Abstract
Abstract. Wetlands are generally accepted as being the largest but least well quantified single source of methane (CH4). The extent of wetland or inundation is a key factor controlling methane emissions, both in nature and in the parameterisations used in large-scale land surface and climate models. Satellite-derived datasets of wetland extent are available on the global scale, but the resolution is rather coarse (>25 km). The purpose of the present study is to assess the capability of active microwave sensors to derive inundation dynamics for use in land surface and climate models of the boreal and tundra environments. The focus is on synthetic aperture radar (SAR) operating in C-band since, among microwave systems, it has comparably high spatial resolution and data availability, and long-term continuity is expected. C-band data from ENVISAT ASAR (Advanced SAR) operating in wide swath mode (150 m resolution) were investigated and an automated detection procedure for deriving open water fraction has been developed. More than 4000 samples (single acquisitions tiled onto 0.5° grid cells) have been analysed for July and August in 2007 and 2008 for a study region in Western Siberia. Simple classification algorithms were applied and found to be robust when the water surface was smooth. Modification of input parameters results in differences below 1 % open water fraction. The major issue to address was the frequent occurrence of waves due to wind and precipitation, which reduces the separability of the water class from other land cover classes. Statistical measures of the backscatter distribution were applied in order to retrieve suitable classification data. The Pearson correlation between each sample dataset and a location specific representation of the bimodal distribution was used. On average only 40 % of acquisitions allow a separation of the open water class. Although satellite data are available every 2–3 days over the Western Siberian study region, the irregular acquisition intervals and periods of unsuitable weather suggest that an update interval of 10 days is more realistic for this domain. SAR data availability is currently limited. Future satellite missions, however, which aim for operational services (such as Sentinel-1 with its C-band SAR instrument), may provide the basis for inundation monitoring for land surface and climate modelling applications.
Highlights
Wetlands and inundated areas are important features of many landscapes because of their roles in hydrological and biogeochemical cycles
More than 2300 samples were used for threshold testing and the results are shown in Fig. 6. 95 % of samples showed an increase of water fraction of less than 0.07 when the threshold was increased by 2 dB (Fig. 6a)
The characteristic bimodal dis- that only the fraction of the grid box where the water table tribution in regions with open water mixed with other land was at or above the surface resulted in net emission of CH4, cover is similar for both tundra and taiga environments
Summary
Wetlands and inundated areas are important features of many landscapes because of their roles in hydrological and biogeochemical cycles. The global annual source strength of CH4 is relatively well constrained (Denman et al, 2007), wetlands are generally accepted as being the largest but least well quantified single source of CH4, with emission estimates ranging from 100 Tg yr−1 (Wuebbles and Hayhoe, 2002) to 231 Tg yr−1(Mikaloff Fletcher et al, 2004). Due to the greater warming expected over high latitudes, they might increase faster and by a large amount than emissions from tropical regions. Large parts are underlain by perennially frozen ground and a total of 1672 Pg of soil carbon is estimated to be stored in northern permafrost regions (Schuur et al, 2008)
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