The reliability or dependability of precipitation is closely related to its variability. Petterssen has defined variability thus: “the variability is defined as the ratio of the mean of deviations from the mean (averaged without regard to sign) to the mean itself.
According to Trewartha “variability may be defined as the deviation from the mean computed for 35 years or more of observations”. A low ratio indicates a high degree of dependability, and a high ratio indicates an erratic behaviour.
Thus, variability of precipitation indicates the deviation of mean annual precipitation of a certain area from its average amount calculated for a long period of time. The average amount of precipitation in a certain area or region differs from year to year, and it invariably differs from its long- term mean. This difference is called the variability of precipitation.
For various regions of the world, we find significant variations in the amount of precipitation, received both in a particular year and between different years. We have noted earlier in this chapter that in the Mediterranean regions, there is more precipitation during the low sun period than during the high sun period.
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Similarly, areas lying between 10° and 20° latitude get more precipitation in summer than in winter. According to Petterssen, when the ratio of variability is less than 15 per cent, there is a high degree of dependability of precipitation, as in the case of the Pacific and Atlantic cyclone belts, and much of the regions near the equator.
In all these regions precipitation may be considered to be reliable. But in areas with 20 to 25 per cent variability, there is a constant fear of prolonged droughts, and the dependability of precipitation is reduced.
All the great deserts lying in the sub-tropical high-pressure belt have more than 30 per cent variability. Thus, it can be concluded that drier a place is on the average, the greater will be the statistical variability in its precipitation. In dry areas, there may be a year with very high amount of rainfall, but for several years the amount of precipitation may be much below the average.
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The variability of precipitation is significant in those semi-arid regions where the total annual amounts of precipitation in normal years are just sufficient for agricultural pursuits. In such areas of marginal rainfall, a slight decrease in the annual totals may prove disastrous.
While preparing plans for agricultural development, this crucial aspect of the distribution of precipitation merits special attention.