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The current state of knowledge of the microwave properties of snow is discussed. Theory behind the microwave emission from snow is reviewed, as are the physical processes of snowpack metamorphism. Field, aircraft, and satellite passive microwave data have been acquired and analyzed for more than 10 years. Results have repeatedly demonstrated the feasibility of employing multifrequency passive microwave data to study snow‐covered area, snow depth, and internal snowpack properties. Radiation emanating from the ground beneath a snowpack is scattered by the snow crystals, and concurrently, the snow itself emits radiation at microwave frequencies. Thus the radiation emerging from the snowpack is the result of a complex series of interactions both within and beneath the snowpack. Future studies recommended by a snowpack properties working group consisting of government and university scientists are discussed in detail. Recommendations include performing extensive laboratory measurements using real and artificial snow, to be coordinated with theoretical modeling and aircraft overflights carrying passive microwave instrumentation. This is considered necessary in order to help to interpret the microwave responses to snow.

Snow accumulation and depletion at specific locations can be monitored from space by observing related variations in microwave brightness temperatures. Using vertically and horizontally polarized brightness temperatures from the Nimbus 6 Electrically Scanning Microwave Radiometer, a discriminant function can be used to separate snow from no snow areas and map snowcovered area on a continental basis. For dry snow conditions on the Canadian high plains significant relationships between snow depth or water equivalent and microwave brightness temperature were developed which could permit remote determination of these snow properties after acquisition of a wider range of data. The presence of melt water in the snowpack causes a marked increase in brightness temperature which can be used to predict snowpack priming and timing of runoff. As the resolutions of satellite microwave sensors improve the application of these results to snow hydrology problems should increase.

Low-resolution meteorological satellite data and simple photo interpretation techniques have been used to map snow-covered areas during early April over the Indus River and Kabul River basins in Pakistan. The early spring snow-covered area was significantly related to April I through July 31 streamflow in regression analyses for each watershed (Indus River, 1969-1973, ? = 0.82, and Kabul River, 1967-1973, ? = 0.89). Predictions of 1974 seasonal streamflow using the regression equations were within 7% of the actual 1974 flow. Because of inadequate hydrometeorological data, conventionally based streamflow predictions are not possible in some of these remote regions, and the satellite-derived runoff estimates have immediate applicability for improved water resources mfinagement. U.S. Department of Interior, Snow mapping and runoff forecasting: Examination of ERTS-1 capabilities and potential benefits from an operational ERS system, interim report, contract 14-08-001-13519, Office of Econ. Anal., Washington, D.C., 1974. Wiesnet, D. R., and M. Matson, Monthly winter snowline variation in the northern hemisphere from satellite records, 1966-1975, Tech. Memo. NESS 74, 21 pp.,

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