The relative growth in urbanization across the three decades in the Tampa-Orlando corridor, southeast Florida (around Miami), and around Jacksonville is most apparent in Supplementary Fig. S1 with a corresponding shrinkage in the rural land cover from 1980 to 2000. Most of this growth in urbanization in Florida is attributed to population growth.22,23,24 Rapid population growth caused changes in land cover from the development of transportation infrastructure, residential, commercial, and industrial complexes, as evident in the growth of the urbanization from 1980 to 2000 in the I–4 corridor connecting Tampa to Orlando (Supplementary Fig. S1a–c). Evaluating a similar time scale at a finer spatial resolution, Kautz et al.25 reported 0.61 million ha of natural and semi-natural land cover types present in Florida in 1985–89 were converted to urban and developed lands (representing 6.2% of all natural land cover types available at the beginning of the period).

The linear trends in the onset date, demise date, length, and seasonal accumulation of rainfall of the wet season over Peninsular Florida are shown in Fig. 1a, b, c, and d respectively. It is apparent from the figure that there are significant heterogeneities in the linear trends both in terms of magnitude and sign of all four variables. For example, strong negative trends in the onset date (Fig. 1a) near Vero Beach (suggestive of a tendency for earlier onset date in recent times relative to earlier periods) is adjacent to positive trends over Orlando just north of it. Likewise, there are apparent hotspots of rising and declining trends in other regions of Peninsular Florida in all four variables (Fig. 1).

Fig. 1 The observed linear trends (shaded) in a onset (days/year), b demise (days/year) dates, c seasonal length (days/year), and d accumulated rainfall (mm/year) of the wet season. The hatched regions indicate passing the Mann-Kendall test for significance (p ≤ 0.05) Full size image

These trends of precipitation are overlaid on the population-interaction zones for agriculture (PIZA) index of 2000 in Supplementary Fig. S2a–d. In considering the distribution of the PIZA index, say for a given year like 2000, we observe that there seems to be a pattern of a trend of later onset date (Supplementary Fig. S2a), earlier demise date (Supplementary Fig. S2b), and thereby shorter seasonal length (Supplementary Fig. S2c) of the wet season over some of the major urban areas (e.g. Tampa, Orlando, Miami, and Jacksonville) in Peninsular Florida. But a pattern in the trends of the seasonal rainfall accumulation of the wet season coherent with the land-surface type is not so apparent (Supplementary Fig. S2d). For example, Tampa region exhibits a declining trend while Orlando exhibits an increasing trend in the seasonal accumulation of the wet season rainfall. Previously, Mishra and Lettenmaier26 found that the rain rates in urban areas show an increasing trend in the US. A similar feature also has been observed in other urban regions when the study was expanded globally.27 However, Mishra and Lettenmaier28 find that unlike the trends in surface temperature the trends of precipitation in urban areas of US show far less frequent statistical significance, which they attribute to the result of higher natural variability on the latter compared to the former.

These patterns of secular changes in the wet season parameters with land type is further confirmed in Fig. 2, which shows the scatter of the linear trends in the onset date, demise date, seasonal length, and seasonal rainfall accumulation of the wet season with the PIZA index for the year 2000. In order to get an unbiased estimate of the linear fit of the trends in the variables of the wet season with the PIZA index, we ensured that there were equal numbers of counties for each PIZA category. We iterated (bootstrapped) the scatter by randomly picking the same (equal) number of counties for each PIZA index for 10,000 iterations to develop the 95% confidence interval for the slope of the linear fit as shown in Fig. 2. It is quite apparent from this figure that a trend of later onset date, early demise date, and thereby shorter seasonal length is preferentially more prominent in counties with PIZA index of 4 (highly urbanized) compared to the more rural regions (PIZA index of 1). Additionally, it may be noted that the rate of increase in the linear trend of the occurrence of later onset date of the wet season with increase in PIZA index is significantly higher (Fig. 2a) than that of the occurrence of the earlier demise date of the wet season with increase in the PIZA index (Fig. 2b). Similarly, we find that the rate of change in the seasonal rainfall accumulation of wet season with PIZA index (Fig. 2d) is proportionately much weaker than that for the rate of change in the seasonal length (Fig. 2c). This implies that the average daily rain rate for the wet season is rising more in the urban regions relative to the rural regions. We know this because despite the comparatively strong shortening of the seasonal length of the wet season in the urban regions the seasonal accumulation continues to decrease far more moderately, thereby causing the average daily rain rate to increase. These findings are consistent with other recent studies that indicate a rising trend in extreme precipitation events in the southeastern United States including Florida.29,30 These studies attribute the likely cause to increasing water vapor content in the atmosphere with rising temperature trend on account of global warming. In another related study, Mishra and Lettenmaier28 find that there is a rising trend in the daily precipitation maximum and in the frequency of extreme daily precipitation in urban areas across the US. They state that the attribution of these trends in urban precipitation is far more complex than a similar rising trend in minimum temperature in urban areas of the US, which is attributed to the urban land cover effect.28,31