Interlinkages of NOAA/AVHRR derived integrated NDVI to seasonal precipitation and transpiration in dryland tropics

The NOAA/AVHRR derived Normalized Difference Vegetation Index (NDVI) offers immense potential to study dryland ecosystems. Its relation with rainfall in semi-arid regions is both sensitive and complex. The physical basis of this relation is vegetation, mainly grown due to soil moisture availability as the result of the seasonal rainfall, which intercepts photosynthetically active radiation (PAR) and hence directly influences the aggregate of NDVI. While the major part of rainfall is lost through direct evaporation from the soil surface, run-off and drainage, a small fraction of rain water which is used by crops for their growth and yield, is a key parameter linking rainfall with NDVI, especially in dryland tropics. The extent of non-utilized rain water depends not only on the local weather parameters but also on optimal land and water management practices along with the cropping pattern having efficient radiation use efficiencies. Although there have been several studies reporting the relation between AVHRR NDVI and rainfall in semi-arid regions the sensitivity of this relation in varying dryland conditions has rarely been analysed quantitatively. The present study aims to interpret seasonal AVHRR NDVI variations with the seasonal aggregate of rainfall and fraction of soil moisture used by vegetation in the six drought prone districts of Karnataka State, India representing very severe, severe and moderate drought conditions. Area averaged seasonal transpiration which has been theoretically linked to integrated NDVI (INDVI), has been realized up to a certain extent in the study areas. The study reveals a closer AVHRR NDVI relation with the water used by vegetation for its growth and yield than the precipitation in dryland ecosystems.