The wetting and drying of a grazed and ungrazed clay soil

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Abstract
SUMMARY: A 4 year field experiment clearly demonstrates the role played by desiccation cracks in the redistribution of rainfall during the autumn re‐wetting of a Windsor Series clay soil. Where topsoil structure is damaged by the trampling of grazing animals, cracks are well developed and a portion of rain water is diverted down them, by‐passing the topsoil. Redistribution takes place from two centres, the bottom of the cracks and the surface. But where structure is unaffected by trampling, the soil accommodates seasonal shrinkage without cracking and redistribution is by diffusion from the surface only. Summer evaporation is shown to be linearly related to Penman's potential evapotranspiration over a wide range of soil moisture deficits, and simple constants are provided to correct E T to obtain E A . The M.A.F.F. method of estimating evapotranspiration is shown to be too conservative and fails to identify the October maximum soil moisture deficit common in this soil.

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