Rainfall in 265 days reached 305 millimeters, whereas it was 333 mm in the same time in the previous year, IRNA reported.
Decline in rainfall has by extension cut the amount of water stored in dams by 4% since last year to reach 40 billion cubic meters.
The worrisome trend is amid rising water consumption due to the spread of the coronavirus in Iran.
Following the spread of Covid-19 in Iran and across the world, and the need to observe personal hygiene and health safety measures, water consumption in the country has jumped because of regular washing of hands and cleaning possessions and the environment.
Gilan Province has registered the highest rainfall so far and the desert province of Yazd in central Iran the lowest. Gilan in the north has received 993 mm of rainfall. The least amount of rain fell in Yazd barely reaching 110 mm.
Iran is divided into six primary and 31 secondary catchment areas.
According to a report by the Iran Water Resource Company, precipitation in the Persian Gulf and Sea of Oman basin in the south, the Caspian Sea in the north, Karakum basin in the northeast and Urmia Lake in the northwest has fallen by 17%, 16%, 7% and 30%, respectively.
Nonetheless, the Central Plateau in Markazi Province and Hamoun Wetlands in the east have saw a rise in rainfall (8% and 30% respectively).
The Persian Gulf and Oman Sea basin in the south have had the highest rainfall at 430 mm so far, but still 23% lower than the year before when rainfall was 530 mm.
Although the Caspian Sea in the north received 510 mm of rainfall in the period (265 days) in 2018, it fell to 430 mm in 2019, down 18%.
Urmia Lake in the northwest with 345 mm also showed a 30% decline in the 265 days compared to 2018 when 490 mm of rainfall was registered. The Karakum basin in the northeast had 287 mm of rain -- down 7% compared to the previous year.
Conversely, two basins received more rain. Over 228 mm of rainfall was registered in the Central Plateau in Markazi Province, 8% higher compared to last year.
Rainfall in the Hamoun Wetlands in the east reached 146 mm, up 30% compared to 2018 when it was 112 mm.
Since last September, the volume of water flowing into the dams has reached 49 billion cubic meters, down 38% compared to 79 bcm last year. The amount of water flowing out of the dams has approached 38 bcm, down 35% compared to 85 bcm last year. The dams at the time of reporting were 78% full.
Of the total 178 dams, 83 are large. Dams in Iran can hold a maximum of 50 billion cubic meters of water.