Evaluation of Effective Rainfall and Irrigation from Ground-water Measurements

Abstract

A FRACTION of the total rainfall that reaches the ground water-table is called the effective rainfall¹. Where there is irrigation, a fraction of the irrigation water also has been found to contribute to the rise of the water-table². Effective values of rainfall and irrigation are usually determined using lysimeters³. The lysimeters, however, need correction for their size and the indications obtained from them are often high. A general figure found at Amsterdam dune water works, where there was no run-off, indicated that 61 per cent of a total rainfall of 659 mm. per annum was effective¹. An actual determination of the rise in water-table over large areas has also been carried out by means of open well water measurements^4,5. The effective values of rainfall and irrigation, however, do not seem to have been arrived at. An analysis of the changes in the ground-water level has therefore been taken up in this investigation, and the effective values of rainfall and irrigation were evaluated from the (fortnightly) measurements of the water-levels recorded for the past 20 yr. in thirteen wells randomly distributed over 565 acres of irrigated farm area of the Indian Agricultural Research Institute, New Delhi.