The technique, which is best suited for practical use, is the ultrasonic method.
It was proven, that also very cheap sensors can measure a distance with centimeter accuracy.
Thats why the HCSR04 is the recommended sensor.
However, this method depends on the temperature, so a calibration is needed.
To measure a distance onto a water surface with the optical sensors leads to a insufficient quality of
these measurements. Either the ground of the waterbody was measured or even small waves lead to a very poor
precision and accuracy of the measurements.
The hydrostatical method is basically suitable for accurately measuring water levels.
But the simplyfid version of the bubble method, that was used in this work, needs more studies to be
ready for practice.
Also the theoretical use of the sensor in a sensor station and the design of a experiment in the field are described.