OPTIMAL USE AND ECONOMIC VALUE OF WEATHER FORECASTS FOR LETTUCE IRRIGATION IN A HUMID CLIMATE

The problem of analyzing a sequence of daily irrigation decisions util izing weather forecast information is formulated for the case of lettu ce grown in central New York state, and solved using a stochastic dyna mic programming algorithm. The crop response is represented using a si mple but physiologically-based model of lettuce growth [van Henten, E. J., 1994. Validation of a dynamic lettuce growth model for greenhouse climate control, Agric. Sys. 45, pp. 55-72], modified to allow the sto matal conductance for CO2 to depend on a simple soil moisture budget. A negative crop response to prolonged wet soil conditions combined wit h warm temperatures is also included in the crop model. Operationally available precipitation and temperature forecasts are incorporated in a way that preserves the effect of time correlation in the weather. Th e results suggest that irrigation is quite viable even in the relative ly humid climate of New York, with the economic value of irrigation (s cheduled according to a conventional, non-optimal rule) vs. no irrigat ion estimated at approximately US$4000-US$5000 per hectare per year fo r lettuce. Optimal use of weather forecasts to schedule irrigations is estimated to provide additional value of approximately US$1000 per he ctare per year, much of which is derived from avoiding crop damage due to excessive soil moisture. (C) 1998 Elsevier Science B.V.