ALOS Data for Land Subsidence Monitoring in Arizona
By Brian Conway, Arizona Department of Water Resources
The Arizona Department of Water Resources (ADWR) has been collecting and processing Synthetic Aperture Radar (SAR) data using interferometry (InSAR) since 2005. The InSAR products produced are used by ADWR for statewide land-deformation monitoring for the purpose of improved water-resource management and more specifically, to determine the spatial extent, magnitude, and time-series history of land-deformation features within south-central and southern Arizona. ADWR has collected, processed, and analyzed C-band SAR data from the European Space Agency’s European Remote Sensing Satellite 1 (ERS-1) and ERS-2, Envisat satellites, and the Canadian Space Agency’s RADARSAT-1 satellite. Recently, ADWR has started to utilize L-band SAR data from the Japanese Advanced Land Observing Satellite (ALOS) provided by the Alaska Satellite Facility (ASF).
Land subsidence has been occurring across Arizona since at least the 1940s when it was first documented. A few areas in Maricopa and Pinal Counties have subsided more than 20 feet since 1940. Land subsidence has damaged roads, sewer lines, flood-retention structures, and many other types of infrastructure throughout Arizona.
Cochise County, located in southeastern Arizona, is home to some of the largest agricultural areas within the State. InSAR products have proven to be an invaluable tool in this region, allowing hydrologists to not only identify the location of land-subsidence features, but to also monitor recurring land subsidence and uplift cycles that are caused by seasonal pumping demands related to crop water needs. The largest annual deformation rate detected (11 centimeters) by ADWR using InSAR products in Arizona is located in Cochise County and was identified using Envisat C-band SAR data between January 20, 2009, and January 05, 2010 (Figure 1). Numerous active earth fissures have been mapped by the Arizona Geological Survey (AZGS) and occur along areas of differential land subsidence that are easily detected with InSAR.
L-band SAR data acquired from ASF have allowed ADWR to complement recent Envisat C-band SAR data and improve coherence within some of the agricultural areas (Figure 2). The interferogram that was formed using L-band data from December 06, 2006, and December 14, 2009, illustrates the extent and magnitude of land subsidence caused by this seasonal, high-demand groundwater use. ADWR depth-to-water data show that groundwater levels have declined as much as 150 feet since 1950 within these active land-subsidence features. The L-band SAR data identifies the highest magnitude of land subsidence over the 3-year period to be 40 centimeters.
Additional L-band data for Cochise County will provide critical information to help us better understand how land subsidence is changing floodplains, altering aquifer hydraulic properties and survey elevations, and contributing to earth-fissure development. Future ADWR InSAR products will not only be used by ADWR, but by engineers, geologists, surveyors, floodplain managers, and other scientists for their own monitoring, modeling, and infrastructure projects.
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