I think you need to back up ...

I think you need to back up one step.  Why are you trying to take readings?  Just for curiosity sake. If your lakes, rivers, reservoirs, are running low are you using too much or is there not enough rainfall? You are entering a classic situation. Do you want to teach or do? What is your pleasure?  Catch rainwater, use, process, reuse again and again.  Collect your waste, process it, make and use compost and compost teas. When soil is black and rich it will hold the moisture twice as long. When you are growing garden food, flowers, or trees they respire. Long term respiration causes weather patterns to change. This increases the rainfall amount.  Take all trash from a country make and use the compost, grow food in a desert  and slowly you will have more and more rain and soon a forest.