Invariance of the design storm in a region under a strong positive trend in annual rainfall in a middle-latitudes region in Argentina

Abstract The design storm is used in water resources for designing hydraulic structures for flood control, or urban stormwater drainage. The design storm is calculated using a time series of annual maximum of daily rainfall. A current issue of scientific and economical interest is whether very intense storms, like the design storm, become more intense in a region where there is a positive trend in rainfall amount along decades. If the design storm would become more intense in the presence of the positive trend in rainfall amount, this effect would have to be taken into account to avoid underdesigning hydraulic structures for flood control. However, if the design storm would remain constant despite the occurrence of such positive trend in annual rainfall amount, then increasing arbitrarily the design storm values for different return periods would cause overdesigning the hydraulic structure and increase substantially and unnecessarily costs. In this research it is analyzed if a climate change in the areal average of annual rainfall implies a climate change in the annual maximum of daily rainfall. Evidence is presented showing that these two types of time series have uncoupled evolution. The mountainous region of study is semi-arid, located in middle latitudes, in central Argentina. The time series of areal average of annual rainfall shows a trend in the mean of 6,4 mm/year during the period 1941-1989. The 95% confidence interval for the rate of change of the mean is (2,3 mm/year; 10,5 mm/year). The residuals of the regression of the areal average of annual rainfall, on time, show a constant variance at the 5% significance level. Increasing number of rainy days accounts for the trend. However, the annual maximum of daily rainfall has a constant mean, at a 5% level of significance. The variance of the annual maximum of daily rainfall remains constant through the period under study. This result indicates that the design storm is invariant in a region of central Argentina where a strong positive trend in annual rainfall is taking place.