 5.86, 
  
336 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B7. Amsterdam 2000.