Three aspects of rainfall variability are the time factor, quantum factor and the areal distribution factor. The effects of these three factors are tremendous.
(i) Time Factors
The length of the rainy season, the date of commencement and withdrawal of monsoon and the length and number of dry spells in between rain events are not definite in India. The onset of summer monsoon may be delayed by two to three weeks and the withdrawal advanced by as much as four weeks.
The occurrences of dry spells accentuate the conditions. The frequency and duration of dry spells is highly variable, thus affecting crop growth and development. This variable nature of rainfall in India adversely affects the agricultural calendar. Selection of crop variety on the basis of term duration is not possible. Contingency plans to counter and neutralize dry spells and rainfall deficits are also difficult to plan and implement without loss of time.
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(ii) Quantum Factor
Quantum of rainfall received during the season and over the entire year is also highly variable. The seasonal distribution of rains as mentioned earlier indicates the predominance of the southwest monsoon in India.
This is also a season of high temperature and high evapo-transpiration. Thus, the effectiveness of precipitation decreased. During dry spell, water stress in crops and plants occur, which adversely affects crop production. Alternatively during this season cloud bursts may occur resulting in flash floods and inundation of fields accompanied by total destruction of crops, settlements, human and livestock population.
(iii) Aerial Distribution Factor
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The real extent of rainfall is not definite. Hence” planning for agricultural activities, viz., preparation of field leveling, ploughing, sowing, wedding etc., precisely is difficult.
All these uncertainties are reflected on the agriculture and crop production of rain-fed regions of India and the consequences of this uncertainty are drought and floods.