Abstract: A solid, annular body embedding naturally occurring radioisotopes, forms a source of radiation bordering the opening of a basin at ground level, into which water percolates from a body of precipitation in the form of snow and ice accumulated on a porous cover closing the opening. Point detectors above the opening and below the basin sense radiation emitted from the source to produce signals attenuated by the accumulated precipitation reflecting changes in density thereof. Other detectors laterally spaced from the source of radiation both inside and outside its signal range, respectively measure changes in soil density and background radiation.