chapter  9
52 Pages

Heterogeneity of Soils in Fields

I. Heterogeneity ............................................................. 352 II. Representative Elementary Volume.......................... 354

A. Criteria for REV.................................................. 354 B. REV for Microstructural Heterogeneity ............ 355 C. REV for Physical Properties of Soils in Fields .. 357 D. REV for Hydraulic Permeability of Soil

with Macropores ................................................. 359 E. Other REVs ......................................................... 361

III. Similar-Media Concept and Scaling .......................... 362 A. Characteristic Length and Scaling..................... 362 B. Similarity of Pressure and Matric

Head of Water ..................................................... 363 C. Similarity of Hydraulic Conductivity................. 366 D. Verification and Limitation of Similar-Media

Concept................................................................ 369 E. Scaling of Natural Soils ...................................... 371

IV. Nonsimilar Media Concept and Scaling ................... 372 A. Characteristic Lengths and Shape Factor ......... 372 B. Deduced Bulk Density ........................................ 374

C. NSMC Model for Aggregated Soils....................... 374 D. Bulk Density Dependence of Air Entry

Suction and Hydraulic Conductivity.................... 376 1. NSMC Model................................................... 376 2. Campbell’s Method ......................................... 376 3. Kozeny-Carman Equation ............................. 377 4. Comparison of Models .................................... 378

E. Determination of the Shape Factor...................... 380 F. Application of the NSMC Model in Fields........... 382 G. Applicability and Limitation of the

NSMC Model......................................................... 383 H. Extension of the NSMC Model to

Unsaturated Soils ................................................. 384 V. Spatial Variability and Geostatistics.......................... 386

A Spatial Variability ................................................. 386 B. Geostatistics .......................................................... 388

1. Semivariogram ............................................... 388 2. Models of Semivariogram............................... 389 3. Spatial Structure in Fields............................. 390 4. Geostatistical Estimation............................... 393

VI. Macropores .................................................................. 395 A. Types of Macropores ............................................. 395 B. Buried Macropores................................................ 396

Appendix: The b-Value Estimation of Campbell’s Method .......................................................... 398

References............................................................................ 399

I. HETEROGENEITY

Uniformity and heterogeneity of soil properties in a field are defined in terms of the spatial variabilities of bulk density, water content, matric potential, soil temperature, soil hardness, solute content, soil particle composition, etc. Since almost all of these soil properties are defined and measured with respect to an element volume, the definition of uniformity and heterogeneity of soils depends on the element size or measurement size.