MAPPING EVAPOTRANSPIRATION FOR DIFFERENT LANDUSE AND LANDCOVER IN THE SAHELIAN REGION OF NIGERIA USING LANDSAT 8 DATASETS

Abstract

The study was conducted to estimate evapotranspiration (ET) for the Lake Chad basin Area from Surface Energy Balance Algorithm for Land (SEBAL) Level 1 Flat model using Landsat-8 data.However, the specific objectives are to estimate the energy balance components and compute the different biophysical parameters required for the SEBAL model, estimate the daily evapotranspiration by SEBAL Algorithm and determine the relative contributions between Land surface temperature (LST), Leaf area index (LAI), Normalize difference vegetation index (NDVI), net surface radiation, sensible heat flux, soil heat flux, Soil adjusted vegetation index (SAVI) to evapotranspiration. SEBAL is a remote sensing-based crop evapotranspiration (ET) model with minimal requirement for ground-based weather data.The study area is Lake Chad basin northwestern Nigeria. It lies between latitude 120 20’N and 140 00’N and longitude 130 10’E and 140 10’E. Landsat-8 imagery with zero percent cloud cover and weather data were acquired from the United States Geological Survey (USGS) and the Nigeria Meteorological Agency (NIMET) respectively. Lake Chad vector map was used to subset the pre-processed Landsat-8 data. Lake Chad basin ET was estimated for 12th April, 2014 using the SEBAL model. The Digital Number (DN) values of the bands were used to estimate the radiance, reflectance, net radiation, soil heat flux and latent heat flux. Other meteorological data like the wind speed, dew point temperature and the maximum temperature were used for the processing. The thermal range of bands were used to estimate temperature related parameters such as sensible heat flux an essential component of energy balance equation. After the estimation of all the energy balance components, the evaporative fraction was computed and this was used to estimate the daily rate of evapotranspiration in the SEBAL algorithm. The study showed that the NDVI varied from -0.20 to 0.778, the net surface radiation varied from 304.906W/m2 to 1468.31W/m2, the sensible heat flux varied from 24.42W/m2 to 250.917W/m2, the SAVI varied from -0.15 to 0.737, the longwave radiation varied from 26.8505W/m2 to 655.504W/m2, the surface albedo varied from 0 to 0.3, the LST varied from 290.42K to 332.14Kand the daily ET varied from 1.42733 and 6.30062 mm/day over the study area. . Estimates of remotely sensed ETusing SEBAL are presented and the spatial variations of ET in terms of different landuse landcover (LULC) types examined.The study revealed that regions with high NDVI values, high net surface radiation, low sensible heat flux, high SAVI, low longwave radiation, low albedo and low surface temperature all have high evapotranspiration values and regions with low NDVI values, low surface radiation, high sensible heat flux, low SAVI, high longwave radiation, high albedo and high surface temperature all have low values of evapotranspiration Overall, wetlands and forests have a higher rate of ET than grass and agricultural lands, and thebare surfaces have the lowest ET. The study concluded that there exist a relationship between the biophysical parameters (NDVI, surface radiation, sensible heat flux, SAVI, longwave radiation, albedo, surface temperature) and evapotranspiration.