Modelling the field drying of hay

Summary The loss of moisture from swaths drying in the field has been investigated using bulk and multi-layer micrometeorological models of an idealized swath. The models produced almost identical results for the variations with time of the swaths' mean moisture contents but the multi-layer version showed in addition the results to be expected from tedding, and the effects of transpiring stubble and evaporation from an underlying moist soil surface on swath drying. The models reproduced in broad detail changes of swath moisture, including rain and dew intercepted by the swath, which had been observed in experiments on grass wilting. However, there appeared to be under-estimation of the rate at which the swath tissue absorbed moisture when the swath contained surface water. The bulk model reduces to a very simple formula when used with typical swath resistances, revealing the importance of both sunshine and vapour pressure deficit in promoting drying, and demonstrating the adverse effect of wind; the model lends itself to the routine classification from daily climatological data of days suitable for hay drying.