Development and Evaluation of a Multi-Year Fractional Surface Water Data Set Derived from Active/Passive Microwave Remote Sensing Data

Ronny Schroeder, Kyle McDonald, Bruce Chapman, Katherine Jensen, Erika Podest, Zachary Tessler, Theodore Bohn, Reiner Zimmermann
<span title="2015-12-09">2015</span> <i title="MDPI AG"> <a target="_blank" rel="noopener" href="" style="color: black;">Remote Sensing</a> </i> &nbsp;
The sensitivity of Earth's wetlands to observed shifts in global precipitation and temperature patterns and their ability to produce large quantities of methane gas are key global change questions. We present a microwave satellite-based approach for mapping fractional surface water (FW) globally at 25-km resolution. The approach employs a land cover-supported, atmospherically-corrected dynamic mixture model applied to 20+ years (1992-2013) of combined, daily, passive/active microwave remote
more &raquo; ... ing data. The resulting product, known as Surface WAter Microwave Product Series (SWAMPS), shows strong microwave sensitivity to sub-grid scale open water and inundated wetlands comprising open plant canopies. SWAMPS' FW compares favorably (R 2 = 91%-94%) with higher-resolution, global-scale maps of open water from MODIS and SRTM-MOD44W. Correspondence of SWAMPS with open water and wetland products from satellite SAR in Alaska and the Amazon deteriorates when exposed wetlands or inundated forests captured by the SAR products were added to the open water fraction reflecting SWAMPS' inability to detect water underneath the soil surface or beneath closed forest canopies. Except for a brief period of drying during the first 4 years of observation, the inundation extent for the global domain excluding the coast was largely stable. Regionally, inundation in North America is advancing while inundation is on the retreat in Tropical Africa and North Eurasia. SWAMPS provides a consistent and long-term global record of daily FW dynamics, with documented accuracies suitable for hydrologic assessment and global change-related investigations. precipitation and temperature trends can be inferred, global CH 4 emissions are typically estimated via process-based models calibrated to individual wetland sites [2] [3] [4] [5] [6] . In this approach, CH 4 emissions from wetland sites are extrapolated via spatially explicit maps of global wetland distribution, combining static and more recently dynamic inundation maps with hydrological and biogeochemical process features thought to be relevant for methanogenesis [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] . Top-down approaches, such as estimation of atmospheric CH 4 using satellite measurements and inverse models [20] [21] [22] [23] are useful tools in constraining ground-based emission inventories, particularly over tropical wetlands which exhibit higher CH 4 emissions than those estimated from forward calculations [24] [25] [26] . This emission discrepancy between model approaches may be explained by both a lack of field data available from tropical wetlands and an incomplete representation of the hydrodynamics of shallow tropical wetland complexes [27] . Development and application of spatially comprehensive and temporally consistent maps of global inundation dynamics as model inputs or optimization targets in global methane models are thus crucial for improving our understanding of the role of wetland emissions in the climate system [16] . Several different remote sensing-based approaches haven been taken towards regional to global-scale satellite monitoring of inundation: passive microwave, active microwave and hybrid approaches. Coarse-resolution (25-km) passive and active microwave (MW) data from satellite instruments (e.g., Advanced Microwave Scanning Radiometer-EOS (AMSR-E), SeaWinds-on-QuikSCAT) are well suited for the global monitoring of inundation patterns because they are sensitive to the distribution of liquid water in the landscape and cover large areas with daily repeat periods at high latitudes. Because MW instruments can operate day and night and are not limited by cloud cover, they provide the unique capability for acquiring temporally consistent and continuous records of inundation dynamics, supporting accurate inference of surface water dynamics and related fresh water storage changes. Differences observed between vertically and horizontally polarized brightness temperatures (T b ) by passive MW instruments have long been shown to be effective in measuring large-scale inundation patterns across densely vegetated landscapes [28] [29] [30] [31] [32] [33] [34] [35] [36] . Common to this approach is the assumption that, across densely vegetated forests, changes in soil moisture and vegetation biomass and structure and associated variations in emissivity caused by both are negligible relative to the influence of inundation. Radar backscatter (σ 0 ) from active MW instruments is also sensitive to inundation conditions and has been used regionally to delineate floods and monitor inundation extent [37] [38] [39] [40] [41] . Its higher sensitivity to vegetation structure, relative to T b , provides a unique capability to characterize biomass variations associated with seasonal plant dynamics [42] [43] [44] [45] [46] [47] [48] . As most major global wetland complexes consist of inundated surfaces that are often vegetated, methods that combine σ 0 and T b provided by coarse-resolution active and passive MW instruments would be ideal for the characterization of surface water dynamics in vegetated ecosystems subject to seasonal plant dynamics.
<span class="external-identifiers"> <a target="_blank" rel="external noopener noreferrer" href="">doi:10.3390/rs71215843</a> <a target="_blank" rel="external noopener" href="">fatcat:t5lgblx5ircrlhi6y55oout32a</a> </span>
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