Tuesday, May 17, 2016

Rationale for Monitoring and Analysis of Green Infrastructure in the Vacant to Vibrant Project

Monitoring and analysis of stormwater capture by green infrastructure in the experimental parcels in each of 3 cities is one of the main tasks of the Vacant to Vibrant Project.  Specifically, we planned to monitor in-site soil stormwater capture and filtration by continuously measuring soil moisture at 5 cm, 10 cm, and 20 cm soil depths and soil temperature at 5 cm at one site in each parcel. We planned to use existing LIDAR datasets to interpret soil moisture in light of local topography and drainage patterns.  We have now completed preliminary analysis of the method for using soil moisture to assess the effectiveness of GI installations in altering stormwater runoff.

Figures 1 and 2 present soil moisture data collected at CWRU’s University Farm at two sites (Forest and Meadow) for 2011 and 2012.  These data were collected with the same type of instruments that we propose using to monitor parcels.  The contrast in patterns of variability at the two sites for the two years is instructive because of the contrast in total annual precipitation between years.  Total precipitation was 65.32 in for 2011 and 44.62 in for 2012.  Furthermore, we anticipate that the experimental parcels will function more like the forest site and the control sites will resemble the meadow site.  Both of these sites at University Farms have similar slopes, but have different soil compaction and plant communities.

Figure 1.  Pattern of variation of soil moisture at two sites in University Farms (Case Western Reserve University, Hunting Valley, Ohio) for 2011.

Figure 2.  Pattern of variation of soil moisture at two sites in University Farms (Case Western Reserve University, Hunting Valley, Ohio) for 2012.

These data demonstrate substantial differences in water retention and processing by the two sites.  The simplest comparison is the level of soil moisture saturation.  These soils have a saturation value of 0.35 m3/m3.  The forest site averaged 58% of saturated soil moisture in 2012 and 66% in 2011.  In contrast, the meadow site averaged 81% of saturation in 2012 and 93% saturation in 2011.  Thus, the forest site’s mean saturation was 0.23 and 0.27 less than the meadow site’s for 2012 and 2011.  In general, higher soil moisture saturation results in greater runoff from a storm event.  The more saturated soils of the meadow site have significantly less capacity to absorb and process individual storm events, particularly in the spring of both 2011 and 2012 and especially in the summer of 2011, when the Cleveland area received nearly double its annual mean precipitation.

The monitoring design for soil moisture monitoring in the Vacant to Vibrant project will provide better estimates of retention and processing of storm water than the monitoring summarized in Figures 1 and 2.  The data in these two figures is from a single, vertically positioned 10 cm probe.  The proposed monitoring will use three probes with horizontal placement at three depths (5, 10, and 20 cm) below the soil surface.  This placement will provide a profile of soil moisture through the root zone and thereby will yield a more accurate assessment of soil moisture dynamics.

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