Rainfall pattern and distribution. Rainfall pattern & distribution in a region is a good index of its water resources.

A considerable portion of the Indian subcontinent belongs to the subtropical zone. However, the region as a whole shares the characteristics of a tropical climate. Literally, the word 'monsoon' means a wind system which undergoes a seasonal 180 degree reversal of direction. Many regions of the world experience the monsoon. In India, however, two factors make it unique. One is the continuous & high mountain mass in the north which forms an effective barrier to the air movement across them. The second is the peninsular shape of the subcontinent with its land in close proximity to the ocean, thereby providing a rich source of moisture.

The rainfall in the country is primarily orographic, associated with tropical depressions originating in the Bay of Bengal & the Arabian sea. The moisture-laden summer monsoon, accounting for the bulk of rainfall in the country, originates from the vast expanse of the Indian Ocean & enters the Indian subcontinent from the south-west as south-westerly current. The physiographic features of the Indian peninsula & the Western Ghats divert the monsoon into two branches, namely, the Arabian Sea branch & the Bay of Bengal branch. The Arabian Sea branch strikes the Western Ghat & precipitates heavily along the Western Ghat from Kerala to Gujarat between the last week of May & the first week of June. After surmounting the Ghats the southern part of the current blows across the Peninsula as a westerly or in places, as a north-westerly wind.The northern portion of the current which crosses the Saurashtra coast blows across Rajasthan as a south-westerly wind & gives rain mostly in the coastal districts near the Aravalli hills & the Punjab Kumaon Hills, but very little in the plains of Rajasthan.

The Bay of Bengal branch turns north & enters Bangladesh, Assam & West Bengal in May as a southerly current. With the progress of summer, this current moves northwards & by about June it strikes the Himalayas & precipitates heavily. On being deflected by the Himalayas, it turns westwards & enters northern India as an easterly current. This occurs in the whole country, except in the western parts of Rajasthan, coming under the influence of the monsoon by about the first week of July. However, the advance of monsoon is not regular every year & is governed by the movement of depressions & low-pressure waves travelling from the Bay of Bengal across the country. Whereas Assam, the western coastal plains & the Western Ghats receive heavy rainfall almost every year, rainfall in other parts of the country is dependent on the number of depressions in the Bay of Bengal. Therefore, the annual monsoon variation in the central, northern & the north-western parts of the country is much more pronounced than in the regions of abundant rainfall.

The monsoon starts withdrawing from Punjab downwards upto Saurashtra & Gujarat in the first week of September & disappears by the middle of November, except in the extreme south & south-east, where the receding monsoon gives rain as the north east monsoon from the Bay of Bengal.

During winter, the northern part of the country gets some rainfall from western disturbances, but these are irregular & not reliable compared with the south-west monsoon. Also, severe cyclonic storms experienced during the transition months of April to June & October to December cause some precipitation. Some parts of the country receive hot-weather rainfall between March & May mainly owing to large-scale thunderstorms called Norwesters. These rains are of substantial importance in West Bengal & Assam.

The annual average rainfall in India is about 120 cm, i.e. slightly more than the global mean of 99 cm. The more important fact to reckon with , however, is its extreme spatial & temporal variation. The regional variation is so large that whereas the Khasi Hills in the north-east get as average annual rainfall of more than 1,000 cm, the average annual rainfall received in parts of the Rajasthan desert is even less than 15 cm. Moreover, nearly 75 percent of the total rainfall is received in the four month period from June-September. Even in this season, the rainfall in most parts of the country is subject to uncertainty of occurence, marked by prolonged dry spells & aberration in the time of commencement & withdrawal, & also the total amount received. This explains the dependence of agriculture in most parts of the country on artificial irrigation & gives an insight into the nature of the country's water resources.

Evaporation and evapo-transpiration. Evaporation is the process by which water vapour escapes from a free water-surface or moist soil. On land, in addition to water lost from the soil, there is also loss of water from plants; the combined loss is termed 'evapo-transpiration'. Evapo-transpiration is an important parameter in determining the water resources of a region. Loss through evapo-transpiration is controlled mainly by two factors, namely, the ability of the atmosphere to supply energy to vaporize water & transport the vapour & enhance the availability of moisture on the earth's surface. The former is determined by such parameters as the short wave & the long-wave radiation, humidity, air temperature, wind movement, albedo & temperature of the evaporating surfaces. The availability of moisture at the earth's surface is influenced by precipitation & the artificial application of water. Evapo-transpiration is also influenced to some extent by such factors as vegetal cover, type of foliage, stage of plant growth, & soil fertility. Because of all these variable complex factors, the estimation of evapo-transpiration becomes very difficult.

A number of theoretical & empirical methods, however, have been used for computing the evapo-transpiration. These include methods based on the energy-budget theory, mass-transfer theory, water-budget approach & empirical methods using various equations, the prominent among which are:(1) Lowry-Johnson;(2) Thornthwaite;(3) Blaney-Criddle;(4) Blaney-Morin;(5) Penman;(6) Hargreaves; and(7) Christiansen. Whereas some of these equations compute the evapo-transpiration directly, others either determine the potential evapo-transpiration directly, others either determine the potential evapo-transpiration (i.e. the rate of evapo-transpiration, if the supply of water is unlimited), or evaporation equivalent to Class A Pan evaporation, & then use empirical relationships between these factors & the evapo-transpiration. The method commonly adopted is to compute evapo-transpiration from Class A Pan evaporation either actually observed on evapori-meters or empirically determined from equations.

In India, the regional values of actual evapo-transpiration have yet not been computed. However, evaporation data based on a network of about 80 stations equipped with wire-mesh covered standard Class A Pan evapori-meters is now available for about 10-15 years. It has been observed that evaporation is lowest in Assam & the adjoining Himalayas & Bengal (the annual value being 150 cm) & the highest value is 387 cm at Jalgaon & Ahmedabad. Evaporation is lowest (from 2mm to 8mm/day) during January & maximum during May (when it ranges from 4 mm to 20 mm/day). The monthly & annual potential evapo-transpiration values have also been computed by applying the Penman formula for about 300 stations in India. The annual variation ranges from 140 cm to 180 cm over a large portion of the country. Values exceeding 200 cm, occur over Western Rajasthan & parts of Saurashtra & Tamil Nadu. This is generally the highest during May (when it ranges from about 10 cm to 30 cm) over most parts of the country. In Rajasthan, it continues to be high in June also. The minimum is usually in December & January, when it ranges from about 3-15 cm per month.