Abstract
The goal of this paper was to develop and demonstrate practical methods forcomputing sub-pixel areas (SPAs) from coarse-resolution satellite sensor data. Themethods were tested and verified using: (a) global irrigated area map (GIAM) at 10-kmresolution based, primarily, on AVHRR data, and (b) irrigated area map for India at 500-mbased, primarily, on MODIS data. The sub-pixel irrigated areas (SPIAs) from coarse-resolution satellite sensor data were estimated by multiplying the full pixel irrigated areas(FPIAs) with irrigated area fractions (IAFs). Three methods were presented for IAFcomputation: (a) Google Earth Estimate (IAF-GEE); (b) High resolution imagery (IAF-HRI); and (c) Sub-pixel de-composition technique (IAF-SPDT). The IAF-GEE involvedthe use of "zoom-in-views" of sub-meter to 4-meter very high resolution imagery (VHRI)from Google Earth and helped determine total area available for irrigation (TAAI) or netirrigated areas that does not consider intensity or seasonality of irrigation. The IAF-HRI isa well known method that uses finer-resolution data to determine SPAs of the coarser-resolution imagery. The IAF-SPDT is a unique and innovative method wherein SPAs aredetermined based on the precise location of every pixel of a class in 2-dimensionalbrightness-greenness-wetness (BGW) feature-space plot of red band versus near-infraredband spectral reflectivity. The SPIAs computed using IAF-SPDT for the GIAM was within2 % of the SPIA computed using well known IAF-HRI. Further the fractions from the 2 methods were significantly correlated. The IAF-HRI and IAF-SPDT help to determine annualized or gross irrigated areas (AIA) that does consider intensity or seasonality (e.g., sum of areas from season 1, season 2, and continuous year-round crops). The national census based irrigated areas for the top 40 irrigated nations (which covers about 90% of global irrigation) was significantly better related (and had lesser uncertainties and errors) when compared to SPIAs than FPIAs derived using 10-km and 500-m data. The SPIAs were closer to actual areas whereas FPIAs grossly over-estimate areas. The research clearly demonstrated the value and the importance of sub-pixel areas as opposed to full pixel areas and presented 3 innovative methods for computing the same.
Highlights
Pixel size plays an important role in area computations, especially when coarser-resolution data are used
The sub-pixel irrigated areas (SPIAs) computed using irrigated area fractions (IAFs)-SPDT for the global irrigated area map (GIAM) was within 2 % of the SPIA computed using well known IAF-high resolution imagery (HRI)
A comparative study for china [1] in estimating the areas derived from Advanced Very High Resolution Radiometer (AVHRR) Version 2.0 International Geosphere-Biospere Programme (IGBP) DIScover [2] dataset showed that about half of the DIScover cropland pixels had less than 60 % fractional cropland cover within a pixel size of 1-km
Summary
Pixel size plays an important role in area computations, especially when coarser-resolution data are used. A comparative study for china [1] in estimating the areas derived from AVHRR Version 2.0 International Geosphere-Biospere Programme (IGBP) DIScover [2] dataset showed that about half of the DIScover cropland pixels had less than 60 % fractional cropland cover within a pixel size of 1-km. The pixel was named “irrigated” because it has certain percentage of area within the pixel which is irrigated- which can, typically, vary from a nominal 10 % to 100 %. It is, thereby, obvious that counting whole pixels can lead to over estimation of actual areas [3]. The implication of using FPAs in place where SPAs need to be reported is of significant importance in many applications such as water use calculations, food production estimates, and global scenario modeling
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
