7.1 limiteD perColation Some landfill sites require little percolation of precipitation into the waste; these sites present the greatest challenge for cover performance.. Although intere
Trang 1The evapotranspiration (ET) landfill cover satisfies wide variations in site needs It applies where the requirements for a cover include little percolation into the waste At the other extreme, its design and construction are flexible and it can allow a large part of average annual precipitation to enter the waste
in order to meet requirements for waste stabilization
The following chapters (Chapters 8 to 13) discuss the design process During site assessment, the planning staff and designers need methods to make an informed initial choice of cover type This chapter focuses on methods to make
an initial assessment about whether an ET cover is appropriate for a particular site
7.1 limiteD perColation
Some landfill sites require little percolation of precipitation into the waste; these sites present the greatest challenge for cover performance Climate is a major determinant
of ET cover performance at a given site and the evaporation-to-precipitation ratio is naturally most favorable in arid and semiarid areas Analysis of readily available cli-mate information provides an inexpensive initial assessment for these landfill sites Although interesting, monthly or annual average values of precipitation, tem-perature, wind, etc., do not produce a satisfactory estimate of the potential for use
of the ET landfill cover Short-term or daily weather events are usually the cause of excess percolation This chapter presents a method for initial assessment; it is based
on estimates of daily weather, including natural daily variability and the resultant size of each term in the water balance at numerous sites It includes extreme events and their effect on annual or average ET, Q, and PRK during a 100 year period
7.1.1 e vaPotranSPIratIon
PET is an easily calculated upper bound for ET The PET varies in response to daily weather factors including humidity, air temperature, and solar radiation PET pro-vides a useful way to evaluate suitability of the ET cover to conditions at a site because it is the upper bound of ET for the site
The ET is less than PET, except for short times when the surface is wet immedi-ately after precipitation Natural stress factors applied to the PET result in estimates
of ET Plant stress due to limited soil water supply is a primary limitation to plant growth; it is a useful indicator of the frequency and duration of dry soil in the ET cover during the year
Trang 27.1.2 c alculatIonS
Hauser and Gimon (2001) calculated daily values of PET, ET, and number of days when soil dryness was the most limiting ingredient for grass growth at 60 locations
in the United States The locations included hot, cold, wet, and dry sites (Figure 7.1) Averages from their data form the basis for general regional maps, indicating the possible level of effectiveness of the ET cover for the continental United States They used the Environmental Policy Integrated Climate (EPIC) model to esti-mate PET, ET, and number of days when soil dryness was the most limiting factor for grass growth at each site The EPIC model and its earlier versions meet the require-ments for ET estimation (Sharpley and Williams 1990; Williams et al 1990; Hauser
et al 2005)
The EPIC model estimates PET for each day, and uses the sum of daily stress factors to estimate ET One stress indicator is the total number of days when soil water content was the most limiting factor for plant growth during each year (water stress days)
The Penman–Monteith method is the most accurate and robust method available for calculating PET; however, it requires a complete climate data set, including solar radiation, daily wind run, and relative humidity (Jensen et al 1990) Available data with adequate record length included only daily precipitation and maximum and minimum air temperatures The Priestly–Taylor and Hargreaves methods estimate PET with acceptable accuracy, if used in regions for which they were developed; they need only the available data (Jensen et al 1990)
Hauser and Gimon (2001) used the Priestly–Taylor ET estimation method east of 100° west longitude and the Hargreaves method for locations west of that line They used the EPIC model to estimate daily values of PET, and from the daily values, they calculated annual estimates of PET for a 100 year period
The EPIC model includes tested climate data for sites near each location It stochastically generated daily values of weather parameters for each location from monthly mean values of rainfall, temperature, wind data, and associated statistics The stochastically generated climate data contains extreme events and has statistical prop-erties similar to measured data (Sharpley and Williams 1990; Williams et al 1990)
figure 7.1 Location of 60 PET evaluation sites.
Trang 3The plant cover consisted of a monoculture of grass that is adapted to the region and climate of each location Each grass has the potential to grow roots 2 m or more into the soil and to extract water from that depth Table 7.1 contains a list
of the grasses and PET equations used Each model estimate used the same soil (Table 7.2)
7.1.3 Pet- to -P recIPItatIon r atIo
The ratio of PET to precipitation is a useful statistic
PET ratio = Annual PET Annual precipitation (7.1) where
Annual PET = average annual total of daily PET
Annual precipitation = average annual total of daily precipitation
taBle 7.1
grass Cover and pet equation used for each region of the united states
Northeast (west to 100°) Russian wild rye grass Priestly–Taylor Southeast (west to 100°) Switch grass Priestly–Taylor Rocky Mountain region (South to AR–NM border) Crested wheat grass Hargreaves Southwest (east to 100°) Range grass mixture Hargreaves West Coast (east to Sierra Nevada and Cascade Mountains) Annual rye grass Hargreaves
taBle 7.2 properties of the soil mixture used for pet and et estimates
soil property value
Sand content 14.2%
Silt content 41.7%
Clay content 44.1%
Bulk density 1.4 Mg/m 3
Wilting point 0.18 v/v Field capacity 0.34 v/v
Organic carbon 1.4%
Cation exchange capacity 21.0 cmol/kg Soil thickness 2.0 m Number of soil layers 10 Hydrologic soil group D
Trang 4Where the PET ratio is large, it is likely that an ET cover will satisfy the require-ments for a landfill cover because evaporation potential greatly exceeds precipita-tion Where the PET ratio is equal to one, evaporation potential is equal to annual precipitation A PET ratio of 1.2 was chosen as the division point between expected satisfactory and unsatisfactory results
The PET ratio is greater than 1.2 for most of the United States (Figure 7.2) In small areas along the Gulf Coast, in Northern New England, and in the coldest cli-mates, the PET ratio indicates caution in using the ET landfill cover However, only
5 of the 60 sites examined had PET ratios less than or equal to 1.2
The PET ratio is greater than 1.5 for the western two thirds of the country with one exception The exception is the cold wet strip extending from the Pacific Ocean
to the coastal range of mountains, between Canada and San Francisco Because aver-age annual precipitation is high in these cool coastal areas, the ET cover should be evaluated for each site It is clear that the ET landfill cover is suitable for use in most
of the country
7.1.4 W ater S treSS d ayS Per y ear
A primary goal of an ET cover is to keep the soil of the cover as dry as possible to provide ample water storage capacity to hold the water produced by extreme events
On days when the soil of the cover is dry enough to limit plant growth and water use, the potential water storage capacity of the cover is high After several days of plant water stress, the soil water reservoir will be empty, or nearly so, and will thus be able to hold storm water to its maximum capacity The number of water stress days per year provides a useful addition to the preliminary selection process It is not an average; it is determined by extreme events
As explained earlier, water stress is one of several stress factors The water-stress-day parameter includes only the days when it was the most limiting factor Water stress existed on other days when other factors were larger As a result, EPIC’s estimate of water stress days is conservative Figure 7.3 shows the geographic distri-bution of water stress days estimated by the EPIC model
High
2.5
>5
1.5
<2.5 >2.5
2.5
<1
<1 1.5
figure 7.2 Average annual PET-to-precipitation ratio.
Trang 5It is desirable for the soil water reservoir within an ET cover to be empty at least once each year Water stress during 10 or more days per year indicates that the soil water reservoir was significantly depleted or nearly empty at those times Ten sites near the Gulf of Mexico or the Atlantic Ocean coasts had about 10 water stress days per year The site with the shortest growing season tested in the United States was located in northern Maine, and it had 20 water stress days per year
For all areas where the number of water stress days exceeds 25, it is likely that the water reservoir in the ET cover is nearly empty more than once each year These data fully support the conclusions based on the PET ratio
7.2 inCreaseD perColation for Waste staBilization
Where decay and rapid stabilization of waste are important goals for landfill reme-diation, it is important for precipitation to move through the cover and into the waste
to maintain the desired water content At these sites, it is easy to reduce the thickness
of the cover or to use soil with lower water-holding capacity to produce the desired infiltration into the waste
At dry sites where the need for a physical cover to confine the waste controls the thickness of the cover, additional measures may be required to produce increased percolation through the cover For example, natural stony soils or fine-textured soil with added gravel will produce greater drainage through the cover than for an equal depth of fine-textured soil Gravel mixed into the cover soil in a dry region may provide two benefits First, the cover may allow adequate deep percolation Second, the grass cover is more robust where coarse material in the soil causes water from a small shower to wet the soil deeper for plant growth, thus leaving less water from the shower in top layers where it may be wasted by surface evaporation
7.3 appropriate use
The ET landfill cover is appropriate for use at almost all sites in the United States, where deep percolation should be limited Evaluate each site in the coastal areas of
25 25
100
>100
<25
<25
figure 7.3 Average number of water stress days per year.
Trang 6Louisiana, Mississippi, and Alabama, and a narrow strip along the west coast from Canada south to San Francisco, California
For sites where a significant portion of rainfall should pass into the waste to hasten decay and landfill stabilization, the ET landfill cover is appropriate for use anywhere in the United States
referenCes
Hauser, V L and Gimon, D M (2001) Vegetated Landfill Covers and Phytostabilization:
The Potential for Evapotranspiration-Based Remediation at Air Force Bases. The Air Force Center for Environmental Excellence (AFCEE), Brooks City Base, San Antonio,
TX http://www.afcee.brooks.af.mil/products/techtrans/landfillcovers/LandfillProto-cols.asp (accessed March 17, 2008).
Hauser, V L., Gimon, D M., Bonta, J V., Howell, T A., Malone, R W., and Williams, J R
(2005) Models for hydrologic design of evapotranspiration landfill covers, Environ
Sci Technol., 39, 7226–7233.
Jensen, M E., Burman, R D., and Allen, R G., Eds (1990) Evapotranspiration and
Irriga-tion Water Requirements ASCE Manual No 70 American Society of Civil Engineers, Reston, VA.
Sharpley, A N and Williams, J R., Eds (1990) Erosion/Productivity Impact Calculator: 1
Model Documentation Technical Bulletin 1768 USDA, Washington, DC.
Williams, J R., Dyke, P T., Fuchs, W W., Benson, V W., Rice, O W., and Taylor, E D
(1990) EPIC—Erosion/Productivity Impact Calculator: 2 User Manual Technical
Bulletin 1768 USDA, Washington, DC.