334 W. C. VISSER 



Some Instances 



From field observations concerning the relation between groundwater 

 depth and yield, during a number of years, collected on different soil 

 profiles, a graphical representation was made for advisory use. It was 

 considered necessary to control whether the graphical representation agreed 

 with the growth equations and growth functions as far as this might be 

 expected from the superficial indications available. 

 In the inserted graph in Fig. 7 the curves are given, which represent all 



Yield depression In °/o 

 O 



120 1 50 200 



15 20 30 40 50 60 70 80 90IOO I40 I80 



Mean ground-water level in cm- surface during growth period 



Fig. 7. The yield curves for increasing groundwater depth (see inserted graph) were 

 constructed with all data available in the Netherlands. By plotting against the 

 logarithm of water depth the curves become to a certain extent parallel, pointing to 

 formula i . 5. The horizontal distance is a measure of the influence of water availability 

 not related to grouiadwater depth. 



the information then available on the relation of the yield to the ground- 

 water depth. The convergent left-hand part of the curves seems to indicate 

 that formula 2.4 might be applicable, or formula 1.4 with a logarithmic 

 scale for the groundwater depth. This logarithm furnishes a close parallel 

 with the pF-concept for soil moisture and seems plausible. The logarithmic 

 scale leads to the main curves of Fig. 7. The parallel left-hand ends of the 

 curves suggest the additive solution of formula 1.5, expressing that the soil 

 moisture in the profile and the logarithm of the groundwater depth may 

 be considered as additive parameters. By shifting the curves horizontally, 

 they will coincide as far as they depict the influence on the yield of the 

 growth factor which is described by the combined effect of ground water 

 depth and profile type. 



