PANAMA CITY – The Panama Canal will expand restrictions on the largest ships crossing the waterway, one of the world’s busiest trade passages, the canal authority’s administrator said on Wednesday, citing shallower waters due to drought.

The measure follows a series of depth restrictions in the 50-mile (80 km) canal since the beginning of the year due to a drought, which authorities had hoped would ease by the start of the Central American country’s rainy season.

Ship traffic, including container ships and oil tankers, using the canal between the Atlantic Ocean and Pacific Ocean accounts for about 3.5% of global trade.

Panamax ships, which use the canal’s older locks, will also face tighter rules on July 9, the port administration told clients in an advisory.

Port administrator Ricuarte Vasquez said the July 19 measure will depend on how much rain pours into the lake system surrounding the canal, calling the current lack of rainfall “worrisome” as the canal basin must also provide additional drinking water for residents.

El Nino, a periodic warming weather phenomenon, has brought a severe drought along the Pacific coast.