Definition of rappel for English Language Learners : to move down a steep cliff, rock, etc., by pushing your feet against its surface and sliding down a rope RAPPEL Defined for Kids
How much water you need to drink to get the most benefits depends on your diet, exercise routine, health and weather. You can start by aiming for the basic recommended amount for adults, which is 8 to 12 cups a day, according to Clemson Cooperative Extension, and making adjustments as needed.
The experiment: It is a fairly simple experiment and doesn't involve a trip to Lourdes, or the Ganges, which would be a bit of a faff. I have to drink 3 litres of water a day for 4 weeks. That is a lot of water, considering at the moment I probably drink about half a cup. I have a bit of a false start as I spend a week counting loads of tea as 3 litres of water - apparently it has to be the pure stuff, so I regroup and start again, really seriously this time.