Abstract
Actual evapotranspiration (AET) can be studied and estimated using remote-sensing-based methods at multiple spatial and temporal scales. Reflectance and Land surface temperature are essential in these methods. However optical and thermal sensors fail to provide these data under overcast conditions and this creates gap in the AET product. Besides, there is a necessity of the AET method that requires less data and estimates AET with better accuracy. In this regard, AET was estimated for all-sky conditions using the vegetation coefficient (VI-Kv) method utilizing microwave, thermal, and optical data. Essential reference evapotranspiration (ET0) under cloudy conditions was estimated using LST-based Penman-Monteith temperature (PMT) and Hargreaves-Samani equations. Furthermore, LST predicted using the microwave polarization difference index (PLST) and LST of moderate resolution imaging spectroradiometer (MODIS) cloud product (MLST) were evaluated with in-situ air temperature (Ta) under cloudy sky conditions. Results revealed that the PLST correlated better with Ta than MLST with correlation coefficient (r) values of 0.71 and 0.81 for day and night times, respectively. Hence, PLST-based solar radiation (Rs) estimation yielded better accuracy with observed Rs with r and root mean square error values of 0.864 and 0.07 for Berambadi station under cloudy conditions, respectively. PMT-based ET0 values corresponded well with the observed ET0 under cloudy sky condition during this study. In addition, AET estimated using the VI-Kv method was compared with the simple two-source energy balance (TSEB) method under clear sky conditions. It was found that the improved VI-Kv method performed better than the TSEB method and could also fairly estimate AET even under cloudy sky conditions.
Highlights
W ATER scarcity is a major problem facing a number of nations in the present time
PLST was found to be more accurate than MLST with correlation coefficient (r) values of 0.71 and 0.82 for day and night times, respectively
In order to check the potentiality of the VI-Kv method, Actual ET (AET) values obtained using this method have been compared with the AET values estimated using widely used and accepted two-source energy balance (TSEB) model for clear sky conditions
Summary
W ATER scarcity is a major problem facing a number of nations in the present time. This is attributed to Manuscript received April 10, 2019; revised November 15, 2019 and February 25, 2020; accepted April 9, 2020. In many areas of the world, where rainfall is too low or insufficient to meet the water demand of the crops, irrigation is a significant component of agricultural (cropping pattern) planning. Actual ET (AET) is measured using ground-based measurements, such as lysimeters, eddy covariance, and Bowen ratio at point scale with high temporal resolution These are difficult to be extended to obtain the spatial distribution of AET at the basin scale, and these involve high installation and maintenance costs. Satellite images provide required data for the estimation of the spatial distribution of AET at fine spatial resolution using satellite-based physical, empirical, and semiempirical models at the basin or regional to global scales
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