Pasture condition and soil desiccation as influences on tunnel-related erosion : a thesis presented in partial fulfilment of the requirements for the degree of Master of Philosophy in Geography at Massey University

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Massey University
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Soil conservation control measures have been developed to ameliorate the effects of tunnel-related erosion on agriculture in areas of New Zealand, usually areas with seasonally dry loessial soils. These control measures have included: mechanical destruction of both tunnels and related gullies, establishment of improved pasture, and maintenance of that pasture by lenient grazing. They were designed with the assumption that tunnels had formed as a result of concentrated water penetration into the subsoil via desiccation-induced shrinkage cracks. Criticism of previous research into the various aspects of tunnel-related erosion was undertaken and enabled the identification of some limitations, contradictions, and wrongly placed emphasis, in the accepted model of tunnel formation in loessial soils in New Zealand. An experiment was designed to measure the effects of grazing intensity upon soil moisture levels. This experiment resulted in the rejection of the accepted mechanism by which leniently grazed pasture was thought to reduce soil drying and subsequent cracking, as soil drying actually increased with longer pasture. It could not be disproved that the supposed effects of lenient grazing were actually duo to the accompanying mechanical treatment or possible climatic changes. However it was shown to be highly likely that the effects of lenient grazing were due to the promotion of the pasture's root growth. Enhanced root growth could restrict both crack development and tunnel initiation, and encourage intact tunnel roof subsidence rather than complete roof destruction - a precursor to gullying. Development of strong root systems is particularly encouraged by lenient grazing in late autumn and early spring. It is recommended that grazing by cattle instead of sheep continue in the areas with seasonally dry loessial soils subject to tunnel-related erosion. Particular care should be taken in late autumn and early spring to ensure that pasture is not overgrazed. This recommendation is qualified by economic considerations which way dictate that cattle grazing is untenable. Further research into the effects of grazing on the development of root systems of pasture species is also recommended. This research would have implications for the control of a number of erosion types throughout New Zealand but has unfortunately been largely ignored in the past.
New Zealand Soil erosion