138 Morgenroth and Buchan: Soil Moisture and Aeration Beneath Pervious and Impervious Pavements most 10 cm (3.9 in). It is thought that this may be a reflection of a distillation effect; overnight cooling of pavement draws moisture upwards, thereby maintaining a high Soil moisture dynamics beneath pervious paving was more soil in the shallow soil layers. closely related to dynamics beneath impervious paving than to that of exposed soils. Soil moisture ranged from c. 32% to 50%, and while early summer depths, the difference diminished to c. 2%–3% by early autumn (Figure 4). Daily depths, often approaching 2%. The magnitude of these fluctua- tions decreased with depth, presumably due to the buffering ef- fect of overlying soil. Under pervious pavement, soil fluctuations were relatively large at shallow ally similar in the uppermost two layers. However, in the final week (week 19), soil moisture at 5 cm was significantly greater than at 10 cm (Table 1). The highest values for soil was gener- tently found at 20 cm. At this depth, soils were significantly wet- ter than soils at 10 cm for the entire growing season other than a month long period during late summer (weeks 14–17). Interest- ingly, soil moisture at 5 cm and 20 cm were statistically similar, despite measured differences. One explanation for the lowest soil moisture in the middle layer (10 cm) is that while gravity draws moisture downwards, distillation draws water vapor upwards, thus water is being pulled in both directions by opposing forces. The measured soil moisture contents beneath both pavement soil were consis- types are extremely high, ranging between c. 4% and 22% higher than the FC of the soil throughout the growing season (Figure 3, Figure 4). In fact, soils beneath pavements likely neared saturation for much of the measurement period. Although small increases in (e.g., days 60, 63, and 78), the quick returns to prerainfall lev- els imply near-saturation and likely drainage into deeper soils. Letey (1985) defined the concept of least-limiting water range (LLWR); this is the soil moisture range within which water, aera- soil beneath pervious paving occurred following precipitation soil varied by up to 8% between different tion, and mechanical resistance do not limit plant growth. The maximum range is bound by the soil’s PWP and FC, but can be reduced if aeration or mechanical resistance become limiting. In this experiment, low 32% in the upper 20 cm of soil. Given that the measured FC [0.1bar (1.5 psi) suction] of the soil was 27.9%, the soil moisture beneath both pavements was above the LLWR, and may have limited aeration. In control plots, however, soil beneath both pavement types rarely fell be- ated within the plant-available water bounds of the LLWR, ex- cept for temporary rises above FC following precipitation events. During days 20–60, soil moisture at 5 cm depth in control plots was generally constant at c. 15% (Figure 2), near the PWP; how- ever, below 5 cm soil typically fluctu- Due to the open nature of pervious paving, it was expected soil increased substantially above the PWP. that underlying soil moisture would differ compared to soils be- neath impervious pavements. However, this was not observed (Table 1). Instead, soil moisture was similar beneath both pave- ment treatments. Water inputs (rainfall) and outputs (evapora- tion) were anticipated to be higher in the pervious pavement treatment, but the results challenge these expectations. While increased soil moisture due to rainfall infiltration was observed (e.g., days 60, 63, and 78), soil moisture quickly returned to prerainfall values. So, increased infiltration did not lead to long- term differences between soils beneath pervious and impervious pavements. Due to the already high (near-saturated) soil mois- ture values below both pervious and impervious paving, these wet soils may not have the ability to retain additional water. It was assumed the large pores in pervious paving would enable relatively high rates of evaporation. However, such large pores can preclude capillary upflow of water through the pavement (Ander- sen et al. 1999). As water is limited to the soil/pavement boundary and not the pavement/atmosphere boundary, evaporation is negli- gible. Together, assumed low evaporation rates and the inability of Figure 3. Soil volumetric water content at three depths beneath impervious pavement. Each data point represents the mean value of five probes (in five replicate plots). The dotted line represents the field capacity of the soil. ©2009 International Society of Arboriculture Figure 4. Soil volumetric water content at three depths beneath pervious pavement. Each data point represents the mean value of five probes (in five replicate plots). The dotted line represents the field capacity of the soil. θ θ θ θ θ θ θ θ θ
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