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Variability of Gardner's α for coarse‐textured sediments
Author(s) -
Khaleel Raziuddin,
Relyea John F.
Publication year - 2001
Publication title -
water resources research
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.863
H-Index - 217
eISSN - 1944-7973
pISSN - 0043-1397
DOI - 10.1029/2000wr900398
Subject(s) - vadose zone , soil water , hydraulic conductivity , log normal distribution , soil science , geology , mineralogy , particle size distribution , range (aeronautics) , hydrology (agriculture) , mathematics , geotechnical engineering , particle size , materials science , statistics , paleontology , composite material
Vadose zone soils in arid regions often contain a high gravel fraction (>2 mm size). This paper examines, for coarse‐textured soils, variability of the slope α of Gardner 's [1958] unsaturated hydraulic conductivity K versus matric potential ψ relation, K = K s e αψ . The steady state head control method was used to obtain unsaturated K for tensions as high as 386 cm. A total of 79 samples were analyzed in the laboratory; the gravel fraction for the 41 gravelly samples ranged from 20 to 71% (by weight); the remaining 38 samples were sandy with no gravel fraction. The measured K values for the gravelly samples fall within a narrow range and are well within the range of measured K values for the sandy samples. The bulk of both gravelly and sandy samples can be described using a single‐slope exponential model; Gardner's α and the intercept K 0 are based on a least squares fit to the late time measurements at lower ψ. The mean and variance of α for the gravelly samples are lower compared with the sandy type. The α values for the gravelly soils follow a normal distribution, whereas the K 0 estimates and the laboratory‐measured K s follow lognormal distributions. For the sandy type, α, K 0 , and K s follow lognormal distributions. The α values appear to be related to the median particle diameter d 50 and the slope of the particle size distribution. A clear reduction in variability of α is apparent with an increase in d 50 , suggesting that the unsaturated K and α variability for the gravelly soils can be estimated within narrow ranges. The flow‐weighted characteristic pore size λ m is clearly dependent on the soil type; λ m values for the gravelly soils are characterized by a smaller mean and variance than for the sandy type. This helps explain the finding that the range in unsaturated K for the gravelly soils is less than for the sandy type. At higher tensions the mean and variance of α and ln K 0 as well as the cross correlation between α and ln K 0 are important inputs to stochastic models of unsaturated flow. The implications of their variability on flow in heterogeneous media are discussed.