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Evaluation of Evapotranspirative Covers for Waste Containment in Arid and Semiarid Regions in the Southwestern USA

^a Jackson School of Geosciences, Bureau of Economic Geology, The University of Texas at Austin, Austin, TX 78758
^b U.S. Dep. of Energy, Sandia Natl. Lab., Albuquerque, NM 87185

View larger version (18K): [in a new window]   Fig. 1. Location of monitored and engineered cover sites in Texas (Sierra Blanca) and New Mexico (Albuquerque) and vertical profiles of texture and materials for the different cover designs evaluated in the study. GCL, geosynthetic clay liner; GABET, GCL/asphalt barrier ET cover; CBET, capillary barrier ET cover; SCL, sandy clay loam; S, sand; MG, muddy gravel; G, gravel; LS, loamy sand. Numbers following textures indicate soil bulk density (Mg m–3). Texas site consisted of topsoil mixed with gravel (24 wt%); New Mexico site includes a 20- to 40-mm-thick gravel surface layer.

View larger version (22K): [in a new window]   Fig. 2. Average monthly precipitation during monitoring periods (lines) and historical records (columns) for (a) Texas site (Sierra Blanca, 29-yr average annual total = 311 mm) and (b) New Mexico site (Albuquerque, 30-yr average annual total = 226 mm).

View larger version (25K): [in a new window]   Fig. 3. Measured cumulative precipitation (P), irrigation (Irr), runoff (Ro), and net infiltration (Net I = P + Irr – Ro) and calculated cumulative ET for the CBET cover system for 1998 through 2001 water years at the Texas site. The long-term (1962–1990) average annual precipitation of 311 mm is shown (± 102 mm 1). Water year 2001 cumulative values are Irr: 2340 mm, Ro: 1866 mm, net I: 673 mm, and ET: 630 mm.

View larger version (24K): [in a new window]   Fig. 4. Daily ET rates to the 1.1-m depth calculated from monthly average values for the GABET and CBET systems and measured leaf area index (LAI) for the CBET system at the Texas site.

View larger version (69K): [in a new window]   Fig. 5. Texas site vegetation response to (a) natural summer precipitation (Aug. 2000) and (b) irrigation (Aug. 2001).

View larger version (30K): [in a new window]   Fig. 6. Average water storage (thick lines) to the 1.1-m depth in the GABET and CBET systems and to the 2.0-m depth in the CBET system at the Texas site. Thin lines represent the coefficient of variation (CV = 100 /µ) of water storage from 10 neutron probe access tube measurement locations in each design.

View larger version (33K): [in a new window]   Fig. 7. Average water content based on data from 10 neutron probe access tubes at selected depths in the CBET system and daily precipitation and irrigation at the Texas site.

View larger version (41K): [in a new window]   Fig. 8. (a) Daily precipitation (P) and irrigation (Irr) depths, (b) and (c) matric potential at selected depths at two locations in the CBET cover system at the Texas site, and (d) water potential monitored with thermocouple psychrometers in the adjacent natural setting at the Texas site.

View larger version (22K): [in a new window]   Fig. 9. Measured cumulative precipitation (P), runoff (Ro), and net infiltration (net I = P – Ro) and calculated cumulative evapotranspiration (ET) for the ET engineered cover to the 1.1-m depth for water years 1998 through 2002 at the New Mexico site. The long-term (1961–1990) average annual precipitation of 226 mm is shown (±55 mm 1 ).

View larger version (36K): [in a new window]   Fig. 10. Average plant cover (columns) and average water storage (thick lines) to the 1.1-m depth in the west and east subplots at the New Mexico site. Thin lines represent the coefficients of variation of five TDR measurement locations for both the west and east subplots.

View larger version (31K): [in a new window]   Fig. 11. Average measured water content for (a) all monitored depths within a given slope position (1: base of slope, 5: top of slope) and (b) all slope positions at a given monitored depth in the east subplot at the New Mexico site.

View larger version (14K): [in a new window]   Fig. 12. Simulated annual water balance results for the 1.1-m Texas profile with a unit gradient lower boundary condition.

View larger version (11K): [in a new window]   Fig. 13. Simulated annual water balance results for the 1.1-m New Mexico profile with a unit gradient lower boundary condition.

View larger version (16K): [in a new window]   Fig. 14. Sensitivity analysis for the Texas site simulation: 25-yr average annual values are shown for the base case simulation and for simulations with a single parameter altered as indicated. PET, potential evapotranspiration; RD, root depth; RLD, root length density; LAI, leaf area index; Ks, saturated hydraulic conductivity; Ku , unsaturated hydraulic conductivity function parameter; n, van Genuchten soil water retention function parameter; NM climate, simulation using New Mexico site climate forcing. Water balance parameters are S, water storage change; T, transpiration; E, evaporation; Ro, runoff; D, drainage.

View larger version (16K): [in a new window]   Fig. 15. Sensitivity analysis for the New Mexico site simulation: 25-yr average annual values are shown for the base case simulation and for simulations with a single parameter altered as indicated. PET, potential evapotranspiration; RD, root depth; RLD, root length density; LAI, leaf area index; Ks, saturated hydraulic conductivity; Ku , unsaturated hydraulic conductivity function parameter; n, van Genuchten soil water retention function parameter; TX climate, simulation using Texas site climate forcing. Water balance parameters are S, water storage change; T, transpiration; E, evaporation; Ro, runoff; D, drainage.

View larger version (21K): [in a new window]   Fig. 16. Calculated water content profiles for the Texas CBET system. All values shown are in millimeters and represent total water storage values over the intervals indicated by associated arrows. Heavy and thin solid line pairs converging at the zero height represent water content profiles for zero and unit downward total head gradient conditions, respectively. The pair of lines converging at 0.34 m3 m–3 water content represent profiles with a capillary barrier located at the zero height having a breakthrough water content equating to –0.3-m head. The two lines converging at 0.21 m3 m–3 water content represent profiles without a capillary barrier and water content at the zero height equating to a prescribed flux of 1 mm yr–1. Dashed lines represent wilting point water content profiles for uniform head conditions ranging from –150 m (long dash) to –500 m (short dash). Abrupt shift in water content near the top of each profile indicates transition from topsoil to subsoil.

View larger version (26K): [in a new window]   Fig. 17. Temporal variability of measured water content with depth in the Texas site CBET system. Calculated water content profiles for zero total head gradient conditions from Fig. 16 are shown in the background for reference. Average water content throughout the monitored period (µ) is shown with error bars, indicating the temporal standard deviation () at the monitored depths. Also shown are water content temporal minimum and maximum values at each depth. The two wettest measured water content profiles are shown (Sept. 1998 and Aug. 2001).