Of the two world's populous nations, India will likely fall in the condition of "water stress" in the first quarter of the 21st century, and China also by 2050. By 2050, there will likely be 15 "water stressed" countries (2.3 billion people), and 39 "water shortage" countries (1.7 billion people), accounting for 24% and 28% of the world population respectively(10).
3. Maximum Allowable Population in view of
Water Resources
(1) Cohen's Maximum Allowable World Population (in view of water resources)
Cohen calculated the number of human beings sustainable by the utilizable replaceable fresh water per annum on the Earth, on the assumption that the use of water is restricted to minimum daily-life requirements such as drinking and irrigation for production of grains(11). His computation is as follows.
First, 2kgs of wheat is required to produce 1kg of flour. Assuming the transpiration rate is 500%, 1,000kgs of water is needed (= 2kgs x 500 = 1m3 = 1 ton). One kilogram of flour generates a energy value of 3,500 Kcal. If a person consumes 1,000kcal a day, the amount of fresh water to produce wheat that can supply the required energy for a year is, therefore, 100 ton.
The amount of fresh water needed for a person to take in the necessary amount of energy for a year from grains produced by irrigation agriculture (Wr m3) can be obtained by the following equation.
where,
D = Energy (kcal) required by a person for his activity per day
10 = The coefficient to denote that 100 ton of water generates 1,000kcal of energy per capita per day.
U = A consumption rate of utilizable replaceable fresh water per annum (%)
L = A rate of grain lost in the processes after production and before consumption
