Influence of Urbanization on Annual and Seasonal Short Duration Rainfall Amounts in Houston

Characteristics of short duration rainfall events have essential application in the design of hydraulic structures. In order to derive various design values, hydrologic data is generally assumed to be time-independent or stationary. Significant changes in land cover could be a source of non-stationarity of flood characteristics and lead to incorrect estimates of flood magnitude and frequency. Past studies of precipitation in urban areas have suggested cities influence spatial and temporal rainfall patterns. However, in the United States, quantification and statistical verification of the modification of long-term trends of rainfall characteristics by urbanization has been difficult because of sparse spatial coverage of rain gages and insufficient record lengths. The data limitations have been reduced during the second half of the twentieth century as numerous cities in the U.S. experienced rapid growth with several having sufficient rain gage coverage in the existing urbanized area, nearby urbanizing areas, and adjacent rural areas to provide a framework to assess urban effects on long-term rainfall patterns. The longer records corresponding to periods before, during and after rapid urbanization permit a more thorough analysis of trends and statistical verification of the effects of urbanization on rainfall variability. The study presented herein involved the analysis of temporal trends of annual and warm season maxima of short duration (1-day, 3-day and 7-day) rainfall amounts recorded in the Houston metropolitan area and climatologically upwind and downwind regions. The observed trends were related to urbanization of Houston in the period of 1950-2003 and the statistical significance was judged by linear regression analysis and nonparametric statistical tests. Results showed that the entire area is likely to experience their heaviest rainfall events in more recent years. Geographic distribution of changes in the annual maximum time series shows that most of the strong rainfall events occur in Houston urban area and its seasonally downwind region.