SOME CHARACTERISTICS AND FREQUENCIES OF OCCURRENCE OF DROUGHTS IN THE DRY CLIMATIC ZONES OF INDIA

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Abstract
The paper describes some results of a climatic study of the characteristics and decennial frequencies of droughts at a few selected stations in the three dry climatic zones—arid, semi-arid and dry sub-humid—of India. The classification of the climates was done according to the 1955 water balance scheme of Thornthwaite, after a careful mapping of the moisture indices for all the climatological observatories in India. Four stations in the arid, six from the semi-arid and from the dry sub-humid zones were selected for the analytical study of droughts. The aridity index of Thornthwaite, viz., ratio of annual water deficiency to the total annual water need, expressed as a percentage, was found to be a useful parameter for the purpose. The values of this parameter were computed for each year of a 65-year period of record for all the selected stations, using the well-known book-keeping procedure of Thornthwaite. From the yearly trends of the index it could be ascertained whether a station was becoming drier, less dry or remaining steady. Taking the amplitude of departure of the index from the normal as a measure of severity of a drought situation, an arbitrary yet practical scheme employing the standard deviation, σ, was adopted for the classification of droughts of different degrees of severity, viz., moderate (< ½ σ), large (½σ to σ), severe (σ to 2 σ) and disastrous (> 2σ). Decennial frequencies (number of drought-years in successive decade intervals) of the above four categories of droughts were worked out for each of the stations and illustrated by means of block-diagrams. By plotting the moisture indices rather than aridity indices against successive years, the climatic shifts at individual stations could also be studied. Considerable shifts in moisture regime, say from arid to first humid at Delhi (normally semi-arid) and from almost arid to second humid at Cuddalore (normally dry sub-humid) were observed signifying that at any station large fluctuations in yearly water balances are quite a common occurrence. Such a study in the generally humid climates may provide important clues to the origin and nature of the so-called contingent droughts and thus enable a rational planning of agricultural and hydrologic projects.

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