Error assessment of airborne snow water equivalent measurements

The National Weather Service maintains an operational Airborne Gamma Radiation Snow Survey Program in the upper Midwest. A network of 300 flight lines, each approximately 15 to 20 km long and 300 m wide, has been established in the region. Natural terrestrial gamma radiation data are collected over the flight lines by a low-flying aircraft and used to infer mean areal snow water equivalent and soil moisture values over the 4.5 to 6.0 km sq area. Errors in the airborne snow water equivalent and soil moisture measurements are due primarily to: (1) uncertainties in the ground soil moisture calibration, (2) errors in the instrumentation used to measure air mass, and (3) limited radiation counting statistics. Nonetheless, the airborne gamma radiation attenuation technique is capable of making a snow water equivalent with an error of less than 1.0 cm and a soil moisture measurement with an error of approximately 3.9 percent soil moisture. Sources of error are discussed using ground and airborne data collected during the 1982 snow season.