Evaluation of vegetation and evapotranspiration changes in Iran using satellite data and ground measurements

Abstract

Evapotranspiration (ET) is a critical component of the water cycle on a global scale. Changes in leaf area index (LAI) influence ET changes. Satellite products are among the effective methods of estimating ET and LAI at local, regional, and global scales. Therefore, the purpose of our study was to evaluate three global remote sensing products of ET and two products of LAI and investigate their simultaneous changes in Iran over the 2000 to 2015 period. Selected ET products include Global Land Evaporation Amsterdam Model (GLEAM), Global Land Data Assimilation System (GLDAS2.0), and Moderate Resolution Imaging Spectroradiometer ET (MODIS ET) products. LAI products include MODIS LAI and Global Land Surface Satellite (GLASS) LAI products. Validation using the 23 selected evaporation stations showed that among the three ET products MODIS has the best performance. Three ET products consistently show an increasing trend in annual ET over a large part of Iran. The mean annual trend values obtained from GLEAM3.0a, GLDAS, and MODIS ET products are 2.022, 1.359, and 0.429 mm/year, respectively. Among LAI products, GLASS had the highest correlation with the selected ET product (0.78). On average, the trend of intra-annual changes of LAI across Iran did not show a significant increasing or decreasing trend. Moreover, in areas where no significant change was observed in LAI, ET changes were unpredictable and depended on other factors. Overall, the results showed a relatively strong correlation between the selected ET and vegetation satellite products in the study area.