A model for estimating soil moisture changes as an aid to irrigation scheduling and crop water-use studies .1. Operational details and description

A model was developed to predict evapotranspiration and soil moisture changes, which could be used either for scheduling irrigation or crop water-use studies. The general form of the model is reported here, and its validation for sugarbeer and potatoes is described in a subsequent paper. The soil characteristics required are depth of topsoil, texture or available water capacity of topsoil and subsoil, and whether a significant slope exists. The plant characteristics required are species and planting date. Meteorological data used to calculate potential evapotranspiration are obtained from the Meteorological Office synoptic network, but local rainfall data are preferred. The model estimates potential evapotranspiration of a reference crop, and uses this to model canopy and root development ibr all crops at each location. Available options allow for observed data on canopy or root development to be incorporated into the simulations. Estimates of potential evapotranspiration for each crop are then adjusted to allow for the effects of water stress, taking soil characteristics, root depth and evapotranspiration demand into account. The model enables growers to reduce the risks of under- or over-watering their crops and has proved successful in irrigation management.