* The information in this Annex has been obtained from FAO irrigation and Drainage Paper 27, Agrometeorological Field Stations, Chapter 6.
The total amount of rainfall over a given period is expressed as the depth of water which would cover a horizontal area if there is no runoff, infiltration and evaporation. This depth is generally expressed in millimetres.
Accuracy of rainfall measurement is mainly affected by wind, by the height of the gauge and exposure. Wind and exposure errors can be very large, even more than 50 percent. The catch of rainfall is a function of the height of the gauge; the more open the location the greater will be the difference in catch with height.
Raingauges have a cylindrical form. The leakproof collector rim is placed above a funnel which leads to a receiver. The receiver should have a narrow neck into which the funnel fits to reduce evaporation loss.
The collector should have a receiving area of 200 to 500 cm2. The rim of the collector should have a sharp edge which falls away vertically inside. The collector is designed so that rain cannot splash out; the walls should therefore be sufficiently deep and the slope of the funnel sufficiently steep (more than 45°).
Raingauges are made of non-corrosive metal, fibreglass or plastic. Since type, diameter of the collector, height and manner in which the gauge is exposed vary considerably from country to country, it is important that the type selected and method of installation should be similar to any other raingauge in the area in order to obtain comparable data. Normal height of exposure is usually 30 cm above ground level. At greater height wind affects the accuracy of measurement. Where the raingauge placement and particularly the siting are very different from local practice, a side by side comparison between the two raingauges may be needed. The graduation of the gauging device (jar or rod) must, however, always be consistent with the size of the collecting area of the raingauge. A number of raingauge types are shown below.
The site must be level and the surrounding ground should be uniform. The ground should preferably be grassed or loose earth. No object such as another instrument, building or trees should be closer than four times their height. Very exposed sites, such as on the top of a hill, should be avoided. For very exposed sites without any natural shelter raingauge shields are sometimes used. The raingauge should be firmly mounted on a concrete base. The rim of the raingauge must always be horizontal.
Measurements should be taken at the same time each morning. A graduated measuring cylinder or a graduated dipstick or rod should be used. The former is preferred. If it is raining at time of observation, measurements should be taken quickly to avoid loss of catch.
A measuring cylinder, standard for the instrument in use, should be of clear glass or plastic. The diameter of the measuring cylinder should not be more than one third of the diameter of the rim of the gauge. Graduations should be clearly engraved in 0.2 mm graduations. The measuring procedure is to pour the rain water from the storage contained into the measure and to read the value from the graduations. If there has been considerable rainfall, this may have to be done in two or more stages. The bottom of the water meniscus should be taken as the defining line. When reading, the cylinder should be held vertically. The empty storage vessel is then returned to the gauge and the collector replaced.
If no special graduated measure adapted to the raingauge in use is available, a measure graduated in cm3 can be used. The procedure is the same but the observed volume should be divided by the surface area of the collector of the gauge in cm2 to find the cm of rainfall. Errors using this type of measurement can be greater.
Rainfall should be observed in units of 0.1 mm. Readings of less than 0.05 mm should be recorded as a "trace". A "trace" is also recorded when there is no sign of precipitation in the gauge but it is known for certain that slight rainfall has occurred since the last raingauge reading.
It is conventional to allocate the 24 hours catch observed in the raingauge before or at 09.00 hours to the previous day. For example, the catch measured at 08.00 hours on 1 December will be shown in the record dated 30 November and be included in the November totals. The hour of observation should, however, still coincide with local practice.
Raingauges should be checked for leakage; dust and leaves should be removed from the collector. The inside should be cleaned but should not be polished. The measuring cylinder should be clean, and should not be dented. A spare measuring cylinder should be available. Plant growth around and above the raingauge should be kept out.