For an operational agricultural zonation, a detailed mapping of agroclimatic conditions is essential.
Potential evapotranspiration, in spite of its importance in evaluating water limitations to crops, is difficult to determine. This is due to the lack of sufficiently dense observations of some essential meteorological elements in Brazil.
To overcome this difficulty, a methodology was developed to estimate potential evapotranspiration in any place, for any month and annually, separately for four geographical regions of Brazil (South, Southeast--Centre-West, Northeast and North). This methodology will permit the use of more than five thousand pluviometric Brazilian stations to evaluate the water balance, whereas today this is possible for only about one hundred meteorological stations.
Linear regression equations relating potential evapotranspiration to altitude, latitude and minimum distance to the Atlantic Ocean were established using a computer program. These equations can be used to estimate potential evapotranspiration as a function of these geographical factors.
In the South, the altitude factor is not important. In this region, for most m.onths, potential evapotranspiration is related to latitude. In all months the minimum distance to the Atlantic Ocean is important, except in the winter months.
In the Southeast--Centre-West, in the dry season, potential evapotranspiration is related only to latitude. In the wet season, altitude is important in all months, latitude in some months and the minimum distance to the Atlantic Ocean only in November.
On the other hand, altitude is important in most months in the Northeast. Here, potential evapotranspiration is related to the minimum distance to the Atlantic Ocean from April to October. Latitude is important from May to December and in March.
The minimum distance to the Atlantic Ocean is the factor correlating most closely with potential evapotranspiration in the Amazonian (North) region. Here, latitude is also important in the months of June, July and August, and altitude is not important.