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4 AUTOMATIC HYDROLOGICAL OBSERVATIONSA description of the various possible procedures as well as different equipmentused in the handling and processing of' hydrological data is given in WHOTechnical Note No.115, Machine processing of hydrometeorological data. Any decisionregarding the eventual treatment of recorded observations will have a very importanteffect an the type of instruments to be used to obtain the data.The use of the computer is most, profitable when handling either_ largequantities or very complicated series of observations. It is usually not justifiableto consider this means of handling the data from a small number of stations. Inthese cases, a relatively small staff can handle the treatment of data with 0 minimumof expense. On the other hand, if expansion of the observation network is envisagedin the near future, it wauld probably be reasonable to immediately start developinga system for automatic processing based on the information collected from the initialsmall number of stations.It should be noted that any existing network, either for hydrological observationsor for other parameters, will have an impact on any decision. If, in the operationof the hydrological network, much money has been invested in the past to installanalogue recorders, it could be more economical not to convert to digital recorders,but to continue using the analogue apparatus and available equipment to convert the analoguerecording to machine input form such as cards and magnetic tapes. Some hydrologistsprefer this latter procedure of having as a basic record an analogue recordingbecause they feel that this type of record permits the person handling the data to havean overall visual check on the information. The existing network should olso be takeninto account to assure that existing interface apparatus be used as much as possible.This would eliminate the purchase of special individual translating equipment to convertpaper tape to cards, magnetic tape to cards, paper tape to magnetic tape etc. Ifa system does not already exist and the number of installed analogue recorders is minimal,it would probably be preferable to initiate digital recorders which can eliminateall human processing of the record.The treatment of hydrological data in itself does not necessarily justify thepurchase or rental of expensive computers so that, before taking a~y decision, astudy should be made of the computer facilities availoble and their cost, and thepossibility of either adding to these facilities or instolling completely new mochineprocessing equipment.If the use of computers is contemplated it isrecording instruments should have an output compatiblefacilities.verywithimportantavailablethat newcomputer1.3 Factors affecting choice of instruments and type of installationfor automatic hydrological observing stationsIn the previous sections the odvantages and disadvantages ofbeen discussed. Once the decision has been made· to automate theautomation havehydrological