Urban Heat Island Mitigation Due to Enhanced Evapotranspiration in an Urban Garden in Saint Paul, Minnesota, Usa

Abstract

As a result of extensive urban development coupled with warming temperatures, urban heat islands (UHI) have become an important factor affecting energy consumption and human health in cities. Prior research has shown that evapotranspiration (ET) from urban vegetation can have a significant cooling effect, but there are relatively few direct measurements from urban vegetable gardens. We compared hourly temperature measurements during two summers (2017 and 2018) in a 750 m 2 research garden at the University of St. Thomas (Saint Paul, Minnesota, USA) to hourly temperatures at the nearby Minneapolis-Saint Paul (MSP) International Airport, located 6 km to the south. We also quantified seasonal ET (June-October) in 132 garden plots and five reference turfgrass plots during the summers of 2017 and 2018. For both years, an increase in temperature of 1.00°C at the MSP airport resulted in an average increase of 0.55°C in the research garden. At temperatures greater than 22°C, the garden was cooler on average compared to MSP airport. ET in the garden plots was significantly higher than in the grass reference plots both years, with means of 46 cm for garden plots compared to 19 cm for grass plots in 2017, and 51 cm for garden plots compared to 33 cm for grass plots in 2018. These results are consistent with other research showing potentially large benefits of cooling through ET from urban gardens that are primarily aimed at crop production.

Investigating potential hydrological ecosystem services in urban gardens through soil amendment experiments and hydrologic models

“…For turfgrass plots, modeled seasonal evapotranspiration was 242 mm in 2017 and 317 in 2018. These values are similar to observed values for these same experimental garden plots of 465 mm (2017) and 510 mm (2018), and values for turfgrass of 195 mm (2017) and 329 mm (2018) reported in Small et al (2020).…”

“…We conducted a multi-year study using the Stewardship Garden at the University of St. Thomas in St. Paul, MN (44°56′17″N, 93°11′46″W) where mean annual temperature is 8.3 ˚C and mean annual precipitation is 803 mm (Small et al 2020). Established in 2011, the research garden contains 32 raised garden beds measuring 4 m 2 and 0.3 m deep (Fig.…”

“…Urban gardens are characterized by high compost inputs (Small et al 2019) and porous soils, potentially resulting in high water storage capacity and ultimately higher ET from crops growing in nutrient-rich garden soil (Qiu et al 2013). Previously documented seasonal ET rates in garden plots were nearly twice as high as in reference turfgrass plots, with lower leachate fluxes in garden plots despite additional water inputs from irrigation (Small et al 2020).…”

High water retention in urban garden soils leads to reduced leachate and elevated evapotranspiration

“…Our results suggest that urban gardens may function in a similar manner to engineered green stormwater infrastructure, with a high capacity for water retention, in ltration, and ET. Through the generation of ET uxes, urban greenspace and gardens also provide some UHI mitigation bene ts (Small et al 2020). These bene ts are notable because urban gardens are constructed and maintained for a variety of other purposes such as crop production, recreation, social and aesthetic goals (McDougall et al 2019), and hydrologic ecosystem services provided by urban gardens are a secondary bene t achieved for free.…”

“…Urban gardens are characterized by high compost inputs (Small et al 2019) and porous spoils, potentially resulting in high water storage capacity and ultimately higher ET from crops growing in nutrient-rich garden soil (Qiu et al 2013). We previously documented seasonal ET rates in garden plots that were nearly twice as high as in reference turfgrass plots, with lower leachate uxes in garden plots despite additional water inputs from irrigation (Small et al 2020).…”