Proximal gamma-ray spectroscopy: An effective tool to discern rain from irrigation

Proximal gamma-ray spectroscopy is a consolidated technology for a continuous and real-time tracing of soil water content at field scale. New developments have shown that this method can also act as an unbiased tool for remotely distinguishing rainwater from irrigation without any meteorological support information. Given a single detector, the simultaneous observation in a gamma spectrum of a transient increase in the214 Pb signal, coupled with a decrease in the40 K signal, acts as an effective proxy for rainfall. A decrease in both214 Pb and40 K signals is, instead, a reliable fingerprint for irrigation. We successfully proved this rationale in two data-taking campaigns performed on an agricultural test field with different crop types (tomato and maize). The soil moisture levels were assessed via the40 K gamma signal on the basis of a one-time setup calibration. The validation against a set of gravimetric measurements showed excellent results on both bare and vegetated soil conditions. Simultaneously, the observed rain-induced increase in the214 Pb signal permitted to identify accurately the rain and irrigation events occurred in the 8852 h of data taking.