It is raining but the reservoirs are not filling

In recent weeks, we have been hearing the same sentence almost every day: “Rainfall has increased in Türkiye, but reservoirs are not filling.” The claim is often taken one step further and turned into the conclusion that “water must be poorly managed.” Although this interpretation may sound reasonable at first glance, it is actually a classic hydrological misunderstanding rooted in a misinterpretation of how natural systems work.

Let us begin with the data. According to official figures from the State Hydraulic Works (DSİ), on December 15, 2024, Türkiye’s reservoirs held 39.2 billion cubic meters of active storage, corresponding to a filling rate of 41.6%. Following a severe drought, by December 15, 2025, stored water had dropped to 31.9 billion cubic meters, and the filling rate declined to 33.8%. In other words, the country entered the rainy season with a significant water deficit.

Then the rains arrived. During the 2026 water year (October 1, 2025 – February 17, 2026), cumulative precipitation across Türkiye increased by 19.4% compared to long-term averages and by more than 70% compared to the same period of the previous year. The effect on reservoirs is clearly visible: by February 17, 2026, active storage had risen to 43.1 billion cubic meters, corresponding to a filling rate of 45.7%. Put simply, billions of cubic meters of water returned to Türkiye’s reservoir system within just two months.

Looking at these numbers, it is impossible to argue that reservoirs are not filling. On the contrary, the system is undergoing the expected recovery following drought conditions. Why, then, does public perception suggest the opposite?

The answer is simple: we tend to confuse meteorology with hydrology.

Rainfall is a meteorological event; reservoir storage is a hydrological outcome. There is a time lag between the two. Expecting reservoir levels to rise immediately after rainfall is like expecting to harvest crops on the same day seeds are planted.

After a prolonged drought, the soil behaves like a dried sponge. Two outcomes may occur. Either the hardened surface generates rapid runoff during intense rainfall, causing water to flow away quickly, or the soil absorbs the incoming water. In either case, rainfall does not immediately flow into reservoirs — in fact, most of the time it does not. First, soil moisture must be restored. Then groundwater reserves begin to recharge. Vegetation replenishes lost water. Only after these processes does surface runoff increase sufficiently for reservoirs to receive significant inflow. Hydrology has long recognized a fundamental principle: the first rains after drought do not primarily fill reservoirs; they repair the system.

The debates surrounding Istanbul provide one of the clearest examples of this misunderstanding. Despite more than 200 millimeters of rainfall since the beginning of the year, reservoir levels increased only by a few percentage points over a short period, leading some commentators to claim that “water is not being stored properly.” In reality, this outcome is neither surprising nor abnormal. After an extended drought, the watershed first compensates for its own water deficit. The rainfall has not disappeared; it simply has not yet reached the reservoirs.

Another overlooked issue is how reservoirs are designed. Reservoirs are not built to fill and empty within a single rainy season. They are engineered to balance multi-year climate variability. Expecting reservoirs to fully recover within a few months after a severe drought is as unrealistic as expecting a country emerging from an economic crisis to repay all its debts with a single paycheck. Hydrological systems have memory, and recovery takes time.

In fact, the official data tell a very clear story. Türkiye experienced a dry period. Rainfall has now returned to above-average levels. Stored water volumes in reservoirs have begun to increase again. The physical system is functioning exactly as it should. The problem lies not in nature, but in our perception of time.

Modern news cycles condition us to interpret everything instantly. When daily rainfall reports are evaluated on the same timescale as seasonal hydrological processes, even normal natural behavior can appear as a crisis. Yet the water cycle does not operate at the pace of social media.

With continued precipitation and the melting of existing snowpack, inflows to reservoirs are expected to become more pronounced in the coming months. Hydrologically meaningful assessment requires observing seasonal trends rather than reacting to individual rainfall events.

In short, the statement “it is raining, but reservoirs are not filling” is scientifically incorrect. Reservoirs are filling — just at the pace allowed by nature. And sometimes, what needs correction is not water management, but our understanding of how nature works.

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