Altitudinal and Temporal Evapotranspiration Dynamics via Vegetation Index-based Modelling over an Andean Páramo Ecosystem of Southern Ecuador

Abstract

The páramo ecosystem is considered the main water supplier for the Andean region. Additionally, in this type of ecosystem, ET is the major source of water loss for the hydrological cycle. However, given the lack of in situ ET measurements in such remote ecosystems, ET dynamics on the hydrological regulation still remain largely unexplored. Therefore, this study, by making use of remote sensing images, assessed the altitudinal and temporal dynamics of actual evapotranspiration using remote sensing images and a vegetation index based on a crop coefficient model in a high altitudinal páramo catchment. It contributes evaluating three vegetation indexes (VI) - Enhanced vegetation index (EVI), Normalized Difference Vegetation Index (NDVI) and Soil-Adjusted Vegetation Index (SAVI) – that were retrieved from clear-sky Landsat satellite imagery. Remote sensing imagery and ground-level meteorological data for a 10 months period (2013 – 2014) were used to create ET maps from each VI. Daily ET time series were obtained with cubic spline interpolation, and then aggregated to monthly time steps in order to validate through the water balance of the study area. Results reveal that EVI- based ET (ETEVI) outperformed the other VI-based ET. ET estimates exhibited a strong dependence on precipitation and land cover distribution. ET variability showed a good fitting with the water balance for the humid months while it was weak in the less-humid months. The percentage error in the less-humid period was two times greater (50%) respect to the humid months (25%). Regarding the altitudinal variability and land cover, ET was higher in the lowlands where evergreen vegetation prevails (65%), while in the highlands ET decreased significantly where tussock grasslands dominate (77%). The study also demonstrated that a fine calibration is needed to obtain reliable ET estimates for the less-humid months. Additionally, it contributes to the evaluation of three VI- based ET, which in turn, contributes to the body of knowledge of ecohydrology of the highland tropical Andes, which is potentially exposed to the negative effects of land use change, agro-industrial activities and global warming.