Abstract
Evapotranspiration (ET) is an essential component of watershed hydrological cycle. Spatial-temporal variations analyses of evapotranspiration and potential evapotranspiration (PET) have remarkable theoretical and practical significances for understanding the interaction between climate changes and hydrological cycle and optimal allocation of water resources under global warming background. The MODIS-estimated ET agreed well with basin evapotranspiration from water balance principle methods in the study. The spatiotemporal variations results based on MOD16 ET data showed the following: (1) multiyear mean ET and PET were 464.2 mm and 1192.2 mm, and annual ET showed an upward trend at a rate of 3.48 mm/a, while PET decreased significantly at a rate of −8.18 mm/a. The annual ET trend showed a complemental relationship with PET; (2) at the seasonal scale, ET was highest in summer and least in winter, while PET was higher in spring and summer. The change of ET and PET in spring and summer had a great contribution to the annual variations; (3) ET and PET in the northern part were significantly stronger than those in the western and southern parts; (4) ET in cropland increased significantly, while PET decreased obviously in grass and forest; (5) changes of ET and PET were closely related to climatic factors. The rise in temperature caused the increase in ET and the decrease of wind speed contributed more to the decrease in PET. The results can provide a scientific basis for water resources planning and management.
Highlights
Evapotranspiration (ET) is a critical component of global hydrological cycle and energy balance, which consumes 60%–65% of precipitation [1]
Evaluations of MOD16 ET Data. ere are scarce meteorological stations of evapotranspiration measurement, which has brought about great inconvenience to evaluate MOD16 ET products using in situ ET observations from stations. erefore, observed runoff and satellite TRMM data are used to calculate annual and seasonal actual ET of the watershed (WBET) according to watershed water balance principle
Correlation coefficient R2 between WBET and MOD16 ET was 0.85, and root mean square error (RMSE) and BIAS were 64.62 mm and 2.01 mm, respectively. ese reasonable statistic indices demonstrated that MODIS data well captured the seasonal variability of WBET and had a good systematic consistence. is study used the pan evapotranspiration from 2001 to 2013 at Luqu station to estimate MOD16 potential evapotranspiration (PET) data, as shown in Figure 4. e annual results showed high consistency between pan evapotranspiration and PET estimated by MOD16
Summary
Evapotranspiration (ET) is a critical component of global hydrological cycle and energy balance, which consumes 60%–65% of precipitation [1] It is composed of evaporation from soil surface and plant transpiration and evaporation from water bodies and sublimation from snow and glacier surface [2]. ET can be measured by using land surface parameters (temperature, net land surface radiation, vegetation index, and soil moisture) at global scale, water balance, or crop growth models for crop ET [9]. These conventional methods cannot represent large-scale terrestrial evapotranspiration due to heterogeneity of land surface and complexity of hydrologic processes [10]. A variety of satellite-based products have provided valuable evapotranspiration data sources to research at different spatial scales, especially for regions with lack of and sparse observations [13, 14]
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