The average storage level of reservoirs supplying Istanbul rose by 14.37 percentage points in four days after several days of heavy rain. According to ISKI data dated October 18, 2026, the overall rate reached 52.43 percent. The same measure stood at 38.06 percent on October 14.
A sharp four-day increase
TRT Haber, Dünya and Diken, citing the Istanbul Water and Sewerage Administration, reported that rainfall began on the evening of October 13, intensified on October 14 and continued over the following days. Rain reaching the catchments was quickly reflected in the combined average. The move from 38.06 percent to 52.43 percent represents a gain of 14.37 points in four days.
The citywide figure does not mean every reservoir is at the same level. Elmalı recorded the highest rate at 77.48 percent. Darlık stood at 75.75 percent, Alibey at 75.03 percent, Istrancalar at 70.48 percent and Ömerli at 66.62 percent.
Terkos was measured at 36.67 percent, Sazlıdere at 30.95 percent and Büyükçekmece at 30.18 percent. Kazandere remained at 8.08 percent, while Pabuçdere was at 3.58 percent. The distribution shows why the overall average cannot fully describe conditions in every basin.
More than double last year’s level
ISKI’s comparison for October 18 shows that the average rate was 24.51 percent in 2025, 34.38 percent in 2024 and 21.29 percent in 2023. The 2026 reading of 52.43 percent is more than twice the level recorded on the same date last year.
The current figure is not the absolute high for the past decade. October 18 readings were 41.35 percent in 2016, 52.66 percent in 2017, 53.63 percent in 2018, 45.4 percent in 2019 and 32.69 percent in 2020. They were 47.58 percent in 2021 and 44.25 percent in 2022. The increase is significant but should be assessed alongside seasonal patterns and longer-term records.
Transfers from Melen and Yeşilçay continue
Istanbul’s supply system is not limited to reservoir storage. ISKI statistics show that 451.54 million cubic metres of water were received from Melen and 120.58 million cubic metres from Yeşilçay from the start of 2026 through the latest reporting period. Together, the two sources supplied 572.12 million cubic metres.
During the same period, drinking-water treatment plants delivered 918.022 million cubic metres to the city. Rainfall accumulated over reservoir catchments since the start of the year was reported at 876.61 kilograms per square metre. The figures show that reservoir storage is important to Istanbul’s water balance but is not the only indicator.
How to read the average
The 52.43 percent figure does not mean all ten main reservoirs have reached the same level or hold equal amounts of usable water. Elmalı, Darlık and Alibey are above 75 percent, while Pabuçdere and Kazandere remain below 10 percent. The combined average compresses substantial variation among catchments into one number, so individual reservoir data also matter for planning.
It can take time for rainfall to reach catchments and appear in measurements, meaning daily figures may continue to change after a storm. Water delivered to the city, transfers from external sources, evaporation and consumption also affect the overall balance.
Daily monitoring will continue
ISKI publishes the current combined rate and individual readings on its official monitoring page. The latest rainfall lifted the citywide average above 50 percent, while Pabuçdere and Kazandere remain at low levels. Rather than treating one short-term rise as a final answer to long-term supply concerns, official data should continue to be monitored and water should still be used efficiently.
What the rainfall increase shows
The four-day rise shows that the recent storms produced a strong response across Istanbul’s water catchments. However, total capacity, daily demand and differences among reservoirs mean a percentage-point increase cannot be converted directly into a fixed number of days of supply. Official statistics allow comparisons on the same calendar date across years, while operating decisions also depend on individual reservoir levels, transfers and the amount delivered through treatment plants. The rapid increase in the average is positive, but it does not remove the need for efficient water use.
