ABSTRACT

NSF has supported the development of Global Positioning System (GPS) networks primarily for geophysical studies. In a broad collaboration with the international community, GPS data from these NSF-sponsored networks are freely available; they are being used for many non-geodetic applications including atmospheric and ionospheric studies. Engineering, telemetry systems, and archiving services already exist to support these continuously operating GPS networks.

For this research, a technique that expands the use of existing continuously operating GPS networks to a new scientific community---the hydrologic community---will be employed. This technique uses signals that reflect off soil beneath continuously operating GPS sites. These reflections (multipath) are intrinsically related to the reflectance of the ground -- and thus soil moisture - and can be observed in GPS signal-to-noise-ratio (SNR) data. A potential advantage of this method is that it could provide an estimate of near-surface soil moisture across an area of ~100 m2 by explicitly measuring the variability of soil moisture that exists at finer scales.

The PIs have compared multipath reflection amplitudes from a GPS site in Tashkent, Uzbekistan to soil moisture from a land surface model. Although the magnitudes of GPS multipath amplitudes and the land surface model soil moisture are not calibrated, they both rise sharply following each rainfall event and slowly decrease over the 70-day period studied. This research will further develop this technique and create a soil moisture calibration site near Boulder, CO. GPS receivers, antennas, and monumentation will be tested to determine which are optimal for soil moisture studies. The dependence of GPS multipath amplitudes on environmental conditions, including temperature, vegetation, soil texture, and vertical distribution of soil moisture, will be studied. Estimated reflectance coefficients will be compared with in situ measurements from volumetric sampling and water content reflectometers.

Broader Impacts
The research could potentially provide daily soil moisture estimates from a large number of existing geodetic GPS sites for continental to global scale climate and hydrologic studies. Soil moisture measurements are critical to study processes, test and calibrate models, and forecast availability of water resources. Although soil moisture networks exist (e.g. http://okmesonet.ocs.ou.edu/), most are still sparsely distributed; many regions in the world still lack standardized instrumentation. GPS SNR based studies could greatly increase the number of soil moisture measurements available to the hydrologic and atmospheric communities. The project will also enhance collaborations between the geodetic, atmospheric, and hydrologic communities in the U.S. These techniques could also be applied to international datasets. The geodetic techniques for this research will be developed by collaborating with hydrologists and transferred to the public domain. A graduate student will be involved in the research, working both with GPS and in situ soil moisture data.

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