The evolution and significance of soil, soil surface, and soil moisture in the ecohydrology of engineered urban green spaces

Bita Alizadehtazi

doi:10.17918/D8C38W

Urbanization causes profound changes in the hydrology of developed watersheds with many negative impacts on local, regional, and global energy and water balances, non-point sources of pollution, loss of stream/riparian structure and function, coastal and terrestrial habitats, and changes in atmospheric composition. In an effort to remedy such conditions, many researchers are trying to quantify the range and extent of ecosystem services like thermal regulation, water regulation, aquifer recharge, water quality improvement, habitat, and cultural services that can be achieved by retrofitting engineered Green Infrastructure (GI) into the urban landscape. These niche ecosystems, often funded as a means of managing urban runoff or compensating for habitat loss, typically include soil and vegetation, arranged in new configurations across formerly impervious spaces. The ability of GI to provide meaningful ecosystem services is both enabled, and constrained, by soil moisture, a key driver of ecosystem function and structure. At a particular site, a soil moisture field emerges as the product of soil-water-climate and vegetation interactions. This field then mediates the local water and energy balance, constrains photosynthesis, supports biota, and controls the redox state. Though billions of dollars are currently being spent on GI, very few researchers have been able to document GI's ability to actually provide ecosystem functions and services in different biophysical and climatic regimes. Such work is needed since engineered GI is fundamentally different from other green kinds spaces (e.g. agricultural plots, forests) in that the imported and/or amended soil, bacteria, and newly transplanted vegetation are both in dynamic, early states of development. This dissertation will shed light on the potential role of soil moisture in determining the ecohydrologic performance of engineered GI, a first step in studying GI's ecosystem services. Specifically, we investigate the role that design decisions can have on spatiotemporally variable soil moisture patterns, and the ecosystem services that depend on them. The research includes observations made in the laboratory and in the field, as well as modeling. It is organized into three papers, as follows: · Paper 1 analyzes several years of soil moisture data collected in two urban GI systems. Relationships between precipitation characteristics, season, and hydraulic loading ratio and soil moisture at different depths are quantified. Potential linkages between GI siting and design decisions and ecosystem services are illustrated by also computing evapotranspiration rates at the sites, speculating that site/or climate factors that lead to higher moisture content, engender greater ES associated with ET. · Paper 2 presents laboratory research conducted using a rainfall simulator to demonstrate the ecohydrological significance of GI surface treatments that buffer the impact of raindrops. This paper attempts to: a) determine if vegetation canopies found in typical GI system can protect the soil surface, and reduce crust formation; b) decreasing post-storm surface ponding, and c) increasing the infiltration of water and recharge, when compered to bare soil surfaces. This study focuses on two different scenarios: 1) bare soil and 2) soil protected by synthetic vegetation canopies. Soil moisture, the surface's penetration resistance, and physical measurements of the volume of infiltrate and runoff are made on all two surface treatments after simulated rainfall events. The results are used to develop recommendations regarding surface treatment in engineered urban GI system designs. · Paper 3 attempts to model the observations reported in the companion papers 1 and 2. Specifically, a customized soil moisture model is developed for use in simulating the spatiotemporal variation in the soil moisture regime engineered GI systems. The model is calibrated to in-situ soil moisture observations made at engineered GI sites, and then used to explore the role that decisions regarding the design, construction, operation and maintenance of these unique systems will influence their ability to provide ecosystem services. Specifically, we investigate how differences in hydraulic loading, surface treatment, and precipitation influence the potential for GI systems to yield runoff and recharge under extreme precipitation conditions.

The evolution and significance of soil, soil surface, and soil moisture in the ecohydrology of engineered urban green spaces

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The evolution and significance of soil, soil surface, and soil moisture in the ecohydrology of engineered urban green spaces

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