Handbook for Agrohydrology (NRI) Chapter 4: Rainfall and other meteorological data 4.1 Rainfall 4.2 Other meteorological data Equipment costs

Handbook for Agrohydrology (NRI)

Chapter 4: Rainfall and other meteorological data

4.1 Rainfall

The collection of snowfall data is not covered here. If detailed information is needed, consult a handbook such as USDA No. 224 or approach the local meteorological service. Ordinary, standard gauges with the collection/funnel component removed and a measured amount of antifreeze added can be used to measure snowfall, but errors due to wind effects can be very large.

Rainfall is the most important single factor in determining whether runoff will or will not occur for a given set of environmental conditions. It determines runoff amount and frequency. There are two measurements of rainfall amount that are commonly collected for hydrological purposes: Daily and (runoff) Event rainfall.

Daily Rainfall is probably the most ubiquitously measured meteorological variable. It is the rain that falls awing a 24 how period starting in the morning of one day (commonly 06:00, 07:00 or 08:00 furs) until measurement is made at the same time the following day. Event Rainfall by contrast is the rainfall occurring awing an unspecified time period usually, but not always less than 24 hours, that can be seen to be responsible for subsequent runoff. The collection and use of each has advantages and disadvantages.

In the case of daily rainfall, data are usually available from many stations, even in countries with only the most basic meteorological network. The equipment to measure daily rainfall is relatively cheap, simple to install, read and maintain. All projects should easily achieve adequate instrumentation. In most cases, many years of historical data will be available for analysis from a variety of sources, in addition to that obtained from meteorological offices: these sources include various government departments, water resource and construction projects, state and private farms, schools and interested individuals. Often basic analyses will have been performed on the data (average monthly and annual totals, spatial distribution, etc.). For the analysis of runoff relations, however, daily rainfall can have one serious drawback. It is the lump sum rainfall awing a 24 how period and in some climatic environments may greatly exaggerate the amount of rainfall thought to be responsible for runoff, but despite this drawback, it is the most commonly used climatic variable in runoff studies.

Event rainfall, obtained from the careful examination of the records of an automatically recording rain gauge, can provide a precise and accurate evaluation of the rainfall responsible for runoff and it is often to be preferred for rainfall/runoff analyses. However, recording rain gauges are not usually in widespread use except at important synoptic stations (especially in developing countries). They are expensive to buy, can be difficult to maintain and staff must have a higher level of expertise to operate them. The analysis of data is more complex and time-consuming.

It is possible to determine whether or not daily rainfall and runoff event rainfall are for all practical purposes, the same, though a number of historical data are necessary to do this. Values of daily rainfall are plotted against values of runoff event rainfall, as illustrated in Figure 4.1. A visible correlation exists for these data from sites around Gaborone in SE Botswana, confirmed by the statistical significance of the coefficient of determination of the relation (R2 = 0.99) The gradient of the line which is for all practical purposes 1:1, the regression equation being y= 1.025x + 0.068. In this case, daily rainfall values could be used instead of event rainfall, without fear of inaccuracy. The ratio of Daily to Event rainfall may not be 1:1, but any relation that is highly significant can be used. Where the relation is not significant, daily rainfall cannot be used to estimate the actual rainfall that caused runoff. The similarity between daily and event rainfall depends on climatic regime but it will be more evident in areas that have convective rainfall, giving heavy, short periods of rain and many thunderstorms. The analysis of rainfall data is discussed in greater detail in Chapter 8.

Rainfall Intensity defines the amount of rain falling during a specified time within the most intense period of the rain. This value is then converted into the amount of rain that would fall in one how at this intensity. For example if 5 mm of rain falls in a 2 minute period, the 2 minute duration rainfall intensity is 150 mm hr-1 and if 27 mm falls in 30 minutes, the 30 minute duration rainfall intensity is 54 mm hr-l. Rainfall intensity can have an important effect on runoff proportion, as it determines the rate at which rain arrives at the soil surface and, consequently, whether the infiltration rate of the soil is sufficient to allow absorption. Automatic recording gauges are needed to measure rainfall intensity The duration under study will be determined to a large extent by catchment size; small plot runoff is often closely related to the 2 or 5 minute intensity durations while large catchment relations are more evident with long durations. The use of daily rainfall becomes a more attractive proposition under these circumstances, especially as the use of long duration intensities can lead to a serious reduction in data, when rainstorms are short-lived.

Figure 4.1: The Relation Between Daily Rainfall and Event Rainfall

4.1.1 Non-Recording (Daily) Rain Gauges

All agrohydrological projects will collect daily rainfall from manually read rain gauges. Even when automatic recording gauges are installed it is important to cross-check the amounts that they measure and ensure that at least daily values are collected if they develop faults. Some automatic gauges have integral manual gauges sited under the recorder to facilitate this. Different models of daily rain gauges are used in different countries. In the case of some developing countries, which can ill-afford even basic meteorological equipment, national networks may use more than one type, having been supplied by different aid agencies or having inherited them from various projects. Usually this does not cause serious problems. It is essential however, to ensure that the correct measuring vessel into which the rain is poured from the gauge (and calibrated only for one particular model) is used to measure the rainfall amount. As a general rule, it is sensible to purchase equipment compatible to that already used by the national Meteorological Service. This has several advantages:

- Data will be strictly compatible.

- In the case of loss or breakage of measuring vessels (glass), a temporary loan may be possible.

- The Service will be familiar with procurement/replacement procedures.

- The Service will be familiar with the most suitable methods of installation.

- Service staff and other gauge readers who may be called upon to read the gauges, will be familiar with the equipment. In some cases it may even be possible to arrange the loan of gauges, if the Service has sufficient reserve.

Types of Gauge

Any open ended vessel can be used as a rain gauge, but uniformity of design to provide consistent splash characteristics necessitates the use of purchased equipment. Most gauges are made of corrosion-resistant metal such as brass.

Gauges with orifices of different sizes measure rainfall with about the same degree of accuracy. Results of tests show that readings are within 1% from gauge openings of between 5 to 50 cm. Differences in measurement usually result from installation and reader error rather than gauge design.

The standard US Weather Bureau non-recording gauge has an 8 inch (20.3 cm) orifice, the UK standard gauge (commonly adopted by former colonies) has a 5 inch (12.7 cm) opening. There is little to choose in design accuracy, but in general, smaller gauges tend to be less expensive. Figure 4.2 shows examples.

Figure 4.2 Non-recording Rain Gauges.

Non-recording gauges usually consist of a collector above a funnel which passes collected rain into a receiving vessel. Important requirements are that the collector walls should be vertical inside and steeply bevelled outside. It should prevent rain splashing in or out by having a sufficiently deep wall and a funnel with steep sides (at least 45 degrees). The area of orifices should be consistent. The receiving vessel should have a narrow neck (to prevent evaporation losses). It is usual to use a larger vessel in the same gauge, where it is impractical to visit the gauge on a daily basis.

The measuring (calibration) vessel should be of clear glass with engraved graduations (usually at 0.1 mm intervals). The type of gauge that it is to be used with, should be clearly marked. To achieve accurate readings for small rainfall amounts, the base will be tapered. Dip rods are sometimes used instead of measuring vessels, but this is unusual.

Plastic rain gauges, often inverted cones in shape and marked with mm gradations, are available in some countries and can provide a good, cheap alternative to expensive standard rain gauges. However, three important facts should be recognized.

- They eventually degrade due exposure to UV light, after one or two seasons.

- They cannot be recommended in areas of frequent hard frost.

- Not all such gauges are produced to accurate specifications and the accuracy of the gradations should be checked and if necessary, calibrated before use.

- They are difficult to install with the orifice exactly horizontal, using a single post.

Installation of Non-recording Gauges

The location of the gauge is the primary consideration in obtaining accurate rainfall measurements and the most serious problem is wind turbulence. Buildings, trees, fences produce eddies and reduce accuracy. Isolated obstructions should not be closer than twice their height to the gauge (further away if possible). However, openings in woods and orchards are suitable places (so long as the trees are no closer than specified); they act as windbreaks and reduce violent air currents. Sloping ground should be avoided and surrounding vegetation should be cut low. In general, smooth artificial surfaces are not suitable as they tend cause splashing and may attain high surface temperatures.

It is very important that the height above ground level of gauge orifices should be the same at all sites. It is preferable that the orifice be as close to the ground as possible. Wind velocity increases with height and the catch of the gauge is reduced thereby underestimating rainfall, however gauges positioned at ground level are prone to in-splashing, which is also undesirable. The UK Meteorological Office recommends that gauges' orifices be located at 305 mm above ground level as a compromise to minimise both of these effects. The influence of wind on rain catch can be expected to be greater in areas of convective rainfall where air turbulence is inherent.

In difficult field conditions however, the practical limitations of accidental disturbance and vegetation growth can result in problems with gauges set close to ground level. From experience, a suitable height is between 0.8-1.2 m. In this range of heights the gauge should be clearly visible and vegetation growth between visits from project staff should not affect readings. A consistent height for all gauges based on local field conditions is probably more important than following hard and fast recommendations. Ground-level gauges can be seen at meteorological stations, but here constant attention can be given to vegetation clearance, animals should not be a problem and checks can easily be made on the equipment. This may not be so at field sites.

The stand supports that hold the gauge should be inexpensive, strong, durable, rigid and easy to build from local materials. Wood is best avoided due to susceptibility to termites and rot. Materials should be easy to transport into the field. Although a concrete base in which stand legs can be set or bolted seems sensible and is often advocated, in areas where animal damage is possible, it is better to install the stands with metal pegs or by digging the legs into the ground to an depth of 50 cm, which is usually adequate. This may lead to the toppling of a gauge if pushed by cattle, but it can be reset immediately and will not be out of operation until the stand is replaced and re-cemented. In all cases, the gauge should be set horizontally by testing with a spirit level.

Gauges may often be placed on private land and it is essential to obtain permission from the owner before doing so. Usually a discussion as to the aims of the project, the (realistic) long-term benefits that are hoped for, and the purpose of the gauge are sufficient to obtain permission. Sometimes and if suitable, land owners can be recruited as gauge readers. It is important to provide an enclosure for the rain gauge to reduce the risk of damage from animals and to prevent interference with normal land use. Barbed wire is most suitable, with strands placed closer together (10-15 cm apart) at the bottom. Not only does this prevent access to animals and deter vandals, but also has no discernible effect on rainfall catch. Cut thorntree branches (e.g. Acacia species) woven into the lowest metre of the fence are a good deterrent against goats and other agile animals, although in areas of termite activity this will need to be replaced each season. For the same reason, enclosure posts are best made of metal, though they are more expensive than wood. If they are too costly or unavailable, wooden posts will last several seasons if soaked in creosote or other preservative. In some areas, termite-resistant tree species can be found for use.

Even though wind turbulence is important in reducing rain gauge accuracy, the use of windshields can generally be discounted. Their main aim is to reduce wind speed, turbulence and splash over the gauge and allow siting close to the ground, but in practice they are not easy to make locally and increase problems of interference by people and animals. Research has shown that differences in shielded and unshielded gauges are relatively small (2-8%).

When locating rain gauges in the field it is important to keep in mind the problems of theft and vandalism. Prevention can be aided by the provision of fencing and enclosures, but the safest action is to enlist the help of local people. Sites placed by the homes of gauge readers are usually convenient and secure. When positioned in more public places, such as common village land, it is advisable to discuss the purpose of the equipment at an open meeting, where the use of rainfall data can be explained. An agreement for safe custody of the equipment can (usually) be made without difficulty.

The UK Met Office and US Weather Bureau recommend that rain gauges in exposed positions be located within a circular turf wall to negate the effects of wind turbulence. Figure 4.3 illustrates the construction of the protective wall which should be kept in a good, clean condition. Note that the rain gauge orifice is level with the top of the wall. It should be stated that in many circumstances, recourse to such a structure will be very difficult.

Figure 4.3 Construction of Turf Wall to Remove the Effect of Wind

Non-Recording Rain Gauges: Routine Data Collection and Site Staff

At field sites that do not have automatic recording gauges, locally-hired gauge readers will almost certainly be required and it is essential that a good working relationship be established and maintained with such gauge readers, observers or site guards. More difficulties in the collection of field data arise from people than from instruments and reliable field staff are an invaluable asset to any project. The selection of suitable field staff can be difficult and common sense and judgement of personality are important criteria in arriving at a suitable choice. The opportunities to recruit will vary from country to country and will depend on social as well as educational factors. The following points can help.

- The person should be reliable and suited for the job. They should be literate and numerate at least to a basic level. It is unlikely that they will need skills sufficient to understand instrument manuals (this can actually be a disadvantage if they are "zealous" in the performance of their duties), but they will be using printed record sheets and measuring and recording numerical values.

- Village school teachers are often used as recorders of rainfall, by national meteorological services. However, they have other responsibilities and often take holidays away from their village. Rainfall records often suffer as a result.

- If possible, it is useful to have someone who can speak your own language, or an interpreter will always be necessary.

- A permanent resident is preferable, even if less educated. Their new responsibilities should not be in conflict with their normal day-to-day business.

- Be aware that the recruitment can give rise to discontent within the family/community by imparting status and/or financial gain to the individual.

- It is important to resist the recruitment of individuals who may be using their status within the community to acquire more and/or who expect financial gain, without being prepared to carry out the work properly.

- The gauge reader should be able to reach the gauge easily, therefore they should live close by and have permission for access if it is sited on someone else's land.

- After selection, the reader should be given a thorough explanation of why the data are being collected and the importance to the village and project.

- They should be tested for competence in all their responsibilities.

A contract should be drawn up covering all duties and rewards in a simple but comprehensive list. For example:

- Reading the gauge at specified times (check that this can be done, or at least that the time of reading can be noted).

- Keeping the record book and measuring vessel at a specified place so that they can be checked on field visits.

- Cleaning the gauge when necessary.

- Noting damage to the gauge, enclosure and keeping down vegetation etc.

- Repairing any such damage when possible.

- Keeping the site clear of vegetation on a routine basis.

- Informing in advance the use of the deputy and expected departures from site.

- Basic pay, holidays or pay in lieu.

- Deductions or action in the event of neglect of duties.

- Best method of contact in the case of unforeseen events.

Any seasonal changes in these duties should be stated, as should a clause covering unsatisfactory performance. A suitable deputy should be appointed as it is inevitable that at some time the gauge reader will have to leave the site. The contract should be translated into the local language if necessary. It is best if payment is made for the services of the gauge reader. This puts the arrangement on a business footing and no favours are asked by either side.

Payment should reflect local employment conditions, bearing in mind that while the work is not arduous, it does restrict the mobility of the gauge reader. All payments should be recorded, copied and signed for without failure. Most of these points will be influenced by local conditions, but a friendly involvement by the project field staff in the reader's work coupled with a direct and business-like approach seem to get best results.

The maintenance of non-recording gauges is straight forward and should be carried out by the gauge reader. The gauge should be checked for any blockage (cobwebs, insects, leaves etc.) in the funnel at each reading by holding the orifice component to the light. Even partial blockages should be cleared. The collection vessel should seen to be watertight and clean and the felt washer on the funnel of the gauge which fits over the vessel mouth (and helps to prevent evaporation) intact. The measuring (calibration) vessel should be clean and in good repair. It is useful to have a spare at site as these are made of glass. It should be ensured that the gauge is correctly seated on or in its stand, often gauges are disturbed when the tightly-fitting collection component is removed to measure the rainfall

Figure 4.4: Routine Data Collection Sheet, Non-Recording Rain Gauges

The stand should be checked for stability and deterioration and repairs effected if necessary. The area around the gauge should be kept clear of vegetation and the enclosure fence kept in good condition. Any repairs to or re-instatement of the gauge, including the date of original installation should be noted in the record book. The field team should carry a book identical to the site record book, with the same headings and columns, so that information can be copied from one to the other with minimum error. A hardback notebook, protected from the weather with stitched pages (rather than a file with loose, detachable pages) is best. Columns to be set out in a form similar to those in Figure 4.4, in addition to the name and address (and telephone number if appropriate) of the project, contact name etc. being clearly printed, in more than one language if necessary, on the book.

Field teams should routinely carry certain items such as a tool kit for impromptu repairs to gauges and stands (depending on the types used), spare record book, spare measuring glass, pay sheets, etc.

It is important to remember that meteorological offices "throw back" daily rainfall records, i.e. to attribute rainfall recorded on a morning to the previous day. This may not be appropriate for some studies.

4.1.2 Recording (Intensity) Rain Gauges

There are three main types of recording rain gauge systems: Tipping bucket, Syphon and Weighing. Recent advances have led to the use of electronic loggers which now frequently replace the usual chart and pen clockwork systems.

Alternatively, very compact intensity gauges measuring to a precision of 1 mm of rain and reputedly accurate to 2% over 2 years are available, though the cost of this equipment is not low. A liquid crystal display readout is given via a connecting cable and daily, weekly and monthly accumulated totals may be collected.

a. Tipping Bucket Gauge

These are commonly seen as in Figure 4.5, below. There are many different types and the manufacturers manual must be followed carefully, as is the case for all recording rain gauges. A dual tipping bucket pivots on a horizontal axis which lies beneath the funnel of the orifice, such that only one bucket receives rainfall at a time. When filled to a preset, calibrated amount (for example 0.2 mm) the bucket tips and is emptied, leaving the second bucket to receive rain. Tips are recorded electronically and individually.

Data are downloaded and analysed by computer software, though hard copies used for manual analysis are sometimes available via a portable printer carried to the field. Alternatively, the data may be recorded on a mechanically driven chart. In general the tipping mechanism works well, but sometimes does not register very light rain in hot climates. It may also under-register during very intense storms, because of the finite time taken for the buckets to exchange positions.

Electronic logger type

Advantages:

- Simple download directly to a computer.

- With computer programs, the analysis of intensity data is quick and easy.

- No problems with mechanical clock, ink, pen etc.

- Very large amounts of data can be stored (32 - 120 kb)

- Options for storing data, time, etc. only when rain occurs, thereby saving memory.

- Can operate for very long periods.

Disadvantages:

- Some designs are new and may not be well field tested.

- If the logger or battery fails, then all the data can be lost.

- Needs computer facilities at base (good electricity supply, etc.).

- Needs spare loggers or portable computer, both of which are expensive.

Maintenance

Buckets must move freely and be oiled on a regular basis in some cases. Mechanism must be checked frequently to ensure that it is horizontal.
Battery and connections must be tested at each visit and replacements made when necessary. Gauges come precalibrated, but they must be re-calibrated at the end of each season in the laboratory. This is a simple process whereby known volumes are emptied into the gauge from a pipette and checked against the record. Check with the manufacturer's instructions.

The tipping motion closes an electrical contact (usually current is provided by a 6 volt dry battery) which registers a pulse on an electrical counter or logger, each pulse representing the bucket contents (the example 0.2 mm of rain). The data are usually in the form of each tip represented by a time (Month: Day: Minute: Second). The opportunity to record the gauge name/number may or may not be offered, so it is wise to keep a careful note of from where the logger came. A 'record only with rain' facility is usual.

Various methods of down-loading can be used. In some instances the logger must be removed and taken to base to be down-loaded via an interface and computer. A replacement must be provided. In some cases the data can be downloaded in to a portable computer and the logger can remain at site. In all cases some sort of set-up procedure is necessary to re-activate the logger once the data are extracted.

Figure 4.5: Tipping Bucket Raingauge, Electronic Logger Type

Mechanical (Clockwork drum and chart)

Some gauges provide a chart record of the tipping instead of an electronic recorder. A permanent pen record is kept of each tip on a clock driven drum chart. Clocks usually work for about 30 days without attention, but this can be altered in most cases, by replacement of parts of the gearing mechanism.

Advantages:

- Permanent record is kept on chart, therefore cannot be lost.

- Less likely to be affected by adverse conditions

Disadvantages:

- Analysis of data is lengthy and must be entered manually into computer storage.

- Needs more frequent visits

- Not as flexible in terms of alteration of the instrument settings

Some recorders offer both electronic and mechanical records. This gives a good back-up facility and some models even record river levels simultaneously. In some cases the tipped water runs to waste, but some gauges have the provision for the rainfall to be collected in a vessel below the gauge via a funnel and so the total rainfall for the period between readings is known. This can be very useful if the gauge or logger develop problems.

Maintenance

This is a little more complicated than for the electronic gauges, though the bucket check is the same. Clock accuracy must be tested on a regular basis, even though charts last 30 days without attention. Chart replacement should be done with an accurate pen reset and any malfunction noted. Chart drum motor may be clockwork or electrical.

b. Tilting Syphon Type

Versions of this type suitable for use in tropical countries are available. Figure 4.6 illustrates a typical syphon instrument.

Rain is collected and falls into chamber A, and raises float B. In response, the pen moves upward and its trace is recorded on the chart fixed to the drum, H. The chamber is on a pivot (C), over-balances when full and empties through the syphon tube (D). The pen is then reset to the zero position while lifted clear of the chart by the rod G. The over- balancing is controlled by the trip, E and the chamber is restored to its original position by F, the counterweight. The siphoning takes approximately 15 seconds.

Figure 4.6: Tilting Syphon Rain Gauge

It is important to test the mechanism regularly by pouring in water through the inlet and to keep the syphon tube and its gauze filter clear of blockage at all times.

c. Weighing Type

This type is less common than the former kinds, but is especially useful where snow is frequently experienced. Precipitation is collected from a funnel into a bucket which, as the frame upon which it stands falls with increasing collection, stretches an isoelastic spring. The movement of the frame is proportional to precipitation and linked to a pen by a series of levers. This records on a clock-driven drum chart.

The maintenance of recording gauges will be the responsibility of visiting field teams who must be trained to a higher level than the field gauge reader responsible for daily gauges. Tools, loggers, charts, pens and ink will be carried routinely. Although gauges are of a type (syphon, tipping bucket, mechanical, electronic etc.), each will vary according to the manufactures' particular specifications and it is impossible here to list specific instructions for all gauges. In addition to the points of particular care noted in the sections describing the types of recording raingauge, the manufacturers instruction manuals should be carried to the field and studied carefully.

Factors Affecting Accuracy of Rain gauges

Many factors affect the accuracy of rain gauges. These include evaporation, adhesion, inclination of the gauge, condensation and splash. However, these are unlikely to cause differences of more than about +/- 1%, whereas wind turbulence at a poorly sited and maintained station can account for much larger errors. The precise effects of wind speed is still contentious despite many years of research. Some authors predict large deficits (for example 17% with winds of 16 km hr^-1 and 60% at 48 km hr^-1) while others (see Figure 4.7 below) expect the effect of wind speed on rain gauge catch to be much less. Agreement that wind effects cause an underestimate of rainfall (and more especially snowfall) is universal, however.

Damage to the instrument during carriage should be avoided, especially denting about the orifice, which can cause discrepancies in readings. Splits and cracks in the receiving vessel can cause serious losses. Care should be taken to ensure that all the water is emptied into the measuring vessel and that the measurement is made accurately at the bottom of the meniscus.

Figure 4.7: Catch versus Wind Speed

A list of errors and causes inherent in measuring rainfall from standard, non-recording gauges are given below. Other factors, notably poor maintenance, faulty resetting of loggers, pens and charts can affect the accuracy of rainfall records obtained from intensity gauges.

Figure 4.8: Errors in the Observation of Point Rainfall

Rain Gauge Networks

Many projects will be adequately served by the installation of rain gauges at each experimental site, when the interpolation of data between sites is unnecessary or can be easily achieved, perhaps using data from the national network. In some circumstances it may even be best to install two daily gauges, to allow for occasions when one gauge may be accidentally inoperative, though of course this must be balanced against cost. If the site area is large, these should be separated, with one gauge in the centre of the site and one at its boundary. When placed on a line at right angles to the prevailing wind, it is sometimes possible to collect information on the distribution of rainfall. If resources allow, the installation of a recording rain gauge at each site, in addition to at least one non-recording gauge, is to be preferred for agrohydrological purposes. Rainfall intensity is an important influence on runoff and its study will undoubtedly play an important part in the research agenda. However, recording rain gauges are very expensive (even "low cost" data-logger versions are currently several hundred pounds sterling, each) and project resources must be considered carefully.

If resources are inadequate for total coverage by recording gauges, then partial cover must be budgeted for. The success of partial cover will depend on a sound instrumentation strategy, which can only decided upon by the staff of each individual project. This will be dependent on the number of sites, their proximity to the base station and each other, the spatial variation of rainfall characteristics, the time between visits and field staff reliability. The two extreme options are:

1. to place all recording instruments at the most distant stations. This will provide event, daily and intensity data from widespread, infrequently visited sites which would otherwise give only rainfall totals from several days using non-recording gauges. However, it is wise to recognise that infrequently visited sites are always the most troublesome. Faults in and damage to the equipment will not be seen for some time, gauges can be interfered with or even stolen. Data and equipment could be totally lost. Access may be impossible at times during the wet season.

2. The second extreme option is to place all such valuable equipment at or near the base station. There is little danger of problems with the equipment not being rectified quickly, but the opportunity to collect data from diverse areas is lost. The best solution, clearly, lies between these two examples and to some extent trial and error (especially becoming familiar with the reliability of the gauges under field conditions) will be needed to determine which outlying stations are most suitable. It is essential to monitor one gauge carefully at the base station and check its readings and reliability of operation.

In many areas of convective rainfall, a statistical randomness means that over time, the average number of storms of a given intensity will be experienced at all locations within the study area. Thus it is reasonable to presume that rainfall intensity can be extrapolated from one site to another, if certain characteristics of a rainstorm (for example the amount and duration of rain) are known. It is convenient if a statistically significant relation exists between rainfall amount and rainfall intensity and allows the substitution of one type of data for another. Figure 4.9 below shows 30 minute duration rainfall intensity against daily rainfall. The significance exceeds the 99.9% level. In some instances, a clustering into groups of data points may be seen with a strong correlation among them, probably indicating that different types of rainfall (for example low intensity frontal and high intensity local convectional) have been experienced. The 2 minute duration intensity against daily rainfall showed no significant relation for the data.

If a comprehensive raingauge network is proposed for an agrohydrological project, the number and density of instruments will depend on several factors. Those relating to the physical environment are:

- Size of area covered

- Prevailing storm type

- Topography and Aspect

- Variation in seasonal rainfall

In general, more gauges will be needed for large areas and denser networks will be needed where storms are convective and localised with high intensities (as opposed to cyclonic areas where rainfall tends to be widespread and of more uniform, low intensity). Convective rainfall is characterised by the predominance of thunderstorms. Mountainous areas, which create orographic rainfall, are expected to have localised rainfall regimes and to need a more dense network than plateau areas (Table 4.1). However in practice, the rainfall in mountainous areas may be of a more regular distribution than extremes of elevation may suggest whereas flat plains dominated by very local convective rain storms often exhibit very large coefficients of variation of rainfall distribution.

It is important to plan with the hydrological characteristics of the area in mind. For instance it is more important to place a denser network of gauges in areas which contribute most to runoff, than in homogeneous areas which contribute little. It is possible to use correlation analysis in determining network densities. For instance if the correlation of daily rainfall between adjacent gauges is high (say, r = 0.90 or greater), a firm basis is provided to reduce the number of gauges in the network. Rainfall is spatially variable to a high degree and even the densest network of gauges can provide no more than an estimate of areal precipitation.

Networks are best planned in the preliminary stage of a project by the use of a map desk study to provide a picture of the overall pattern of gauge distribution. The distribution should not be random; random events are studied by a systematic arrangement of sampling points. Minor revisions of the network pattern can be made during installation, if unexpected problems of siting are found. It is useful to place some gauges outside the study area to ensure that extrapolation is possible to the boundaries of the study area. More gauges are needed if results are to be taken to other areas, rather than limited to the original study area.

Figure 4.9: Daily Rainfall versus Rainfall Intensity

As stated above, project objectives and resources will determine, to a great extent, the level of instrumentation. A watershed study that relates total precipitation and total annual runoff yield would need fewer gauges than a study of rainfall on a storm-by-storm basis. The USDA regards the following gauge densities as suitable (see Table 4.1), but states clearly that the size of the study area is the only criterion used to determine them. Factors such as climate and topography are not considered. These recommendations can be compared to those of WMO which take into account, in a limited way, climate and topography. However, it is likely that the needs of the project will be of paramount importance when compared to such general recommendations.

Table 4.1 Recommended Density of Daily Rain Gauges by Size of Study Area

4.2 Other meteorological data

Rainfall data collected for hydrological purposes will also be useful to other project members, such as agronomists and soil physicists. The same is true of other meteorological data which may be important, for example in assessing crop performance under varying climatic conditions. These other data will help categorise climate in general and are essential to estimate evapotranspiration and soil moisture conditions which can have an important effect on runoff production, soil moisture availability and crop growth. Indeed, the calculation of evapotranspiration is one the most important uses to which comprehensive climatic data are put during agrohydrological investigations. This chapter concentrates on instrumentation; the uses and analysis of data are covered in chapter 8.

Site of Meteorological Stations

The installation of climatic instruments requires a suitable site which should be representative of the macroclimate of the study area. Where climate varies greatly, perhaps due to topography, several stations may be necessary, though the spatial variability of some meteorological variables is greater than others. The site should not be in an exposed position on a steep slope, nor should it be within the distance of four times the height of any nearby trees or buildings. In semi-arid areas, sparsely vegetated open areas make good, representative sites. The site will probably represent the greatest concentration of instruments for a project and it is essential that a suitable, secure location be selected. Advice on instrumentation should be sought from the local Meteorological service and where possible instruments of the same manufacture should be acquired.

The site can be instrumented according to particular need, but special attention should be paid to such details as shading from elevated posts and other instruments. Doors to equipment should be away from direct sunlight, areas of artificial surfaces should be kept to a minimum. The area should be well fenced and gated, not higher than 1m, with wire mesh which is fine at the foot of the fence, to deter animals. Birds can be prevented from roosting on instruments by the provision of alternative high perches. Fence posts should be of metal to avoid termite damage. It is useful to retain extra space within the compound, for instruments that may be added at a later date. Where possible recording instruments should be used. Experience will show that records obtained manually, during holidays and weekends often appear suspiciously inconsistent when compared to weekday readings.

Records of equipment should be kept secure at the station. Loose-leafed books are more prone to damage and loss than those with permanent bindings. A site map of water pipes, cables etc. is useful if the station acquires permanent buildings. Longitude and latitude should be noted on any such map. Records of instruments added or removed are very useful as are schedules for routine repair, painting and grass mowing. Files should be kept which contain notes on the instruments; when bought, invoices, serial numbers, calibration tests, instruction manuals, repairs etc. Having this information easily at hand can save a great deal of time and frustration. Such details should be kept separate from the day-to-day records of measurements, and in a secure place.

4.2.1 Air Temperature

Air temperature is one of the most commonly measured meteorological variables. Maximum and minimum temperatures are used to calculate mean daily values for use in evapotranspiration estimations. Figure 4.10 illustrates the installation of maximum/minimum thermometers.

Figure 4.10: Maximum and Minimum Thermometers

They should be housed within a screened building or box.

The temperature readings are obtained from two separate thermometers. The maximum thermometer is of mercury in glass, secured so that the bulb end is 5° above horizontal. The minimum thermometer is alcohol-filled, with the bulb end about 5° below the horizontal.

As temperature drops, the alcohol retreats into the bulb, inducing an index (a small, dark dumbbell) to move within the bore of the thermometer, until the minimum temperature is reached. When the temperature rises, this index is left behind to give the minimum reading. This reading is taken at the end of the index furthest away from the alcohol-filled bulb. Both temperature readings are usually taken at about 08:00 each day and the instruments are then reset. In the case of the mercury-filled maximum thermometer, a rise in temperature forces the expanding mercury through a constriction above the reservior. The mercury cannot return when the temperature falls and the maximum temperature is shown. The mercury must be shaken gently back into place after the reading is taken.

4.2.2 Humidity

The moisture status of the air has a strong influence on rates of soil evapotranspiration (Et) and open water evaporation, both of which are greater when the humidity of the air is low. Relative humidity values are widely used in evapotranspiration equations and two main methods are used to measure humidity:

The first uses thermodynamic principles and measures temperature differences between wet and dry thermometers ("psychrometers"). They are set together, with the wet thermometer slightly lower than the dry and are usually housed as one unit in a secure metal frame. Around the bulb of the wet thermometer is placed a wick sheath, which trails into a container of clean, distilled water. The wick should fit tightly; dust, dirt and insects are sometimes a problem and the wick may need replacement or cleaning each week. The simplest and most common type of psychrometer (sometimes also called a "hygrometer") is housed in a screened box. The natural flow of air around the wet bulb thermometer results in it registering a lower temperature than the dry bulb thermometer. The use of psychometric tables converts the readings into dew point temperatures and relative humidity (RH). Hand-held versions are available for spot readings, these being whirled around on a handle to encourage ventilation; others use the assistance of electric fans to achieve this effect.

In the second case the instrument uses the hygroscopic properties of a material (usually human hair) to determine humidity, and is called a "hair hygrometer". A series of hairs expand and contract according to atmospheric moisture and oscillate a pen which marks a trace on a chart, moved by a clockwork drum. Adjustments to the instrument can be made by altering the arrangement of linking levers.

Hygrographs are usually placed on the floor to ensure stability. Shelves used for such instruments increase the possibility of readings being affected by vibrations and for this reason the housing fabric should be strong and rigid. The chart can be annotated with date of chart replacement, station, reader etc. Checks for correct readings should be made against psychrometer values when the humidity is high (early morning or during a rainy period) and low (mid afternoon). Hairs that become dirty should be cleaned with a soft brush, but eventual replacement will be necessary. Very often this instrument is linked to a temperature sensor that gives a continuous record on the chart and can be used as a check or back-up to the maximum and minimum thermometers. In this case the instrument is called a hygrothermograph. Less costly, non-recording hair hygrometers are also available.

4.2.3 Wind Speed and Direction

Wind also has an important effect on levels of evaporation and evapotranspiration. It removes humid surface air layers from above land and water and can physically damage crops. Anemometers are used to measure wind speed and duration and thereby windrun, in km day -1. A standard anemometer has three cups mounted at 120° to each other on a vertical axis. The movement of this rotor closes an electrical contact which measures and records a standard distance of wind movement. A continuous record of wind speed and direction is also provided. Wind direction is obtained by the operation of a single-panel vane. Pen and chart recorders are usual, but electronic recording of these data on data loggers is now common.

A site that is relatively level is to be preferred, with no obstructions within 100 x the height of the nearest obstacle. The World Meteorological Organisation's recommended height is 10m for general speeds, but this height imposes the need for larger and more expensive mast structures. For use in the calculation of Et values by the widely-used Penman method, 2m is recommended. Fortunately an estimate of wind speeds for levels other than of the instrument can be obtained by the formula:

u₂ = (ln z₂/ln z₁) a_u1 where (4.1)

u₂ is the estimate of wind speed

u₁ is the known speed at instrument height

z₁ and z₂ are the heights in cm of the known and estimated wind speeds

a is an exponent between 1 and 0.6 according to ground surface roughness

Empirically this can be stated as Hellmann's formula:

Velocity at height 'h' / Velocity at 10 m = 0.233 + 0.656 log₁₀ (h + 4.75) (4.2)

Hand-held anemometers with digital displays of wind speed are available, but these do not incorporate a wind direction sensor. Relatively compact, portable systems that can be quickly assembled at site, can be purchased. It is useful to note that accurate readings of wind speeds less than 5 km per hour are difficult to achieve.

4.2.4 Solar Radiation

Solar radiation provides energy for evaporation and plant development. Several methods of calculating Et use solar radiation as a key parameter, often converted to net radiation, which takes into account the portion of solar radiation that is reflected back into the atmosphere. Total incoming shortwave radiation is measured by solarimeters, sometimes called pyranometers, which sense the intensity of radiation from the sun and sky, that falls in a horizontal plane.

The portion of all radiation that is transformed into other forms of energy is called "net radiation". Net radiation is measured as the difference between incoming (downward) and outgoing (upward) radiation of all wavelengths by the net radiometer. As radiation varies between night and day, counters can be linked to the radiation measuring device to record these values separately for easy reading. Electronic pulses may be recorded on a strip chart, but increasingly (especially with small, automatic meteorological stations) the data are recorded on an electronic logger and can be downloaded directly in digital form on to a computer, for viewing and analysis.

Sites should be clear of obstructions, with a view to the horizon that is not affected by nearby trees or buildings. Under no circumstances should shading occur and artificial surfaces that can direct radiation to the instrument should be avoided. The site should be typical of conditions under study, but compromise is inevitable where conditions of vegetation type and cover, soil reflection, etc. vary from place to place within the local area. Placing the instrument high, perhaps at 3m, increases the field of reception which is useful in areas that are heterogeneous. Cultivated field situations are more likely to give representative values that rangeland areas which tend toward heterogeneity. As a guide, an instrument set at X metres above the ground will receive 90 and 99% of its upward flux from ground areas with radii 3X and 10X respectively. Figure 4.12(b) shows a typical net radiometer that would be one component of a small, automatic weather station.

It is very important to keep the glass or plastic domes clean and undamaged. The presence of dust is a common problem. A photographer's air brush is very useful for cleaning, but if not available, soft tissues can be used. Care should be exercised as the domes are prone to scratching. The instrument should be kept horizontal at all times. Calibration is important because of deterioration of the domes and black reception faces and should be carried out every few months. However, this involves the use of a replacement radiometer to continue the record and it may only be possible to check the upper and lower sensors during an off-season period, when a break in the record may not be important. This is best done at a time of steady radiation, when the faces of the radiometer are inverted for 10-minute periods. Averages are taken and both faces should give readings within 5% of each other. Alternatively, a second instrument can be kept in good storage conditions and used only as a standard for the field instrument. As different ground conditions may be measured by the two adjacent instruments, it is as well to exchange their positions around to check the first results.

Solarimeters (pyranometers) measure only incoming short-wave radiation and are sometimes used at meteorological stations. These can be used to calibrate radiometers. They should be shaded from direct solar radiation by placing the shadow of a black matte disc (about 1m away and held by a thin support) over the instruments. The response by both sensors should be the same such that D_r/ C_r= D_s/C_s. As D_r and D_s, the change in response of the radiometer and solarimeter are measured (in mv) and C_s, the calibration constant of the solarimeter is known, C_r can be found. This should be done several times.

Sunshine:

Sunshine hours are commonly recorded where the cost and practicability of maintaining radiation meters are limiting. The widely-used Campbell-Stokes recorder consists of a glass sphere mounted on a pedestal which concentrates bright sunlight onto a chart and so burns a trace along it, thus sunlit periods are recorded. Instruments which are specified by ranges of operation latitude (for example 0° to 60° N or S) and the correct charts for the N or S hemisphere should be used.

Electronic meters that measure photosynthetically active radiation (400 - 700 nm) are also available.

4.2.5 Evaporation

Evaporation Pans:

The measurement of free surface water evaporation depends on air temperature, wind, humidity and solar radiation. It is a commonly measured index that integrates these meteorological factors and to some extent, illustrates the behaviour of evaporation from water bodies and evapotranspiration from wet soil, where the availability of water is not limiting.

The most commonly used instrument, which is an international reference instrument, is the US Weather Bureau 1.22 m (4 foot diameter) A-pan. This can be purchased complete, or made from local material such as a suitable gauge, galvanised steel sheet. Pressed seams should not be allowed to cause buckling. Welded seams should be treated to prevent rusting The pan should be mounted horizontally on a wooden platform with tamped soil below, to allow a 13 mm air space (half an inch). In humid climates the areas around the pan will be grass, whereas in arid areas, vegetation will come and go according to season. However, vegetation should not be allowed to grow above the level of the pan. Locations near areas with artificial surfaces, boggy areas or water surfaces should be avoided.

The water level in the pan can be measured by an inclined, graduated gauge staff, but accurate reading in this manner is more difficult than by using a stilling well and micrometer hook gauge. The level is measured with the hook tip, lowered below the water and then raised until the tip just pierces the surface. The mechanism is removed from the stilling well and the reading taken from the graduated vernier scale. Water levels should be kept at 5 cm below the rim of the pan (+/- 2.5 cm) and water should be added or removed to maintain this level. A reading should be taken before and after this has been done. The differences in daily levels give evaporation, with additions and removals of water and daily rainfall (measured nearby) being taken into account. Readings are taken at the same time each day, normally 08:00 hours, though alternatively a WLR could be used if the cost is not prohibitive.

Daily maximum and minimum temperatures of the pan water are often taken by floating thermometers, kept at least 30 cm from the side. Problems can occur with birds and animals drinking from the pan. Fences will keep out larger animals, but a wire screen fixed over the pan itself, may be necessary. This can effect readings by the suppression of evaporation and in semi-arid climates a correction of 16% is made to measurements. Algal growth can be prevented by a small addition of copper sulphate to the water and the pan should be kept clean of debris and insects. Figure 4.13 shows a hook gauge version of the Type A evaporation pan. Data may be lost during periods of heavy rainfall and over-topping.

Lysimeters:

Lysimeters most commonly measure evapotranspiration by changes in the weight of containers filled with soil, to which water is added. Losses by evapotranspiration are then calculated. Lysimeters can be very large, weighing several tons, while others used in field locations may only measure water losses from a few kilogrammes of soil. Crops and vegetation may or may not be grown in them. Large containers are weighed by permanent pressure transducers, though the construction of large lysimeters is normally beyond the scope of many agrohydrological projects and is not practicable under field conditions. Meteorological services may find it useful to install them.

Evapotranspiration from small lysimeters is measured by their removal and these instruments are more commonly used for field research. Suitable ones can be made from PVC water pipe with a diameter of 15 cm and a length of 20 30 cm. They are filled by attaching a steel cutting edge to the lower edge of the plastic, and a metal ring to the top. The latter prevents damage when they are hammered into the ground at the required location, when soils are at field capacity and drainage has ceased. Jacking lysimeters into the ground may be necessary, but the appropriate equipment must be available.

The lysimeter is removed and the soil is retained by a wire mesh (5 mm is suitable), which is screwed into the base. They are then replaced into their holes which have been fitted with tubes, a few millimetres larger in diameter and deeper than the lysimeters. Lysimeters are removed and weighed every hour or so throughout the day. Because the cores are isolated from root activity, new cores should be taken every 2 to 3 days, to account for plant extraction.

There are other problems of representing true conditions with lysimeters: filling with soil can disturb the profile, edge-effects are great especially with small models, isolation leads to hydraulic continuity being lost at the sides and affected at the base. Intermittent, unexpected rainstorms can affect readings.

4.2.6 Soil Temperatures

Soil temperatures have direct effects on the germination and root growth of crops and natural vegetation, the state of which can greatly affect runoff. Soil temperatures determine the micro climate of the overlying air and are important for assessing the growing environment of crops. Soil temperatures are not only dependent on incoming and outgoing radiation, but also on the thermal properties of the soil which can change greatly with the addition of water by rain and its removal by evapotranspiration. Temperatures reach maximum some time after local noon and minimum after midnight.

Continuous records of temperature can be taken with thermocouples (thermographs) linked to pen and chart or electronic loggers. Thermocouples should be calibrated with mercury thermometers at least twice a season, at the beginning and end, and any corrections must be noted. Bent-stem mercury thermometers for 10 and 20 cm depths and encased mercury types with bulbs in crystalline wax and suspended in steel tubes for 50 and 100 cm depths, are used where recording instruments are not available. These depths are recommended by WMO. Readings are normally taken at 08:00, 14:00 and 20:00 Local Time. Good contact with the soil is necessary and accidental trampling should be prevented.

4.2.7 Automatic Weather Stations

Research applications may demand the collection of climatic information at sites in addition to base stations. In such cases the use of automatic weather stations may be more suitable than an array of individual instruments. Electronic logging is used to keep records (usually on a multi-channel data logger) and to avoid the need for frequent visits. The period between visits is determined by the number of instruments used, the frequency of record and the memory size of the logger.

Different formats for the presentation of data will be used according to manufacturer.

Figure 4.14 Example Meteorological Data sheet

Stations should be enclosed by fences in a suitable position, as discussed in the sections on individual instruments. Considerable thought should be given to the possible problems of vandalism and theft because of the cost of automatic weather stations and the ease with which the array of instruments can be damaged.

Figure 4.15: Typical Weather Station Layout

Figure 4.14 shows an example data sheet for the meteorological variables discussed above.

Figure 4.15 shows a plan of suitable weather station site, equipped with a basic list of individually installed instruments. Note that sufficient space is left within the compound to accommodate instruments that may be installed at a later date.

It is important to select automatic weather stations according to particular project needs, for example some stations place anemometers below the recommended 10 m elevation, and if wind speed and direction are important factors in the research agenda, this may not be suitable. Automatic weather stations can be very cost effective when their prices are compared to those of collections of individual instruments. Generally, the seven following meteorological parameters are measured:

- rainfall and relative humidity
- air and soil temperature
- wind speed and direction
- solar radiation.

Equipment costs

All costs of locally made equipment are approximate. The costs of raw materials and especially labour are highly variable from country to country, but a good idea of cost magnitude can be gained from the figures quoted below. The costs of manufactured equipment are based on 1993 prices. Shipping, agents' fees and fluctuations in exchange rate cannot be taken into account.

Table