There are various ways of measuring head at your hydro site as of a certain point in time. Head varies at any site usually, therefore a head measurement should be accompanied by the date it was taken and if possible how long it’s duration was.
“Head” is the difference between upper water level and lower water level, usually measured in meters or in feet. You need to measure the head wherever you plan on putting in a hydro system, because the head or the amount of distance the water falls determines how much energy there is available in the water to extract.
One thing to keep in mind about head; it’s always changing. Both the upper water level and lower water level usually changes with seasonal changes. In rainy wet seasons flows increase causing the upper water levels to rise and usually the lower waters also. The amount of these changes depends on the specific site and on the particular water course but they have a significant impact on a hydro system and need to be planned for.
Therefore you should never think of head of water as a static number.
The hydro installation you install may be designed for a particular amount of head after considering all the range and duration of heads, but always think of the amount of head as changing, therefore you must think in terms of the duration or length of time each head measurement is present at a site over the course of a year. This is usually presented in the form of a head duration curve.