In this paper we briefly review some aspects of the soil water balance, highlighting the possible connections and feedbacks among the various processes as well as pointing out the manifold repercussions of soil moisture dynamics on vegetation response and biogeochemical cycles. The discussion is not intended to be complete and reflects the authors' interests, but we hope that it may be helpful to highlight the multidisciplinary nature of the hydrologic research as well as to suggest useful avenues for an improved and coherent description of the realm of the phenomena that are controlled by the apparently simple fact that water incessantly moves through the soils.

Increasing urbanization is likely to intensify the urban heat island effect, decrease outdoor thermal comfort, and enhance runoff generation in cities. Urban green spaces are often proposed as a mitigation strategy to counteract these adverse effects, and many recent developments of urban climate models focus on the inclusion of green and blue infrastructure to inform urban planning. However, many models still lack the ability to account for different plant types and oversimplify the interactions between the built environment, vegetation, and hydrology. In this study, we present an urban ecohydrological model, Urban Tethys-Chloris (UT&C), that combines principles of ecosystem modelling with an urban canopy scheme accounting for the biophysical and ecophysiological characteristics of roof vegetation, ground vegetation, and urban trees. UT&C is a fully coupled energy and water balance model that calculates 2 m air temperature, 2 m humidity, and surface temperatures based on the infinite urban canyon approach. It further calculates the urban hydrological fluxes in the absence of snow, including transpiration as a function of plant photosynthesis. Hence, UT&C accounts for the effects of different plant types on the urban climate and hydrology, as well as the effects of the urban environment on plant well-being and performance. UT&C performs well when compared against energy flux measurements of eddy-covariance towers located in three cities in different climates (Singapore, Melbourne, and Phoenix). A sensitivity analysis, performed as a proof of concept for the city of Singapore, shows a mean decrease in 2 m air temperature of 1.1 ^∘C for fully grass-covered ground, 0.2 ^∘C for high values of leaf area index (LAI), and 0.3 ^∘C for high values of V_c,max (an expression of photosynthetic capacity). These reductions in temperature were combined with a simultaneous increase in relative humidity by 6.5 %, 2.1 %, and 1.6 %, for fully grass-covered ground, high values of LAI, and high values of V_c,max, respectively. Furthermore, the increase of pervious vegetated ground is able to significantly reduce surface runoff.