Sensors for measurement of the quality and quantity of both surface water, including the oceans, and groundwater are similar in principle, and so it makes for greater clarity if this chapter is organised by sensor type rather than by application.

Measuring water level

Staff gauges

Graduated staff gauges are widely used for the manual measurement of rivers, lakes and sea level. They are usually installed vertically in the river bed or fixed to a weir (Fig. 10.1(a), (b)), bridge or harbour wall. Boards are made in one metre and two metre lengths and are about 15 cm wide, fixed one above the other to cover greater depths, and marked to span up to 12 metres, or more. Alternatively, several may be installed, each progressively higher up the bank of a river if there is no structure to which to fix them and the river is deep and wide. They are graduated in a variety of ways, some every centimetre, others every 10 or 20 centimetres – as in the case of some sea level gauges. Boards are also available for fixing at an angle of 45 or 30 degrees, laid flat on river banks, their markings being stretched to compensate. Most are graduated from bottom to top, but others are made with an inverted scale for situations where levels below a reference point are needed. Gauging boards have the advantage of cheapness and simplicity, although care is needed in reading them.