Of all the components of the hydrological cycle, the elements of precipitation, particularly rain and snow, are the most commonly measured. Sevruk and Klemm (1989) have estimated that there are 150 000 storage rain gauges in use worldwide. It would appear to be a straightforward procedure to catch rain as it falls and the depth of snow lying can be determined easily by readings on a graduated rod. People have been making these simple measurements for more than 2000 years; indeed, the first recorded mention of rainfall measurement came from India as early as 400 BC. The first rain gauges were used in Korea in the 1400s AD (as a means to plan farming and set taxes), and 200 years later, in ca. 1680 in England, Sir Christopher Wren and Robert Hooke described designs for the self-recording rain gauge.
Climatologists and water engineers appreciate that making an acceptable precipitation measurement is not as easy as it may first appear. It is not physically possible to catch all the rainfall or snowfall over a catchment; the precipitation over the area can only be sampled by rain gauges. The measurements are made at several selected points representative of the area and values of the total volume (Ml) or equivalent areal depth (mm) over the catchment are calculated later. Such are the problems in obtaining representative samples of the precipitation reaching the ground that, over the years, a comprehensive set of rules has evolved. The principal aim of these rules is to ensure that all measurements are comparable and consistent. All observers are recommended to use standard instruments installed uniformly in representative locations and to adopt regular observational procedures (as set within the particular country).
Many investigations carried out in England into the problems of rainfall measurement owe their origin to the enthusiasm of one man, G. J. Symons. Symons, a civil servant in the Meteorological Department of the Board of Trade in the 1850s, instigated and encouraged formal scientific experiments by such volunteers as retired army officers or clergymen whose spare time interests included observations of the weather and measurements of meteorological variables (Mill, 1901). The results of this work were incorporated by Symons into his Rules for Rainfall Observers. Symons' rules continue to form the basis of the practice of precipitation measurement in the UK today (Met Office, 2006).
The Met Office, which in 1919 inherited the advisory functions of Symons and his successors in the British Rainfall Organization, has approved instruments of several designs having the salient features recommended as a result of the early experiments. These include various types of storage rain gauge and automatic rain gauges. For the assessment of water resources, monthly totals may suffice; for evaluating flood peaks in urban areas (Chapters 16 and 18), rainfall intensities over an hour or even minutes could be required, so automatic rain gauges are used.
3.1 Non-recording (storage) rain gauges
Rain gauges vary in capacity depending on whether they are to be read daily or monthly. The period most generally sampled is the day, and most precipitation measurements are the accumulated depths of water caught in simple storage gauges over 24 h.
For many years, the UK's recognized standard daily rain gauge has been the Met Office Mark II instrument (Fig. 3.1a; Met Office, 1980; British Standard 7843, 1996). The gauge has a sampling orifice of diameter 127mm. The 12.7mm rim is made of brass, the traditional material for precision instruments, and the sharply tooled knife edge defines a permanent accurate orifice. The Snowdon funnel forming the top part of the gauge has a special design. A straight-sided drop of 102 mm above the funnel prevents losses from out-splash in heavy rain. Sleet and light snowfall also collect readily in the deep funnel and, except in very low temperatures, the melted water runs down to join the rain in the collector. The Snowdon funnel, the main outer casing of the gauge and an inner can are all made of copper, a material that has a smooth surface, wets easily and whose surface, once oxidized, does not change. The inside of the collecting orifice funnel should never be painted, since the paint soon cracks, water adheres to the resulting rough surface and there are subsequent losses by evaporation. The main collector of the rain water is a glass bottle with a narrow neck to limit evaporation losses. The gauge is set into the ground with its rim level at 300 mm above the ground surface, which should ideally be covered with short grass, chippings or gravel to prevent in-splash in heavy rain.
During very wet weather, the rain collected in the bottle may overflow into the inner can. Bottle and can together hold the equivalent of 150 mm rainfall depth.
The inner can is easily removed from the outer casing and its contents can be emptied and measured without disturbing the installation.
The gauge is inspected each day at 0900 h GMT, even if it is thought that no precipitation has occurred. Any water in the bottle and inner can is poured into a glass measure (Fig. 3.2) and the reading taken at the lowest point of the meniscus. The glass measure is graduated in relation to the orifice area of the rain gauge and so gives a direct reading of the depth of rain that fell on the area contained by the brass rim. The glass measure has a capacity of 10 mm; if more than 10 mm of rain has fallen, the water in the gauge must be measured in two or more operations. The glass measure is tapered at the bottom so that small quantities can be measured accurately. If no water is found in the gauge and precipitation is known to have fallen, this should be noted as a 'trace' in the records. The glass bottle and inner can should be quite empty before they are returned to the outer case. It is advisable to check the instrument regularly for any signs of external damage, or general wear and tear. Severe frosts can sometimes loosen the joints of the copper casing and, if this is suspected, testing for leaks should be carried out.
The Snowdon gauge (Fig. 3.1b), a Met Office Mark I instrument, remains in favour among private observers in the UK, since without the splayed base it is easily maintained in a garden lawn. It is, however, more difficult to keep rigid with the rim level. Globally, the daily storage gauge in most common use is the German Hellmann gauge, with over 30 000 gauges of this type in use (Sevruk and Klemm, 1989). This gauge is similar in design to the Snowdon gauge, but with a larger funnel diameter of 159.6 mm.
Monthly rain gauges hold larger quantities of precipitation than daily gauges. The catch is measured using an appropriately graduated glass measure holding 50 mm. Monthly gauges are designed for remote mountain areas and are invaluable on the higher parts of reservoired catchments. Measurements are made on the first day of each month to give the previous month's total and corrections may need to be made
Fig. 3.3 An Octapent monthly storage gauge in use in the UK. Units outside of the parentheses are the original design in inches, while those in parentheses are the equivalent millimetres.
to readings obtained from remote gauges recorded late in the day in wet weather. The Octapent monthly rain gauge (Fig. 3.3), a hybrid of the 5-inch and old 8-inch diameter gauges, is made in two sizes with capacities of 685 mm and 1270 mm.
The need for the continuous recording of precipitation arose from the need to know not just how much rain has fallen, but when it fell and over what period. Numerous instruments have been invented with two main types being widely used: the tilting-siphon rain recorder developed by Dines, and the tipping-bucket gauge, which had its origins with Sir Christopher Wren and Robert Hooke.
The Dines tilting-syphon rain recorder (Fig. 3.4) is installed with its rim 500 mm above ground level. The rain falling into the 287 mm diameter funnel is led down to a collecting chamber containing a float. A pen attached to the top of the plastic float marks a chart on a revolving drum driven by clockwork. The collecting chamber is balanced on a knife edge. When there is no rain falling, the pen draws a continuous horizontal line on the chart; during rainfall, the float rises and the pen trace on the chart slopes upwards according to the intensity of the rainfall. When the chamber is full, the pen arm lifts off the top of the chart and the rising float releases a trigger disturbing the balance of the chamber, which tips over and activates the syphon. A counterweight brings the empty chamber back into the upright position and the pen returns to the bottom of the chart. With double syphon tubes, syphoning should be completed within 8 s, but the rain trap reduces the loss during heavy rainfall. It is recommended, however, that a standard daily storage gauge is installed nearby and that quantities recorded are amended to match the daily total. Each filling of the float chamber is equivalent to 5 mm of precipitation.
Charts are normally record by the day, but modifications to the instrument can allow a strip chart to be used which gives continuous measurements for as long as a month and which has an extended timescale for intense falls over very short periods. In cold weather, the contents of the float chamber may freeze and special insulation with thermostatically controlled heating equipment, the simplest being a low-wattage bulb, can be installed. The provision of heating assists in the melting of snow, but in very cold weather or during heavy snow, existing low-powered heating devices will not be adequate and there will be a time lag in the melted water being recorded on the chart. Care must be taken to avoid too much heating since evaporation of the melted snow would result in low measurements. Adequate drainage below the gauge should be provided during installation, especially in heavy clay soils and in areas liable to heavy storms, for the syphon system will fail if the delivery pipe enters flood water in the soak away. A model for use in the tropics has a 128-mm diameter receiving aperture and the filling of the float chamber represents 25 mm on the chart. Despite the increased use of tipping-bucket rain gauges with data loggers, tilting-syphon gauges with charts remain in widespread use throughout the tropics.
The principle of the tipping-bucket rain gauge is shown in Fig. 3.5. Rain is led down a funnel into a wedge-shaped bucket of fixed capacity. When full, the bucket tips to empty and a twin adjoining bucket begins to fill. At each tip, a magnet attached to the connecting pivot closes a circuit and the ensuing pulse is recorded on a data logger. The mechanism can be used in a variety of gauges. The 15 g of water in one bucketful represents 1 mm of rain caught in a 150 cm2 gauge, and 0.2 mm in a 750 cm2 gauge. A small adjustment allows the tipping buckets to be calibrated precisely. It is advisable to install an adjacent storage gauge (sometimes called a 'check gauge') so that a day's or month's total can be measured if the recording mechanism fails. Ceramic resistors connected to a high-capacity battery can be used to reduce the likelihood of the mechanism freezing during cold weather. Other errors specific to tipping-bucket rain gauges are detailed within Hodgkinson et al. (2005).
Many other types of automatic rain gauge are either in development or use within the UK, including electronic weighing rain gauges, capacitance rain gauges, drop-counting rain gauges and optical present weather detectors. Disdrometers are particularly notable, given that they give raindrop size distribution as well as rainfall intensity, which is useful for rainfall radar calibration and erosion studies (Strangeways, 2003, 2007).
Choosing a suitable site for a rain gauge is not easy. The amount measured by the gauge should be representative of the rainfall on the surrounding area. What is actually caught as a sample is the amount that falls over the orifice area of a standard gauge, that is, 150 cm2. Compared with the area of even a small river catchment of 15 km2, for example, this 'point' measurement represents only a 1 in 109 fraction of the total catchment area. Thus even a small error in the gauged measurement due to poor siting represents a very substantial volume of water over a catchment.
It is best to find some level ground if possible, definitely avoiding steep hillsides, especially those sloping down towards the prevailing wind. In the UK, the wind comes mainly from westerly directions. A sheltered, but not over-sheltered, site is the ideal (Fig. 3.6). It is advisable to measure the height of sheltering objects in determining the best site, taking into account anticipated growth of surrounding vegetation.
In over-exposed locations on moorlands, plateaus and extensive plains, where natural shelter may be scarce, a turf wall of the kind designed by Hudleston is recommended (Fig. 3.7; Hudleston, 1934). The surrounding small embankment prevents wind eddies, which can inhibit rain drops from falling into an unprotected gauge. The disadvantages of this enclosure are that drifting snow may engulf the gauge and very heavy rain may flood it if there is no drainage channel beneath the wall.
It has always been appreciated in the UK that the compromise setting of the gauge rim 300 mm above the ground surface is not altogether satisfactory. Hydrologists have led the move to require gauges to be set with the rim at ground level and various methods to prevent in-splash have been developed. The most acceptable installation is one in which the gauge is set in the centre of a pit about 1 m square, which is then covered with a metal grid with a hole in the middle to accommodate the funnel. Ideally, the square grid slats should be less than 1 mm thick at the top edge and be 50 mm deep with 50 mm spacing. This installation is known as an Institute of Hydrology Ground-Level Gauge (Fig. 3.8; Institute of Hydrology, 1977).
It has been shown by several researchers that a standard daily gauge in its conventional setting typically catches 6-8 per cent less rain than a properly installed ground-level gauge, though undercatch can be considerably more on windy mountain slopes (Rodda, 1967; Sevruk and Hamon, 1984). If turf-wall or ground-level installations are not used, then wind speed data should be used to correct rainfall totals dynamically (Sevruk, 1996).
Rain-gauge sites should be examined occasionally to note any possible changes in the exposure of the instrument. Removal of neighbouring trees or the growths of adjacent plants are modifications of the natural surroundings that could affect the rain gauge record. Observers should be encouraged to report any major structural changes to buildings near the gauge because they could result in changing wind patterns in the vicinity of the instrument which could also affect the homogeneity of the catch record. When inconsistencies in a record caused by such changes in the exposure of a gauge are reported or discovered, the data processors make suitable amendments to the measurements (World Meteorological Organization; WMO, 2008).
3.4 Horizontal rain and occult precipitation gauges
On steep mountain slopes, particularly close to ridge tops, a significant proportion of the precipitation can have a horizontal component that is intercepted by trees but poorly measured by conventional rain gauges. Where these mountains slopes are in excess of 1500 to 1200 m in height, the total precipitation comprises a small but measurable quantity of occult precipitation or fog interception. The modified Juvik fog gauge can be used to measure both wind-driven horizontal rainfall and fog interception (Frumau et al., 2006); however, there are no permanent installations of these gauges within the UK.
There are various solid forms of precipitation, and all except hail require the surface air temperature to be lower than about 4° C if they are to reach the ground.
Small quantities of snow, sleet or ice particles fall into a rain gauge and eventually melt to yield their water equivalent. If the snow remains in the collecting funnel, it must be melted to combine the catch with any liquid in the gauge. If practicable, the gauge may be taken indoors to aid melting but any loss by evaporation should be avoided. Alternatively, a quantity of warm water measured in the graduated rain measure for the rain gauge type can be added to the snow in the funnel and this amount subtracted from the measured total.
Large wind errors arise with the use of rain gauges to measure snowfall. The WMO designed the octagonal, vertical, double-fence shield (or double-fence inter-comparison reference) to reduce this effect and act as the international reference (WMO, 1998).
When snow has accumulated on the ground, its depth can be measured. A representative smooth cover of the ground, not subject to drifting is selected and sample depths are taken with a metre stick held vertically. For a rough estimate of water equivalent, the average snow depth is converted taking 300 mm of fresh snow equal to 25 mm of rain. However, the density of fresh snow may range between 50 and 200 gL-1 according to the character of the snow flakes. When compacted snow lies for several days and there are subsequent accumulations, the observer is advised to take density measurements at selected points over the higher parts of important catchment areas. The density of snow increases with compaction to around 300 gL-1. Sample volumes of the snow are taken at different depths (WMO, 2008), are weighed and the density calculated. Thence the water equivalent of the total depth of snow can be obtained. If the snow is not too deep, a total sample core can be weighed to give an overall density or melted to give the water equivalent directly.
A continuous monitoring of the water equivalent of lying snow is essential to promote warnings against flooding if a sudden thaw occurs. Snow pillows can be used continuously to monitor the weight of accumulated snow (Archer and Stewart, 1995). These devices typically use pressure transducers to measure the snow weight and hence accumulated water, but can be difficult to maintain.
Unlike satellite estimates of rainfall, radar provides a direct measurement. There are several types of rainfall radar. The operational system within the UK uses a conical radar beam in the C-band (3-6 GHz) and measures the energy reflected and scattered back from the precipitation. The simplest empirical relation between rainfall rate, R, and radar reflectivity, Z, has the form:
Z = aRb where b varies between 1.4 and 1.7, and a varies from 140 for drizzle to 500 for heavy showers. There are, however, many environmental factors, such as bright band (from melting atmospheric snow) and permanent ground obstructions that need to be taken into account to provide the correct calibrations. These calibrations incorporate adjustments to observations from tipping-bucket rain gauges and Disdrometers; see Collier (1989) for further discussion. The present operational network of radar stations measuring rainfall in the UK is shown in Fig. 3.9; and the calibrated coloured displays are seen regularly on TV weather programmes.
The detailed rainfall intensity displays are received by the regional offices of the Environment Agency and Scottish Environmental Protection Agency and provide front-line information for flood warning. The central advantage of the rainfall radar method to the hydrologist is that it produces a measure of the rainfall over the whole of a catchment area as it is falling.
Thermal-infrared and visible wavelength spectrometers mounted on satellite platforms have the ability to measure cloud-top brightness, temperature and texture. These characteristics help distinguish cloud type, and this knowledge can improve the real-time calibration of ground-based radar. Within the Met Office Nimrod system, these spectra from the Meteosat satellite are combined with the ground-based radar
and telemetered tipping-bucket rainfall data to produce 1 km resolution rainfall for the whole of the UK. These are available to hydrologists via the British Atmospheric Data Centre, BADC (http://badc.nerc.ac.uk/data/nimrod/). Additionally, the more specialised Met Office GANDOLF system provides 2 km resolution data for periods when convective thunderstorms are present. These data are also integrated with a nowcast (12-h forecast) modelling system called STEPS (Pierce et al., 2004).
Outside of the UK, Meteosat data have been used to observe rainfall without the use of ground-based radar (Symeonakis et al., 2008). The first radar system designed to measure reflectivity of rainfall from space has been operational since 1997. Mounted on the TRMM (Tropical Rainfall Measuring Mission) satellite, this radar provides rainfall intensity estimates for tropical regions, with a pass of each location every 3 h.
The proportion of precipitation that reaches the ground beneath vegetation (i.e. that which is not lost by wet-canopy evaporation; Section 10.3) is called net precipitation. This precipitation reaches the ground either by running down vegetation stems, and is called stemflow, or drips through or off vegetation canopies as throughfall. Stemflow can be measured using a stemflow collar that directs the flow into a tipping-bucket rain gauge (Institute of Hydrology, 1977) or into a larger (1-L) tipping-bucket mechanism. Throughfall can be measured using a network of storage or tipping-bucket rain gauges placed beneath the canopy. The greater spatial variability of stemflow and throughfall in comparison to rainfall means that 50 such gauges are needed for comparison with rainfalls measured in the open or above the canopy. Alternatively, a smaller number of the larger throughfall troughs (4 m in length, 0.1m width and 0.3 m depth) or plastic-sheet net rainfall gauges (Institute of Hydrology, 1977) can be used.
1 http://nora.nerc.ac.uk/5770/ References
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