Abstract
Estimating daily evapotranspiration is challenging when ground observation data are not available or scarce. Remote sensing can be used to estimate the meteorological data necessary for calculating reference evapotranspiration ETₒ. Here, we assessed the accuracy of daily ETₒ estimates derived from remote sensing (ETₒ-RS) compared with those derived from four ground-based stations (ETₒ-G) in Kurdistan (Iraq) over the period 2010–2014. Near surface air temperature, relative humidity and cloud cover fraction were derived from the Atmospheric Infrared Sounder/Advanced Microwave Sounding Unit (AIRS/AMSU), and wind speed at 10 m height from MERRA (Modern-Era Retrospective Analysis for Research and Application). Four methods were used to estimate ETₒ: Hargreaves–Samani (HS), Jensen–Haise (JH), McGuinness–Bordne (MB) and the FAO Penman Monteith equation (PM). ETₒ-G (PM) was adopted as the main benchmark. HS underestimated ETₒ by 2%–3% (R2 = 0.86 to 0.90; RMSE = 0.95 to 1.2 mm day−1 at different stations). JH and MB overestimated ETₒ by 8% to 40% (R2= 0.85 to 0.92; RMSE from 1.18 to 2.18 mm day−1). The annual average values of ETₒ estimated using RS data and ground-based data were similar to one another reflecting low bias in daily estimates. They ranged between 1153 and 1893 mm year−1 for ETₒ-G and between 1176 and 1859 mm year−1 for ETₒ-RS for the different stations. Our results suggest that ETₒ-RS (HS) can yield accurate and unbiased ETₒ estimates for semi-arid regions which can be usefully employed in water resources management.
Highlights
Evapotranspiration (ET) is one of the main components of the hydrological cycle
Energy is required to convert liquid water to the vapour state. Most of this energy comes from absorbed radiation which depends on latitude, season, cloud cover, air temperature and surface albedo
Comparison between Meteorological Variables Estimated from Remote Sensing with Station Data
Summary
Its quantification is essential for water resource management [1] It is arguably the most difficult process to measure, especially in arid and semi-arid areas where losses of water tend to be spatially and temporally highly variable [2,3]. Evapotranspiration (ET) consists of two main component processes: evaporation and transpiration [4,5]. Energy is required to convert liquid water to the vapour state Most of this energy comes from absorbed radiation which depends (inter alia) on latitude, season, cloud cover, air temperature and surface albedo (the fraction of solar shortwave radiation reflected from the earth back into space, which is affected by surface conditions and soil moisture [4,6]). Transpiration (T) occurs when water absorbed by plant roots is transferred to the leaves via the vascular system and returned to the atmosphere
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