The higher the altitude becomes, the more the rainfall increases and on the southern slopes from the Fuego Volcano, rainfall reaches the maximum. On the other hand, rainfall decreases in the high lands in Guacalate River Basin that are situated behind the Fuego, Acatenango and Agua volcanoes because of the loss of humidity on the slope. At San Jose (4.0 m MSL), located at the Pacific Coast, the mean annual rainfall is 1,100 mm. At Ceylan Station (1,100 m MSL) and Los Tarros Station (700 m MSL), which are located at the southern slope of Fuego Volcano, the rainfall reaches to 4,400 mm and 4,800 mm a year, respectively. At Los Aposentos (1,776 m MSL) which is located in the highland, mean annual rainfall is only 930 mm.
- Location of meteorological stations.
Some ninety percent of the annual rainfall takes place in the rainy season and especially, June and September have a relatively high monthly rainfall compared to other months in the rainy season. Rainfall characteristics at seven stations in the area which are considered to represent the rainfall characteristics in the basin are tabulated in the following table.
- Isohyet of mean annual rainfall.
Annual maximum basin rainfall were calculated for the basins of the three reference points to estimate probable rainfall. The annual maximum daily rainfall for the basins for 22 years are shown in the following table. The largest values of daily basin rainfall for the 22-year period are 184.6 mm/day, 140.2 mm/day, and 170.0 mm/day for Achiguate Reference Point I, Achiguate Reference Point II, and Pantaleon Reference Point, respectively, all of which were caused by Hurricane Francelia on September 5, 1969.
- Monthly mean rainfall and rainy days.
The probable rainfall for the basins of the three reference points were estimated by the use of the Gumbel Method with the Thomas plotting position. The daily basin rainfall on September 5, 1969 for the three reference points corresponds to a 30-year return period.
- Calculated probable rainfall for the three basins.
(Flood Control Project No. 69, 1985)