DEM has been widely used for hydrological analysis. With ASTER DEM available, the higher resolution of 30m by 30m elevation data make the hydrological and flood risk analysis applicable at regional level. For example, Thanapakpawin et al. 2006; Arthur et al., 2005, Wyatt 2003, Brakebill and Preston 2003 used a 30-m DEM to obtain topography and flow network. In this study, two steps analysis is taken: (1) A hydrologic analysis of Malawi is conducted based on ASTERDEM data, and Flood risk map is generated at a TA municipal area using an integrated flooding risk index. Enumerator areas (EA), which are based 2008 census map, are the smallest geographic unit of the census. TA is a regional level of municipality area. Each TA is consisted of a number of EA areas. There are 368 TAs in the whole country. Precipitation data are obtained from “WorldClim” Global Climate data. It is interpolations of observed data, representative of 1950-2000 (http://www.worldclim.org/), with a resolution of 30 arc-seconds (~900m). The heavy rainfall happens in the middle east of the country during January, but highest in March in northeast of the country. We calculate the average rainfall of the five months in the rain seasons, January to April and December, and use it as an indicator for flood risk evaluation. (2) Following the analysis of flooding risk at TA level, we evaluate the risk of five selected cities – Lilongwe, Blantyre, Zomba, Mzuzu and Karonga. Using Karonga city as an example, we tentatively map the potential floodplains by applying a method of various buffer area based on elevation.
Hydrological analysis based on DEM is used in water availability studies, water quality projects, flood forecasting programs, as well as many other engineering and public policy applications. From DEM (Figure 1A), a raster data of flow sinks are identified. Flow sinks is an area surrounded by higher elevation values, and is also referred to as a depression or pit, which may potentially be landscapes of lakes, rivers and streams. Using flow sinks, flow direction (Figure 1B), flow accumulation, and stream network were derived. Flow Direction records the direction of flow out of each cell (Figure 2). Flow accumulation, a new grid that contains the number of cells that contribute to each cell. Stream network (Figure 1C) is generated by defining a flow accumulation greater than 5000 cells (4.5 km2). It means that any cell with more than 5000 cells flowing into it represents a stream pathway. 4.5km2 is commonly chosen as a minimum flow accumulation threshold for the identification of a stream. This threshold can be found in previous research such as Arthur et al., 2005, Wyatt 2003, Brakebill and Preston 2003. Based on the stream segments and flow direction, watershed is mapped for whole Malawi (Figure 1D). Catchments or watersheds are land areas that drain to a Hydro Network. The determination of their boundaries is necessary when modeling a hydrologic system.
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