Remote sensing in microwave and gamma ranges for the monitoring of soil water content of the root zone

Knowledge of vertical distribution of the soil water content in the root zone W plays a key role in the optimization of irrigation and, hence, for water saving. Therefore, in this study, synergistic use of the frequency modulated continuous-wave P-band (λ = 68 cm) scatterometer (SC) and the gamma-ray radiometer (GR) for the range of 50-3000 keV, both low in sensitivity to soil surface roughness and vegetation, was proposed. This combination allowed the determination of W at the three depth ranges: 0-5, 0-30, and 5-30 cm. This considerably specified the vertical distribution of W. Both instruments were developed as an integrated remote-sensing system that was installed onboard a light aircraft and tested on an irrigated agricultural region of the Negev desert, Israel. For processing and analysing the results, analytical models for both reflection coefficient (Ṙ) and intensity of the natural gamma radiation (I_s) were developed for various types of soils and vertical distributions of W. Using these models, the problem of the inverse retrieval of W was solved, and the depth of sampling z was evaluated. SC allowed the measurement of average W for the depth of 0.5-5.0 cm depending on W and dW/dz at the soil surface. For GR, the sampling depth varied from 20 to 30 cm depending on the mean value of W in the top soil layer. Field experiments showed that W retrieved by the developed system was in good agreement (r² ≥ 0.9) with ground measurements, thus indicating sufficient accuracy of both instruments.