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Direct acquisition of rainfall data on site

Divisions Calcul et Hydrologie - CEMAGREF - Antony

Abstract

Rainfall readings convey a great deal of information, but their execution requires manual or semi-automatic recording (digitalizing table) and a substantial amount of time which is often incompatible with available staff resources. To automatise the discretisation phase, coding of rainfall conditions directly on-site on magnetic cassette tape has been undertaken using off-line coders. The magnetic recorder of precipitation takes the form of a box (33 cm x 22 cm x 20 cm) ; it is a multi-channel device (6) for digital and analogue inputs. Its quite flexible basic layout makes it possible to adapt the six input channels to the user's requirements according to the parameters to be recorded. Our purpose being to record rainfall conditions, a layout has been selected in the light of ECMA norms 341 to obtain the greatest possible autonomy and the most reliability in reading the magnetic cassette tapes. This layout corresponds to a single digital input channel served by the six electronic meters of the six "strappe" channels; observation time is six minutes and each record contains 26 bytes. The opera ting principle is the cumulation in each meter (6) of the impulses generated during six minutes at each tilt of the cup. After 36 minutes the contents of the meters are transferred in a single block to magnetic storage. The logic of the transfer to tape increments the various blocks progressively by one unit. Data processing involves the use of a cassette deck and software designed on TEKTRONIX 16 K (read-in) and on IRIS 80 (processing), which are themselves integrated in the basic processing programmes on hydrology and, therefore, articulated with the subsequent automatic processing. This coder-recorder system, which meets the various requirements, is in conformity with the layout described above. The system meets the following conditions as regards the acquisition and processing of the rainfall for the experimental catchment basic of the Orgeval : - environment conditions (- 75 °C < T °C < 40 °C) ; - autonomy (power source for three months, cassette recording for 2.5 months) : - observation time (6 minutes), which yields good knowledge of rainfall conditions over shorter periods (seven observation stretches of two seconds to sixty minutes) ; - (negligible) time drift which guarantees adequate synchronization of rainfall events acquired by the various sensors c a given dense system.