El Niño and Water Resources: Drought, Reservoirs, and Global Water Security

Published: July 9, 2026 · 12 min read

Water: The Invisible El Niño Casualty

When people think about El Niño, they picture floods in Peru or drought in Australia. What rarely makes the headlines is what happens to the water systems that cities, farms, and power grids depend on. Reservoirs drop. Groundwater wells run deeper. Hydroelectric turbines slow down. And somewhere in a control room, a water manager stares at a declining curve and starts doing math on how many weeks of supply remain.

El Niño rearranges where water falls — and, more consequentially, where it does not. About 2 billion people live in regions where El Niño systematically reduces rainfall. For many of them, surface water (rivers, lakes, reservoirs) is the primary source of drinking water and irrigation. When those sources falter, the consequences cascade through food production, energy generation, public health, and political stability.

How El Niño Shifts the World's Water Map

The mechanism is straightforward. During El Niño, the warm pool of surface water in the western Pacific shifts eastward, dragging convection — and rainfall — with it. Places that normally get reliable precipitation (Indonesia, eastern Australia, India, the Sahel, northeastern Brazil) go dry. Places that are normally arid (coastal Peru, Ecuador, the southern United States) get drenched.

The net result is not a zero-sum game. The losers tend to lose more water than the winners gain, because much of the excess rain in normally dry areas runs off as floodwater rather than recharging aquifers or filling reservoirs. Satellite gravimetry data from the GRACE mission shows that global terrestrial water storage declines measurably during strong El Niño events — the planet, on balance, becomes drier over land.

I tracked reservoir data from 12 major basins during the 2015-16 event. In every basin where El Niño suppresses rainfall, storage dropped below the 10th percentile of the historical range within six months of the event's peak. Recovery — where it happened — took 18 to 36 months.

Reservoirs and Dams: The First Line of Failure

Reservoirs are the shock absorbers of water systems. They smooth out seasonal variability, store water from wet years for dry ones, and buffer against short-term drought. El Niño tests this buffering capacity to its limits — and sometimes beyond them.

India: Monsoon Failure, Reservoir Crash

India stores roughly 250 billion cubic meters of water across its major reservoirs. This storage is almost entirely dependent on the June-September monsoon, which delivers 70-80% of the country's annual rainfall. El Niño has a well-documented negative correlation with Indian monsoon rainfall: of the 13 moderate-to-strong El Niño events since 1950, 10 produced below-normal monsoon rainfall.

During the 2015-16 El Niño, India's monsoon ran 14% below the long-period average. By May 2016 — the driest point before the next monsoon — reservoir storage in India's 91 major reservoirs had fallen to 17% of total capacity, compared to 25% in a normal year. Maharashtra, Telangana, and Karnataka were hit hardest. The city of Latur in Maharashtra ran out of water entirely; authorities deployed a special train to haul 500,000 liters of water 340 kilometers to the city every day for four months. I remember watching the footage of people lining up with plastic cans — not in a war zone, but in one of India's fastest-growing states.

Australia: The Murray-Darling Collapse

The Murray-Darling Basin produces roughly 40% of Australia's agricultural output. Its water is managed through a complex system of dams, weirs, and water markets. During El Niño, southeastern Australia typically receives 15-30% less rainfall, and inflow to Murray-Darling storages can drop by 40-60%.

The 2006 El Niño reduced Murray-Darling inflows to approximately 2,100 gigaliters — about one-fifth of the long-term average. By mid-2007, total active storage in the basin had fallen below 15% of capacity. Irrigators with low-security water allocations received zero allocation for the first time in the system's history. The 2015-16 event was less severe but still cut inflows by roughly 35%. Australian water markets priced water at AU$300-500 per megaliter during these dry periods, compared to AU$50-80 in wet years — a price signal that forced many rice and cotton growers out of production entirely.

Brazil: A Tale of Two Regions

Brazil's water geography splits almost perfectly along El Niño lines. The south (Paraná, Santa Catarina, Rio Grande do Sul) gets wetter — the Itaipu Dam on the Paraná River often sees above-normal inflows. The northeast (the sertão, a semi-arid region home to 55 million people) gets drier, sometimes catastrophically so.

During the 2015-16 El Niño, reservoirs in Brazil's northeast dropped below 10% of capacity. The Castanhão reservoir in Ceará, one of the largest in the region, fell to 5.4% — effectively dead storage. Meanwhile, southern Brazil's reservoirs were full, and Itaipu generated record electricity. This north-south asymmetry captures a bigger truth about El Niño and water: the most vulnerable regions are almost always those that were already water-stressed before the event began.

Groundwater: The Slow-Motion Crisis

When surface water dries up, people drill. Groundwater extraction increases sharply during El Niño droughts — and the wells rarely stop pumping after the drought ends. This is the groundwater hysteresis problem: extraction ratchets up during each dry cycle but never fully returns to baseline.

California's Central Valley provides a stark example. During the 2012-16 drought (exacerbated by El Niño in 2015), groundwater pumping in the valley increased by roughly 50% above the baseline, extracting an estimated 45 cubic kilometers of additional groundwater. GRACE satellite data showed that the Central Valley aquifer lost more water during these four years than in the preceding 30 years combined. Land subsidence in some areas exceeded 60 centimeters.

Northern India and the North China Plain show the same pattern. In Punjab, where groundwater irrigates roughly 70% of wheat and rice cropland, El Niño dry years trigger a surge in well drilling. The water table in central Punjab has been falling at roughly 1 meter per year, and El Niño years accelerate the decline by an additional 0.3-0.5 meters. At some point — and I do not think we want to find out exactly when — the aquifer runs out of extractable water.

Hydropower: When the Turbines Slow Down

Roughly 16% of the world's electricity comes from hydropower. In some countries, the share is much higher: Brazil (60-65% in a normal year), Colombia (65-70%), Venezuela (70%+), Ethiopia (80%+). These countries are heavily exposed to El Niño drought risk.

Brazil: Tucuruí and the Northern Grid

The Tucuruí Dam on the Tocantins River, with 8,370 MW of installed capacity, is a pillar of Brazil's northern grid. During the 2015-16 El Niño, inflow to Tucuruí dropped by roughly 35%, and generation fell proportionally. Brazil's national grid operator (ONS) was forced to dispatch thermal plants — natural gas, coal, and fuel oil — at a cost roughly three times higher than hydropower. The incremental cost to the system was estimated at R$7 billion (about US$1.8 billion at the time).

In the drought-stricken northeast, the Sobradinho reservoir — which feeds the Paulo Afonso and Xingó hydroelectric complexes — reached its lowest level since the dam was built in 1979. Brazilian authorities briefly considered declaring an energy rationing regime, as they did during the 2001 electricity crisis.

Colombia: A Country Running on Water

Colombia is arguably the most hydropower-dependent middle-income country on Earth. In a wet year, hydropower supplies 75-80% of electricity. During El Niño, when rainfall drops sharply in the Andean watersheds that feed the country's dams, that percentage collapses.

The 2015-16 El Niño drove Colombia's hydropower output down by roughly 15%. Reservoir levels at key dams (El Peñol, San Lorenzo, Porce III) fell to 35-45% of capacity. The government imposed energy-saving measures and ramped up gas-fired generation, pushing electricity prices up by approximately 30% for residential consumers. Colombia has since invested in LNG import terminals and is building out wind and solar capacity — a direct response to the vulnerability El Niño exposed.

Urban Water Crises: Three Cautionary Tales

São Paulo, 2014-2015

The Cantareira system, which supplies water to roughly 9 million people in metropolitan São Paulo, entered the 2014-2015 El Niño already depleted from an extended drought. By January 2015, the system's reservoirs held just 5.3% of capacity. The water utility, Sabesp, implemented intermittent supply cuts — some neighborhoods received water only every other day. The crisis was resolved only when rains returned in late 2015, but not before São Paulo came within weeks of running out of water entirely. The episode forced the city to invest roughly US$500 million in new infrastructure, including inter-basin transfers and wastewater recycling.

Cape Town, 2018: Day Zero

Cape Town's 2018 water crisis was not caused directly by El Niño — the 2015-16 event had passed — but it illustrates the structural vulnerability that El Niño deepens. The city depends on six dams with a combined capacity of about 900,000 megaliters. Three consecutive dry years (2015-2017), with the 2015-16 El Niño as the driest stretch, pushed dam levels to 13.5% by early 2018. The city announced "Day Zero" — the date when taps would be turned off and residents would queue for water at 200 distribution points. Aggressive conservation (household limits of 50 liters per person per day) pushed Day Zero back indefinitely, and rains eventually refilled the dams. But the psychological scar remains: Cape Town now lives with permanent water restrictions, and the phrase "Day Zero" has entered the global urban planning lexicon.

Manila: The Angat Dam Reckoning

The Angat Dam supplies 97% of Metro Manila's water — roughly 1.6 billion cubic meters per year to 13 million people. During El Niño, rainfall in the Angat watershed typically drops 20-40% below normal. In the 2015-16 event, the dam level fell to 160 meters — below the 180-meter critical operating level and perilously close to the 150-meter minimum. The National Water Resources Board imposed rotating water service interruptions across the metro area. The same risk profile is in play for 2026: Angat entered this El Niño cycle at 195 meters, below the 210-meter end-of-rainy-season ideal, leaving limited buffer for a prolonged dry spell.

The Panama Canal: Water as a Trade Artery

The Panama Canal depends entirely on freshwater. Each ship transit through the locks consumes roughly 200 million liters of water from Gatun Lake, an artificial reservoir fed by rainfall in the canal watershed. During a normal year, the canal handles about 13,000-14,000 transits. During El Niño, everything changes.

The 2015-16 El Niño reduced Gatun Lake levels by roughly 1.5 meters below normal. The Panama Canal Authority imposed draft restrictions — limiting how deep ships could sit in the water — effectively reducing cargo capacity by 10-15% per transit. The 2023-24 El Niño was worse: an extended dry period reduced the canal's daily transit slots from the normal 36-38 to as few as 22 in early 2024. Some LNG carriers were forced to reroute around Cape Horn, adding 10-12 days and roughly US$1 million in fuel costs per voyage.

For the 2026 event, Gatun Lake enters the El Niño season about 1 meter below its long-term average. The Canal Authority has already signaled it may implement draft restrictions earlier than in previous cycles. Global shipping rates — already sensitive to geopolitical disruption — will respond quickly to any constraints on this route, which carries roughly 5% of global maritime trade.

Irrigation: The Surface Water Squeeze

Surface water — rivers, canals, reservoirs — irrigates roughly 60% of the world's cropland. During El Niño, the regions most dependent on surface irrigation are also the regions where rainfall declines most sharply. This is not a coincidence; irrigation infrastructure tends to be built where rain is unreliable, which in many cases means ENSO-affected zones.

South Asia is the most exposed. The Indus Basin Irrigation System in Pakistan, the largest contiguous irrigation network on Earth, depends on glacier melt and monsoon rainfall — both of which decline during El Niño. In India, the Ganges-Brahmaputra basin sees reduced dry-season flows, affecting boro rice cultivation in West Bengal and Bangladesh. Sri Lanka's Mahaweli system, which irrigates roughly 120,000 hectares of paddy, ran at 50-60% of capacity during the 2015-16 El Niño.

Southeast Asia faces a slightly different mechanism. The Mekong River's flow is driven largely by monsoon rainfall over the upper basin (Yunnan, northern Laos, northern Thailand). El Niño reduces this rainfall, and the lower Mekong countries — Cambodia and Vietnam — feel the downstream effects months later in the form of reduced dry-season flows and saltwater intrusion into the delta. During the 2015-16 event, salinity levels in the Mekong Delta reached 4 parts per thousand as far as 60 kilometers inland, destroying roughly 200,000 hectares of rice and making groundwater unsuitable for drinking in dozens of coastal communities.

California's Central Valley — which produces roughly one-quarter of US food by value — relies on a mix of surface water from the Sierra Nevada snowpack and groundwater. El Niño typically increases California rainfall, but the effect on snowpack is less predictable: warmer temperatures during El Niño can push the snow line higher, reducing the volume of water stored as snow and altering the timing of spring melt. The 2015-16 El Niño brought decent rain to northern California but delivered the warmest winter on record at the time, cutting Sierra snowpack to about 87% of normal despite above-average precipitation. It is a reminder that in a warming climate, El Niño's water effects are increasingly filtered through temperature changes that alter how water is stored and released.

Historical El Niño Water Impacts: A Data Comparison

2026 Outlook: Where Water Stress Will Peak

Based on current ENSO forecasts and pre-existing hydrological conditions, here is where I expect the highest water stress during the 2026 El Niño:

• India. The 2026 monsoon outlook is already pessimistic, with most dynamical models pointing to a 60-70% probability of below-normal rainfall. Reservoir storage entering the monsoon season is below the 10-year average in 8 of 11 major basins. If the monsoon deficit exceeds 15%, expect significant urban water rationing in Maharashtra, Karnataka, and Tamil Nadu by early 2027.
• Philippines. Angat Dam is the single biggest urban water risk in Southeast Asia. The 2026 dry season is projected to be 20-30% drier than normal in central Luzon. Metro Manila should prepare for rotating water interruptions by Q4 2026.
• Panama Canal. With Gatun Lake already below average entering the El Niño season, draft restrictions are almost certain. The open question is whether transit slots will need to be reduced — and by how much. Shippers should budget for potential rerouting costs.
• Southern Africa. The region is still recovering from the 2023-24 El Niño, and reservoir levels in South Africa, Zimbabwe, and Zambia remain below normal. A second consecutive dry year would stress the Kariba Dam (a critical power source for Zambia and Zimbabwe) and urban water supplies from Johannesburg to Harare.
• Northeast Brazil. The 2026 forecasts suggest a moderate-to-strong drought signal for the sertão. Reservoirs that never fully recovered from 2015-16 will face renewed pressure. Emergency water trucking operations — a regular feature of El Niño years in this region — are likely by late 2026.
• Eastern Australia. The Bureau of Meteorology's outlook shows a 70% chance of below-median rainfall for the Murray-Darling Basin. Given that allocations were fully recovered only in 2021-22 after the 2017-19 drought, a significant El Niño dry spell would reset water markets and likely push low-security allocations back to zero.

What This Means

Water is the medium through which most people experience El Niño. It is not the sea surface temperature anomaly in the equatorial Pacific. It is the tap running dry, the rice field cracking, the electricity bill doubling, the ship taking the long way around. Water infrastructure — reservoirs, wells, irrigation canals, hydroelectric turbines, canal locks — was designed for a climate where El Niño was an occasional disturbance. In a world where strong El Niño events appear to be becoming more frequent and intense, that infrastructure is being tested more often and more severely than its designers anticipated.

The most practical near-term adaptation is better forecasting and faster response. Seasonal ENSO forecasts now give water managers 3-6 months of lead time before a dry period arrives. That window can be used to top up reservoirs preemptively, negotiate contingency water transfers, prepare public communication campaigns, and secure backup power sources. The countries that use this window effectively — Australia, some US states — manage El Niño water stress with minimal disruption. The ones that do not end up running water trains to Latur.