Interferometric SAR for Hydrology Applications

Land subsidence has plagued many portions of Arizona since highly efficient means of groundwater extraction were developed in the early to mid 1900s. As groundwater extraction exceeded natural aquifer recharge, groundwater levels dropped by as much as 500 feet and the dynamics needed to initiate land subsidence were put in place. According to Land Subsidence, Earth Fissures Change Arizona's Landscape published by the University of Arizona Water Resources Research Center "with the increasing occurrence of land subsidence and resultant earth fissures in certain areas of the state, the consequences of dropping water tables become distinct, physical and sometimes dramatically visible. Land subsidence and fissuring provide tangible evidence that the over withdrawal of groundwater has geological as well as public policy consequences."

SAR interferometry has been extremely successful in measuring ground motion in this arid environment. Results from this technology have provided important information about the health and dynamics of the aquifer in the Phoenix region. In addition, these ground motion measurements have directly impacted the management of infrastructure projects in the area such as pointing to areas of likely fissure risk and indicating points of critical sinking in the vicinity of aqueducts. The Arizona Department of Water Resources currently uses DifSAR measurements operationally.

Figure 1

SAR interferometry has been used extensively in the Phoenix valley. This ground motion measurement from the Glendale-Peoria vicinity was created from a single pair of SAR acquisitions. Shown as an overlay on Google Earth (TM) imagery, the measurement corresponds to the time period from May 1995 to February 1998 and was derived from ERS 1/2 SAR satellite constellation. The higher density fringes in the southeastern feature reflect a ground motion of about 20 cm during this period. Modified water withdrawal practices in this area have largely mitigated subsidence at this location, although subsidence remains an issue in other parts of the valley.

Figure 2

Portions of the Phoenix area have undergone enormous ground subsidence during the past century. The Luke Air Force Base area west of Phoenix has witnessed as much as 18 ft of subsidence, with localized differential subsidence reversing storm-water drainage near the base. As a result, a monsoon rain event in September 1992 flooded the base, resulting in its temporary closure and ~$3 million in damage. This image, taken in 1991, shows the change in the ground elevation during the 35 preceding years - the top of the pole indicates the ground elevation in 1957. This 5.5 m subsidence resulted from heavy agricultural pumping.

Figure 3

Long term subsidence can be a symptom of a decline in the aquifer's ability to store water. In addition, ground motion represents a distinct threat to infrastructure. For example, the horizontal strain arising from subsiding ground can result in ground fissures which can further erode with rain. This image shows a fissure, found in the western Phoenix valley, that arose over the period of several days. Fissuring is a very significant concern surrounding overproduction of aquifers and the resulting subsidence.

For more information, contact David Cohen or see this overview.