The surface elevation of a shrinking soil provided good estimates of the change in profile water content under alfalfa. Surface shrinkage was found to change linearly with water content as measured by neutron and gamma attenuation devices. Extracted soil cores were tested in the laboratory to obtain the linear shrinkage ratio, b. In the field, shrinkage was calculated as the average elevation change of small ceramic plates. Compaction of the soil by harvesting machinery had no adverse effect on the linearity of the shrinkage/water loss relationship, but lack of compaction resulted in nonlinear shrinkage as the soil was unconsolidated and may have exhibited subsurface horizontal cracking. These cracks were attributed to roots which anchored the soil during shrinkage. In a second study, local values of swelling (b') from initial measurements before and after irrigation were superior estimators of measured water loss compared to a single universal b value from independent tests.
Research on the swelling behavior of vertisols has led to the concept of using the shrinkage of the soil surface to predict water content (Yule 1984a). Application of this concept under field conditions may be influenced by compaction caused by machinery traffic. Compaction would be expected to affect the cracking properties upon drying. This paper first addresses the basic principles underlying the interactions between water content and shrinkage to provide a basis to analyze field data obtained under different traffic regimes. The first experiment has as its objectives: (1) to appraise the utility of using surface elevation as a means for estimating changes in water content throughout the entire soil profile, and (2) to compare field shrinkage-water loss relationships with those of extracted soil cores. The purpose of the second experiment reported here was to validate the method of using surface elevation