The historic drought in neighboring Sonora is threatening a major source of water for the capital Hermosillo.

The El Novillo reservoir supplies nearly a third of the city’s water. But it’s now sitting at less than a quarter full, the lowest level registered in its history.

On Monday, interim Hermosillo Mayor Fermín González told Sonoran outlet Uniradio Noticias that — without significant rainfall — it could become impossible to continue drawing water from the reservoir within the next 25 days.

Much of Mexico is currently experiencing drought, but Sonora is among the states most impacted. At this point last year, the El Novillo reservoir was at nearly 70% of its capacity, according to federal data.