Satellites To Estimate Reference Evapotranspiration And Air Temperature

Peninsular rivers in India are heavily dependent on monsoons. Climate change is likely to worsen the situation as monsoon patterns change. Due to the enormous demand for water from various sectors, water resources have become limited and led to overexploitation. Precise estimation of reference evapotranspiration (ETo) plays a vital role in water resources management.

ETo has become the agrometeorological standard because of its extensive application in irrigation demands and scheduling, climatological and environmental impact assessment, and water resource management, with conflicting objectives. ETo is a climatic parameter and measures the evaporative demand of the atmosphere, independent of crop type, crop development, and management practices.

Owing to the difficulty in obtaining a direct measurement of ETo, the Food and Agricultural Organization (FAO) proposed the Penman-Monteith method (FAO56-PM). However, all climatic variables required in the FAO56-PM are difficult to obtain from meteorological stations. Therefore, researchers have developed alternative models, such as temperature and radiation type, which require fewer climatic variables (air temperature and radiation) and named them based on their requirements of climatic variables.

Traditional meteorological stations fail to provide climatic variables required in the alternative models at fine spatial and temporal resolutions because dense networks of them are costly to be installed and maintained in many developing countries. In this context, satellites play an important role in providing climatic variables at various spatio-temporal resolutions from the regional to the global scale.

Air temperature (Ta) is an indispensable parameter for ETo estimation. In this study, Ta was estimated using remote sensing-based temperature & vegetation index (TVX) and advanced statistical (ASA) approaches over the Cauvery River basin.  Land surface temperature (LST) and vegetation information are essential parameters in Ta estimation, and these were derived from a Moderate Resolution Imaging Spectroradiometer (MODIS) sensor. This sensor was launched in 2002 and was transported on the NASA (US National Aeronautics and Space Administration) polar-orbiting Aqua satellite. In addition, digital elevation data from the Shuttle Radar Topography Mission (SRTM) was also used. Estimated Ta was evaluated with the observed values of Automatic Weather Stations (AWS) installed by the Indian Space Research Organization (ISRO). Results indicated that ASA approach predicted better Ta than TVX approach.

Consequently, Ta is employed in temperature [Hargreaves-Samani, (HS), simple LST based equation (SLBE), and Penman-Monteith temperature (PMT)] and radiation (Makkink) type models to estimate ETo. Solar radiation needed in the Makkink model was obtained from the Indian geostationary satellite Kalpana-1, which was operational from 2002 to 2017. Furthermore, LST was utilized in place of Ta in ETo models to check its potency to estimate ETo. These alternative models were evaluated with the FAO56-PM ETo estimated using climatic variables information from Automatic Weather Stations (AWS) at point scale. Temperature and radiation type models with LST as input overestimated ETo, excepting SLBE. Statistical analysis implied that the PMT model with Ta as input performed better than the other ETo models for various land cover classes and for various climatic conditions in the Cauvery basin. Ta estimated using the ASA approach by minimizing errors between LST and Ta resulted in better performance of the ETo models with Ta as input.

Our study has demonstrated the practical applicability of satellite-based Ta and ETo estimation over an Indian river basin that had not been examined previously. This study will be valuable for selecting proper satellite-based ETo and Ta models for water resource management, irrigation scheduling, and climate change studies.

These findings are described in the article entitled Performance evaluation of satellite-based approaches for the estimation of daily air temperature and reference evapotranspiration, recently published in the Hydrological Sciences Journal. This work was carried out by H. R. Shwetha and D. Nagesh Kumar from the Department of Civil Engineering, Indian Institute of Science, Bangalore, India.