Document Type : Research Article
Authors
1 Drainage and Irrigation Engineering, University of Shahre kord, Shahre kord
2 shahrekord university
Abstract
Introduction Management of agricultural practices plays a vital role in reducing the use of limited water resources in arid and semi-arid regions which could result in their sustainability. Proper management of these resources requires accurate information of component of the water budget. Recently, due to the problems of water deficit and drought in most parts of the country, evaporation of reservoirs of dams, lakes and other water bodies as one of the water losses has been considered and therefore, a more accurate estimation of evaporation rate from these water bodies And its reduction strategies have become very important. Evaporation which is the most important water output from terminal lakes plays a significant role in the lakes water balance. It can also vary chemical compositions of lakes. Conventional techniques of Evaporation Estimation likely entail substantial observation errors during bad weather and other conditions. These methods, therefore, cannot represent large-scale terrestrial Evaporation. Remote sensing methods for calculating evaporation are used. However, remote sensing data combined with some meteorological data provide a means to estimate regional Evaporation, given the advances in remote sensing technology. Some land surface variables, such as surface albedo, surface emissivity, and land surface temperature, can be estimated directly by remote sensing data. Then Evaporation can be estimated by a set of equations hierarchically, which converts spectral radiances derived from satellites or airplanes images. One of the models based on remotely sensed data is the Surface Energy Balance Algorithm for Land (SEBAL) model, in which the land surface temperature, albedo, emissivity, and normalized difference vegetation index (NDVI) are of significance to estimating Evaporation.
Materials and Methods In the present research, Zayandeh Rood dam reservoir and its lake with area of 54 square kilometer were selected and to estimate the evaporation from this reservoir, various empirical methods including Mayer, Marciano, Shahtin, Henfer, Ivanov, Tikhomirov and USBR were used. Also Surface Energy Balance Algorithm for Land (SEBAL) was implemented on 8 satellite images of Landsat 8 from June to September 2017. For this purpose, the main components of the energy balance equation, including net radiation flux, soil heat flux and sensible heat flux to the air for each image, have been calculated and the instantaneous evapotranspiration flux for each pixel is estimated as the residual energy balance equation. To improve the non-dependency on ground data, a general equation was therefore used. The Net Radiation is the electromagnetic balance of all incoming and outgoing fluxes reaching and leaving a flat surface. The amount of shortwave radiation (RS↓) that remains available at the surface is a function of the surface albedo (α). Surface albedo is a reflection coefficient defined as the ratio of the reflected radiant flux to the incident radiant flux over the solar spectrum. It was calculated using satellite image information on spectral radiance for each satellite and the incoming shortwave radiation (RS↓) was computed using the solar constant, the solar incidence angle, a relative earth-sun distance, and a computed atmospheric transmissivity. The incoming longwave radiation (RL↓) was computed using a modified Stefan-Boltzmann equation with atmospheric transmissivity and a selected surface reference temperature. Outgoing longwave radiation (RL↑) was computed using the Stefan-Boltzmann equation with a calculated surface emissivity and surface temperature. Surface temperatures were computed from satellite image information on thermal radiance. The surface emissivity is the ratio of the actual radiation emitted by a surface to that emitted by a black body at the same surface temperature. Soil heat flux was empirically calculated using vegetation indices, surface temperature, and surface albedo. Sensible heat flux was computed using wind speed observations, estimated surface roughness, and surface to air temperature differences. Sensible heat flux is the part of internal energy of a substance that is proportional to the substance’s temperature.
Results and Discussion Accordingly, the SEBAL model in the study area has the maximum and minimum daily evapotranspiration in the pictures of June 25 and September 11, equal to 14.13 and 10.4 mm/day. The results evaluate with the corresponding pan evaporate measurements on reservoir bank. The results showed that none of the empirical methods including Mayer, Marciano, Shahtin, Henfer, Ivanov, Tikhomirov and USBR have been able to have acceptable correlation with the reference method. In contrast, SEBAL method in addition to display spatial distribution of evaporation in the reservoir has an R-square coefficient of 0.88, RMSE and MAE with 0.28 and 0.31 respectively, which shows high accuracy of the results of modeling rather than empirical methods. Also according to the high error percent of empirical methods, it would be necessary to calibrate coefficients and parameters relative to different climatic conditions. Alternatively, this algorithm can be used to replace time-consuming and costly methods of calculating evapotranspiration at different surfaces.
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