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ES 470 SAMPLING AND ANALYSIS OF HYDROLOGICAL DATA Manoj K. Shukla, Ph.D. Assistant Professor

ES 470 SAMPLING AND ANALYSIS OF HYDROLOGICAL DATA Manoj K. Shukla, Ph.D. Assistant Professor Environmental Soil Physics. FEBRUARY 09, 2006, (W147, 3 - 5 PM). J. H. Dane G.C. Topp (Editors) Methods of Soil Analysis- Part 4, Physical Methods. ES-470. Scales of Variability.

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ES 470 SAMPLING AND ANALYSIS OF HYDROLOGICAL DATA Manoj K. Shukla, Ph.D. Assistant Professor

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  1. ES 470 SAMPLING AND ANALYSIS OF HYDROLOGICAL DATA Manoj K. Shukla, Ph.D. Assistant Professor Environmental Soil Physics FEBRUARY 09, 2006, (W147, 3 - 5 PM)

  2. J. H. Dane G.C. Topp (Editors) Methods of Soil Analysis- Part 4, Physical Methods ES-470

  3. Scales of Variability Particles or Pore Aggregate Molecules Column or Horizon Field or Watershed Regional Pedosphere ES-470

  4. Variability Spatial: variability with increasing distance (space) from a location Temporal: variability with increasing duration/time We will limit our discussion to field scale ES-470

  5. Agriculture Field ??? • In situ soil exhibits large degree of variability or heterogeneity • Changes in soil types need to be accounted for in the composite sampling • The composite sample must maintain the heterogeneity of the insitu soil ES-470

  6. Sources Intrinsic Factors: Soil forming factors, time, soil texture, mineralogy, pedogenesis (geological, hydrological, biological factors) The intrinsic variables have a distinct component that can be called regionalized, i.e., it varies in space, with nearby areas tending to be alike Extrinsic Factors: Land use and management, fertilizer application, other amendments, drainage, tillage ES-470

  7. Structure of Variability Random sampling is done to ensure that estimates are unbiased Meet the criterion of independent sampling under identical conditions Yi = m + ei where Yi is the realization of a soil attribute at location i, m is the mean value for the spatial domain, and ei is a random error term ES-470

  8. An attribute (i.e., bulk density, nitrate concentration, etc.) is described through two statistical parameters E [Yi] = m First moment or Mean E [(Yi - m)2] = s2 Second moment or Variance ES-470

  9. E [Yi] = m E [(Yi - m)2] = s2 Mean and variance or first and second moment are often assumed to be the parameters of a normal (Gaussian) probability distribution function; and Allow for a series of sophisticated statistical analysis Arithmetic mean = m = (x1 + x2 + x3) / 3 Geometric mean = m = (x1* x2* x3)1/3 Harmonic mean = m = (1/x1 + 1/x2 + 1/x3)* (1/n) Variance (s2) = (1/n) * ∑(xi – xm)2 ES-470

  10. Soil N content data Mean = 1.35 g kg-1 Variance = 0 E [Yi] = 1.35 Mean = 1.339 g kg-1 Variance = 0.0003 E [Yi] = 1.339 ± (0.0003)0.5 ES-470

  11. Normal (Gaussian) Distribution Mean The function is symmetric about the mean, it gains its maximum value at the mean, the minimum value is at plus and minus infinity ES-470

  12. Histogram for Sand Content Sigma Plot 8.0 Normal distribution ES-470

  13. Histogram for Saturated Hydraulic Conductivity Skewed distribution- Positive ES-470

  14. Skewed distribution- Positive Skewed distribution- negative One of the tail is longer than other- Distribution is skewed ES-470

  15. Different Data Structures ES-470

  16. So in place of E [Yi] = m E [Yi] = m + b(xi) + ei An Appropriate model Where b(xi) can be a constant or a function, both dependent on a spatial or temporal scale Therefore, simple randomization may not be sufficient Stratified sampling will be better Stratified sampling- the area is divided into sub areas called strata ES-470

  17. Case Study • Formulate objectives • Formulate hypotheses • Design a sampling scheme • Collect data • Data Interpretation Objective: Determine the relative magnitude of statistical and spatial variability at Field scale ES-470

  18. Sampling Design? • Simple random • Stratified • Two-stage • Cluster • Systematic 4 1 2 -3 5 ES-470

  19. How many samples? Sample size for simple random sampling Relative error should be smaller than a chosen limit (r) Where m1-a/2 = (1-a/2) quartile of the standard normal distribution; S- standard deviation of y in the area; is mean Standard deviation or coefficient of variation is known Absolute error to be smaller than a chosen limit d Time and Resources ???? ES-470

  20. Students t-table df = degree of freedom; p is probability level ES-470

  21. Example data of N concentration: 1.10, 1.11, 1.12, 1.13, 1.13, 1.14, 1.16, 1.17, 1.19, 1.20, 1.23, 1.24, 1.25 Relative error = 0.01 g kg-1 Mean of Y = 1.17 g kg-1 Standard deviation = 0.05 Alpha = 0.05 Degree of freedom = 13-1 = 12 t Students (table) = 1.782 ES-470

  22. Relative error (r) = 0.02 g kg-1 Alpha = 0.10 Degree of freedom = 13-1 = 12 T Students (table) = 1.782 r = 0.02 r = 0.01 ES-470

  23. E(Yi)s = Ym Var(Yi) =0 Deterministic parameters Variation in properties Stochastic parameters Mean value and an uncertainty statistics Variance Semi variogram function Var(Yi)s = s2s Var[(Yi)s-(Yi+h)s]= 2g(h) • It is always implied: • Domain is first- or second- order stationary • Process is adequately characterized by a mean value and an uncertainty statistics ES-470

  24. We will use a data collected on a grid of 20 x 20 cm in a field seeded to grass for last 20 years ES-470

  25. Variability can be expressed by coefficient of variation • Where: • x = an individual value • n = the number of test values • = the mean of n values Standard deviation of two independent sets where: n1 = number of values in the first set; s1 = standard deviation of the first set of values; n2 = number of values in second set; s2 = standard deviation of second set of values ES-470

  26. Statistical variability of soil properties at local scale Water Transmission Textural Coefficient of variation (CV) AWC- Available water content (cm) VTP - Volume of transport pores (qs-q6) (%) VSP - Volume of storage pores (%) ic - Steady state infiltration rate (cm/min) Ks - Sat. hydraulic conductivity (cm/min) I - Cumulative infiltration (cm) I5 - Infiltration rate at 5 min (cm/min) ES-470 Shukla et al. 2004

  27. Descriptive statistics (or CV) cannot discriminate between intrinsic (natural variations) and extrinsic (imposed) sources of variability Geostatistical analysis- grid based or spatial sampling For example-20 m x 20 m ES-470

  28. Range (a) Partial Sill (C1) ArC View Variowin Nugget (C0) Lag (h; m) Pannatier, 1996 ES-470

  29. Note: • g increases with increasing lag or separation distance • A small non-zero value may exist at g = 0 • This limiting value is known as nugget variance • It results from various sources of unexplained errors, such as measurement error or variability occurring at scales too small to characterize given the available data • At large h, many variograms have another limiting value • This limiting value is known as sill • Theoretically, it is equal to the variance of data • The value for h where sill occurs is known as range ES-470

  30. Variogram • The most common function used in geostatistical studies to characterize spatial correlation is the variogram • The variogram, g(h), is defined as one-half the variance of the difference between the sample values for all points separated by the distance h where var [ ] indicate variance and E { } expected value ES-470

  31. Estimator for the variogram is calculated from data using where N(h) is total number of pairs of observations separated by a distance h. Caution- variograms can be strongly affected by outliers in the data ES-470

  32. Variogram Model • Variogram model is a mathematical description of the relationship between the variance and the separation distance (or lag), h • There are four widely used equations ES-470

  33. Isotropic Models Linear Model Spherical Model Exponential Model Gaussian Model ES-470

  34. Sill C0, Nugget a, Range Linear Model Spherical Model Does not have a sill or range and the variance is undefined Precisely defined sill or range ES-470

  35. b ~ a/3 b ~ a/30.5 Exponential Model Gaussian Model Range is 1/3 of the range for spherical model Range is 1/sqrt(3) of the range for spherical model ES-470

  36. Variogram is constructed by • Calculating the squared differences for each pair of observations (xj - xk) • Determining the distance between each pair of observation • Averaging the squared differences for those pairs of observations with the same separation distance If observations are evenly spaced on a transect, separation distances are multiple of the smallest distance h1 = 2 m; h2 = 4m; h3 = 6 m …… ES-470

  37. When observations are placed on an irregular pattern, variograms are : • constructed by assigning appropriate lag interval • Binning procedure • B ins are created with interval centers at distances • h1 = (1-2) m; h2 = (2-4) m; h3 =(4-6) m ………………….. ES-470

  38. Important considerations when calculating a variogram: • As separation distance becomes too large, spurious results occur because fewer pairs of observation exist for large separations due to finite boundary • Width of lag interval can affect the sample variogram due to number of samples and variation in the separation distances that fall into a particular lag interval • Uncorrelated and correlated data show different nugget effects • Number of datasets used influence on variogram ES-470

  39. Before you start spatial analysis: Check for normal distribution WSA- water stability of aggregates (%) sand- sand content (%) Ic- saturated hydraulic conductivity (cm/h) ES-470

  40. Use of descriptive statistics Mean, median (most middle), skewness, etc. ES-470

  41. Plot the data to see the structure Y Saturated Hydraulic Conductivity X ES-470

  42. Estimator variance Example Variance = 13.7 Variance = 15.5 Variance = 16.1 ES-470

  43. Sand Content Saturated Hydraulic Conductivity ES-470

  44. Modeling of Variogram Sand Content Spherical Model SS = 0.04598 Nugget = 0 Range = 37.92 m Sill = 16.0 Spherical Model SS = 0.00994 Nugget = 3.04 Range = 49.77 m Sill = 16.0 ES-470

  45. Saturated Hydraulic Conductivity Spherical Model SS = 0.0494 Nugget = 0 Range = 19.8 m Sill = 0.0384 Spherical Model SS = 0.04938 Nugget = 0.004 Range = 19.8 m Sill = 0.0384 ES-470

  46. Parameters for spherical variogram model for soil properties ES-470

  47. Spatial variability: nugget – total sill ratio (NSR) Lower NSR – higher spatial dependence Water Transmission Textural Nugget to total sill ratio NSR < 0.25 highly spatial variable NSR > 0.75 less spatial variable Cambardella et al., 1994 Shukla et al. 2004 ES-470

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