Evaluation of drought hazard

The incomplete T-curve has been fitted to frequency distributions of n-week rainfall totals. The parameters in this equation were estimated by the method of maximum likelihood. This curve, when checked against the frequency histograms by the chi-square test, showed a good fit in most cases. Only when the frequencies exhibited a definite bi-modal tendency were the chi-square values greater than would be expected from random fluctuation. More years of record would probably erase much of this tendency. From the probability integrals of the fitted curves the likelihood of occurrence of a certain class interval of n-week rainfall totals can be estimated for each station. The probabilities of like amounts, x(a), or less for giver durations at each station have been computed and, likewise, the probabilities of the proper base amounts, x(d) or less (different for each station) have been estimated. These probabilities afford a basis for comparison of different areas with regard to rainfall distribution alone and also with regard to the likelihood of receiving the minimum amount of precipitation needed to produce a normal corn crop. Similarly, the probability of occurrence of any class of rainfall totals could be determined; e.g., the probability of receiving between 8.0 and 10.0 inches of precipitation at Ames during the 3-week period beginning July 5 could be estimated. The use of the distribution function is by no means limited to the drought problem. The probabilities determined point to distinct differences in the drought hazards experienced at various stations. Southern Iowa, while enjoying a rainfall contingency comparing favorably with other areas in the state, has rainfall requirements so high that drought is much more likely in the southern counties than in the central and northern sections. While less pronounced, western Iowa presents more of a drought problem than does the eastern part of the state. It is supposed that soil characteristics and evaporation conditions are responsible for the greater needs of the southern and western counties.