Urbanization and increases in impervious area are known to increase stream runoff and flashiness and several
indices have been developed to quantify flashiness in temperate streams. The effectiveness of these indices in the humid
tropics and how urbanization influences the flashiness of tropical streams that are known for their inherent flashiness is
poorly understood and documented. This study compares two existing flashiness indices and a new approach that quantifies
flashiness by the average time between large events on 13 urban to forested streams in Northeastern Puerto Rico. The average
time between events of specific magnitudes, the Richards-Baker Flashiness index, and a Frequency of Stage Change
approach were calculated and compared using average daily, hourly, and 15 minute discharge data. All analysis was based on
USGS discharge records for the same 10 year period, 2000 through 2009. The results indicate that when flashiness is based
on average daily stream flow, there was little to no discernible difference in the flashiness of the urban and forested streams.
This results because average daily discharge records miss or underestimates the magnitude, duration, and the frequency of
most events. When comparing the time between events of a given magnitude using hourly or 15 minute discharge series, the
urban drainages have a shorter time period between peaks of a given magnitude than rural drainages. The Frequency of Stage
Change approach also indicates that high density urban drainages are flashier than drainages with forested or mixed land use.
For the average days and the stage frequency change method, flashiness indices based on hourly time series are similar to
those based on the 15 minute series. The analysis indicates that tropical urban streams are flashier than their rural
counterparts; however the difference is less than has been reported in temperate studies and is only statistically apparent
when using high resolution discharge records.