Utilizing space-based GPS technology to determine hydrological properties of soils

Competition for limited water resources is one of the most critical issues being faced by irrigated agriculture in the United States. Site-specific irrigation applies irrigation water to match the needs of individual management zones within a field, significantly reducing water consumption, runoff, and nutrient leaching in ground water. Remote sensing for real-time and continuous soil moisture measurements at specific depths is essential for success of site-specific irrigation system.The overall objective of this study was to investigate the feasibility of utilizing a GPS-based sensor technology to determine site-specific information such as the soil moisture condition by recording the GPS signal reflected from the earth's surface. A modified GPS Delay Mapping Receiver (DMR) tracks and measures the direct, line-of-sight, Right-Hand-Circularly Polarized signal of a GPS satellite. It also simultaneously measures the delayed, earth-reflected, near-specular, Left-Hand-Circularly Polarized GPS signal. These measurements can be used to estimate the surface scattering coefficient and path delays between the direct and reflected GPS signals. Over land, scattering coefficients can be used to estimate changes in soil moisture contents.Our results showed that the space-based technology has a great potential for determining soil volumetric moisture contents in the pursuit of site-specific irrigation management. There were strong correlations between the GPS reflectivity measurements and soil moisture contents. The GPS reflectivity increased as the soil moisture contents increased. Careful analysis of the test data showed very conclusively that the sensitivity of L-Band signal (1.575 GHz) to soil moisture contents changed with soil type and sampling depth. The sensitivity decreased with sampling depth in light soils and increased in heavy soils.

► Strong correlations between the GPS reflectivity measurements and soil moisture contents.
► GPS reflectivity increased as the soil moisture contents increased.
► The sensitivity of L-Band signal (1.575 GHz) to soil moisture contents changed with soil type and sampling depth.
► Sensitivity decreased with sampling depth in light soils and increased in heavy soils.