Central Waterpower, Irrigation and Navigation Commission, Government of India.
This paper deals.with the determination of runoff from rainfall. The forecast of water yield is based on the rational concept that run-off is the residual of rainfall after deduction of evaporation and transpiration “ loss “. An analysis of data shows that this “ loss “ during any period is a function of the mean temperature during that period, but limited to the amount of rainfall, if the latter is less than the “ loss “ so calculated.
The author offers, what he believes to be, a universal relationship (subject to certain limitations) between mean monthly temperature (Tm). Fah. and monthly evaporation loss (Lm) inches, viz.,
For Tm <40°F., the loss (Lm) rnay provisionally be assumed as below :—
Tm 40° 30° 20° 10° 0° Fah. Lm 0.84 0.70 0.60 0.50 0.40 inches; The corresponding monthly rainfall (Pm) runoff (Rm) relationship will be :—
Rm = P — Lm
The formula for annual rainfall (PA) and annual run-off (RA) with mean annual temperature (TA) will be
RA = PA - XTA
where X is a constant for a given catchment.
On the basis of the author’s formula the annual water resources of India work out to nearly 1,307 million acre feet or a perennial discharge of 1.8 million cubic feet per second.
