Trees in Pavement: Modelling Water and Heat Flux in Street Tree Plantings Using HYDRUS

Street trees are an important mediator of water and heat fluxes in urban environments. Pervious pavement is increasingly being installed as an alternative to mulch or bare soil around urban trees in paved areas to create level walking surfaces while allowing infiltration of storm water. However, it is unknown how these new surface materials influence water and heat fluxes and the subsequent effect on tree health. We address this question by using experimental observations of root distribution, tree growth, soil water and temperature, and tree water uptake, to calibrate a numerical modelling environment, HYDRUS-1D. This model simulates water, heat, and solute movement in agricultural soils, but has not been used with the engineered layers common in urban systems. We evaluated the ability of HYDRUS to estimate root depth distribution in response to these layered pavement/soil profiles. We hypothesize that 1) HYDRUS-1D can be further calibrated to predict water and heat fluxes in engineered urban soil systems that include pervious pavement, a gravel base course, and geotextile layers; 2) upper soil layers under pervious pavement will have greater water content, resulting in shallower root systems, compared to those in bare soil. To generate the observational data, in November 2014 we built 24 simulated urban tree pits, half of which were covered with pervious pavement, comprising a non-woven geotextile, base course and resin-bound gravel. We planted Platanus x acerifolia ‘Bloodgood’ whips in 12 of the 24 pits resulting in 4 treatments: tree-pavement, no tree-pavement, tree-soil, no tree-soil. We measured saturated hydraulic conductivity, texture, pore size distribution, and bulk density at 10- and 60-cm depths. We recorded stem diameter and root distribution and continuously monitored soil moisture and temperature at different intervals throughout the year. These values, together with layer thickness and the hydraulic conductivity of the geotextile, base course and pervious pavement, were used to characterize the behavior of each layer in HYDRUS-1D. After one growing season, trees in pavement had 59% greater stem diameters than those in bare soil and more superficial root systems. Root distribution was associated with greater soil moisture, suggesting that a root growth model is possible. Results have implications for pavement section design to increase street tree resilience to climate change as well as to reduce root-pavement conflicts.