Advanced Materials Research Vols. 610-613 (2013) pp 3013-3016 © (2013) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMR.610-613.3013
Online: 2012-12-13
Characteristics of Reference Evapotranspiration of Wetland Based on Penman-Monteith and Hargreaves Methods Xianghu Li1,2,a, Qi Zhang2,b, Yunliang Li2 1
State Key Laboratory of Hydrology-Water Resources and Hydraulic Engineering, Hohai University, Nanjing, 210098, China 2 State Key Laboratory of Lake Science and Environment, Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, Nanjing, 210008, China a
b
[email protected],
[email protected] Keywords: Reference evapotranspiration; Penman-Monteith equation; Hargreaves equation; Wetland
Abstract. Evapotranspiration (ET) constitutes the dominant water loss from many different types of wetlands. The relative importance of ET is apparent in its influence over water depth, temperature and salinity. However, direct measurement of ET, especially for wetland, is difficult, costly, and rarely available. In this study, the Penman-Monteith model was selected to estimate the reference evapotranspiration for short and tall canopies, moreover, Hargreaves equation was also used to simulate the ETref and test the precision of Penman-Monteith method. The results show that the reference evapotranspiration are maximal between June and August and minimal in winter, and ETrs is larger than ETos in the whole simulation periods. H ET0 and ETos have the same variation trend, but the former was smaller than the latter during May to September. Finally, there is a strong correlation between ETos and ETrs, with the correlation coefficient are 0.98 and 0.99 at daily and monthly means scale. Introduction Evapotranspiration (ET) constitutes the dominant water loss from many different types of wetlands [1]. The latent heat flux process also represents the chief wetland energy sink [2]. The relative importance of ET is apparent in its influence over water depth, temperature and salinity [3] as well as areal extent of water coverage and inundation duration. The accurate estimation of water loss by ET is very important for assessing water availability and requirements [4], making proper water resources plans [5], and calibrating and improving hydrologic models [6]. Reference evapotranspiration (ETref) can be measured by lysimeters. However, the use of lysimeters is generally limited to specific research purposes due to difficult and expensive construction. Thus, the development of ET model has always been one subject of research on ET. In the last two decades, many ET models have been developed and validated, from single climatic variable driven equations to energy balance and aerodynamic principle combination methods [7]. Many studies evaluated various equations for calculating the hourly ETref. The California Irrigation Management Information System (CIMIS) Penman equation was used for estimating hourly ETref in several studies [8-10]. The original Penman equation calculates ETref for periods not shorter than one week. The locally adjusted Penman equation performed quite well in a high advection area in Southern Spain [11]. Reference [12] compared hourly ETref estimates by the CIMIS Penman and Penman-Monteith equations to hourly lysimeter observations from Davis and Five Points, California. A comparison shows that Penman-Monteith equation gives better agreement with measured ETref. Many studies have also indicated the superiority of the Penman-Monteith equation for estimating hourly ETref [13-17]. As such, the Penman-Monteith model was selected to estimate the reference evapotranspiration (ETref) for short (ETos) and tall (ETrs) canopies using daily weather data. In addition, Several researches have proposed the empirical Hargreaves equation as the best alternative for areas in which data are scarce, such as those where only daily air temperature data are available. So, the Hargreaves method was also used in the study to calculate ETref and compare with the results of Penman-Monteith equation. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, www.ttp.net. (ID: 128.210.126.199, Purdue University Libraries, West Lafayette, USA-11/07/15,10:00:31)
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Study area and Model Description Study area. Poyang Lake basin is located in the middle and lower reaches of the Yangtze River, China and the lake receives water flows mainly from the five rivers: Xiushui River, Ganjiang River, Fuhe River, Xinjiang River and Raohe River and discharge into the Yangtze River through a channel in its northern part. The total drainage area of the water systems is 16.22×104 km2, accounting for 9% of the drainage area of the Yangtze River basin. Poyang Lake basin has a subtropical wet climate characterized by a mean annual precipitation of 1680 mm and annual mean temperature of 17.5°C. Annual precipitation shows a wet and a dry season and a short transition period in between. In response to annual cycle of precipitation, the Poyang Lake can expands large water surface of 3800 km2 in the wet season, but shrinks to little more than a river during the dry season and exposes extensive floodplains and wetland areas. Penman–Monteith method. The Penman–Monteith equation in the following form is used to estimate daily or hourly ET0 for two reference surfaces: 900 0.408∆( Rn − G ) + γ u 2 (e s − e a ) T + 273 ET0 = (1) ∆ + γ [1 + 0.34u 2 ] where ET0 is the reference evapotranspiration (mm/day); Rn is the net radiation at the crop surface (MJ/(m2⋅d)); G is the substrate soil heat flux (MJ/(m2⋅d)); T is the average air temperature at 2 m height (oC); u2 is the wind speed at 2 m height(m/s)；es is the saturation vapor pressure(kPa)；ea is the actual vapor pressure (kPa)；es-ea is the saturation vapor pressure deficit (kPa); ∆ is the slope of the saturated water vapor pressure curve (kPa/oC)；and γ is the psychrometric constant (kPa/oC). According to FAO Irrigation and Drainage Paper no. 56 [18], the reference surface is a 0.12m height (short crop), cool season extensive grass such as perennial fescue or ryegrass. A second reference surface, recommended by the American Society of Civil Engineers, is given by a crop with an approximate height of 0.50m (tall crop), similar to alfalfa. Hargreaves method. As an alternative when solar radiation data are missing, daily ET0 can be estimated using the Hargreaves equation [19,20]. An adjusted version of this equation, according to Reference [18], is given: 1 T + Tmin ET0 = A + B ⋅ ⋅ 0.0023 ⋅ max + 17.8 ⋅ Tmax − Tmin ⋅ Ra (2) λ 2 where ET0 is the reference evapotranspiration (mm/day); parameters A (intercept) and B (slope) are calibrated coefficients, to be determined on a monthly or yearly basis by regression analysis or visual fitting; λ is the latent heat of water vapourisation (MJ/kg); Tmax and Tmin are maximal and minimal air temperature (oC). Results and Discussions The model was carried out from January 1 to December 30, 2011 using meteorological data at Wucheng. Estimated daily ETref for tall (denoted by ETrs) and short (denoted by ETos) canopies by Penman-Monteith equation are shown in Fig.1. It is obvious that the reference evapotranspiration are maximal (about 6~8 mm) between June and August and minimal (about 1 mm) in winter, and ETrs is larger than ETos in the whole simulation periods. At the same time, the comparison of ET0 calculated by the Hargreaves equation (denoted by H ET0) and ETos was shown in Fig.2. Although the H ET0 and ETos have the same variation trend, the former was smaller than the latter during May to September and larger in other periods.
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Fig.1 Estimated daily ETref for tall and short canopies using Penman-Monteith equation
Fig.2 Comparison of daily ETref for short canopies using Hargreaves and Penman-Monteith equation
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Fig.3 shows the comparison of monthly means of daily ETref using Penman-Monteith and Hargreaves equation. The ETrs, ETos and H ET0 have the same trend, but ETos is more close to H ET0 than ETrs. This indicate that Hargreaves method has a close precision with Penman-Monteith equation for short canopies. 8 ETrs 6
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Fig.3 Comparison of monthly means of daily ETref using Penman-Monteith and Hargreaves equation Fig.4 shows the scatter plots of ETrs and ETos at daily and monthly means scale. It is obvious that there is a strong correlation between ETos and ETrs, the correlation coefficient are 0.98 and 0.99 at daily and monthly means scale. Furthermore, the slopes are smaller than 1, it is mean that the reference evapotranspiration for short canopies are smaller than the tall canopies. Conclusions The reference evapotranspiration for short and tall canopies were estimated by the Penman-Monteith model. Hargreaves equation was also used to simulate the ETref and test the precision of Penman-Monteith method. The results show that the reference evapotranspiration are maximal
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between June and August and minimal in winter, and ETrs is larger than ETos in the whole simulation periods. H ET0 and ETos have the same variation trend, but the former was smaller than the latter during May to September and larger in other periods. ETos is more close to H ET0 than ETrs, which indicate that Hargreaves method has a close precision with Penman-Monteith equation for short canopies. Finally, there is a strong correlation between ETos and ETrs, the correlation coefficient are 0.98 and 0.99 at daily and monthly means scale. 9
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Fig.4 Scatter plots of ETrs and ETos at daily (left) and monthly scale (right) Acknowledgements This work is jointly supported by the Open Foundation of State Key Laboratory of Hydrology-Water Resources and Hydraulic Engineering (2010490811), the National Natural Science Foundation of China (41101024) and the Open Research Fund of State Key Laboratory of Simulation and Regulation of Water Cycle in River Basin (China Institute of Water Resources and Hydropower Research), Grant NO: IWHR-SKL-201111. References [1] Z.D. Judy, L.S. Richard, D. Spano, et al: Hydrol. Process. Vol. 18 (2004). P. 2071 [2] D.A. Wessel, W.R. Rouse: Boundary-layer Meteorology, Vol. 68 (1994). P. 109 [3] G.G. Burba, S.B. Verma, J. Kim: Agricultural and Forest Meteorology, Vol. 94 (1999). P. 31 [4] K. Parasuraman, A. Elshorbagy, S.K. Carey: Hydrol. Sci. J. Vol. 52(3) (2007). P. 563 [5] J.Z. Drexler, F.E.Anderson, R.L. Snyder: Hydrol. Process. Vol. 22(6) (2008). P. 725 [6] D.M. Sumner, J.M. Jacobs: J. Hydrol. Vol. 308(1-4) (2005). P. 81 [7] M.C. Zhou, H. Ishidaira, H.P. Hapuarachchi, et al: J. Hydrol. Vol. 327(1-2) (2006). P. 151 [8] R.L. Snyder, W.O. Pruitt: Irrigation & Drainage Session Proc/Water Forum, Baltimore, MD, (1992). P. 128 [9] S.O. Ortega-Farias, R.H. Cuenca, M. English: J. Irrig. Drain. Eng. Vol. 121(6) (1995). P. 369 [10] F.Ventura, D. Spano, P. Duce, et al: Irrigation Sci. Vol. 18(4) (1999). P. 163 [11] J. Berengena, P. Gavilan: J. Irrig. Drain. Eng. Vol. 131(2) (2005). P. 147 [12] P.J. Vaughan, T.J. Trout, J.E. Ayars: Agric. Water Manage. Vol. 88(1–3) (2007). P. 141 [13] S. Lecina, A. Martinez-Cob, P.J. Perez, et al: Agric. Water Manage. Vol. 60(3) (2003). P. 181 [14] R.G. Allen, W.O. Pruitt, J.L. Wright, et al: Agric. Water Manage. Vol. 81(1–2) (2006). P. 1 [15] R. Lopez-Urrea, F.M. de Santa Olalla, C. Fabeiro, et al: Agricultural Water Manag. Vol. 86(3) (2006). P. 277 [16] P.J. Perez, S. Lecina, F. Castevllvi, et al: Hydrol. Process. Vol. 20(3) (2006). P. 515 [17] P. Gavilan, J. Berengena, R.G. Allen: Agric. Water Manage. Vol. 89(3) (2007). P. 275 [18] R.G. Allen, L.S. Pereira, D. Raes, et al: Irr. & Drain. Paper 56. UN-FAO, Rome, Italy (1998) [19] G.H. Hargreaves, Z.A. Samani: Appl. Eng. Agric. Vol. 1 (1985). P. 96 [20] G.H. Hargreaves, R.G. Allen: J. Irrig. Drain. Vol. 129 (2003). P. 53
Progress in Environmental Science and Engineering 10.4028/www.scientific.net/AMR.610-613
Characteristics of Reference Evapotranspiration of Wetland Based on Penman-Monteith and Hargreaves Methods 10.4028/www.scientific.net/AMR.610-613.3013
