Winter water regimes of clay soils

Autumnal recharge of water in a clay soil is prolonged, taking >3 months. Once thoroughly re-wetted, mean winter water contents lie within 0.5% v/v year by year at each site, and variability during the winter period is low, with SE = 0.2%. The uniformity of winter soil water content reflects the important part played by macropores as rapid transmission pathways for the disposal of rainwater. Weekly changes in water content are small but are inversely related to antecedent levels and the soil may become marginally drier from one week to the next despite substantial falls of rain if the soil begins the period at moisture contents higher than the seasonal mean. This behaviour has been observed in other clays and in lighter textured soils. The constancy of the mean winter soil water content provides a practical measure of the upper limit of plant-available water, i.e. field capacity. It can be established by field sampling on as few as six occasions, and is free from the problems associated with plot irrigation and tension table methods of determining field capacity.