Several simple methods such as simple averaging, Thiessen polygons,
isohyetal and Inverse Distance Weighting (IDW) have been used so far as traditional methods in spatial estimation of precipitation (Thiessen, 1911; Shepard, 1968; Tabios & Salas, 1985; McCuen, 1989).
To determine the total water availability of the study area, first the average precipitation (P) is estimated by applying the method of
isohyetal curves (Jimenez, 1992; Monsalve, 1995; MAVDT, 2004) from rainfall average monthly record multiyear (common period between 1980-2005 at stations) of hydroclimate stations managed and operated by the National Coffee Research Center (CENICAFE), the Institute of Hydrology, Meteorology and Environmental Studies (IDEAM) and the Central Hidroelectrica de Caldas (CHEC).