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Institute o! Hydrology, Walling]ord, Berkshire, United Kingdom The length o.f flowing channel in a small clay basin in southeast England was observed once per week over a period of I year. The main controls of the total length of flowing channel were found to be the effective rainfall during the previous week and the length 'of stream flowing at the time of the previous week's observation. Variations in the lengths of streams of different orders were studied, first-order streams being found to vary most.The study of drainage channel networks has been approached in three m.ain ways. First, methods of characterizing the geometry and topology of networks have been developed by Horton [1932, 1945], Strahler [1952, 1957], Shreve [1966], and others. Second, various ways of modeling networks have been devised, .such as those by Leopold and Langbein [1962] and by Howard [1971]. Third, relations have been

Forestry is generally considered to be a land use giving low sediment yields. However, in Britain most forestry is on land that requires extensive drainage, and it was found that the drainage of a small upland catchment resulted in sediment yields over the following five years equivalent to nearly half a century's load at pre-drainage rates. Subsequent sediment yields did not decline to pre-drainage levels, but remained about four times higher, as a result of erosion of the drains.

The aim of FLOODNET is to provide a communications and data distribution facility speci®cally designed to meet the demanding temporal requirements of¯ood monitoring within the European Union (EU). Currently, remotely sensed data are not fully utilized for¯ood applications because potential users are not familiar with the procedure for acquiring the data and do not have a de®ned route for obtaining help in processing and interpreting the data. FLOODNET will identify the potential user groups within the EU and will, by demonstration, education and the use of telematics, increase the awareness of users to the capabilities of earth observation (EO) and the means by which they can acquire EO data. FLOODNET will act as a ®lter between users and satellite operation planners to help assign priorities for data acquisition against previously agreed criteria. The network will encourage a user community and will facilitate cross-sector information transfer, particularly between`¯ood experts' and administrative decision makers. The requirement for two levels of¯ood mapping is identi®ed: (1) a rapid,`broad-brush' approach to assess the general¯ood situation and identify areas at greatest risk and in need of immediate assistance; (2) a detailed mapping approach, less critical in time, suitable for input to hydrological models or for¯ood risk evaluation. A likely networking technology is outlined, the basic functionality of a FLOODNET demonstrator is described and some of the economic bene®ts of the network are identi®ed.

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