In 1978 a simulation model of the water-plant-soil type was developed by researchers of the Brazilian Agricultural Research Enterprise (EMBRAPA), (Garagorry et al., 1982). This model was the main element of a computerized system for analysis of agroclimatic data, used to meet the needs of researchers of the Center for Agricultural Research in the Semi-Arid Tropics (CPATSA, located in Petrolina, State of Pernambuco). These needs were related to:

    (1) The determination of the optimal planting period for annual crops in NEB.
     
    (2) The production of risk estimations associated with the several crops.
     
    (3) The preparation of maps of agroclimatic zones.
     
    (4) The appraisal of potential in different projects for the use of excess water in the soil.
     
    (5) The development of proposals for improved agricultural systems at the farm level.