**2.3. Wetlands**

global lakes and reservoirs, HydroLakes, was developed and includes hydrologic attributes, such as volume and residence time, using a geo-statistical model [42] (**Figure 5**). Within the US, the NHDplus (1:100k) dataset provides coverage of lakes and areas as polygons, an area

farm ponds. The NHD high-resolution (1:24k) dataset estimates lake and reservoir area cover-

; however, this dataset misses small waterbodies, especially

, almost 3.5 times higher than that of NHDplus.

estimated at almost 250,000 km2

64 Pure and Applied Biogeography

**Figure 4.** Global lake abundance estimated by several different studies.

**Figure 5.** HydroLakes database depiction of global lakes and reservoirs.

age as approximately 890,000 km2

Wetlands are transitional systems by nature, making them difficult to distinguish from other waterbodies. A distinction is provided by the U.S. Fish and Wildlife Service (USFWS) [45], which defines wetlands as "lands transitional between terrestrial and aquatic systems where the water table is usually at or near the surface or the land is covered by shallow water". USFWS [45] goes on to list three main attributes of wetlands: "(1) at least periodically, the land supports predominantly hydrophytes, (2) the substrate is predominantly undrained hydric soil, and (3) the substrate is non soil and is saturated with water or covered by shallow water at some time during the growing season of each year." In contrast, the International Union for the Conservation of Nature (IUCN) broadens the definition of wetlands to be all-inclusive of "areas of marsh, fen, peatland or water, whether natural or artificial, permanent or temporary, with water that is static or flowing, fresh, brackish or salt, including areas of marine water the depth of which at low tide does not exceed 6 m" [46]. For our purposes, we include wetlands as any waterbody or part of the landscape that falls within the definitions above, but cannot be distinguished as a lake, reservoir, pond, river or stream.

Unfortunately, there is little consistency in the nomenclature distinguishing among various waterbodies in the spatial datasets used to estimate global coverage of wetlands. The GLWD is commonly used in representations of wetlands across the globe (**Figure 6**). Many of the spatial datasets contributing to the GLWD, however, have contrasting naming conventions for waterbodies [21]. In particular, the DCW does not distinguish between vectors portraying lakes, reservoirs, larger rivers, and wetlands [26]. In comparison, the Wetlands Map of the World Conservation Monitoring Center (WCMC) includes 20,000 wetland and lake polygons classified into 21 types and represents the most comprehensive and accurate vector map of the world's wetlands [47]. As opposed to representing wetlands as vectors or polygons, other mapping efforts display wetlands as raster maps. For example, the US Geological Survey Global Land Cover Characteristics (GLCC) database [48] and MODerate resolution imaging spectroradiometer (MODIS) data [49] provides classification of global landcover, including wetlands, as 30 second grids (MODIS). Others have developed global wetland land cover maps at coarser resolutions using varied methodologies [50–52]. Because of the uncertainties on global wetland extents and inventories, the Ramsar Convention on Wetlands has promoted new efforts and advanced remote sensing technologies to provide new and improved global wetland inventories [53, 54].

Similar to other freshwater systems, estimates of the global coverage of wetlands have increased over time with advances in higher-resolution spatially comprehensive datasets.

**Figure 6.** Map of global waterbodies based on the Global Lakes and wetlands database (GLWD).

Early estimates (pre-2000) ranged from 4.3 to 5.3 million km2 whereas current estimates approach almost 13 million km2 (**Table 3**). However, the highest estimate may be an overestimate inclusive of lake and reservoir waterbodies [57] relative to the reference [21] estimate of 9.2 million km2 . Within the US, wetlands are depicted by a few vector and raster datasets. For the conterminous US, the Multi-Resolution Land Characteristics Consortium (MRLC) provides National Land Cover Databases (NLCD) as raster images [58]. According to the 2011 NLCD data, the area classified as woody or herbaceous wetlands sums to 417,442 km2 . Open water constitutes almost the same spatial area, 422,111 km2 . The USFWS maintains the


**Table 3** Global areal estimates of wetland coverages according to different studies.

A Synopsis of Global Mapping of Freshwater Habitats and Biodiversity: Implications... http://dx.doi.org/10.5772/intechopen.70296 67

**Figure 7.** Comparison of wetland maps derived from the National Wetlands Inventory (NWI), the National Land Cover Database (NLCD), and the Global Lakes and Wetlands Database (GLWD) for a coastal portion of the State of North Carolina located in the eastern United States. Examples of types of wetland databases available in the conterminous US.

National Wetland Inventory (NWI), a database of polygons and associated very detailed classification framework for the conterminous US [59] (**Figure 7**). The NWI provides a status update of the nation's wetlands every five years with the latest 2009 report indicating there were 445,559 km2 of wetlands, 95% of which are freshwater systems [60]. The difference of 28,118 km2 between NWI and NLCD estimates of wetland area for the entire conterminous US suggests differences in the approaches taken to classify wetlands (**Figure 7**). Both of these datasets, however, far exceed the spatial granularity of wetlands depicted by the GLWD (**Figure 7**).
