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Information and reporting system for
water and agriculture in Asian monsoon areas - Spatial information

Paddy FieldIn the framework of the "Information and reporting system for water and agriculture in Asian monsoon areas", two spatial datasets have been developed that are of key importance to assess the impact of water use for agriculture on the available water resources. One dataset describes the extent of irrigation as percentage of total land area in South East Asia, and the other is a delineation of river basins and sub-basin boundaries.

The map of irrigation for South East Asia has been developed according to the same methodology as the Global Map of Irrigated Areas, and is fully compatible with this dataset. The map with river basins is adapted from the HYDRO1k geographic database as developed by the US Geological Survey's EROS Data Center in cooperation with UNEP/GRID Sioux Falls.

Behind the map of major river basins and sub-basins in South East Asia

By investigating the path of surface water over the topography of the land surface, watershed boundaries can be delineated from a digital elevation model (DEM) by using GIS software. This study aims to investigate the integrity of the U.S. Geological Survey’s (USGS) HYDRO1k DEM for South East Asia, to adjust it where errors are perceived, and to derive a vector map of river basins and their sub-basins named after the largest tributary within its limits. The documentation provided can be used as a guideline for similar work in the future, requiring the use of ESRI ArcINFO version 8.2, with Spatial Analyst and ArcHydro extensions.

Behind the map of Irrigated Areas for South East Asia

The Land and Water Development Division of Food and Agriculture Organization of the United Nations and the Johann Wolfgang Goethe Universität, Frankfurt am Main are currently co-operating in the development of a global irrigation mapping facility. The map of irrigation for South East Asia has been developed according to the same methodology as the Global Map of Irrigated Areas, and is fully compatible with this dataset. The irrigation map inventory for South East Asia includes sub-national irrigation statistics for most countries. The target year for the statistics is the year 2000 and refers to areas equipped for irrigation.

To distribute the irrigation statistics per sub-national unit digital spatial data layers and printed maps were used. Irrigation maps were derived from project reports, irrigation sub-sector studies or books related to irrigation and drainage. After on-screen digitizing of the maps the boundaries of the digitized irrigated areas were compared to satellite images in many regions. The shape and size of the digitized areas was adjusted if necessary. Unfortunately for many sub-national units irrigation maps are not available. This occurs in particular in areas with low irrigation density dominated by small scale private schemes. Here additional information was used to find areas were irrigation is likely (e.g. land use maps that show the location of the main irrigated crops). By assigning weights to digitized areas it was possible to distribute irrigation statistics over digitized irrigated areas within the sub-national units. The methodology is described in detail in previous reports of the irrigation mapping project (e.g. A Digital Global Map of Irrigated Areas - An Update for Latin America and Europe, Siebert and Döll, 2001). The map resolution during the generation process was 0.01 degree which was aggregated for use on a continental scale to a final resolution of 5 minutes (0.0833 degrees). The full documentation of this update is described by Stefan Siebert, Sebastian Feick and Jippe Hoogeveen in: "A digital global map of irrigated areas - An update for Asia" (2005). A summary per country is given below.

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