Water resources management in agricultural watersheds using remote sensing

The goal of this study is a contribution to the water resources management through the detailed study of evapotranspiration estimated by calculating the surface energy balance. The surface energy balance is examined in a new way with the use of satellite remote sensing techniques deriving, finally, crop coefficients values (Kc) giving much emphasis to the prevailing local conditions. During this thesis satellite-based energy balance for Mapping EvapoTRanspiration with Internalized Calibration (METRIC) methodology is successfully modified for the region of Thessaly, Greece. At the same time, a new land use map is produced, necessary for the next steps of the described water balance methodologies. Furthermore, field measurements (spectral measurements) for the year 2012 for selected crops at the Lake Karla basin were recorded. Hundreds of surface reflectivity values are filtered and then converted according to the specific wavelengths of Landsat 7 bands. This is achieved by applying the method of interpolation in order to yield values in steps of 1 nm. These values are then filtered according to the relative spectral profiles provided by the satellite manufacturer. Finally, using the Relative Spectral Response (RSR) of Landsat TM/ETM+, and applying interpolation, surface reflectivity values equivalent to channels 1, 2, 3 and 4 of Landsat TM/ETM+ for selected crops of Lake Karla basin, namely cotton, wheat, maize, alfalfa and sugar beet are generated. As a result, vegetation indices can be estimated with high accuracy without the need for atmospheric correction. It is therefore possible now to incorporate accurate values of vegetation indices into METRIC methodology to Lake Karla watershed and finally get separate Kc values for each crop with a spatial resolution of 30 m x 30 m. Kc values can be finally introduced into FAO CROPWAT model computing water needs for each crop. The next step is the application of specialized downscaling techniques, where fine spatial resolution from Landsat imagery, is combined with fine temporal resolution from AQUA/MODIS sensors, resulting to an "artificial" ETa and/or Kc image map with improved spatiotemporal characteristics. This methodology can be applied again for every crop as previously. The reason for doing this procedure is the need for Institutional Repository Library & Information Centre University of Thessaly 17/09/2020 03:20:00 EEST 54.70. 40.11 xxiv improvement of Landsat’s temporal resolution which is sixteen (16) days. The processing results show that it is possible to predict a 30 m x 30 m ETa Landsat image map for Lake Karla watershed simply applying a linear model (linear regression) derived from MODIS images with a 250 m x 250 m spatial resolution. The basic assumption of this methodology is that the fine-scale variability inside a MODIS pixel (250 m x 250 m) is assumed to be fixed between the times of the first and the second Landsat image acquisition. After that, an estimation of the required amount of irrigation water requirements using CROPWAT (FAO) model is generated, using specific land use. At last, a new model incorporating all the previous applied methodologies is established. The new model uses field measurements, downscaling techniques between Landsat and MODIS derived NDVI values and linear regression in order to finally produce daily crop coefficient values with 30 m x 30 m spatial resolution. Validation of the proposed methodologies which have been developed and applied at this study is not possible through reliable measurements. This happens both to evapotranspiration and crop water requirement measurements. The reason is the absence of lysimeters and/or a water consumption measurement system respectively. For this reason, an internal valiation is performed instead. Initially, values of actual evapotranspiration produced by METRIC methodology are checked against the theoretical Penman-Monteith methodology. The procedure is performed at the regions close to the available meteorological stations of the study area. Then, the proposed methodology is validated independently utilizing new Landsat 7 and Landsat 8 images not used initially at the proposed methodology, by comparing the predicted values when applying the proposed methodology with the computed values of evapotranspiration based on METRIC methodology. Validation is satisfactory for both cases. Finally, a sensitivity analysis of the assessment of crop irrigation requirements is performed. CROPWAT outputs are checked against the possible error of crop coefficients derived from the proposed methodology. The results are again ecouraging. Institutional Repository Library & Information Centre University of Thessaly 17/09/2020 03:20:00 EEST 54.70. 40.11 xxv ABSTRACT IN GREEK ΕΚΤΕΤΑΜΕΝΗ ΠΕΡΙΛΗΨΗ