**1. Introduction**

When people talk about the climate, the urban heat island (UHI) is not a new terminology. This began to be relevant decades ago when the cities began to be megacities built without any environmental planning and when the trees and grass were converted to concrete and asphalt. According to the United States Environmental Protection Agency, an urban heat island is created in developed areas where the built surfaces absorb and retain radiation from the sun. The impacts of UHI include increased energy consumption, higher concentration of air pollutants, and increase of air temperature level and flash flood.

Temperature is the most significant atmospheric parameter in researches about the effects of heat island. Studies have documented that urban areas have air and surface temperatures that are, on average, 1.8–5.4°F higher than temperatures in surrounding rural areas and there is potential for up to a 22°F difference in more extreme situations [1]. According to the Intergovernmental Panel on Climate Change (IPCC), global average temperatures have risen by 0.6°C (1.1°F) since 1970 and can be expected to rise another 1–4°C (1.8–7.2°F) by the end of the twenty-first century, depending on future societal practices and the amount of greenhouse gas emissions released into the atmosphere. The 2009 US Global Change Research Program report entitled "Global Climate Change Impacts in the United States"

illustrated that the average mainland US temperature has increased by 1.1°C (2°F) since about 1960, precipitation has increased by 5%, and the frequency of heavy precipitation has also increased by a factor of two [2].

Another important aspect to be studied is evapotranspiration (ET) because the increase of impervious surfaces prevents the movement of air and water which are key in the cooling processes of evaporation and transpiration and creates a typical "heat urban islands" of warmer temperatures, while the green vegetation in the surrounding rural areas better regulate surface temperatures.

This could be evident with the analysis between cities such as San Juan, Puerto Rico (urban), and Gurabo, Puerto Rico (rural). A decrease of evapotranspiration energy from the rural area to the suburbs and finally to the urban area is expected. This change will coincide with a decrease in vegetation coverage. Without the immediately available energy outlet of evaporation, urban and suburban areas must store more energy during the day. The stored energy is subsequently released to the atmosphere at night, primarily through higher radiant emissions and to a lesser extent via increased convection [3].

The flash flood is a consequence of an urban heat island effects. Most people consider that sudden floods are the product of weather phenomena such as tropical waves through even hurricanes. Previous studies have shown that most of flash floods are caused by anthropogenic behavior generally named anthropogenic heat, which is generated by human activity and comes from many sources, such as buildings, industrial processes, and change in land use from pervious to impervious.
