According to the data, of the 59 reservoirs monitored, 34 had water availability greater than 80 percent of the total volume at the end of May.

On the last day of May and compared with the last day of the previous month there was an increase in the volume stored in five river basins and a decrease in seven.
The Barlavento river basin had the lowest water availability at the end of May, with 31.7 percent, followed by Mira (49.3 percent).

According to SNIRH data, the basins of Mondego (89.3 percent), Tagus (87.7 percent), Guadiana (86.6 percent), Douro (83.7 percent), Cávado and Ave (81.3 percent), Oeste (79.5 percent), Arade (72.8 percent), Lima (70 percent) and Sado (68 percent) had the highest storage levels at the end of May.

The May 2021 storages by river basin are higher than the May storage averages (1990/91 to 2019/20), except for the Lima, Mondego, Mira and Ribeiras do Algarve river basins.
Each river basin may have more than one reservoir.