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Challenges in African Hydrology and Water Resources (Proceedings of the Harare Symposium, July 1984). 1AHS Publ. no. 144.

Continuous monitoring of rainfall, streamflow and suspended sediment concentration in semiarid environments A. VAN B. WEAVER & D. A. HUGHES Geography Department and Hydrological Unit, Rhodes University, PO Box 94, Grahamstown 6140, South Africa

Research

ABSTRACT The occurrence of rainfall and runoff events in semiarid environments of southern African can be highly erratic. Due to the short term variability of these processes, low frequency sampling of hydrological processes is often meaningless and precludes their accurate quantification. Some design considerations that are particularly important for semiarid instrument systems are outlined and illustrated with reference to the continuous monitoring of rainfall, streamflow and sediment concentration using microprocessor controlled electronic instruments in a research basin of the eastern Cape Province, South Africa. Surveillance continue des précipitations, du débit, et de la concentration de sédiments en suspension dans des milieux semiarides RESUME Les précipitations et les écoulements qui en résultent dans les milieux semi-arides de l'Afrique méridionale risquent d'être extrêmement irréguliers. Puisque ces processus sont si variables dans un temps très court , des observations ou mesures peu fréquentes relatives aux processus hydrologiques sont souvent sans signification et rendent impossible leur quantification précise. Nous insistons sur certaines considérations relatives à la conception qui sont surtout importantes pour des systèmes d'instruments destinés aux milieux semi-arides. Nos observations sont illustrées en se référant à une surveillance et un contrôle continu des précipitations, de l'écoulement et de la concentration en sédiments grâce à des instruments électroniques contrôlés par un microprocesseur dans un bassin de recherche a l'est de la province du Cap, Afrique du Sud.

INTRODUCTION A large proportion of southern Africa experiences an arid to semiarid climate and these regions are characterized by the highly erratic nature of their hydrological regimes. In a worldwide study of streamflow data, McMahon (1979) concluded that the variability of annual streamflow and peak floods for arid zones is about twice that for continental areas as a whole. Gorgens & Hughes (1982) report a comprehensive study of streamflow records for 30 semiarid drainage 363

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B.Weaver

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TABLE 1 Summary of streamflow semiarid South African drainage Gbrgens & Hughes, 1982)

characteristics basins (extracted

Mean Catchment area (km2) Period of complete years of record (years) Mean annual runoff (MAR, mm) Coefficient of variation (annual flows) Max. annual total (% MAR) Min. annual total (% MAR) Zero flow months as % of total analysed Monthly coefficient of variation* NOTE: a few valueless.

isolated

high

values

for

30 from

Range

7434

28-25500

34 19.1

13-55 0.6-158.0

1.11 527 6

0.63-3.01 238-1660 0-26

37

0.5-86.3

5.6

(median)

render

the

1.94-1264.80 mean

basins within South Africa. Table 1 summarizes some of the calculated statistics and there is ample evidence to attest to the extremely variable nature of both the amount and occurrence of streamflow. The unpredictability of runoff events, coupled with the difficulty of establishing readily accessible measurement sites in these sparsely populated regions, present major problems in the acquisition of data of sufficient resolution for detailed hydrological analyses. The common occurrence of rapidly developing events with short duration precludes the possibility of adequate hand sampling at all but those sites close to a centre of operation. The detailed study of sediment load or yield relationships presents a further problem in that the availability of sediment rather than the carrying capacity of streams has been shown to be the limiting factor in determining the sediment load of rivers in semiarid regions of South Africa (Rooseboom, 1978; Rooseboom & Harmse , 1979). While a clearly defined relationship may exist between sediment load and runoff for one flow event following a prolonged dry spell, there is no evidence to suggest that this relationship will hold for subsequent events (Weaver, 1980). The antecedent sequence of rainfall and runoff events, which affects the amount of sediment available for transport at any one time, is the limiting factor. It must therefore be concluded that for any semiarid river section, there is no general relationship between sediment load and water discharge (VanSickle & Beschta, 1983). It is therefore not possible to extrapolate values based on a limited sampling procedure as is sometimes done in humid environments (Walling, 1977). Sediment concentration levels in semiarid rivers can also show considerable variation over short time periods within one flow event. For example, sediment concentration during a 50 min flow event from a 1.1 km2 basin varied between 4 and 40 g l - 1 . This event resulted from a total of 15 mm of rain falling in 24 min

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(average intensity of 37.5 mm h ). However, the 1 min intensities within the storm varied between 6 and 216 mm h (data from the Ecea River, eastern Cape, South Africa). It is clear that low frequency sampling of hydrological processes in a semiarid environment is often meaningless and precludes their accurate quantification. Some form of continuously recording instrumentation that can assure a high sampling frequency during events is therefore required. This paper attempts to identify the important considerations involved in the design of instruments suitable for semiarid regions and to illustrate their use with examples from research drainage basins in the eastern Cape Province of South Africa.

INSTRUMENT RECORDING SYSTEMS FOR SEMIARID ENVIRONMENTS - DESIGN CONSIDERATIONS There are many design considerations that are to some extent common to a variety of hydrological instruments. Table 2 summarizes a number of these under the general headings of instrument hardware, recording hardware and software, and retrieval system. The headings relate specifically to microprocessor controlled electronic logging devices but most of the considerations are equally applicable to clockwork or battery operated strip chart mechanisms. No attempt is

TABLE 2 systems

Design

considerations

for

Instrument sensor hardware

Recording hardware

Robustness Range of operation Calibration efficiency Repeatability Resolution

Storage capacity Power supply lifespan Reliability Ease of operation

hydrological

Recording software

instrumentation

Retrieval system

Recording interval Resolution Data format Ease of operation Security and permanence storage

of

made here to discuss all of these considerations in detail; instead, some of the elements of Table 2 that are specifically relevant to semiarid conditions are highlighted. The extreme range of semiarid processes has already been referred to and it is essential that instrument sensors are capable of being calibrated (where necessary) and of measuring over the widest range that might be experienced. The necessity to record both small and large events accurately is also of prime importance in the design of the recording system. For example, to enable the peak sediment concentration of a flow event from a small basin to be identified, it may be necessary to record at short time intervals of 10 min or less, but to record continuously at 10 min intervals may be extremely

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wasteful of recording capacity considering the number of zero flow periods. Consequently, some form of variable recording interval is desirable that combines the advantages of high resolution recording during rapidly changing conditions, and long recording intervals during slow or zero changing conditions, in order to preserve storage space. Several considerations in Table 2 are related to the necessity for remote observation stations commonly found in semiarid areas. The storage capacity and power supply lifespan of any instrument must be sufficient for the frequency of site visits to be kept to an acceptable minimum. A remote station may also be visited by an unskilled operator who has only a modest understanding of the system. Consequently, the field operation of the instrument should be as simple as possible in order that operational errors causing loss of data are kept to a minimum. It is clear that the specialized design criteria for semiarid instrumentation systems cannot be ideally satisfied by using conventional clockwork recording systems. The relatively recent application of microprocessor technology to the field of hydrological instrumentation provides an alternative that theoretically should be able to meet the demands of most research investigatipns.

DRAINAGE BASIN INSTRUMENTATION - A SEMIARID EXAMPLE The Hydrological Research Unit (HRU) of the Department of Geography at Rhodes University has been monitoring rainfall, streamflow and evaporation in a series of five nested drainage basins (1.1-73 km ) of the Ecca River (33°12'S, 26°40'E) in the eastern Cape Province since 1975 (Roberts, 1978; Murray & Gbrgens, 1983). Until recently, all continuous recording instruments were of the clockwork stripchart type. A proposed expansion of the research programme to incorporate detailed sediment and dissolved solid concentration studies led to a re-assessment of the instrumentation commercially available in South Africa. The HRU has a microcomputer system used for retrieval of data from paper strip charts (rainfall and streamflow). It was considered desirable to extend this system to be able to read solid state memory devices and to make use of microprocessor controlled data loggers in the new instruments where possible. These have the advantage that the recording system is often flexible and can be pre-programmed according to the users requirements. They also have the advantage of eliminating at least one stage of lengthy data processing, for example, digitizing raingauge charts to obtain digital data. The use of solid state data logging systems is discussed below with respect to monitoring rainfall, channel stage and sediment concentration in the Ecca basin. Rainfall Hughes & Guthrie (1984) discuss the disadvantages of syphon type gauges and clockwork recording devices, a combination commonly in use today. The alternative sensor device is the tipping bucket gauge. These are simpler in operation, less prone to mechanical failure than the syphon type and lend themselves to use with

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microprocessor controlled memory devices more easily than other raingauge types. Such a system, designed and built at Rhodes University, is described in detail in Hughes & Guthrie (1984) and a summary of the software design is reported here as it is directly related to the requirements of semiarid data collection. The logging technique has been designed to maximize the resolution of measurement during periods of high rainfall, while minimizing the amount of memory space that is used. Thus for low intensity rainfall (1.2-12 mm h~ ) the logging interval is 10 min; between 12 and 180 mm h~ the interval is variable, the time being written at the nearest integer minute following the completion of at least 10 bucket tips (2 mm). For intensities of greater than 180 mm h - 1 the time resolution increases to 1 min. Should no tips be registered in a complete 10 min period, then the logger effectively switches off until a new rainfall event begins. Real time (month, day, hour, minute) is recorded followed by the next series of data pairs (tips, time). While memory space is preserved during zero rainfall, extended periods of light rainfall (