Geographical characteristics of China’s wetlands derived from remotely sensed data

Abstract

In this paper, we report the first wetland mapping of the entire China using Landsat enhanced thematic mapper plus (ETM+) data. These data were obtained from the Global Land Cover Facility at the University of Maryland spanning from 1999 to 2002. A total of 597 scenes of Landsat images were georeferenced and mosaiced. Manual image interpretation of satellite images was aided with elevation data, soil data, land cover/land use data and Google Earth. The minimum mapping unit is 10 pixel × 10 pixel, equivalent to 9 ha. The aim of our first round of mapping was only targeted at the boundary delineation of any type of wetland except those wetlands that are under agricultural use (i.e., paddy fields), which has already been well mapped by others. Our interpretation results indicate that a total of 359478 km 2 of wetlands are of non-agricultural use. Among our preliminarily mapped wetland, 339353 km 2 are inland wetland, 2786 km 2 are non-agricultural artificial wetland, and 17609 km 2 are coastal wetland. Because low-tide is rarely captured in satellite images, an under-estimation of coastal wetland is inevitable. We conducted some statistics based on our mapped wetlands and compared them with those previously obtained from a number of sources including a land cover/land use map made with satellite images during the late 1990s and early 2000s, a marshland map developed in approximately the same period, survey data of coastal wetland in early 1980s, and area data for approximately 400 larger patches of marshland in China compiled in 1996. Because some inconsistencies exist in the guidelines of those different wetland surveys, difference in area is expected. Some further comparison indicates that the wetland distributions derived from the preliminary wetland map are reasonable and more objective than other sources. The mapping process also indicated that the method adopted by us was efficient and cost-effective. We also found that in order to ensure comparability of the wetland maps developed at different times, a set of standard guidelines on the wetland categories to be mapped, and the mapping methods to be used must be well conceived, developed and effectively employed. We carried out some initial geographical analysis on the distribution of wetlands.China wetlands, remote sensing, wetland mapping Wetlands host a biodiversity far beyond any other ecosystem. Along with oceans and forests, wetlands form the three major ecosystems on earth [1] . In the past two hundred years, human activity and settlement has drastically reduced wetlands [2] . From the 1780s to 1980s, America's wetlands were reduced by 53% [3] . In England, since the Rome times, 23% of the river deltas, 50% of marsh and 40% of the bog have disappeared (http:// www.ramsar.org/about/about_wetland_loss.htm, 1996) [4] .

Preliminary estimation of the organic carbon pool in China’s wetlands

“…There is great uncertainty about the carbon sequestrated in wetland soils in China. The results of Yu et al [41] and Niu et al [5] provided results which were different by 3-4 times. Based on the MODIS classification with a spatial resolution of 500 m [79], Yu et al [41,76] noted that the area of wetland soil in China was 727900 km 2 and its carbon pool was 12.20 Pg.…”

“…The improved inventory approach was used to calculate the organic carbon storage of wetland mainly including soil, vegetation and water bodies based on previous study [5].…”

“…The organic carbon density of wetland soils in China was available from some data including organic carbon depth distribution and from some limited references, and the first national wetland resources census data (2000-2003) was added to evaluate wetland soil organic carbon pool in China, giving a result of 8-10 Pg [44]. Based on the studies of Yu et al and Shi et al, Niu et al used the ETM + data instead of MODIS data [79], and calculated that the total organic carbon pool of wetland soils in China was 3.67 Pg [5].…”

“…Accurate estimation of the size of wetland carbon pools is a pre-requisite for protection of wetland resources and implementation of carbon sink enhancement plans, and its importance and urgency have been gradually recognized by scientists and politicians. However, experimental study on estimation of wetland carbon pools is just in the initial stages, and there are still many problems to be solved [5]. Since soil is a major component of wetland ecosystems for carbon sequestration, estimates of carbon pools in the soils of China are essential for accurately appraising global terrestrial carbon inventories and are critical for optimizing strategies to mitigate the accumulation of CO 2 in the atmosphere.…”

Lake Fluctuation Effectively Regulates Wetland Evapotranspiration: A Case Study of the Largest Freshwater Lake in China

“…However, a comprehensive assessment of ET remains a challenge over lakes or wetlands due to the limited on-site access to inundated areas, the complexity of wetland cover and ecosystem composition [21]. Remote sensing provides an effective means to estimate regional ET [22][23][24][25], but few studies have investigated the spatial distribution of wetland ET [26]. It remains unclear to what extent the wetland ET varies with lake fluctuation and other environmental influences, which is of great importance to effective management of water resources and a comprehensive understanding of wetland hydrology under a changing climate.…”