GROWTH AND WATER USE OF VEGETABLES 



253 



Dry weight »/. 55cm 

 45cm 



15 



10 



o 



.fo ^^^^A^y' 



± 



_L 



_L 



_L 



10 15 



Fresh weight / mm v/ateruse 



Fig. 5. Relation between dry-weight percentage and the ratio fresh- weight/total water 

 use. The curves are valid for the indicated depths of the ground-water table. For key 

 see Fig. 3. 



in direct evaporation from the soil. From the relation y = mx between 

 dry-weight yield (y) and total water use (x), the tangent is 



}ii= o-oo3o62r +0-594 

 + 0-00099 ± 0-039 



where z is the depth of the ground-water level. 



Some remarks might be made when comparing yield and water use in 

 the greenhouse with that under field conditions. From a physical point of 

 view, evaporation has been studied in many ways on a basis of energy 

 balance, in which net-radiation equals sensible and latent heat. Because 



