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  International Water Management Institute (IWMI)
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Frequently Asked Questions (FAQ)  
 
   
   
1. There are too many acronyms!  
Answer: This goes with remote sensing territory. But do not worry. In the documentation look for acronyms and abbreviations and your problems will be solved!.  
 
 
   
2. Why maps and images? What is the difference?  
Answer: There is a huge difference between maps and images. Maps are output products that are ready to go. Images are finished products as well, but offer the flexibility to manipulate. You can for example, change class names, colours, even combine classes, and perform additional analysis using images.  
 
 
   
3. What formats are images and maps?  
Answer: Most images are in ERDAS Imagine (.img) format. Some of the mega-file primary datasets are in ERMapper (.ers) formats. ERDAS and ERMapper are two of the leading image processing software. Maps are either in JPEG or TIFF formats.  
 
 
   
4. Is this the first Global Irrigated Area Map?  
Answer: This is the first satellite sensor derived Global irrigated area map (GIAM). There is, however, another Global irrigated area map based on National statistics produced by FAO/Kassel University.  
 
 
   
5. What are the unique features of IWMI’s Global Irrigated Area Map (GIAM)?  
Answer: First and foremost, IWMI’s GIAM is NOT just a map. It has a product-line that is made accessible through the web-link (http://www.iwmigiam.org). There are several other very unique features in IWMI’s GIAM. Please refer to conclusions and abstract in the documentation of GIAM for details and discussions on this issue.  
 
 
   
6. Are the data and products free?  
Answer: Of course, yes!.  
 
 
   
7. What is the scale of IWMI’s Global Irrigated Area Map (GIAM)?  
Answer: There are wide array of scales or pixel resolutions used and they vary between 100-m to 10-km- even though it is realistic to say the scale is between 1-10km. The overwhelming proportion of the Global data used in the study is 10-km AVHRR. The 1-km SPOT vegetation data has been used for the entire World as well. The secondary data, which covered the entire World, of elevation and forest cover were in 1-km, rainfall at 50-km, and temperature at 10-km. The 100-m JERS SAR data was used only for the rainforests of Africa and South America.  
 
   
8. Is there documentation?  
Answer: Yes, ofcouse!. Please look into documentation. The GIAM results are published as IWMI research report. The GIAM methods are published as IWMI working paper. Both the results and the methods are published in the GIAM web portal (http://www.iwmigiam.org).  
 
 
   
9. What are Irrigated Areas?  
Answer: When crop water requirements are met by artificial means, we say the area is irrigated. Irrigation is practiced in several forms such as gravity flow from reservoirs, ground water pumping, sprinkler, drip, and even manual watering.

Naturally, crop water requirements are met by rainfall.
 
 
 
   
10. What is supplemental irrigation?  
Answer: When rainfed agriculture is supported by irrigation to ensure crop water requirements are met, then we call it supplemental irrigation. When there is enough rainfall to meet the crop water requirements, then no irrigation will be provided. During rainfall deficit periods, irrigation comes in.  
 
 
   
11. Why different classes of irrigation?  
Answer: The different classes of irrigation imply differences in factors such as cropping intensity (e.g., single crop, double crop), crop calendar (e.g., main cropping season starting in June or in October), crop types (e.g., rice dominant), or agro climatology (e.g., dry land irrigation, humid land irrigation). Each irrigated areas have a spectral characteristics that are distinctive and tell a story about the class.  
 
   
12. Why estimate areas for different season?  
Answer: When a particular area is mapped as irrigated, the question that needs to be asked is whether the crops is for a single season, two seasons, or even 3 or 4 seasons. If a particular area has 3 crops a year and another just 1 crop a year, there is a huge difference in crop water demand and the total area irrigated in a given year.

The total annualized irrigated areas of the World are 480 million hectares (or Mha). Globally, the total area available for irrigation is 401 Mha (nearest equivalent of FAOs equipped area for irrigation). Annualized area takes into consideration irrigated areas during different seasons over same areas within a given year. Of the total annualized area of 480 Mha, a total of 75 percent (375 million hectares) of all irrigated areas of the world is in Asia, followed by Europe with 8 percent North America with 7 percent, South America 4 percent, Africa 2 percent, and Australia 2 percent. The irrigated areas spread across the season are: (a) 263 Mha for season 1, (b) 176 Mha for season 2, and (c) 41 Mha for continuous.
 
 
 
   
13. Why differences in area estimate for different seasons?  
Answer: Some lands have continuous cropping, some only one crop, and some 2 or more crops. As a result the standing area under irrigation will vary throughout the year and hence the water requirements of crops and evapotranspiration from crops and agricultural lands.  
 
 
   
14. What is sub-pixel decomposition technique (SP-DCT)?  
Answer: Suppose we label a pixel or a group of pixels as “irrigated”. There is a tendency to think that the entire pixel area is irrigated. So, the full pixel area (FPA) is taken as irrigated area. But in reality, the only a certain proportion of the pixel is actually irrigated unless it is one of those rare “pure” pixel.

So a pixel is de-composed to determine what percent of the pixel is actually irrigated and this can vary between very low proportions to a 100% for pure pixels. The technique that deals with de-composition of a pixel and assigning the sub-pixel area (SPA) or the actual area is called SP-DCT. Please see area story for further details.

The SP-DT is one of the three methods of sub-pixel area calculations.
 
 
 
   
15. What are spectral matching techniques (SMTs)?  
Answer: Matching the spectra of a class with spectra of a “known or target” class. The target spectra can be obtained from ideal locations (e.g., areas where we absolutely are sure it is irrigated) . In absence of ideal locations, we can group spectra of many classes that have nearly same spectral characteristics in time-series data. Such comparisons are made using quantitative or qualitative approaches as explained in detail in SMT methods.  
 
 
   
16. What are the main differences between GIAM, GMRCA, GMLULC and Generic IWMI 951 class maps?  
Answer: The GIAM is a global map of irrigated area. It has 28 classes- 1-10 are surface water and 11-28 are groundwater\conjunctive use. Since the conjunctive use classes are overwhelmingly (about 90 percent or more) ground water, they can be clubbed under groundwater.

The GMRCA is a global map of Rainfed Cropland Areas. Like GIAM these are agricultural lands, but purely rainfall dependant.

The GMLULC is a global map of land use/land cover areas. This has irrigated, rainfed, and other (e.g., forests, rangelands) LULC classes.

The Generic IWMI map of 951 classes is the most disaggregated map consisting of detailed LULC classes. This map was used to produce GIAM, GMRCA and GMLULCA
 
 
 
17. What is a mega-file?  
Answer: When hundreds or even thousands of data layers are composed into a single file, it is called a mega-file. The mega-files are ideal for time-series data analysis where images acquired on different dates are all composed into single file called mega-file. The mega-files enable study of time history of a pixel or any location with a click of a mouse. The advantages of mega-files include ease of handling and analyzing 100s or 1000s of images over time periods.  
 
   
18. What do terms TAAI, AIA, IAF, SPDT etc., means?  
Answer:
AIA - Annualized irrigated area. Does consider intensity. The AIA is defined as the sum of the irrigated areas from different seasons in a 12-month period. Typically, it is sum of irrigated areas during the season 1, season 2, and continuous and\or season 3.
Boundaries - The source of administrative boundaries for the Global map is ESRI. For the Individual countries, the boundaries provided by the country.
FPA - Full Pixel Aarea.
GIA - Gross irrigated area = about the same as annualized irrigated areas.
GIAM 10 Km - Actually, GIAM 10km product uses 10 km time-series AVHRR, 1-km SPOT vegetation, and a multitude of secondary data and ground truth data. In certain places of the World, it also uses finer resolution satellite sensor data such as JERS SAR 150 m data for the African and Amazonian rainforests. Since AVHRR 10km is the overwhelming proportion of the data used, the product is referred to as GIAM 10 km.
GIAM 500 M - Global Irrigated Mapping at 500m using MODIS 500m, 7-band, every 8-day, time-series for the years 2001-03.
GIAM 30 M - GIAM 30 M using LANDSAT ETM+ data for nominal year for 2000 and secondary datasets. In addition, GIAM 30m product also uses SRTM 90 m data and MODIS time-series for 1 year.
IAF - irrigated area fraction. The IAF is required to compute SPA from FPA.
IAF-GEE - IAF from Google earth estimate.
IAF-HRI - IAF high-resolution imagery.
IAF-SPDT - IAF sub-pixel de-composition technique.
IIA - Intensity of irrigated areas = refers to the utilization of given irrigated area in a 12- month period. So, if 1 hectare has crop only once in 12-month period, the intensity is 100 percent. If it has crops during 2 seasons (each time for the entire area), then 2hectare/1 hectare = 200 percent intensity.
NIA - Net irrigated area = about the same as TAAI.
SPA - sub-pixel area. This is defined as FPA * IAF
TAAC - Total area available croplans (TAAC). TAAC is the sum of the available areas from irrigated (TAAI) plus rainfed (TAAR).
TAAI - Total Area Available for Irrigation. Does not consider intensity. Defined as the areas irrigated at any given point of time plus the areas left fallow at the same point of time. This is the nearest equivalent to FAO’s "areas equipped for irrigation" and the "net irrigated areas" (NIA) reported in the national statistics. TAAI is closer to NIAcreated.
TAAR - Total area available for rainfed-crops (TAAR). TAAR is the area available for rainfed crop at any given point of time. It includes area actually cultivated and dependant on rainfall plus area that is left fallow.
Zero - It indicates no data.
 
 
   
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