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Rainfall measurements by farmers in their fields

Last modified May 23, 2011 08:59

One source has it that the son of King Sejong the Great, who reigned in Korea from 1418 to 1445, invented the first rain gauge in 1441. Droughts plagued the kingdom, and the king directed every village to measure the amount of rainfall. King Sejong sent a rain gauge to every village, and they were used as an official tool to measure the farmer's potential harvest. They also used these measurements to determine what the farmer's land taxes should be. The story does not reveal how tampering with the data was prevented!

Kees Stigter (Agrometeorologist, Agromet Vision: Bondowoso, East Java, Indonesia and Bruchem, Netherlands) (cjstigter@usa.net)

Yunita T. Winarto (Anthropologist and Academy Professorship Indonesia in Social Sciences & Humanities, KNAW-AIPI, Universitas Indonesia: Jakarta, Indonesia) (yunita.winarto@gmail.com)

Tanya Stathers (Researcher in collective learning for agricultural adaptation to change, Natural Resources Institute, University of Greenwich, Chatham, UK) (tstathers@aol.com)

 

History

One source (http://www.rmets.org/weather/observing/raingauge.php) has it that the son of King Sejong the Great, who reigned in Korea from 1418 to 1445, invented the first rain gauge in 1441. Droughts plagued the kingdom, and the king directed every village to measure the amount of rainfall. King Sejong sent a rain gauge to every village, and they were used as an official tool to measure the farmer's potential harvest. They also used these measurements to determine what the farmer's land taxes should be. The story does not reveal how tampering with the data was prevented!

[Note added in May 2011: Simon Winchester (2009) revealed that Joseph Needham, the author of the 24 Volumes of “Science and Civilisation in China” (1954-2004, Cambridge University Press), discovered in the 1950s a reference to what turned out to be the first rain gauge ever made, in a book on mathematics in the Yuan dynasty (1279 – 1368). Reference: Simon Winchester, 2009. Bomb, Book, Compass. Joseph Needham and the great secrets of China. Penguin Books, London/New York, p. 188. KS.]

 

The most common problem measuring rainfall on-farm

The advantage of commercially obtained “farmer rain gauges” is that they have a scale from which the rainfall can be directly read and noted down in mm. No further calculations are necessary because the (in this case rectangular) surface area of the orifice of the rain gauge was taken into account in the scaling.

In the same rain, rain gauges with large orifices catch more water than rain gauges with smaller openings. The water caught is proportional to the orifice surface. Measuring that water as a depth in mm of rain that has fallen on a horizontal surface therefore demands calibration with that surface area of the orifice.

This is of course not necessary when the rain gauge is a complete cylinder and the depth of the water caught is made visible. One of the latter type, but without a scale, made by farmers in Indonesia (Figure 1) was not deep enough. Also the water was measured with a measuring glass unrelated to the surface of that gauge. This last error is frequently encountered with many farmer rain gauge proposals. It unnecessarily complicates the issue.

 

Rainfall measurements by farmers in their fields 

Figure 1. Home made rain gauge in Indonesia. It is not deep enough and the contents were wrongly measured. (Photo Esti Anantasari)

 

It is therefore proposed that for all rain measurements by farmers in their own fields, the problem to know the surface area of the orifice of the rain gauge should be removed. This means that either simple scales should be engraved on the rain gauges plastic surface or a combination of a complete and deep enough cylinder with a wettable dipstick should be used (see further below).

With lower accuracy, any other circular container, such as buckets or cut off parts of plastic bottles, may be used as farmer rain gauge, but they are almost never cylindrical throughout. If the containers are too wide, evaporation of water caught will diminish the accuracy of the measurements even more.

In former French colonies, in remote or otherwise unprotected areas, officially so called “rainfall totalisators” are in use. In these gauges the water caught is measured each week or each month from a tap fitted at the bottom. Inside, the water falls through a layer of oil. Water is taken out through the tap till some oil appears. In an FAO project in Madagascar in 1969, the taps were protected from interference by inverse beakers with a key lock. The totalisator is too rough a method for most daily data and here again a measuring glass is needed calibrated with the surface of the orifice.

 

Recently reported suitable gauges

Mali

In the Malian pilot projects, organized by the National Meteorological Services, plastic rain gauges were used of a more sophisticated type developed by a Dutch/French company, Agromet Applications Associates, Ferney-Voltaire, France. They were locally manufactured from a mall by a private company. Construction details were so far not made available by Mali outside WMO but the gauges have the scale engraved.

We feel that this is a solution when the local National Meteorological Services want to be involved in large scale country wide on-farm rainfall measurements in various local projects. In that case WMO could be officially approached by them for advice on local construction of plastic rain gauges and their use.

Indonesia

In Indonesia, ten transparent plastic wedge shaped so called “farmer rain gauges”, such as tested in Dar es Salaam, were installed on-farm since early November 2008 by alumni farmers of a Climate Field School in Gunungkidul, Wonosari, near Yogyakarta, Central Java (Figure 2). These were bought in the USA (http://www.benmeadows.com/store/Weather/Portable_Equipment/Rain_Gauges/) and shipped surface mail to Yogyakarta for a total of 25 $ each. This would become a few dollars less when bought in larger numbers.

 

 

Rainfall measurements by farmers in their fields 

Figure 2. Commercially available farmer rain gauge with engraved scale in use in Gunungkidul, Indonesia. A researcher advices on the farmer’s readings. (Photo Esti Anantasari)

A simple and worthwhile alternative, described by Stigter in 1970 for the Dutch NGO Agromisa, is the use of a fully cylindrical receptacle. This should be made of plastic or metal, with a diameter between 10 and 30 cm and a height of between 30 and 50 cm, but with the larger diameters not in combination with the smaller heights. An example from Indramayu, Indonesia, is in Figure 3. Gauges shallower than 30 cm will run the risk of water splashing out and evaporation of caught water may also be high.

 

Rainfall measurements by farmers in their fields 

Figure 3. Fully cylindrical rain gauge in use in Indramayu, Indonesia, for which the depth of rain fallen can be determined with a simple dipstick. (Photo Yunita T. Winarto)

Because it is cylindrical, the bottom has the same dimensions as the orifice and the depth of rain can be measured with a dip stick. This stick should have mm indications from its lower end upwards or the wet part should be measured with a ruler (Figures 4 and 5 for the Indramayu case).

 

Rainfall measurements by farmers in their fields 

 

 

Figure 4. The dipstick in use in Indramayu, Indonesia, made of aluminium painted with dark red wall-paint. (Photo Yunita T. Winarto)

 

 

Rainfall measurements by farmers in their fields

 

 

 

 

 

 

 

 

 

Figure 5. Using a ruler to measure the wetted part of the dipstick, indicating how many mm of water had fallen over the last 24 hours. (Photo Yunita T. Winarto)

The dipstick surface should be made of a material on which it is well visible till where it was in the water. It should not absorb water. For example cedar wood with a metal bottom protection plate is good. In Indramayu the dipstick was made of aluminium. A surface painted with a darkly coloured wall-paint showed the wetness best (Figures 4 and 5). This method is somewhat less accurate for small amounts of rainfall compared with the wedge shaped “farmer rain gauge” but still of sufficient accuracy in agricultural applications.

 

 

Numbers of rain gauges to be used

Due to the variability of rainfall in time and space, a large number of simple rain gauges is much more useful than a small number of more accurate ones. Each farmer should in fact have one or more rain gauges on each of his/her plots, depending on sizes of the plots, distances and rainfall gradients.

If a large enough number of gauges is available, as should be the case, it might be wise to keep sufficient spares for positioning them at places that seem insufficiently sampled after some time, due to gradients measured. Only if measurements have gone actually wrong or readings from places appear close to identical over a long time, some gauges may be discontinued and used at more suitable places.

 

Mounting of the rain gauges

It should be understood that wind speed causes the largest errors in rainfall measurements, because droplets miss the rain gauge. The smaller the droplets in the rain, the larger this error is, and wind speed always increases with height. Where the wind becomes more turbulent, such as around buildings and other high obstacles, this error also increases.

Farmer rain gauges are therefore advised to be mounted in the open on a pole with the orifice at a height of about 1.5 m, levelled and above the pole (Figures 1 and 2). When transparent with a scale, it can be read with the eyes at the height of the water level and emptied before remounting. Deep cylinders may be put on level ground or slightly raised surfaces, but the chances of interference by curious by-passers or animals are greater.

Gauges should not be mounted on roofs and other high mountings or under or close to any surface interfering with the rain. This includes plant surfaces that could drip or splash water into the gauge, so gauges should be kept away from trees (also still small ones) or other high growing vegetation. Work performed by Stigter in the Netherlands in the 1970s showed that rainfall measurements above crops are fine as long as the orifice is al least 30 cm above the highest protruding plant parts. Gauges should not be mounted with their orifices more than 1.5 m above bare soil or surfaces covered with grass/low crops, because the errors due to higher wind speeds may become more substantial.

 

Positioning of the rain gauges

It is generally advised to position the rain gauges not too close to roads or to otherwise too easily accessible places, because people are tempted to interfere. It is, however, also a nuisance if the distances that have to be covered are too large or gauges are otherwise difficult to reach, particularly in or after bad weather. In the first period in Gunungkidul, emptying as well as filling of the “farmer rain gauges” by by-passers was occasionally noted.

“Vandalism” and theft can nowhere be completely prevented. However, talking about the rain gauges (and any other project equipment) and the intended learning attached to them, at neighbouring schools, public meetings (barazas) and private gatherings, helped improve understanding and awareness and diminish interference in project activities in Sudan, Kenya and Nigeria.

 

Details of observations

In Gunungkidul the farmers decided on which early hour of the day all measurements would be taken each day. If comparison with existing meteorological stations is important, times and heights should be adapted to the routine at those stations.

If calamities prevent a measurement from being made and no stand in observer can be organized, the following day the gauge should be read at the usual hour, indicating the missed day of observation clearly on the observation sheets. There is a tendency to skip observations in heavy rainfall but in fact the need then is high. Umbrellas may be donated to observers. If a rain gauge has overflown, this should also be clearly mentioned on the data sheet.

2008/09 rainfall sheet for decadal observations

Sedio Mulyo Farmers-Group, Wareng IV hamlet, Gunungkidul

 

Farmer’s Name : ………………………………………

Location : ………………………………………

Month:

No.

Day

Date

Time

Amount of rainfall (mm)

Explanations (if any)

 

1

 

 

 

 

 

 

2

 

 

 

 

 

 

3

 

 

 

 

 

 

4

 

 

 

 

 

 

5

 

 

 

 

 

 

6

 

 

 

 

 

 

7

 

 

 

 

 

 

8

 

 

 

 

 

 

9

 

 

 

 

 

 

10

 

 

 

 

 

 

Figure 6. Decadal data collection sheet for rainfall in Gunungkidul, Indonesia.

An example of a data collection sheet used in Gunungkidul is shown in Figure 6. An example of farmer collected data in Gunungkidul is given in Figure 7. It may be observed that there is a need for involvement of at least one researcher or trained observer (e.g. a field school facilitator) to work with the farmers on the data.

 Rainfall measurements by farmers in their fields

 

Figure 7. Example of rainfall in mm obtained in Gunungkidul, Indonesia. Graph created by Hestu Prahara for various locations (Universitas Indonesia, Jakarta).

It will also be of much help if a researcher/facilitator could assess regional and plot data collected in a project. This should be done at least after each year of measurements but preferably earlier and more regularly. It would assist in the interpretation, detection of possible errors and advising on additional gauges to be mounted. In Mali data were reported daily, by intermediaries that collected them from the farmer observers, to the National Weather Service. There they were quality controlled and used by a team in agrometeorological advisories/services to the farmers.

Farmers in Gunungkidul appeared to be proud to be able to talk about rainfall quantitatively. They quickly related their observations to the growth behaviour of their crops. After some years a feel for the agricultural meaning of these rainfall data will definitely have strengthened and the data will become even more helpful in their decision making.

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