Evaluation of actual and potential evapotranspiration in global products across Iran's sub basins

Document Type : Research Paper

Authors

Department of Water Science and Engineering, Arak University, Arak, Iran.

Abstract

The purpose of the research is to evaluate the global gridded products database for two variables of actual (ETa) and potential evapotranspiration (ETp) across Iran's sub-basins. For this purpose, the daily meteorological data were collected from 100 synoptic stations around the Iran during 1987-2019. The six products including TERRA, CRU, ERA5, GLEAM, GLDAS and MERRA with 0.5×0.5 spatial resolution were compared to ETa and ETp in 30 sub basins. To assess the performance of six popular ET products, across Iran's sub basins on monthly scale, the Mean Bias Error (MBE), Pearson correlation coefficient (R) and Normalized Root Mean Square Error (NRMSE), of each product were calculated at the basin scale. The results show that more accuracy of products in ETp in comparison with ETa especially in July, August and September months. The more underestimate of ETa was accrued in hyper arid sub basins (e.g. Baluchistan with MBE equal to -34 mm per month). While, the more underestimate of ETp products were achieved in humid basins e.g. Haraz sub basin with MBE= -13 mm per month. The high underestimate of ETp were calculated to GLEAM product in June, July and August with -100 mm month-1. The high correlation between ETa observed and reanalysis are belonging to TERRA and ERA5 in Winter and Spring-Summer period, respectively. Based on NRMSE, results indicated that more accuracy of ETp comparison with ETa products with 0.19 (Excellent and Good) and 0.89 (poorly weak), respectively.

Keywords

Main Subjects

Evaluation of actual and potential evapotranspiration in global products across Iran's subbasins

 

EXTENDED ABSTRACT

 

Introduction

Evapotranspiration (ET) is a critical variable in the hydrological cycle, and plays a key role in designing and operating irrigation systems. The main aim of the research is to evaluate actual and potential evapotranspiration (ETa and ETp) achieved from the global gridded products database across subbasins of Iran.

 

Material and Methods

Daily meteorological variables from 100 synoptic stations during 1987-2019 were used for this purpose. ETa and ETp were calculated based on measured data using complementary relationship (CR) and Penman equation as target actual and potential ET.  Thiessen polygon method was used to extend the monthly time series of ETa and ETp in synoptic stations to sub basin scale. The six global gridded products including TERRA, CRU, ERA5, GLEAM, GLDAS and MERRA with 0.5×0.5 spatial resolution were compared to ETa and ETp in 30 subbasins. The Mean Bias Error (MBE), Pearson correlation coefficient (R) and Normalized Root Mean Square Error (NRMSE) were used to assess the accuracy of these products, for each subbasins on monthly scale.

 

Results and Discussion

The results indicated that more accuracy of products in ETp in comparison to ETa specially in summer season (i.e. July, August and September). The CR equation generally overestimates ETa in the study regions in comparison to ETa achieved from products. The MBE of all products from May to September (Spring and summer) indicate underestimate of ETa equal to -23, -40, -47, -42 and -27 mm per month, respectively. The TERRA datasets show high deviation in ETa during inter annual scale with MBE varied from -57 to +18 mm month-1 in July and December, respectively. However, the more underestimate of ETp products were achieved in humid basins e.g. Haraz subbasin with MBE= -13 mm per month, the more underestimate of ETa was accrued in hyper arid subbasins (e.g. Baluchistan with MBE equal to -34 mm per month). Between studied products, the GLEAM shows the high underestimate of ETp specially in June, July and August with -100 mm month-1. The high correlation between observed ETa and reanalysis are belonging to TERRA and ERA5 in Winter and Spring-Summer period, respectively. Moreover, the correlation is more in northern subbasins e.g. Aras subbasin with R equal to 0.67 in all of products. Based on NRMSE, results indicated that more accuracy of ETa products in Spring and Summer months in humid sub basins. Overall, the ranking of these six products indicates their tendency to better performance of ETa products for Iran's subbasins (ERA5 > GLEAM > MERRA > GLDAS > TERRA). Besides, CRU and GLEAM are more accurate products for ETp variable in Iran's subbasins.

 

Conclusion

Generally, this study presents a comprehensive evaluation of popular available ET datasets over Iran's subbasins at monthly time scale. The findings of the research can provide comprehensive information beneficial to the researchers who want to apply scientific findings to select suitable product for ET variable in Iran's subbasins.

 

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Iranian Journal of Soil and Water Research
Volume 54, Issue 5
August 2023
Pages 789-810

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