In winter the tunnel can operate in reverse, conveying surplus water from the Berg River into Theewaterskloof.

Rainfall in 2017 remained well below average, and by early 2018 the dam was again approaching critically low level, resulting in water consumption being limited to only 50 litres per person per day, and plans for a possible "Day Zero" in April 2018 when Cape Town's municipal water supply was predicted to be shut off.

[7] Theewaterskloof dam has a capacity of 480,406 megalitres (16,965.4×10^6 cu ft) of water, and when full the reservoir covers an area of 5,059 hectares (12,500 acres).

[2] The catchment area of 500 square kilometres (190 sq mi) is served by streams emanating in the Hottentots Holland mountain range.

[8] Historically[9] records show we are in a dry period[10] 1mm of rain per square meter equates to 500 000 000 litres falling on this area.

Hence it requires a full metre of rainfall with 100% runoff to fill the dam from totally empty, which is unlikely to happen in any single year.