1 Department of Physics, Imo State University, Owerri, Nigeria 2 Agroecological Studies Unit, International Institute of Tropical Agriculture (IITA), PMB Ibadan, Nigeria 3 Department of Physics, University of Ibadan, Ibadan, Nigeria Applicability of a Weather Simulation Model Based on Observed Daily Meteorological Data in Humid Tropical Climate T. C. Chineke 1 , S. S. Jagtap 2 , and J. I. Aina 3 With 7 Figures Received July 10, 1996 Revised September 11, 1998 Summary Long time series of daily meteorological data that are needed in various applications are not always available or appropriate for use at many locations. The Weather Generation Model (WGEN) developed by Richardson and Wright (1984) was evaluated as a substitute for daily observed data at 17 sites located in the main climatic zones of Nigeria. The Wilcoxon-Mann-Whitney U-test has shown that the number of months per year, for which the differences between the long-term monthly mean observed meteorological and WGEN simulated data, were signifi- cant, was less than 4 at most of the study sites. 1. Introduction The large and apparently stochastic (random) variability displayed by daily weather means that it is best generated by an appropriate stochastic model. Daily weather data are needed in many applications; to aid in design of hydraulic structures, to evaluate the effects of watershed changes on hydrology within of runoff areas, water quality, or erosion (Richardson and Wright, 1984). The weather variables needed for most weather generator models include rainfall, max- imum and minimum temperatures, solar radia- tion and potential evapotranspiration (Bruhn et al., 1980; Richardson, 1981; Keller, 1987). Year to year variation in the daily weather patterns causes large variations in crop yields. Crop models that use daily data in simulating crop yields have the potential for being used to assess the risk of producing a given crop in a particular soil- climate regime, and for assisting in management decisions that minimize the risks of crop production. To further advance generation tech- nology for crop models and its applicability in agricultural practice and field management, in evaluating weather impacts on crop production and in forecasting crop performance, weather generation models must be fully developed in conjunction with crop models. These weather models (Bruhn et al., 1980; Nicks and Harp, 1980; Larsen and Pense, 1982; Richardson and Wright, 1984; Geng and Auburn, 1987; and Keller, 1987) provide useful tools for analyzing the impact of weather and climatic fluctuations on agriculture and on the risks associated with alternative management decisions. However, they have the restriction of requiring many years of daily observed data to generate the estimates of the meteorological data, and these daily data may not be readily available or accessible. This prevents their application to sites where such data are unavailable (Hutchinson, 1991). Richardson and Wright (1984) developed the weather generation model (WGEN), using at least 20 years of rainfall records at 139 locations and a minimum of 10 years each of solar Theor. Appl. Climatol. 64, 15–25 (1999)