Multi-site calibration and validation of SWAT with satellite-based evapotranspiration in a data sparse catchment in southwestern Nigeria

Abstract

Abstract. The main objective of this study was to calibrate and validate the eco-hydrological model Soil and Water Assessment Tool (SWAT) with satellite based actual evapotranspiration (AET) data (Global Land Evaporation Amsterdam Model (GLEAM_v3.0a) and Moderate Resolution Imaging Spectroradiometer Global Evaporation (MOD16) for the Ogun River Basin (20 292 km2) located in southwestern Nigeria. The novelty of the study is the use of freely available satellite derived AET data for calibration/validation of each of the SWAT delineated subbasins, thereby obtaining a better performing model at the local scale as well as at the whole watershed level. The Sequential Uncertainty Fitting technique (SUFI-2) in the SWAT-Calibration and Uncertainty Program was used for the sensitivity analysis, model calibration, validation, and uncertainty analysis. Three different structures of the SWAT model were used in which each model structure was a set-up of SWAT with a different potential evapotranspiration (PET) equation. The two global AET products (GLEAM_v3.0a and MOD16) were subsequently used to calibrate the SWAT simulated AET outputs from each model structure resulting in six calibration/validation procedures at a monthly time scale. The model performance for the three SWAT model structures was evaluated for each of the 53 subbasins through the six calibrations/validations, which enabled the best model structure with the highest performing AET product to be chosen. A verification of the simulated AET variable was carried out by: (i) comparing the simulated AET of the calibrated model to GLEAM_v3.0b AET, this is a product that has a different forcing data to version of GLEAM used for the calibration, and (ii) assessing the long-term average annual and average monthly water balances at the outlet of the watershed. Overall, the SWAT model structure composed of Hargreaves PET equation and calibrated using the GLEAM_v3.0a data performed well for the simulation of AET and provided a good level of confidence for using the SWAT model as a decision support tool. The 95% uncertainty of the SWAT simulated variable bracketed most of the satellite based AET data in each subbasin. The SWAT model also proved efficient in capturing the seasonal variability of the water balance components at the outlet of the watershed. This study demonstrated the potential to use remotely sensed evapotranspiration data for hydrological model calibration and validation in a sparsely gauged large river basin with reasonable accuracy.