The relationship between desiccation cracking and soil moisture changes in a clay soil at Tinui, Wairarapa

Abstract

The aim of this thesis is to study the relationship between desiccation cracking and soil moisture changes in a clay soil at Tinui, in eastern Wairarapa. The physico-chemical characteristics of Whakaroro clay loam (the soil under study), are discussed with reference to the theory of clay shrinkage. The soil may be regarded as a cracking clay due to the combined effect of high clay contents, a high percentage of the montmorillonite clay mineral, and a high cation-exchange capacity, which produce high swelling potentials and desiccation cracks. The behaviour of cracks in the study area over the period, summer 1972/73 to summer 1974/75, is discussed and related to site factors such as vegetation and topography, in so far as these reflect soil moisture changes. Several types of crack development took place, depending on soil moisture, topography and vegetation density. The frequency and size of cracks was affected by the amount and frequency of rainfall and the density of vegetation; sparsely vegetated areas having more cracks of a larger dimension. The effect of cracks on soil moisture is gauged and the variation in crack behaviour is explained by an index of the soil moisture regime. Soil adjacent to cracks was subjected to the greatest extremes in soil moisture and moistened more rapidly and thus to a greater depth than soil a short distance from the crack. During winter cracks provided a continuous pathway for water movement. The rate of change of soil moisture tension with depth was accepted as a good predictor of the cracking behaviour of the soil.