The maximum allowable population in the case of the greatest water consumption (U = 20%, D = 10,000kcal, and L = 40%) is, therefore, 4.9 billion, 1,7 billion or 1,1 billion depending on the value of Wa. Needless to say, the actual world population is far above any of these figures. This implies that it is almost impossible, in view of the water resources, for all the countries in the world to enjoy as luxurious dietary lives as the US of today. On the other hand, in the condition of the least water consumption (U = 100%, D = 2,350kcal and L = 20%), the Pmax value is 137.5 billion, 47 billion or 30.2 billion depending on the value of Wa. However, it is impossible to use 100% of utilizable fresh water, and these figures are so unrealistic that the [realistic] maximum allowable world population would, of course, be much below them.
In the standard condition which can be considered relatively close to the current situation of the world's water consumption (i.e., U = 20%, D = 2,350kcal, and L = 40%), the Pmax value is 20.5 billion, 7 billion or 4.7 billion. As far as the condition of water resources is concerned, the maximum allowable level of the world population is not specially high, which implies that the world as a whole may face tight constraints in the 21st century, depending on efficiency in use of water resources.
(2) Maximum Allowable Populations of Major Regions in the "standard condition" and the "75% efficiency-improvement condition"
Column (2) of Table 3 shows the maximum allowable populations of the world major regions, obtained by the above Cohen equation where U = 20%, D = 2,350kcal, L = 40% and Wa = an utilizable amount of replaceable fresh water per annum (41,000 km3 if for the world as a whole).
If we compare the maximum allowable population in regions in the "standard condition" with the present and future populations (Column (1) of Table 4), the 1995 populations in all the regions are below the maximum allowable level.
