Observed variability in soil moisture in engineered urban green infrastructure systems and linkages to ecosystem services

Abstract

Soil-water-climate-vegetation interactions jointly determine the ability of landscapes to provide ecosystem functions and services. In particular, spatio-temporal patterns in soil moisture underpin landscape ecohydrology. Though these patterns have been of interest to researchers for some time, there is new interest in the topic today as city managers engineer green infrastructure (GI) into urban landscapes. This paper presents soil moisture data collected from 2012 to 2014, and weighing lysimeter observations continuing through 2016, in two urban GI systems. Relationships between precipitation history, season, soil depth, hydraulic loading ratio (HLR) on the frequency and magnitude of soil moisture responses are described quantitatively. A logistic regression model is used to quantify the odds that each of these variables triggers a detectable soil moisture response. The results suggest that the higher HLR site (Site 2, HLR = 3.8) had 129.7% higher odds of a soil moisture response than Site 1 (HLR = 1). The results also indicate that there are 82.9% lower odds of a response in summer than in winter. Moreover, the odds of a response decrease with increasing soil depth. The linkage between GI siting and design decisions that impact soil moisture and ecosystem services is illustrated by also reporting evapotranspiration (ET) rates at the sites as determined by the lysimeter. Higher ET observed during wetter conditions supports the hypothesis that GI siting and design factors that lead to higher moisture content can engender greater ecosystem services associated with this hydrologic process. Indeed, the higher HLR of Site 2 sustained higher soil moisture levels during the summer compared to Site 1.