Interferometric radar measurements of water level changes on the Amazon flood plain

Measurements of water levels in the main channels of rivers, upland tributa ries and floodplain lakes are necessary for understanding flooding hazards, methane production, sediment transport and nutrient exchange. But most rem ote river basins have only a few gauging stations and these tend to be rest ricted to large river channels. Although radar remote sensing techniques us ing interferometric phase measurements have the potential to greatly improv e spatial sampling, the phase is temporally incoherent over open water and has therefore not been used to determine water levels. Here we use interfer ometric synthetic aperture radar (SAR) data(1-3), acquired over the central Amazon by the Space Shuttle imaging radar mission(4), to measure subtle wa ter level changes in an area of flooded vegetation on the Amazon flood plai n. The technique makes use of the fact that flooded forests and floodplain lakes with emergent shrubs permit radar double-bounce returns from water an d vegetation surfaces(5,6), thus allowing coherence to be maintained. Our i nterferometric phase observations show decreases in water levels of 7-11 cm per day for tributaries and lakes within similar to 20 km of a main channe l and 2-5 cm per day at distances of similar to 80 km. Proximal floodplain observations are in close agreement with main-channel gauge records, indica ting a rapid response of the flood plain to decreases in river stage. With additional data from future satellite missions, the technique described her e should provide direct observations important for understanding flood dyna mics and hydrologic exchange between rivers and flood plains.