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The geographical location of Panama, its size, shape, orientation and relief determina the temporary and spacial distribution of the rain, and of the flows as well as the performances in the various regions in the country.
The geomorphic, geologic and soil use characteristics influence in the length, slope and orientation of the water flow, as well as the basing retention capacity.
The continental division is constituted by a series of mountainous chain that extend from East to West. This chain makes rivers flow into two slopes: The Pacific one, that covers 70% of the national territory and the Caribbean or Atlantic slope, that covers the remaining 30%. In general terms, the rivers are short and are usually oriented in a perpendicular direction to the coasts.
The average length in the Caribbean slope is 56 Km with an average slope of 2.5% and in the Pacific slope the average length is 106 Km with an average slope of 2.27%.
Chiriqui River, upstream of the "La Esperanza" hidrologic station
In the background, deflect Dam towards Barrigon Reservoir.
Valle Risco Station - Changuinola River
The average annual flow in the Panama total surfacem including the continental and insular territory is aproximatelly 4,222 m³/s (cubic meters per second) that matches an erosion of 1,764 mm; Towards the Pacific is discharged aproximatelly 60%. The average erosion coefficient is 60.3%.
For its high unit performances stand out the Changuinola, Guarumo, Cricamola and Calovebora Rivers basins in the Caribbean slope and the Chiriqui, Fonseca, Tabasara and San Pablo Rivers in the Pacific slope, with performances higher than 72 l/s/Km² (liters per second per square kilometer). The Pacific slope posseses the best water resources in the country, concentrated in the Chiriqui province. In the Bocas del Toro province we can find the most abundant resourses in the Caribbean region. The oriental portion in the Azuero peninsula and the Cocle flats present the lowest resources in the country.
Cable way for capacities - Calovebora River
There's a marked difference in the temporary distribution of the flows between the two slopes. The Caribbean presents a higher natural regulation with 20% to 30% of the flow in the months from January to April, and with 70% to 80% of the erosion in the rainy season, from May to December. In the Pacific slope only between 7% and 15% of the annual share is between the months of January and April, and the remaining 85% to 93% is in the rainy season, from May to December. You can observe that the variation of the monthly flows in the Caribbean region is much smaller than the Pacific region.
In general, the month with the biggest flow is October, although some Caribbean sector stations register maximmums in November and December. The oriental region basins, of Darien Mamoni, register the maximmum average monthly flow in November.
The biggest rivers in the country are: Changuinola, Bayano, Chiriqui, Chucunaque, Tabasara, Tuira, Santa Maria, Cocle del Norte and Fonseca.
The most important reservoirs are: Alajuela 50.2 Km2 (643 Mm3), Gatun 436.2 Km2 (769 Mm3), Bayano 353 Km2 (4,787 Mm3) and Fortuna 10.9 Km2 (221.7 Mm3).
Meteorological Factors that Affect the Flows
The magnitude and behavior of the flows in Panama is determined by diverse regional and global meteorological factors that explain its behavior. From the most common and significant factors we have the following:
Hurricane season in the Atlantic: Depending of its coordinates and magnitude, the hurricane can indirectly bring about, bad weather conditions, or by the contrary, good weather conditions. The rains can be so intense that in some occasions cause floods in the rivers.
Cold Fronts: When the cold air push comming from the north is moved to latitudes near the panamanian territory, generates inestability, activating the intertropical convergence zone and the low preassure systems, producing a great rainfall, great rises and floods.
Western waves: These waves in their trajectory to the west, when they're preety active, cause great precipitations and rises.
Intertropical Convergence Zone (ZCIT): When the ZCIT is active, near or over the Panama territory, is common that the rain frequency increases.
Orography and Trade Winds: Is frequent that the trade winds when they collide with the skirts of the mountain chain that conforms the continental dividing activate the clouds and generate the rainfall, mainly in barlovento.
Rise of Caldera River (November 2008)
Convection Rains: These are characterized for its intensity and short duration. They usually cause sudden rises and dangerous floods located in rivers and ravines of small basins.
El Niño & La Niña Phenomena: There's a close relation between the magnitude of the average monthly flows in Panama and the occurence of the El Niño and La Niña events. During the development of the El Niño event, in the Pacific slope we can observe a decrease in the flows and a increase in the Caribbean slope. The opposite usually happens during the development of La Niña event.
Eventually, during the occurency of El Niño events, extraordinary rises occur in the Pacific slope, that, in some cases, for their magnitude, make that the average for a specific month exceed its historic average.
It's also important to point out that we've observed that the flow decrease during the El Niño events is variable. This decrease can be moderate or can be critical for El Niño events classified as weak and moderate. In the other hand, for a strong El Niño event the flow decrease is always critical.
We have to point out the particular case of the the high basin of the Chiriqui River, where is located the Fortuna Hydroelectric, which behaviour obbeys to the conditions developed in the Pacific Ocean slope, but also is exposed to the meteorological conditions that occur in the Caribbean. That's why in this basin you can register flows over the average during the development of an El Niño event.