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Global map of irrigation areas
This section describes the background and history of the project that develops and maintains the "Global Map of Irrigation Areas". It includes links to the documentation of earlier versions of the map.
Agriculture is by far the largest water use at global level, with about 70% of water withdrawal and 90% of water consumption. In several developing countries, irrigation represents up to 95% of all water uses, and plays a major role in food production and food security. Future agricultural development strategies of most of these countries depend on the possibility to maintain, improve and expand irrigated agriculture. On the other hand, the increasing pressure on water resources by agriculture faces competition from other water use sectors and represents a threat to the environment in an increasing number of regions.
With the advent of communication technology and the development of an increasing number of global datasets, major efforts have been made in the last decade by the international community to assess the different elements of the water balance and predict current and future water needs for the different use sectors. Large uncertainty remain, however, on the extent and distribution of irrigated land in the world, and on agricultural water use, making it therefore difficult to adequately monitor the irrigation sector. A quality coverage of irrigated areas in the world, available in GIS, is the single most important information needed to improve future global studies on water and agriculture.
Global Map of Irrigated Areas, version 1.0
The first version of the "Digital Global Map of Irrigated Areas" was published as a raster map with a resolution of 0.5° by 0.5°. For the whole land area of the globe (except Antarctica), the data set provides the percentage of each 0.5 ° by 0.5 ° cell area that was equipped for irrigation around 1995, the so-called irrigation density. The area actually irrigated in 1995 was smaller, but is unknown for most countries. In the next sections, the areas equipped for irrigation will be refered to as "irrigated areas". The methodology to generate this first version of the map is explained in detail in Döll and Siebert (1999), and in an article by Döll and Siebert in ICID-journal 2000, vol. 49 no. 2. This section provides only a short summary.
The generation of the digital map includes a variety of steps depending on the type of data for the respective country. First, the location of areas equipped for irrigation within each country was determined, mainly by digitizing irrigation maps. Then, the irrigation density was modelled on a 5' raster based on information on the total area equipped for irrigation within a spatial unit (e.g. a country), and finally the information was aggregated to a 0.5° raster. In this process the following main steps can be distinguished:
- Creating digital maps with irrigated areas per country. For several countries paper maps are available with the outline of irrigated areas within the country. As these maps originate from different sources, the quality and type of the information varies considerably.
- For many countries no geographical information regarding the outline of irrigated areas is easily available. Sometimes descriptive information regarding the distribution of irrigated areas can been found in the literature, in these cases it is assumed that irrigation occurs in specific areas (for example valleys of rivers). If such information cannot be found it is assumed that the irrigation is equally distributed over the area.
- In GIS, the different country maps with irrigated areas are rasterised to a grid with cells of 5' geographical longitude and 5' geographical latitude. For each cell is indicated whether the cell include irrigated areas.
- The irrigation density of the grid cells is calculated as percentage of the total area of the grid cell by equally distributing FAO’s AQUASTAT country figures over the grid cells that have assigned irrigation to them. The figures from AQUASTAT do not refer to one specific year, but refer to the most recent, reliable, information available during the period the survey was done. In practice, this means that the AQUASTAT figures refer to areas equipped for irrigation around the year 1995.
- Several countries collect statistics regarding the irrigated area per sub-national boundary (federal states, provinces, counties, or river basins). If maps with the same sub-national boundaries are available, a more detailed estimation of the sub-national distribution of irrigated areas can be made.
- For many countries the 5' by 5' raster map suggests an accuracy that is higher than the accuracy of the source data. Therefore the maps are aggregated to a 0.5° by 0.5° grid. The aggregation is done by summing up all the irrigated areas within the 36 5' cells within each 0.5° cell.
Developments since version 1.0
Since the publication of the first version of the "Digital Global Map of Irrigated Areas", many improvements to the map have been made. In 2001 Siebert and Döll published "A Digital Global Map of Irrigated Areas - An Update for Latin America and Europe". For the update of the map for Latin America, information is used from the AQUASTAT survey on irrigation in Latin America that was done in 1999/2000. The update of the map for Europe is based on recent data on total irrigated area per sub-national units collected from national information and on new geographical information like the CORINE land cover data set (European Environmental Agency, 2000). Next to more detailed data, also the method to generate the map was improved. These latest map generating methods are described in detail in Siebert and Döll (2001). As a whole the improvements made to the map were considerable and justified publication of version two of the "Digital Global Map of Irrigated Areas".
Improvements in map generation and continental updates
With the update for Latin America and Europe several improvements have been made to the map generation method (Siebert and Döll, 2001). A summary of the improvements of the map generation method compared of the first version of the map is presented below:
- For version one of the Global Map of Irrigated Areas, paper maps were used with polygons indicating where the irrigated areas in the countries were located. For version two of the map not only polygon maps were used, but also maps with point information regarding the location and size of irrigation projects as well as interpreted satellite images.
- In version one of the map there was generally only one map per country available with the outline of the irrigated areas. For version two there were often more maps available. Therefore rules were developed to distribute the total irrigated area within (sub-)national units to the various geographic records within this unit. In order to facilitate the distribution, priority levels were assigned to the maps, based on the type and the reliability of the information.
In general the highest is given to point records (irrigation projects) with known irrigated area, followed by point records without size. Polygon records (outlines of large irrigated areas) with size information get the next lower priority, which is again higher than that of polygon records without size information.
Additional information like interpreted satellite images with information on agricultural areas compared to built-up areas, open water, desert or woodland, was used to distribute irrigated areas within (sub-)national units if no information with higher priorities was available.
- In version one the irrigation density was calculated on a working resolution of 5’ (0.0833°). For Latin America and The Caribbean, Europe and Africa the working resolution was increased to a 0.01° raster.
In 2002, FAO used the improved map generation method to update the map of irrigated areas for the African continent and Stefan Siebert updated the continent of Oceania. As this update was generated after publishing version 2.0, the updated version of the map is included in the "Digital Global Map of Irrigated Areas, version 2.1".
Early 2005, an update for the continent of Asia came available. This update is included in the "Digital Global Map of Irrigated Areas, version 2.2". In April of the same year an update for North America was produced. With this last update, the map for the whole globe was generated by using the same methodology, and therefore it was decided to call the new version 3.0.
After publication of version 3.0, a methodology was developed to assess the quality of the map. The results of this quality assessment was published in Siebert et al. (2005). At that time it was also decided to change the name from "Global Map of Irrigated Areas" to "Global Map of Irrigation Areas". The reason is that the data on the map refer to the physical "area equipped for irrigation" according to the AQUASTAT definition. The expression "irrigated area" could create confusion since it is often used to define the "actually irrigated area" (the part of the area equipped for irrigation that is actually irrigated) or the "harvested irrigated cropped area" (which would take into account double-cropping).
In 2006, FAO and the University of Frankfurt presented a pre-release of version 4.0 at the International Workshop on Global Irrigated Area Mapping, as organised by the International Water Management Institute (IWMI) and held September 25-27 in Colombo, Sri Lanka. The full documentation of this map came available early 2007. In comparison to the pre-release of map version 4.0, small adjustments were made for the countries Cape Verde, Montenegro and Morocco. The current version of the map is therefore 4.0.1.
The output of the irrigation mapping project consist of a spatial database in a GIS-environment with different data-layers containing raw data on the locations of areas equipped for irrigation, and tools to convert these raw data into one data layer containing the amount of area equipped for irrigation per grid-cell. The data in the spatial database is organized in such a way that newly available data can be easily incorporated allowing quick updates of the global map. FAO and the University of Frankfurt maintain a tool to organize spatial data that can generate a global map with the most up to date data available at that moment. The maps with areas equipped for irrigation in different formats are products that are distributed to users through the Internet. The tools to generate the map are installed and available at both institutions but will not distributed.
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