High resolution global grids of revised Priestley-Taylor and Hargreaves-Samani coefficients for assessing ASCE-standardized reference crop evapotranspiration and solar radiation

Other literature type English OPEN
Aschonitis, Vassilis G. ; Papamichail, Dimitris ; Demertzi, Kleoniki ; Colombani, Nicolo ; Mastrocicco, Micol ; Ghirardini, Andrea ; Castaldelli, Giuseppe ; Fano, Elisa Anna (2016)

The objective of the study is to provide high resolution global grids of revised annual coefficients for the Priestley-Taylor (P-T) and Hargreaves-Samani (H-S) evapotranspiration methods after calibration based on ASCE-standardized Penman-Monteith method (ASCE method includes two reference crops: short clipped grass and tall alfalfa). The analysis also includes the derivation of global grids of revised annual coefficients for solar radiation <i>R<sub>s</sub></i> estimations using the respective <i>R<sub>s</sub></i> formula of H-S. The analysis was based on global gridded climatic data of the period 1950&ndash;2000. The method for deriving annual coefficients of P-T and H-S methods was based on partial weighted averages (p.w.a.) of their mean monthly values, which eliminate the effect of monthly coefficients that occur during periods where <i>ET<sub>o</sub></i> and <i>R<sub>s</sub></i> fall below a specific threshold. Five resolution global grids (30 arc-sec, 2.5, 5, 10 arc-min and 0.5 deg) of annual coefficients for each method were developed. The new coefficients were validated based on data from 140 stations located at various climatic zones of USA and Australia with expanded observations up to 2016. Nine statistical criteria including Taylor diagrams were used in the validation procedure. The validation procedure for <i>ET<sub>o</sub></i> estimations of short reference crop showed that the P-T and H-S methods with the new revised coefficients outperformed in comparison to the typical methods reducing the <i>ET<sub>o</sub></i> <i>RMSE</i> of estimated values by 39&thinsp;% and 36&thinsp;%, respectively. The estimations of Rs using the H-S formula with revised coefficients reduced the <i>RMSE</i> by 30% in comparison to the typical H-S radiation formula (the given results are based on the finer resolution grid). All the statistical criteria indicated better performance of the revised coefficients of all resolutions versus the typical coefficients used in the original methods. Finally, a raster database was built consisting of: a) global maps of revised annual coefficients for the ETo methods of P-T and H-S for both reference crops and the <i>R<sub>s</sub></i> H-S formula, b) global maps which indicate the optimum locations for using the original P-T and H-S methods and their expected error based on reference values. The provision of the database aims to improve <i>ET<sub>o</sub></i> and <i>R<sub>s</sub></i> estimations which are used in hydrologic/climatic applications when climatic data are limited. The datasets produced in this study are archived in PANGAEA database (<a href="https://doi.pangaea.de/10.1594/PANGAEA.868808" target="_blank">doi:10.1594/PANGAEA.868808</a>, <a href="https://doi.org/10.1594/PANGAEA.868808" target="_blank">doi:10.1594/PANGAEA.868808</a>) and in ESRN-database (<a href="http://www.esrn-database.org"target="_blank">http://www.esrn-database.org</a> or <a href=" http://esrn-database.weebly.com/"target="_blank">http://esrn-database.weebly.com/</a>).
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