During the season, each time you analyse your water, you note that the total alkalinity value is continually dropping.
There is a perfectly natural reason for this: evaporation and agitation of your water leads to a drop in the total alkalinity.
If you consider the composition of the total alkalinity in detail, it constitutes the water’s carbonate and bicarbonate content. Therefore it partly consists of dissolved carbon dioxide (CO2) gas. For further details, see the article on the water balance.
When the water evaporates in the pool, it is not only pure water (the H2O molecule) that is released, but also dissolved CO2. This is what is called the water degassing phenomenon.
The removal of this dissolved CO2 from the water will have a direct impact on the total alkalinity, and will cause its value to fall.
Thus hot water or strongly mixed water (created by jet stream fittings, a spa pool, etc.), waterfalls and bad weather (storms, rain, etc.) and high heat levels will increase the water evaporation and degassing phenomena.
For all these reasons, the total alkalinity in your swimming pool will fall continually during the season.
Consequently, it is recommended that you balance the pool water at the start of the season by having a sufficiently high total alkalinity value (200-250 mg/l) in order to have a sufficient buffering effect and reduce the impact of the gradual falling of the methyl-orange alkalinity on the water balance.