Calibrating Priestley-Taylor coefficient to estimate free water surface evaporation (Case Study: Mahabad Dam Reservoir)

Document Type : Research Article


1 MSc. Student, Department of Environmental Engineering, College of Environmental , West Tehran Branch, Islamic Azad University, Tehran, Iran

2 Associate Professor, Water Sciences and Engineering Department, Faculty of Engineering and Technology, Imam Khomeini International University, Qazvin, Iran

3 Associate Professor, Environment and Energy Department, Science and Research Branch, Islamic Azad University, Tehran, Iran


Evapotranspiration is one of the important components of basin water balance which cannot be measured directly at the basin scale. Therefore, it is inevitably estimated through indirect methods. In this regard, the Advection Aridity model, one of the widely used models of complementary relationship, has attracted lots of attentions. Due to the existence of Priestley-Taylor equation in the Advection Aridity model, it is required to calibrate Priestley-Taylor coefficient to increase the accuracy of the model. The current research aims at calibrating Priestley-Taylor coefficient in estimation of potential evaporation through Penman method to apply it in the Advection Aridity model in the studied area of Mahabad dam reservoir, Iran. The required data were collected for a period of 26 years (1986-2012) from Mahabad 1st order meteorological station which is located a short distance from Mahabad reservoir. The results showed that Priestley-Taylor coefficient undergoes monthly changes during a year and decreases during the warm months of the year. Therefore, it is better to use its monthly values in calculations. In the area under study, its minimum and maximum averages were 1.01 and 1.68, respectively. Moreover, the long term average of this coefficient during a period of 26 years has been calculated to be 1.25.


Main Subjects


    1. Valizadeh Kamran Kh, Jahanbakhsh S, Zahedi M, Rezaee Banafsheh M. Actual evapotranspiration and its relation to land use analysis in GIS case study Meshkinshar city. Journal of Geographic Space. 2012; 37: 39-54 [Persian].
    2. Poormohamadi S, Dastourani MT, Cheraghi SAM, Mokhtari MH, Rahimian MH. Evaluation and estimation of water balance components in dry areas by using remote sensing and GIS (Case Study: Yazd Manshad watershed). Journal of Water and Wastewater. 2011; 22(3): 99-108.
    3. Bouchet RJ. Evapotranspiration Reelle et Potentielle Signification Climatique. International Assosciaton of Hydrological Sciences. 1963; 62: 134-142.
    4. Brutsaert W, Stricker H. An advection aridity approach to estimate actual regional evapotranspiration.Water Resources Research. 1979; 15(2): 443-449.
    5. Priestley CHB, Taylor RJ. On the assessment of surface heat flux and evaporation using large-scale parameters. Monthly Weather Review. 1972; 100(2): 81-92.
    6. Arasteh PD, Tajrishy M. Calibrating Priestley Taylor model to estimate open water evaporation under regional advection using volume balance method case study Chahnimeh reservoir Iran. Journal of Applied Sciences. 2008; 8(22): 4097-4104.
    7. Eichinger WE, Parlange MB, Strickler H. On the concept of equilibrium evaporation and the value of the Priestley Taylor coefficient. Water Resources Research.1996; 32(1): 161-164.
    8. Lhomme JP. A theoretical basis for the Priestley Taylor coefficient. Boundary Layer Meteorology. 1997; 82(2): 179–191.
    9. Castellvi F, Stockle CO, Perez PJ, Ibanez M. Comparison of methods for applying the Priestley Taylor equation at a regional scale. Hydrological Process. 2001; 15(9): 1609–1620.
    10. Pereira AR. The Priestley Taylor parameter and the decoupling factor for estimating reference crop evapotranspiration. Agricultural and Forest Meteorology. 2004; 125(3-4): 305–313.
    11. Fisher JB, De Biase TA, Qi Y, Mu M, Goldstein AH. Evapotranspiration models compared on a Sierra Nevada forest ecosystem. Environmental Modeling Software. 2005; 20(6): 783-796.
    12. Komatsu H. Forest categorization according to dry canopy evaporation rates in the growing season: comparison of the Priestley Taylor coefficient values from various observation sites. Hydrological Processes. 2005; 19(19): 3873-3896.
    13. Allen RG, Pereira LS, Raes D, Smith M. Crop evapotranspiration guidelines for computing crop water requirements. Irrigation and Drainage FAO56. Rome. FAO. 1998.
    15. Ulgen K, Hepbasli A. Solar radiation models Part 1: a review. Energy Sources. 2004; 26(5): 507-520.
    16. Kamali GH, Moradi I. Solar radiation: fundamental and applications in agriculture and renewable energy. Atmospheric & Meteorological Research Center (ASMERC) Tehran. 2004 [Persian].
    17. Penman HL. Natural evaporation from open water, bare soil and grass. Proceedings of the Royal Society of London Series A –Mathematical and Physical Sciences. 1948; 193: 193- 120.
    18. Valiantzas J. Simplified versions for the Penman evaporation equation using routine weather data. Journal of Hydrology. 2006; 331(3-4): 690-702.
    19. Szilagyi J, Parlange MB, Katul GG. Assessment of the Priestley-Taylor parameter value from ERA-Interim global reanalysis data. Journal of Hydrology and Environment Research. 2014; 2(1): 1-7.
    20. Vourlitis GL, Hayashi M, De Nogueira SJ, Caseiro FT, Campelo JH. Seasonal variations in the evapotranspiration of a transitional tropical forest of Mato Grosso Brazil. Water Resources Research. 2002; 38(6): 30-1-30-11.
    21. De Bruin HAR, Keijman JQ. The Priestley-Taylor evaporation model applied to a large shallow lake in the Netherlands. Journal of Applied Meteorology. 1979; 18(7): 898-903.



Volume 4, Issue 3
September 2017
Pages 803-815
  • Receive Date: 08 February 2017
  • Revise Date: 28 April 2017
  • Accept Date: 14 April 2017
  • First Publish Date: 23 September 2017