A SOIL AND AGROCLIMATIC MODEL FOR ESTIMATING MACHINERY WORK-DAYS - THE BASIC MODEL AND CLIMATIC SENSITIVITY

A model is outlined which estimates the number of days when different soils are workable (for ploughing or seed-bed preparation) at different locations during the spring or autumn tillage periods. The procedure integrates soil properties with an agroclimatic parameter, meteorological field capacity. Field capacity, estimated from a simple water balance model, defines the period in days each year when the maximum water holding capacity of the soil is reached and drainage occurs. This period of field capacity is used as a first approximation for soil conditions unsuited to workability. The soil factors considered are wetness class, which quantifies the depth and duration of waterlogging in the soil profile, and retained water capacity, which is a function of texture, organic matter content and bulk density. These are used to assign a workability rating to soils, which reflects their relative suitability for field work. This rating is converted into a number of days that are added to, or subtracted from, the initial climatic estimate of the workable period to derive a final estimate of good machinery work-days (Wd). The spatial distribution of work-days is illustrated for England and Wales. Workability assessments are derived for the dominant soil series within 5 km x 5 km grid squares and this information is integrated with the corresponding, altitude adjusted, climatic data. Upper and lower quartile start and end dates for field capacity were used in the model to indicate the sensitivity of machinery work-days to potential variations in climate. During wet years most of the country is shown to have no spring work-days, but the situation improves dramatically in a dry year. This indicates why spring-sown crops have been less favoured under the climatic conditions of the past 50 years. Conversely, most of the country experiences at least a few autumn work-days even during a wet year. However, soil tillage in the north and west of England and Wales would benefit greatly if the climate was comparable with that during a dry quartile year.