Evapo-transpiration and water balance. Apart from severe drought leading to crop failures,dry spells of varying lengths occur in the monsoon season.They cause water stress in plants, and the growth & yields are adversely affected, if such stress periods occur during the critical phases of the growth of the crops. The extent to which the water needs of crops are met in a locality depends on the rainfall input,the losses due to run off & the evapo-transpiration-evaporation from the soil surface & transpiration by the plants. It is important in water-balance studies to know the extent of potential demands on water through evapo-transpiration.The evaporation from a free water surface & the potential evapo-transpiration depend upon the radiation-incoming as well as outgoing-and the evaporative power of the atmosphere,which again is largely dependent on the saturation deficit & the air movement.In the case of potential evapo-transpiration, the extra roughness of the vegetation cover in relation to water surface is also considered.The areas of high annual potential evapo-transpiration are the extreme west of Rajasthan(Jaisalmer) and the extreme south of Tamil Nadu (Tuticorin).

In large parts of the country where rainfall is meagre, a better exploitation of irrigation potential through major & minor projects, the intensive use of ground-water resources & a more efficient conservation & management of water are necessary to stabilise agricultural yields. The growing of drought-resistant varieties in low-rainfall areas, the methods of soil management & cropping that reduce run-off & increase the storage of soil moisture, such as strip-cropping,contour-bunding,mulches & wind breaks, are some of the ameliorative measures that can profitably be adopted.

Temperature. The air temperature is a decisive factor in plant growth, especially in the subtropics.The growing season in these latitudes, where frost conditions set a limit,is defined on the basis of air temperature. In India, high temperatures are also of special importance, as they may have damaging effects on crops during the periods of water stress in the arid or semi-arid regions.Each crop is known to have its own optimum,maximum & minimum temperature conditions.Air temperatures bear a close relationship with the incoming solar radiation & the energy available for the growth of plants.

The maximum temperatures in the country are high in March,April & May over northern Deccan & central parts of India.Later in June & July, the highest values shift northwards & north-westwards.In July & August ,Tamil Nadu also experiences high maximum temperatures.

Cold waves and frosts. The cold season begins over India by early December.During the winter months, under the influence of western disturbances, which are followed by cold dry northernly winds the night temperatures fall rapidly by 7-8 degree centigrade below the normal & sometimes more in regions north of the Deccan Plateau & by about 5 degree centigrade in the peninsular parts of the country.The ground temperatures are usually much lower than air temperatures.The crops may be subject to sub-zero temperatures & damage due to chilling or ground frost is likely to take place.The frost hazard is greatest in northern Punjab,being 10-20 days each in December.January & February. Southwards & eastwards of this area, frost occurence decreases rapidly.

Cold waves & frost warnings are issued by the regional & state forecasting offices of the Indian Meteorological Department to enable the farmers to adopt timely protective measures.Irrigation or spraying with water helps to mitigate frost damage in the fields.The use of heaters in orchards will help to raise the temperatures by as much as 5 degree centigrade.Valleys in which cold air stagnates are subject to frost to a greater extent, especially in the grape fields of Madhya Pradesh & Maharashtra.While laying out orchards & tea-gardens , such valleys should be avoided. Trees planted as shelter belts or as shade-trees protect the crops during the periods of mild frost. When the standard screen temperature is 5-7 degree centigrade,some plant injury is possible even if the stage of ground frost is not reached.Valuable crops can also be protected by coveringthe ground with suitable material, e.g. paper, plastic or celophane, to reduce the cooling of the ground through radiation.

Sunshine. Sunshine is not a limiting factor anywhere in India. Even during the monsoon, it is not the lack of sunshine but excessive rain that is responsible far an occasional set-back to plant growth. However, continued cloudiness during flowering is injurious to all plants.It also creates conditions favourable to the multiplication of pests & diseases. High humidity & warm temperatures are conducive to most plant diseases.

Winds. High winds or squalls are experienced in association with cyclones,depressions & dust-storms & thunder-storms. High winds cause mechanical damage to crops, such as breaking or lodging. Light winds blowing from colder parts will bring about a drop in temperature & create frosty conditions. Warm & dry winds cause greater losses through evaporation & result in water stress. Windbreaks planted across the prevailing wind directions afford protection to the plants against damage owing to winds. They are recommended as a useful means to reduce the evapo-transpirative losses,frost damage,& soil erosion by wind.

Crop-yield formulation. Weather elements, e.g. precipitation, temperature & sunshine, influence the yield of crops. In India,weather data are available over long periods for a large network of stations. Yield data for different crops for periods ranging from 25-65 years are also available from different states. With the help of these data, formulae have been evolved to forecast the yield of principal food crops(rice & wheat) subdivision-wise by using regression techniques.

The linear correlation between rainfall & other recorded meteorological parameters & the yield of a particularcrop over a large number of years has been worked out for overlapping periods of 7-90 days on a very fast electronic computer. The significant periods, thus located, have been used for final regression analysis.

These studies reveal that technology comprising improved methods of farming , the use of hybrid seeds, fertilisers,etc. have enhanced the yields in almost all the sub-divisions of the country, significantly in recent years. As regards weather, it had been noticed that whereas the distribution of rainfall during the crop-growing season plays a very important role increasing the yield, the kharif rice temperature plays only a minor role. In respect of wheat, the pre-season precipitation during September-October & the temperatures during the growing season are the main parameters which control the yield. The in-season precipitation is of less importance.

Such forecasts are expected to help the Government to estimate the production of major crops well before the harvest season.Formulae have been developed for a few major crops over a large part of the country & test forecasts are being issued to the Directorate of Economics & Statistics of the Ministry of Agriculture & Irrigation. Pilot studies to develop similar formulae for other major food crops are also in progress.

Long-range forecasts of weather and crop yields. Indian agriculture mainly depends on the monsoon rains.The farmers anxiously await the onset of the monsoon & a favourable distribution of rainfall during the rainy season. As there is a considerable variation from year to year in the advent of the monsoon over different divisions, the predicting of the dates of its onset & giving advance indications to the probable distribution of rainfall are of great economic significance.

Since 1886, the Indian Meteorological Department has been issuing long-range or seasonal forecasts for the monsoon rainfall during June-September every year. The method consists in making use of certain weather factors over different parts of the world that have significant relationship with the subsequent monsoon rainfall in India. The first forecast is issued in June for the period from June to September, covering the Peninsula, including Maharashtra,Gujarat,Madhya Pradesh,Telangana,interior Mysore(Karnataka north), the coastal Andhra Pradesh & the coastal Mysore (excluding South Kanara) & northwestern India comprising Jammu & Kashmir, western Uttar Pradesh,Haryana, Punjab,Himachal Pradesh,Delhi,Chandigarh & Rajasthan. The second forecast is issued for the latter half of the monsoon season (August-September) for each of the Divisions, the Peninsula & north-western India. The third forecast in January is for the probable amount of rain & snow during January to March in northern India. These forecasts are, however, of limited use, since they cover large areas, such as northern India & the peninsula as a whole. Since the monsoon is a major atmospheric circulation over the globe, a dynamic approach by taking into consideration the global factors is necessary. It is also essential to understand the role of the oceanic picking up of moisture & the underlying oceanic currents in order to enable long term predictions being made with some degree of confidence.

The Division of Agricultural Meteorology, Poona, prepares every year charts, showing the progress of important weather factors, such as rainfall, temperature & humidity. An outlook on crop situation in India based on these charts, is issued once a month to the Directorate of Economics & Statistics of the Agriculture & Irrigation Ministry for preparing crop yield stastistics for India.