Water · 21 October 2026 · 7 min readFrom cloud to tap: how the Yungas supply Tucumán
The rivers crossing Tucumán's plain are born on a forested mountain. What we know, sourced, and what we don't.
By Yungas tucumanas
Tucumán climbs from a plain at about 200 metres to peaks above 5,000 metres in roughly 100 kilometres. That slope means the mountain rules the province's water: what rains and condenses up high feeds what is irrigated, drunk and generated below. An analysis in La Gaceta argues that this supply depends on the condition of the mountain ecosystem. The Yungas are also the wettest ecosystem in the province and the main catchment for the rivers that sustain regional agribusiness.[9, 2]
Part of that moisture never falls as rain. In the montane forest belt (about 900 to 1,600 m) mean annual precipitation is around 1,800 mm and, according to a paper citing Hunzinger (1997), fog can add up to 100% on top of vertical precipitation. Globally, a review by Bruijnzeel and colleagues recorded between 22 and 1,990 mm a year of intercepted cloud water across 15 tropical cloud-forest sites; models put it below 5% of precipitation in wet areas and above 75% in dry ones. We found no direct fog measurements for Tucumán, so these numbers are references, not local data. Argentina's National Parks Administration does note that in the Yungas of Salta and Jujuy, horizontal precipitation feeds rivers and streams.[1, 5, 10]
That water descends through rivers born in the range. The Salí is formed by what the Calchaquí mountains capture and adds streams from the Aconquija; it receives twelve major tributaries, among them the Loro, Lules, Gastona, Medina and Marapa. Aconquija National Park protects the headwaters of the Jaya and Las Pavas rivers, which empty into Tucumán's valleys and farmland. Its boundaries were drawn around a hydroelectric and irrigation project on the Las Cañas, Gastona and Medina rivers, with the Potrero del Clavillo and El Naranjal reservoirs.[3, 11, 7, 8]
How many people depend on them? Malizia and colleagues (2012) state that the Yungas' main ecosystem service is supplying water for irrigation and human consumption: about 400,000 hectares of crops and more than 2 million people, a figure they attribute to Brown and colleagues (2002), whose original we could not read. Zamora Nasca and colleagues (2014) speak of roughly 1.5 million people, and a secondary source says nearly 2.5 million depend on the Salí River, in a basin of about 57,000 km² across five provinces. The differences reflect different geographic cuts, not a consensus.[1, 2, 3]
El Cadillal is the hub of the system. Its contributing basin covers 4,700 km². According to Contexto Tucumán, it supplies drinking water to 56% of Greater Tucumán (about 600,000 people), irrigates some 40,000 hectares, serves four sugar mills and a citrus plant, generates about 5% of the province's electricity and dampens floods.[4]
The problem is what travels down with the water. The same report says sediment has already consumed 52% of the original capacity and another 1% is lost each year; engineer Franklin Adler projected it would become useless in 23 years. La Gaceta cites sedimentation of "almost 50%". Neither piece blames deforestation for the silting, so these sources do not let us claim that link.[4, 9]
What happens when forest is removed? The answer has nuance. Bruijnzeel and colleagues estimate that converting lower montane cloud forest to pasture probably increases water yield; for upper cloud forest the change would be modest. More water does not mean steadier water. In the Sierra de San Javier, soil under privet-invaded forest held 32.5% and 24% less moisture in two dry seasons than under native forest. In eastern Tucumán, the authors link the loss of riparian forest to recurrent overflows. The good news: in the Yungas, 90% of the transformed land is flat, under 5% slope, so hillsides are in better shape. Today 19% of Tucumán (428,300 ha, 14 areas) is under protection.[5, 2, 6, 1, 12]
What we still don’t know
- We found no direct measurements of intercepted fog in Tucumán’s Yungas: the figures quoted are references from other sites.
- We could not trace the “more than 2 million” people figure to a primary source, and other works give between 1.5 and 2.5 million depending on how the area is drawn.
- No source we read links El Cadillal’s silting to deforestation in the upper basin.
Protected areas of Tucumán → How much forest is left → Back to the notes →
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