Relationship between the variability of primary production and the variability of annual precipitation in world arid lands

In arid and semi-arid rangelands, primary production, hence carrying capacity, is closely linked to the amount and distribution of rains, as shown by many authors. But variability in annual production appears to be relatively greater than variability in annual rain. The authors have used two indicators: The Rain Use Efficiency factor (RUE) which is the quotient of annual primary production (kg DM/ha/year) by rainfall (mm/year) and the Production to Rain Variability Ratio (PRVR) which is the ratio of the coefficient of variation (standard deviation divided by the mean) of annual production to the coefficient of variation of annual rainfall. The analysis of 77 series of annual data on production and rain, with 895 pairs of observations from various arid zones of the world, shows the following approximate values: RUE = 4·0 (S.E.M. = 0·3) PRVR = 1·5 (S.E.M. = 0·07). Thus, each mm of rain produces an average 4 kg of above ground dry matter per ha/year in these 77 series, while variability in production is, on the average, 1·5 times greater than variability in rainfall. Using these two criteria in conjunction with rainfall distribution percentiles the authors, via an empirical mathematical formula, predict variation in range production from rainfall records. A concrete case study is given as an example.

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