Two Rainfall-Characteristic Indices Proposed as Suitable for Agricultural Planning in Puerto Rico

Abstract

A statistical evaluation was made of the daily rainfall at Caguas and Fajardo, covering 70- and 60-year periods. The purposes of this evaluation were: 1, To investigate the character, if any, of the rainfall distribution and of its deficiency (drought) characteristics; and 2, in the event that any character was found in rainfall in the sense of distribution and deficiency, to develop a Rainfall Deficiency Index which would be useful to agricultural and other programs and inventories. The study first consisted of determining the least rainfall in 60, 120, and 180 consecutive days, and the maximum rainfall in 30 and 90-day periods of each year. This analysis suggested the occurrence of most deficient and least deficient rainfall in 120 (or 180) day cycles during the drier part of the year within 10- to 11-year periods; the Caguas data beginning in 1899 and in 1909 at Fajardo. The suggested cycles appear to be more defined at Caguas than Fajardo. It appears that years ending in 6 or 7 almost invariably seemed to be among the driest in their respective decades; 1899 to 1968. Years ending in 1 or 2 seemed identified with least deficient and better distributed rainfall. This preliminary analysis was based mainly on the least rainfall in 120 days parameter. It was found that the ratio of the mean least rainfall in 120 days of the dry years (ending in 6 or 7) to the "wet" years (ending in 1 or 2) is 0.364 for Caguas and 0.603 for Fajardo. Also, the coefficient of variation of the data for dry and "wet" years was lower than when calculated for all years. Contrast between dry and wet years is apparent even for the least rainfall in 180 days (6 months of the year). In this respect the ratios of dry to "wet" years are 0.494 and 0.595 for Caguas and Fajardo, respectively. There is no apparent contrast, however, when the annual, or even maximum, rainfall in 90 days at the two locations is compared for "wet" and dry years. A Rainfall Deficiency Index (DEF) is proposed. Such deficiency is normally distributed for Caguas and Fajardo. This index integrates the deficiency and distribution characteristics of the annual rainfall. It can be used, therefore, as a guide for evaluating the rainfall characteristics in the agricultural regions of Puerto Rico and for the determination of priorities for supplemental irrigation. A Rainfall Distribution Coefficient (DIS) was developed as a direct means of describing the distribution characteristics of annual rainfall, and it accounts indirectly for the rainfall deficiency within a given area or areas of similar annual rainfall. DEF accounted for 74.0 and 64.4 percent of the variations in DIS at both Caguas and Fajardo.