Recent research studies have investigated an airborne gamma radiation detection system to determine the water equivalent of snowpacks in nonmountainous areas. Snow attenuates natural gamma emissions from the soil, and the magnitude of attenuation is related to the mass of the water blanket between the soil and the detector. Gamma spectral and total counting rates are collected and recorded by an airborne system using 14 4‐ by 4‐inch sodium iodide (NaI (Tl)) crystals. These data are corrected for soil moisture, background radiation, and effects of air density. Extensive snow depth and density measurements were taken to determine ‘ground truth’ water equivalent under the flight path. Results of the first year of research indicate that gamma spectral data may be expected to give areal measurement of snow water equivalent within at least 0.2–0.5 inch over favorable terrain. The use of total count data is even more promising but requires methodology still under development for eliminating background interference.

The attenuation by snow cover of natural gamma radiation emitted from the soil serves as an excellent index to the water equivalent of the snow cover. A small portable gamma ray detector was installed on a boom about 2 meters above the ground at the National Oceanic and Atmospheric Administration‐Agricultural Research Service (ARS) cooperative snow study site at the ARS Sleepers River watershed near Danville, Vermont, for the 1970–1971 snow season. Comparison of gamma ray count rates with snow measurements taken at the site indicates that the small unshielded gage could be used to measure snow water equivalent (range 5–40 cm) with a standard error of 1.5 cm without preliminary editing of gamma ray count rates. A major source of this error was the deposition of radioactive aerosols on the snow surface by precipitation. The deviation of gamma ray count rates due to precipitation events is short‐lived, and a simple editing procedure on the count rate time trace reduced the snow season standard error to 1.1 cm. The edited count rate yielded 6% error in the 5‐ to 13‐cm water equivalent range, decreasing to 4% in the 25‐ to 40‐cm water equivalent range. This measurement method could be extremely valuable in providing unmanned measurement of snow water equivalents at remote locations.

Research studies measuring terrestrial gamma radiation from aircraft to determine snowpack water equivalent are reported. Both spectral and total count data were collected using 10·16 em by 10·16 em (4 in.) NaI (T1) scintillation crystals. Extensive ground truth data were used in conjunction with count rates obtained to develop empirical procedures relating count attenuation with snow water equivalent. Soil moisture, altitude, and air density corrections were made in the course of development of the method. Spectral relations are compared with theoretical. Significant limitations on data collection and interpretation imposed by the presence of radon gas are discussed. Because of radon gas interference in the total count, the spectral method gives the best measurement of water equivalent in the snow-flood sensitive North-Central United States. RESUMEOn rend compte des etudes faites en vue d'evaluer, a partir d'un avion, la radiation gamma terrestre et pour determiner la hauteur d'eau equivalente des couches de neige. Les donnees spectrales ainsi que les donnees de calcul total ont ete recueillies au moyen de cristaux de scintillation de 10·16 em sur 10·16 em (4 in.) NaI(Tl). Des donnees etendues sur la verite terrestre furent utilisees de concert avec les taux de calculs obtenus pour developper les processus empiriques qui ont un rapport entre I'attenuation et la hauteur equivalente d'eau de neige. Des corrections sur I'humidite du sol, l'altitude et la densite de I 'air ont ete faites au cours du developpement de la methode et les rapports spectraux ont ete compares aux rapports theoriques. On discuta des limitations significatives sur la collection et I'interpretation des donnees imposees par la presence du gaz radon. A cause de l'intervention du gaz radon dans Ie calcul total, la methodespectrale fournit la meilleure evaluation de la hauteur d'eau equivalente dans le Nord et le Centre des Etats-Unis qui sont sujets aux crues des neiges. .

The attenuation of highly penetrating cosmic radiation shows promise as a means of measuring the water equivalent of snow cover. The attenuation of cosmic radiation by water is sufficient to make the method practicable, especially for deep snow. As an example, statistical counting errors in a two‐detector setup (using 10 cm by 10 cm Nal(Tl) scintillation detectors, one above the snow and one beneath the snow) would produce a water equivalent measurement accuracy of better than 1% in measuring 100 cm of water with a 24‐hour measurement time.