Resetting the fundamental hydrological processes of infiltration and evapotranspiration within the urban environment will help mitigate the human and ecological stress of the urban heat island (UHI) (Oke, 1973). In our accelerated rate of urban growth (Beach, 2002; Elvidge et al., 2004), we are building streets which shed stormwater and absorb solar radiation. Not only are new population records continually set in these urban areas (United Nations, 2004) but air temperatures are also setting new records (Hansen et al., 2005). The Intergovernmental Panel on Climate Change, 3rd Assessment, estimates increases in air temperature from 1β’4 to 5β’8 Β°C from 1990 to 2100 (McCarthy et al., 2001), and separate studies show UHI processes cause additional temperature increases for cities of 0β’1-0β’4 Β°C per decade (Akbari et al., 2001). In UHI characterization work by Rosenzweig et al. (2005) in Newark, NJ, minimum temperatures were 3 Β°C higher than surrounding non-urban minima, and Akbari (2006) has noted urban-non-urban differences can range from 2β’5 to 4β’5 Β°C for ten US regions. With the increase in heat, humans suffer from heat stress (McCarthy et al., 2001), and chemical reactions accelerate to increase human exposure to elevated levels of atmospheric pollutants (Taha et al., 1996). Innovative stormwater research has the capacity to address both human discomfort and health issues related to the UHI, as well as improve the urban ecosystem. Several studies have shown that increasing tree coverage in urban areas will mitigate the UHI effect (Heisler et al., 1994; Taha et al., 1996; McPherson et al., 2005; Solecki et al., 2005); however, none have completed a water budget to ensure tree systems have adequate water to provide the cooling services of shade (i.e. direct cooling) and evapotranspiration (i.e. indirect cooling). In a study of UHI cooling options for Sacramento, CA, Akbari (2002) noted the city could increase tree cover from 13 to 36%, and in Los Angeles, CA, there is an initiative to plant 1 million trees to address UHI and air pollution problems (USDA, 2006). For both these cases, as well as future scenarios, research into stormwater irrigation of trees is complementary, providing a low-cost, self-organized method of sustaining tree-cooling services. Naturalization of urban watersheds has been approached by the lowimpact development design called bioretention basins (USEPA, 1999), which reset hydrological flow paths. The bioretention basin name combines the biological systems of living trees and soils (bio) with shallow, short-term ponding (retention) and subsequent infiltration. In keeping with bioretention standard design (Winogradoff and Coffman, 2001), bioretention basins are conceived as street-side tree plots that intentionally receive stormwater runoff from directly connected impervious areas as well as water from direct precipitation. As illustrated in the earlier tree-based urban cooling research, urban trees that receive water contributions from these bioretention basins can directly reduce insolation striking the street (Heisler and Wang, 1998), as well as indirectly lower ambient temperatures through evaporative cooling (Akbari and Taha, 1992; Stull, 2000).