İstanbul’s dams are well over half full.
61.9%Monday 9 November 2026, 09:10Measured 11 hours ago. Source: İSKİ. No newer reading has been published since.
That is about 537 million cubic metres of water.
About 61 mm of rain fell on the dam catchments over 27 October – 3 November. A week ago the dams were 53.7% full.
We call 60–100% “safe”.
- In 14 days 23 November
- 69–81% most likely 74%
- In 30 days 9 December
- 71–96% most likely 83%
There’s an 80% chance the level stays inside each range.
Over the next month the water will most likely rise.
How well these forecasts hold up
The most likely level a month ahead has moved up 2.6 points since yesterday morning (80%, now 83%). Main reason: the updated rain forecast.
8 forecasts have a result, and 7 landed inside the range. With so few, we don’t give a hit rate. In most of the 120 short runs we made in recent days, the model put the level higher than it turned out. Our value for a month ahead may be high too.
Hindcast: We re-ran 2011–2023, each time with only the data known on that day. The 30-day forecast was off by 3.7 points on average, and 82% of outcomes landed inside the range (the target is 80%).
We make the forecast again every morning.
The live archive started on 19 October 2026. It holds 22 mornings’ forecasts so far. 7 days ago the most likely level a month out was 75%. In today’s forecast it is 83%. The forecast has moved up 8 points.
How rain shapes the water forecast
Catchment rainfall and reservoir fill, on the same dates. Some rain never reaches the lakes; the level depends on all the water flowing in and out.
About 57 mm of rain is expected over the catchments in the next 14 days (38–86 mm). In the same days of the previous 26 years the average was 30 mm; the expected amount is above that.
The first 14 days use the ECMWF weather forecast. After that, the model tries weather from the same season in past years. A 30-day water forecast is therefore not a 30-day weather forecast.
- The soil holds some rain.
- The rest runs into the lake. Dry ground absorbs more water. The same rainfall can bring more water to the dams when the ground is already wet; its effect can take time.
- Rain is only part of the balance.
- City use and losses lower the level; transfers from Melen and Yeşilçay raise it. Temperature affects use. Evaporation is modelled together with other losses.
Daily chart values
| Date | Rain range (mm) | Past average (mm) | Fill range |
|---|---|---|---|
| 9 November | 0–0 (median 0) | 1 | – |
| 10 November | 4–7 (median 5) | 3 | 63–63% (median 62.7%) |
| 11 November | 5–8 (median 6) | 6 | 63–64% (median 63.2%) |
| 12 November | 5–10 (median 6) | 8 | 63–65% (median 63.6%) |
| 13 November | 5–11 (median 7) | 11 | 63–65% (median 63.9%) |
| 14 November | 7–29 (median 15) | 12 | 63–67% (median 64.6%) |
| 15 November | 23–43 (median 33) | 14 | 65–69% (median 66.4%) |
| 16 November | 24–45 (median 33) | 16 | 66–71% (median 67.8%) |
| 17 November | 24–45 (median 34) | 17 | 66–72% (median 68.8%) |
| 18 November | 25–45 (median 35) | 20 | 66–73% (median 69.3%) |
| 19 November | 30–53 (median 38) | 23 | 67–74% (median 69.8%) |
| 20 November | 32–59 (median 43) | 26 | 68–75% (median 70.5%) |
| 21 November | 33–67 (median 48) | 28 | 68–76% (median 71.5%) |
| 22 November | 38–86 (median 57) | 30 | 68–78% (median 72.8%) |
| 23 November | – | – | 69–81% (median 73.8%) |
| 24 November | – | – | 69–82% (median 74.6%) |
| 25 November | – | – | 69–84% (median 75.3%) |
| 26 November | – | – | 69–85% (median 75.9%) |
| 27 November | – | – | 70–86% (median 76.5%) |
| 28 November | – | – | 70–87% (median 77.1%) |
| 29 November | – | – | 70–88% (median 78.0%) |
| 30 November | – | – | 70–89% (median 78.7%) |
| 1 December | – | – | 70–90% (median 79.3%) |
| 2 December | – | – | 70–91% (median 79.7%) |
| 3 December | – | – | 71–92% (median 80.1%) |
| 4 December | – | – | 71–93% (median 80.6%) |
| 5 December | – | – | 70–93% (median 81.0%) |
| 6 December | – | – | 70–94% (median 81.5%) |
| 7 December | – | – | 71–94% (median 81.8%) |
| 8 December | – | – | 71–95% (median 82.3%) |
| 9 December | – | – | 71–96% (median 82.6%) |
Rainfall accuracy and water-forecast accuracy are different. The Scorecard checks predicted fill against measurements; it does not give a separate accuracy score for the weather forecast.
The city uses about 3.2 million cubic metres of water a day.
That’s enough to fill the Basilica Cistern 40 times over, every day.
In the past week the water in the dams rose by about 71.2 million cubic metres. That is what the city uses in 22.5 days.
The dams hold 868 million cubic metres when full. With no rain and nothing from the pipelines, the fill level would drop by about 0.4 percentage points a day.
İstanbul is not on a big river, so it stores rain. That is what cisterns like the Basilica Cistern and the Ottoman dams in the Belgrade Forest were built for.
Daily use is an approximate value read off İSKİ’s yearly charts. The Basilica Cistern holds about 80,000 m³ (yerebatan.com).
On 5 November 2007 the dams were down to just 80 million cubic metres.
It is the lowest day on record. The dams were 9.2% full. The level fell until this day, then rose again with the autumn rain.
Today the dams hold about 6.7 times as much.
The same drought ran on into 2008.
Some of the water at the bottom of a dam never reaches the pipes. İSKİ doesn’t say how much. A rule of thumb often quoted in the press is 10% of capacity. If İSKİ’s percentages already leave this water out, that share should not be counted. Until İSKİ tells us, we show both.
On this date the dams are fuller than in most years.
The record has readings for 9 November in 25 years. In 21 of them the dams held less water than today, and in 4 they held more. The last year they were fuller on this date was 2015 (67.8%).
There is no reading for this date in 2024, so that year is left out of the comparison.
Pick a year. The water in the drawing moves to that year’s level on this date, and the red mark stays at today’s level.
- 2024: no reading
Average 46.5%. Lowest 2007 (9.5%), highest 2009 (87.8%).
Water reaches the dams two ways and leaves them two ways.
Rain and the pipelines from the Melen and Yeşilçay rivers fill the lakes. The city’s taps and evaporation drain them. Rain is the least certain part of the sum.
- Rain
- Dry soil keeps some of it. The rest runs into the lakes. 27 October – 3 November: about 61 mm.
- Pipelines
- About 1.4 million m³ a day. İSKİ decides how much.
- The city
- About 3.2 million m³ a day.
- Evaporation
- Water lost off the lake surface. More in summer.
Daily amounts are approximate, derived from the yearly totals İSKİ publishes.