Extracting A-L Relationship for Urmia Lake, Iran Using MODIS NDVI/NDWI Indices
A technique to extract volume-area-elevation relationships was developed using MODIS–Terra images in the absence of accurate bathymetric data and was tested over the Urmia Lake, Iran, which is facing a very great degree of water shortage difficulty due to remarkably water decline over the past 20 years. Lake areas were estimated using satellite data and two simple image computation methods: Normalized Difference Vegetation Index (NDVI) and Normalized Difference Water Index (NDWI). The area computed from the imageries was compared with those retrieved from satellite images. The area-elevation relationships were created by fitting a third order polynomial trend line to each dataset. Results revealed that NDVI A-L relationship provides slightly a better accuracy (R2=0.97) than the NDWI A-L relationship (R2=0.96). A comparison between long-term variation of lake area provided by the reference A-L equation and NDVI A-L equation showed that radar altimetry information shows more considerable inaccuracy when the lake becomes drier and the reference A-L equation overestimates the lake area in comparison with the NDVI A-L equation. This weakness can be improved by combination with MODIS images when the lake becomes drier. The V-A equation of Urmia Lake was developed using NDVI area estimations and computed volumes of the lake by the reference V-L equation at concurrent dataset of the lake level. Our analysis shows that to restore Urmia Lake to a level to maintain ecological benefits, 12500 MCM of water is required that this is in fair agreement with those obtained by the reference V-L equation.