International Journal of Phytoremediation, 17: 159–164, 2015 C Taylor & Francis Group, LLC Copyright ISSN: 1522-6514 print / 1549-7879 online DOI: 10.1080/15226514.2013.862210
Field Note: Comparative Efficacy of A Woody Evapotranspiration Landfill Cover Following the Removal of Aboveground Biomass WILLIAM SCHNABEL1, JENS MUNK2, and AMANDA BYRD3 1
University of Alaska Fairbanks, Water and Environmental Research Center, Fairbanks, Alaska, USA University of Alaska Anchorage, School of Engineering, Providence Drive, ENGR 201, Anchorage, Alaska, USA 3 University of Alaska Fairbanks, Alaska Center for Energy and Power, Fairbanks, Alaska, USA 2
Woody vegetation cultivated for moisture management on evapotranspiration (ET) landfill covers could potentially serve a secondary function as a biomass crop. However, research is required to evaluate the extent to which trees could be harvested from ET covers without significantly impacting their moisture management function. This study investigated the drainage through a six-year-old, primarily poplar/cottonwood ET test cover for a period of one year following the harvest of all woody biomass exceeding a height of 30 cm above ground surface. Results were compared to previously reported drainage observed during the years leading up to the coppice event. In the first year following coppice, the ET cover was found to be 93% effective at redirecting moisture during the spring/summer season, and 95% effective during the subsequent fall/winter season. This was slightly lower than the 95% and 100% efficacy observed in the spring/summer and fall/winter seasons, respectively, during the final measured year prior to coppice. However, the post-coppice efficacy was higher than the efficacy observed during the first three years following establishment of the cover. While additional longer-term studies are recommended, this project demonstrated that woody ET covers could potentially produce harvestable biomass while still effectively managing aerial moisture. Keywords: ET cover, water balance cover, alternative cover, biomass energy, lysimetry
Introduction Evapotranspiration (ET) covers represent a promising technology for the final closure of municipal solid waste landfills or contaminated sites in many arid and semi-arid locations. An ET cover can be defined as a relatively thick layer of minimally compacted soil placed over waste, usually planted with vegetation, that inhibits the downward migration of aerial moisture to the waste through the combined processes of soil storage, soil evaporation, and vegetative transpiration. Conventional covers, by comparison, are designed to inhibit downward migration of aerial moisture through the action of a minimally permeable synthetic, composite, or compacted soil barrier. Demonstrations of ET technology are widely reported in the literature, frequently in the context of relative efficacy compared to more conventional capping techniques (Abichou et al. 2011; Albright et al. 2004; Barnswell and Dwyer 2012;
Address correspondence to William Schnabel, University of Alaska Fairbanks, PO Box 755860, Fairbanks, AK 99775-5860. E-mail: [email protected] Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/bijp.
Fayer and Gee 2006; Rock, Myers, and Fiedler 2011; Scanlon et al. 2005). While the primary function of ET covers is to inhibit percolation of aerial moisture to the underlying waste, many could potentially produce marketable biomass as a by-product of their action. Thus, there exists a potential to realize secondary economic benefits to help offset the cost of the ET cover itself. Indeed, Licht and Isebrands (2005) outlined a number of case studies illustrating how such benefits could be realized by similar phytoremediation projects. In cold regions, ET cover biomass could potentially be treated as a short rotation coppice crop (SRCC) and used as a community heating source. While the concept of growing SRCC at conventional landfill locations has been proposed (Ettala 1988; Nixon et al. 2001), the effects of this practice upon the functionality of ET covers has not been well studied. The purpose of this study was to investigate the functionality of an ET cover after harvesting all woody vegetation at 30cm above ground surface. This was intended to simulate the use of ET cover trees as short rotation coppice crops in the context of biomass energy production. Drainage results following coppice were compared to the performance of the ET cover in the years leading up to the coppice event, as well as to the performance of a conventional cover employed as a control.
160 Experimental Methods The study was conducted at a lysimeter installation located at Joint Base Elmendorf Richardson (JBER) in Anchorage, Alaska. Detailed descriptions of the lysimeter facilities, installation techniques, cover specifications, soil properties, vegetation makeup, and initial results were previously reported (Munk et al. 2011; Schnabel et al. 2012a; Schnabel et al. 2012b). In short, two adjacent basin lysimeters 10 m × 20 m × 1.6 m deep were installed to compare the drainage response of a compacted soil cover (CSC) to that of an ET cover in 2004. The CSC cover consisted of three 15 cm lifts of silt (USCS-ML) compacted to yield a saturated hydraulic conductivity less than 10−5 cm/sec, overlain by one 15 cm lift of topsoil planted with local grasses. The ET cover consisted of one 60cm layer of silt and silty sand (USCS-ML and USCS-SM), compacted using low ground pressure equipment to 80–90% of maximum proctor density, planted with a mixture of 80% black cottonwood/balsam poplar (Populus trichocarpa/Populus balsamifera), 10% quaking aspen (Populus tremuloides, and 10% little leaf/golden willow (Salix alba vitel, arbusculoides). The saplings were spaced in a square grid at approximate 1.2 m intervals, with the aspen and willow randomly mixed into the poplar. As the transplanted saplings were originally harvested from an area where balsam poplar and black cottonwood are known to coexist and hybridize, no attempt was made to distinguish between these two species. Both the ET and CSC
W. Schnabel et al. covers within their respective lysimeters were underlain by a 120 cm layer of common fill material. Aerial precipitation was measured at an onsite weather station during the months of May through September of each year, and obtained from the Ted Stevens International Airport weather station, approximately 12 km to the west, during the periods from October through April. Drainage through each cover was collected through a drain in the bottom of each lysimeter, piped to a nearby manhole, and measured via tipping bucket rain gauge. Lysimeter drainage in this study was used as a proxy for drainage through a field-scale cover into an underlying waste layer; hence used as a measure of cover effectiveness. Numerous other parameters including runoff volume, soil temperature, and detailed climatic observations were collected in conjunction with the drainage and precipitation data, and were reported previously for the initial portion of the study (Schnabel et al. 2012b). As described, the covers were installed and planted in summer 2004. Instrumented comparison of the two covers began in August 2005, after the covers were allowed one year to expel any excess moisture accumulated during the construction process. The initial period of observation ran from August 2005 through July 2009, the results of which have been previously reported (Schnabel et al. 2012b). They are summarized in this manuscript to provide a basis of comparison against the final phase of the study. Due to a failure of the moisture metering instrumentation, the results from August 2009
Fig. 1. ET lysimeter (background) and CSC lysimeter (foreground) in July 2010.
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through July 2010 are not presented. The instrumentation was repaired/replaced in Summer 2010, allowing for accurate presentation of drainage results from August 2010 through March 2012. In March 2011, all 144 of the ET lysimeter trees were coppiced at a height of 30cm above ground surface. Thus, the period spanning April 2011 through March 2012 provides drainage results from the period in which the ET cover was recovering from the coppice event. In April 2012, the moisture metering instrumentation failed again, thus preventing us from reporting drainage results from that point forward. The regrowth of the coppiced ET cover trees was observed until October 2012, at which point they were harvested again and measured for biomass production.
Results and Discussion ET Cover Growth Summary Over the course of the initial phase of the study, the rooted saplings on the ET cover matured into a closed canopy stand measuring up to 5 m in height (Figure 1). The woody biomass (above 30 cm) production rate between summer 2004 and fall 2010 was measured to be 3.1 tonnes/ha-yr (dry weight). After the March 2011 coppice event, the woody vegetation sprouted new growth from the cut stems and root suckers the following May. After a single season of regrowth, multiple stems emerging from each cut stem reached approximately 2m in height. Following the second season of regrowth, the canopy had nearly closed, and the new growth exceeded 3m in height (Figure 2). The woody biomass (above 30 cm) production rate over the two-season regrowth period was measured to be 5.5 tonnes/ha-yr (dry weight). The increased biomass production rate observed in the regrowth compared to the original growth was not surprising, given that the trees following coppice arose from intact, well-developed root systems. Biomass production on the CSC cover was not integral to the study, and was therefore not measured. However we did observe that the density of grasses on the CSC remained relatively constant throughout the entire study period (Figure 1, foreground). ET and CSC Drainage Results The total monthly drainage from the ET and CSC lysimeters for each year of the project are presented in Figure 3, along with the measured monthly precipitation (rain or snowwater equivalents). For ease of comparison, the vertical axes in Figures 3A–3E are scaled consistently. Figures 3A–3D represent the initial phase of the study, prior to coppice. Again, the results summarized in Figures 3A–3D are presented and discussed in detail elsewhere (Schnabel et al. 2012b). Figure 3E encompasses the period leading up to and following the coppice event. As illustrated in Figure 3, the drainage from both lysimeters was seasonal in nature. The lysimeters tended to drain in the autumn following the seasonal rains, would taper off during the winter season, then drain again during and shortly after the spring snowmelt event. Lysimeter drainage tended to taper off again during the summer period.
Fig. 2. Coppiced ET lysimeter following two seasons of regrowth, October 2012.
In order to capture the observed seasonality of lysimeter drainage in the tabulated results, the data were parsed into two categories; each category representing one of the two annual drainage events. Thus, the drainage and efficacy results for the ET and CSC lysimeters, summed over six-month periods of observation, are presented in Table 1. Efficacy, in this case, represents the percentage of precipitation applied to the covers over a given period that did not result in drainage over the same period. In Table 1, the period termed fall/winter of each year represents the period ranging from August of the listed year through January of the following year. The spring/summer category represents the drainage results ranging from February through July of the listed year. When evaluating the results, readers should note that some amount of moisture remained in the soil profile from one period to the next. Thus, the drainage results reported over a given period could potentially have been influenced by the precipitation received during the previous period. To minimize this effect, the spring/summer and winter/fall categories were each initiated during periods when the lysimeter drainage was at its annual minimum, and the stored soil moisture was assumed to be at its lowest annual point. The relative efficacy of the ET versus CSC cover can be observed visually in Figure 3, as well as through an evaluation of the values presented in Table 1. For instance, while the
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fall/winter drainage was similar between the two cover types for the first two years of the study (Figures 3A and 3B), the ET cover clearly restricted fall/winter drainage more effectively than the CSC cover during the third and fourth years of the study (Figures 3C and 3D). This is evident in Table 1, as the ET cover was observed to be only 3% and 1% more effective than the CSC cover in 2005 and 2006, respectively, then 11% and 17% more effective during the two subsequent fall/winter periods. Indeed, by 2010, the fall/winter ET cover efficacy was 100%, while the CSC efficacy was 81% (Table 1). Thus, in the years leading up the coppice event, the ET cover appeared to be increasingly effective compared to the CSC cover during the fall/winter months. This disparity of cover efficacy was not as apparent during the spring/summer months. Indeed, the ET cover was found to be less effective than the CSC cover during spring/summer 2006, and only moderately more effective during the remaining spring/summer seasons prior to 2011 (Table 1). Hence, the covers tended to drain at relatively
comparable levels following the spring snowmelt events in the years leading up to the coppice. Drainage during the period following the March 2011 coppice event is illustrated in Figure 3E, and summarized in the final two rows of Table 1. As illustrated in Figure 3E, the ET cover drainage during the April/May 2011 snowmelt event was minimal compared to that of the CSC. Thus the ET cover efficacy was 36% higher than that of the CSC cover during that spring/summer period (Table 1). This was due in large part to the relative decrease in efficacy witnessed in the CSC during spring/summer 2011 compared to previous years (Table 1). The ET cover, by comparison, was almost equally effective in spring/summer 2011 compared to the previous measured year (2009), and more effective than all other years. That the ET lysimeter did not drain appreciably more than previous years immediately following the coppice event is not surprising. As noted in a previous report, the magnitude of the snowmelt drainage event in these cold region lysimeters is highly
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Table 1. Lysimeter drainage and efficacy, 2005–2011.
Season∗
Year
Pre-Coppice Results 2004 Spring/Summer 2004 Fall/Winter 2005 Spring/Summer 2005 Fall/Winter 2006 Spring/Summer 2006 Fall/Winter 2007 Spring/Summer 2007 Fall/Winter 2008 Spring/Summer 2008 Fall/Winter 2009 Spring/Summer 2009 Fall/Winter 2010 Spring/Summer 2010 Fall/Winter Post-Coppice Results 2011 Spring/Summer 2011 Fall/Winter
Precipitation (mm rain or snow-water equivalents)
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200 110 320
ET Drainage (mm)
ET Efficacy∗∗ (%)
CSC Drainage (mm)
CSC Efficacy (%)
Saplings transplanted, Summer 2004 Equilibration period. Drainage data not reported. Equilibration period. Drainage data not reported. 30 88 37 85 14 90 2 99 68 81 73 80 27 72 32 67 23 92 55 80 26 86 32 83 5 98 41 81 41 95 136 83 Instrumentation failure. Drainage data not available. Instrumentation failure. Drainage data note available. 0 100 37 81 7 17
93 95
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19 36 4
∗ Fall/Winter ∗∗ Efficacy
= August – January; Spring/Summer = February-July. = (Precipitation-Drainage)/Precipitation expressed as a percentage.
influenced by the moisture stored in the soil following the previous autumn rains (Schnabel et al. 2012b). As the ET lysimeter did not appreciably drain during fall/winter 2010, there was a component of moisture storage capacity available within the cover soils to help mitigate the spring snowmelt. During fall/winter 2011, drainage in the ET cover was similar to that of the CSC cover (Figure 3E), and the difference in efficacy between the two covers was only 4% (Table 1). As the ET cover had been increasingly more effective than the CSC during the fall/winter seasons leading up to the coppice, this represents an abrupt reversal in the previous trend. However, this result was due in part to an increased observed efficacy in the CSC cover in fall/winter 2011 compared to previous years (Table 1). The ET cover was 95% effective in fall/winter 2011, compared to 100% in 2010 and 98% in 2008. Given that the site received approximately 50% more fall/winter precipitation in 2011 compared to the two previous measured years (Table 1), this result was somewhat surprising. In the relatively wet fall/winter season following coppice, the ET cover efficacy was higher than it had been during the first three fall/winter seasons of the study, and only moderately lower than during the two relatively dry fall/winter seasons prior to coppice. Thus, there appeared to be little, if any, discernible impact of the coppice event upon ET cover function during the subsequent fall/winter season.
through the cover during the one-year period following the coppice event. Given the relatively robust regrowth of the trees following the coppice event, we expect that the ET cover would continue to perform well in subsequent years after the study period, with respect to moisture management. While the duration of this study was not sufficient to provide long-term data on post-coppice drainage, we conclude that utilizing fastgrowing ET cover trees as short rotation biomass crops is a promising strategy that merits further study.
Conclusions
Funding
This study found that the removal of top growth from a closed-canopy, primarily poplar/cottonwood ET cover in Anchorage, Alaska had little discernible effect on the drainage
This project was made possible through generous funding from the U.S. Air Force Office of Scientific Research, Award number FA9550-11-1-006.
Acknowledgments Critical logistical assistance was generously provided by the 673rd Civil Engineering Group, Joint Base Elmendorf Richardson. We also thank the Air Force Center for Engineering and the Environment, Weston Solutions, William Lee (University of Alaska Anchorage/Fairbanks), and Dr. Tarek Abichou (Florida State University) for their technical support during the construction and initial operational phases of the lysimeter facility. Finally, we appreciate the technical and field support provided by the Alaska Center for Energy and Power, along with Dr. Steve Sparrow and Darleen Masiak from the UAF School of Natural Resources and Agricultural Sciences, and Dr. David Barnes and Ken Irving from the UAF Water and Environmental Research Center.
164 References Abichou T, Musagasa J, Yuan L, Chanton J, Tawfiq K, Rockwood D, Licht L. 2011. Field performance of alternative landfill covers vegetated with cottonwood and eucalyptus trees. Int J Phytorem 14(sup1):47–60. Albright W, Benson C, Gee G, Roesler A, Abichou T, Apiwantragoon P, Lyles B, Rock S. 2004. Field water balance of landfill final covers. J Environ Qual 33:2317–2332. Barnswell KD, Dwyer DF. 2012. Two-year performance by evapotranspiration covers for municipal solid waste landfills in northwest Ohio. Waste Manage 32(12):2336–2341. Ettala MO. 1988. Short-rotation tree plantations at sanitary landfills. Waste Manage Res 6(3):291–302. Fayer MJ, Gee GW. 2006. Multiple-Yyear water balance of soil covers in a semiarid setting. J Environ Qual 35(1):366–377. Licht LA, Isebrands JG. 2005. Linking phytoremediated pollutant removal to biomass economic opportunities. Biomass Bioenergy 28(2):203–218.
W. Schnabel et al. Munk J, Schnabel WE, Barnes D, Lee W. 2011. Atmospheric loading effects on free-draining lysimeters. Water Resour Res 47 (W05541). Nixon DJ, Stephens W, Tyrrel SF, Brierley EDR. 2001. The potential for short rotation energy forestry on restored landfill caps. Bioresour Technol 77(3):237–245. Rock S, Myers B, Fiedler L. 2011. Evapotranspiration (ET) Covers. Int J Phytorem 14(sup1):1–25. Scanlon BR, Reedy RC, Keese KE, Dwyer SF. 2005. Evaluation of evapotranspirative covers for waste containment in arid and semiarid regions in the southwestern USA. Vadose Zone J 4(1): 55–71. Schnabel WE, Munk J, Abichou T, Barnes D, Lee W, Pape B. 2012a. Assessing the performance of a cold region evapotranspiration landfill cover using lysimetry and electrical resistivity tomography. Int J Phytorem 14(sup1):61–75. Schnabel WE, Munk J, Lee WJ, Barnes DL. 2012b. Four-year performance evaluation of a pilot-scale evapotranspiration landfill cover in Southcentral Alaska. Cold Region Sci Technol 82(0):1–7.
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Field Note: Comparative Efficacy of A Woody Evapotranspiration Landfill Cover Following the Removal of Aboveground Biomass WILLIAM SCHNABEL1, JENS MUNK2, and AMANDA BYRD3 1
University of Alaska Fairbanks, Water and Environmental Research Center, Fairbanks, Alaska, USA University of Alaska Anchorage, School of Engineering, Providence Drive, ENGR 201, Anchorage, Alaska, USA 3 University of Alaska Fairbanks, Alaska Center for Energy and Power, Fairbanks, Alaska, USA 2
Woody vegetation cultivated for moisture management on evapotranspiration (ET) landfill covers could potentially serve a secondary function as a biomass crop. However, research is required to evaluate the extent to which trees could be harvested from ET covers without significantly impacting their moisture management function. This study investigated the drainage through a six-year-old, primarily poplar/cottonwood ET test cover for a period of one year following the harvest of all woody biomass exceeding a height of 30 cm above ground surface. Results were compared to previously reported drainage observed during the years leading up to the coppice event. In the first year following coppice, the ET cover was found to be 93% effective at redirecting moisture during the spring/summer season, and 95% effective during the subsequent fall/winter season. This was slightly lower than the 95% and 100% efficacy observed in the spring/summer and fall/winter seasons, respectively, during the final measured year prior to coppice. However, the post-coppice efficacy was higher than the efficacy observed during the first three years following establishment of the cover. While additional longer-term studies are recommended, this project demonstrated that woody ET covers could potentially produce harvestable biomass while still effectively managing aerial moisture. Keywords: ET cover, water balance cover, alternative cover, biomass energy, lysimetry
Introduction Evapotranspiration (ET) covers represent a promising technology for the final closure of municipal solid waste landfills or contaminated sites in many arid and semi-arid locations. An ET cover can be defined as a relatively thick layer of minimally compacted soil placed over waste, usually planted with vegetation, that inhibits the downward migration of aerial moisture to the waste through the combined processes of soil storage, soil evaporation, and vegetative transpiration. Conventional covers, by comparison, are designed to inhibit downward migration of aerial moisture through the action of a minimally permeable synthetic, composite, or compacted soil barrier. Demonstrations of ET technology are widely reported in the literature, frequently in the context of relative efficacy compared to more conventional capping techniques (Abichou et al. 2011; Albright et al. 2004; Barnswell and Dwyer 2012;
Address correspondence to William Schnabel, University of Alaska Fairbanks, PO Box 755860, Fairbanks, AK 99775-5860. E-mail: [email protected] Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/bijp.
Fayer and Gee 2006; Rock, Myers, and Fiedler 2011; Scanlon et al. 2005). While the primary function of ET covers is to inhibit percolation of aerial moisture to the underlying waste, many could potentially produce marketable biomass as a by-product of their action. Thus, there exists a potential to realize secondary economic benefits to help offset the cost of the ET cover itself. Indeed, Licht and Isebrands (2005) outlined a number of case studies illustrating how such benefits could be realized by similar phytoremediation projects. In cold regions, ET cover biomass could potentially be treated as a short rotation coppice crop (SRCC) and used as a community heating source. While the concept of growing SRCC at conventional landfill locations has been proposed (Ettala 1988; Nixon et al. 2001), the effects of this practice upon the functionality of ET covers has not been well studied. The purpose of this study was to investigate the functionality of an ET cover after harvesting all woody vegetation at 30cm above ground surface. This was intended to simulate the use of ET cover trees as short rotation coppice crops in the context of biomass energy production. Drainage results following coppice were compared to the performance of the ET cover in the years leading up to the coppice event, as well as to the performance of a conventional cover employed as a control.
160 Experimental Methods The study was conducted at a lysimeter installation located at Joint Base Elmendorf Richardson (JBER) in Anchorage, Alaska. Detailed descriptions of the lysimeter facilities, installation techniques, cover specifications, soil properties, vegetation makeup, and initial results were previously reported (Munk et al. 2011; Schnabel et al. 2012a; Schnabel et al. 2012b). In short, two adjacent basin lysimeters 10 m × 20 m × 1.6 m deep were installed to compare the drainage response of a compacted soil cover (CSC) to that of an ET cover in 2004. The CSC cover consisted of three 15 cm lifts of silt (USCS-ML) compacted to yield a saturated hydraulic conductivity less than 10−5 cm/sec, overlain by one 15 cm lift of topsoil planted with local grasses. The ET cover consisted of one 60cm layer of silt and silty sand (USCS-ML and USCS-SM), compacted using low ground pressure equipment to 80–90% of maximum proctor density, planted with a mixture of 80% black cottonwood/balsam poplar (Populus trichocarpa/Populus balsamifera), 10% quaking aspen (Populus tremuloides, and 10% little leaf/golden willow (Salix alba vitel, arbusculoides). The saplings were spaced in a square grid at approximate 1.2 m intervals, with the aspen and willow randomly mixed into the poplar. As the transplanted saplings were originally harvested from an area where balsam poplar and black cottonwood are known to coexist and hybridize, no attempt was made to distinguish between these two species. Both the ET and CSC
W. Schnabel et al. covers within their respective lysimeters were underlain by a 120 cm layer of common fill material. Aerial precipitation was measured at an onsite weather station during the months of May through September of each year, and obtained from the Ted Stevens International Airport weather station, approximately 12 km to the west, during the periods from October through April. Drainage through each cover was collected through a drain in the bottom of each lysimeter, piped to a nearby manhole, and measured via tipping bucket rain gauge. Lysimeter drainage in this study was used as a proxy for drainage through a field-scale cover into an underlying waste layer; hence used as a measure of cover effectiveness. Numerous other parameters including runoff volume, soil temperature, and detailed climatic observations were collected in conjunction with the drainage and precipitation data, and were reported previously for the initial portion of the study (Schnabel et al. 2012b). As described, the covers were installed and planted in summer 2004. Instrumented comparison of the two covers began in August 2005, after the covers were allowed one year to expel any excess moisture accumulated during the construction process. The initial period of observation ran from August 2005 through July 2009, the results of which have been previously reported (Schnabel et al. 2012b). They are summarized in this manuscript to provide a basis of comparison against the final phase of the study. Due to a failure of the moisture metering instrumentation, the results from August 2009
Fig. 1. ET lysimeter (background) and CSC lysimeter (foreground) in July 2010.
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through July 2010 are not presented. The instrumentation was repaired/replaced in Summer 2010, allowing for accurate presentation of drainage results from August 2010 through March 2012. In March 2011, all 144 of the ET lysimeter trees were coppiced at a height of 30cm above ground surface. Thus, the period spanning April 2011 through March 2012 provides drainage results from the period in which the ET cover was recovering from the coppice event. In April 2012, the moisture metering instrumentation failed again, thus preventing us from reporting drainage results from that point forward. The regrowth of the coppiced ET cover trees was observed until October 2012, at which point they were harvested again and measured for biomass production.
Results and Discussion ET Cover Growth Summary Over the course of the initial phase of the study, the rooted saplings on the ET cover matured into a closed canopy stand measuring up to 5 m in height (Figure 1). The woody biomass (above 30 cm) production rate between summer 2004 and fall 2010 was measured to be 3.1 tonnes/ha-yr (dry weight). After the March 2011 coppice event, the woody vegetation sprouted new growth from the cut stems and root suckers the following May. After a single season of regrowth, multiple stems emerging from each cut stem reached approximately 2m in height. Following the second season of regrowth, the canopy had nearly closed, and the new growth exceeded 3m in height (Figure 2). The woody biomass (above 30 cm) production rate over the two-season regrowth period was measured to be 5.5 tonnes/ha-yr (dry weight). The increased biomass production rate observed in the regrowth compared to the original growth was not surprising, given that the trees following coppice arose from intact, well-developed root systems. Biomass production on the CSC cover was not integral to the study, and was therefore not measured. However we did observe that the density of grasses on the CSC remained relatively constant throughout the entire study period (Figure 1, foreground). ET and CSC Drainage Results The total monthly drainage from the ET and CSC lysimeters for each year of the project are presented in Figure 3, along with the measured monthly precipitation (rain or snowwater equivalents). For ease of comparison, the vertical axes in Figures 3A–3E are scaled consistently. Figures 3A–3D represent the initial phase of the study, prior to coppice. Again, the results summarized in Figures 3A–3D are presented and discussed in detail elsewhere (Schnabel et al. 2012b). Figure 3E encompasses the period leading up to and following the coppice event. As illustrated in Figure 3, the drainage from both lysimeters was seasonal in nature. The lysimeters tended to drain in the autumn following the seasonal rains, would taper off during the winter season, then drain again during and shortly after the spring snowmelt event. Lysimeter drainage tended to taper off again during the summer period.
Fig. 2. Coppiced ET lysimeter following two seasons of regrowth, October 2012.
In order to capture the observed seasonality of lysimeter drainage in the tabulated results, the data were parsed into two categories; each category representing one of the two annual drainage events. Thus, the drainage and efficacy results for the ET and CSC lysimeters, summed over six-month periods of observation, are presented in Table 1. Efficacy, in this case, represents the percentage of precipitation applied to the covers over a given period that did not result in drainage over the same period. In Table 1, the period termed fall/winter of each year represents the period ranging from August of the listed year through January of the following year. The spring/summer category represents the drainage results ranging from February through July of the listed year. When evaluating the results, readers should note that some amount of moisture remained in the soil profile from one period to the next. Thus, the drainage results reported over a given period could potentially have been influenced by the precipitation received during the previous period. To minimize this effect, the spring/summer and winter/fall categories were each initiated during periods when the lysimeter drainage was at its annual minimum, and the stored soil moisture was assumed to be at its lowest annual point. The relative efficacy of the ET versus CSC cover can be observed visually in Figure 3, as well as through an evaluation of the values presented in Table 1. For instance, while the
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Month (2006-2007) ET Drainage (mm)
CSC Drainage (mm)
Precipitaon (cm) 14
40
Figure 3D
30
9
20 4
10
-1
0 A
S
O N
D
J
F M A M
J
J
Month (2008-2009)
CSC Drainage (mm)
Precipitaon (cm) 14
40
Figure 3E
Coppice Event
30
9
20 4
10 0
-1 A
S
O
N
D
J
F
M
A
M
J
Precipitaon (cm)
Figure 3A
30
Precipitaon (cm)
14
Precipitaon (cm)
Lysimeter Drainage (mm)
40
ET Drainage (mm)
J
A
S
O
N
D
J
F
Precipitaon (cm)
Precipitaon (cm) Lysimeter Drainage (mm)
CSC Drainage (mm)
Lysimeter Drainage (mm)
ET Drainage (mm)
Precipitaon (cm)
162
M
Month (2010-2012)
Fig. 3. Lysimeter drainage (mm) and precipitation (cm) results prior to the coppice event (A–D), and following the coppice event (E).
fall/winter drainage was similar between the two cover types for the first two years of the study (Figures 3A and 3B), the ET cover clearly restricted fall/winter drainage more effectively than the CSC cover during the third and fourth years of the study (Figures 3C and 3D). This is evident in Table 1, as the ET cover was observed to be only 3% and 1% more effective than the CSC cover in 2005 and 2006, respectively, then 11% and 17% more effective during the two subsequent fall/winter periods. Indeed, by 2010, the fall/winter ET cover efficacy was 100%, while the CSC efficacy was 81% (Table 1). Thus, in the years leading up the coppice event, the ET cover appeared to be increasingly effective compared to the CSC cover during the fall/winter months. This disparity of cover efficacy was not as apparent during the spring/summer months. Indeed, the ET cover was found to be less effective than the CSC cover during spring/summer 2006, and only moderately more effective during the remaining spring/summer seasons prior to 2011 (Table 1). Hence, the covers tended to drain at relatively
comparable levels following the spring snowmelt events in the years leading up to the coppice. Drainage during the period following the March 2011 coppice event is illustrated in Figure 3E, and summarized in the final two rows of Table 1. As illustrated in Figure 3E, the ET cover drainage during the April/May 2011 snowmelt event was minimal compared to that of the CSC. Thus the ET cover efficacy was 36% higher than that of the CSC cover during that spring/summer period (Table 1). This was due in large part to the relative decrease in efficacy witnessed in the CSC during spring/summer 2011 compared to previous years (Table 1). The ET cover, by comparison, was almost equally effective in spring/summer 2011 compared to the previous measured year (2009), and more effective than all other years. That the ET lysimeter did not drain appreciably more than previous years immediately following the coppice event is not surprising. As noted in a previous report, the magnitude of the snowmelt drainage event in these cold region lysimeters is highly
Coppice Effects on Evapotranspiration Cover Function
163
Table 1. Lysimeter drainage and efficacy, 2005–2011.
Season∗
Year
Pre-Coppice Results 2004 Spring/Summer 2004 Fall/Winter 2005 Spring/Summer 2005 Fall/Winter 2006 Spring/Summer 2006 Fall/Winter 2007 Spring/Summer 2007 Fall/Winter 2008 Spring/Summer 2008 Fall/Winter 2009 Spring/Summer 2009 Fall/Winter 2010 Spring/Summer 2010 Fall/Winter Post-Coppice Results 2011 Spring/Summer 2011 Fall/Winter
Precipitation (mm rain or snow-water equivalents)
250 140 370 100 280 180 220 800
200 110 320
ET Drainage (mm)
ET Efficacy∗∗ (%)
CSC Drainage (mm)
CSC Efficacy (%)
Saplings transplanted, Summer 2004 Equilibration period. Drainage data not reported. Equilibration period. Drainage data not reported. 30 88 37 85 14 90 2 99 68 81 73 80 27 72 32 67 23 92 55 80 26 86 32 83 5 98 41 81 41 95 136 83 Instrumentation failure. Drainage data not available. Instrumentation failure. Drainage data note available. 0 100 37 81 7 17
93 95
46 30
57 91
ET – CSC Efficacy (%)
3 -9 1 5 11 3 17 12
19 36 4
∗ Fall/Winter ∗∗ Efficacy
= August – January; Spring/Summer = February-July. = (Precipitation-Drainage)/Precipitation expressed as a percentage.
influenced by the moisture stored in the soil following the previous autumn rains (Schnabel et al. 2012b). As the ET lysimeter did not appreciably drain during fall/winter 2010, there was a component of moisture storage capacity available within the cover soils to help mitigate the spring snowmelt. During fall/winter 2011, drainage in the ET cover was similar to that of the CSC cover (Figure 3E), and the difference in efficacy between the two covers was only 4% (Table 1). As the ET cover had been increasingly more effective than the CSC during the fall/winter seasons leading up to the coppice, this represents an abrupt reversal in the previous trend. However, this result was due in part to an increased observed efficacy in the CSC cover in fall/winter 2011 compared to previous years (Table 1). The ET cover was 95% effective in fall/winter 2011, compared to 100% in 2010 and 98% in 2008. Given that the site received approximately 50% more fall/winter precipitation in 2011 compared to the two previous measured years (Table 1), this result was somewhat surprising. In the relatively wet fall/winter season following coppice, the ET cover efficacy was higher than it had been during the first three fall/winter seasons of the study, and only moderately lower than during the two relatively dry fall/winter seasons prior to coppice. Thus, there appeared to be little, if any, discernible impact of the coppice event upon ET cover function during the subsequent fall/winter season.
through the cover during the one-year period following the coppice event. Given the relatively robust regrowth of the trees following the coppice event, we expect that the ET cover would continue to perform well in subsequent years after the study period, with respect to moisture management. While the duration of this study was not sufficient to provide long-term data on post-coppice drainage, we conclude that utilizing fastgrowing ET cover trees as short rotation biomass crops is a promising strategy that merits further study.
Conclusions
Funding
This study found that the removal of top growth from a closed-canopy, primarily poplar/cottonwood ET cover in Anchorage, Alaska had little discernible effect on the drainage
This project was made possible through generous funding from the U.S. Air Force Office of Scientific Research, Award number FA9550-11-1-006.
Acknowledgments Critical logistical assistance was generously provided by the 673rd Civil Engineering Group, Joint Base Elmendorf Richardson. We also thank the Air Force Center for Engineering and the Environment, Weston Solutions, William Lee (University of Alaska Anchorage/Fairbanks), and Dr. Tarek Abichou (Florida State University) for their technical support during the construction and initial operational phases of the lysimeter facility. Finally, we appreciate the technical and field support provided by the Alaska Center for Energy and Power, along with Dr. Steve Sparrow and Darleen Masiak from the UAF School of Natural Resources and Agricultural Sciences, and Dr. David Barnes and Ken Irving from the UAF Water and Environmental Research Center.
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