Figures show the outflows from the reserve after a summer with somewhat lower rains.
Thanks to Hannes Marais and Jannie Coetzee, we have the next lot of water data from Verloren Valei. As the data accumulates, it will become more and more valuable for assessing the reserve’s contribution to the country. Since 2021, they have concentrated on collecting two readings annually, one at the end of the dry season (typically September/ October) and one at the end of the wet season (February/ March).
Between July 2022 and March 2023, the reserve experienced lower than usual rain – 586mm at the office, 669mm at the Witpad gauge and 958 at Witrand, close to the western boundary. The annual mean is 1078mm.
Despite the lower rainfall, the stream flow as measured in March 23 was 476 litres per second, or 41 megalitres per 24-hour period. As the table shows, the results over the years vary wildly.
|Wet season||Flow per 24 hours|
|January 1996||149.5 megalitres|
|March 2021||600 megalitres|
|February 2022||29 megalitres|
|March 2023||41 megalitres|
Jannie Coetzee makes the point that wet season readings tend to fluctuate wildly because they are directly correlated to how much rain fell just prior to the reading as this is only source of Verloren Valei’s water. Thus if a reading is preceded by heavy rain, it will be abnormally high, and the opposite if no rain happened to fall in the preceding few days.
For example, a reading taken soon after our former manager took this picture of the Lunsklip in flood (summer of 2020-21) would show an abnormally high flow rate!
Logically, Jannie argues, the really important measurement is the stream flow at the end of the dry season, because that indicates just how much water the wetlands can store during the wet months. The data from the last four dry seasons shows a fairly constant outflow of approximately 13 megalitres per 24 hours
|Dry season||Flow per 24 hours|
|October 1995||14.2 megalitres|
|July 1998||13 megalitres|
|September 2021||12.8 megalitres|
|October 2022||7 megalitres|
However, cause for concern is Jannie’s observation that the rainfall seems to be on a downward trend at present.
More data will help to clarify the picture and identify trends.