AIRBORNE MEASUREMENT OF SURFICIAL SOIL MOISTURE USING NATURAL TERRESTRIAL GAMMA RADIATION.

Research has been conducted since 1969 to develop a technique using natural terrestrial gamma radiation attenuation to measure snow water equivalent and soil moisture from a low flying aircraft. Relevant activities are reported. Based on their results, it is concluded that the gamma radiation attenuation technique is capable of measuring the soil moisture flux in the upper 20 cm of the soil surface with the accuracy necessary for operational applications. Ground-based soil moisture data collected from a network of 240 flight lines in the upper Midwest indicate mean areal soil moisture values can be calculated with an rms error of 3. 2 % soil moisture. The airborne soil moisture values tend to be 1. 5 % less than the ground sampled values. The airborne gamma radiation technique is capable of measuring reliable, real-time mean areal soil moisture values over large areas with the accuracy required for operational hydrologic and agricultural applications.