Use of Piche evaporation in a Stevenson Screen to estimate reference evapotranspiration in the Sudan

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1 Sudanese Journal of Agricultural Sciences (2014) 1, Use of Piche evaporation in a Stevenson Screen to estimate reference evapotranspiration in the Sudan Hussein Suliman Adam a, Shamseddin Musa Ahmed b * a Water management specialist, Former Dean, Water Management and Irrigation Institute, University of Gezira,Wad Medani, Sudan b Water Management and Irrigation Institute, University of Gezira, Wad Medani, Sudan Abstract Direct measurements of evaporation from open water have not been satisfactory since its adoption in the early days. The class A pan proved to be unsatisfactory especially in dry areas due to high advected energy from the surrounding areas. The Piche tube needs a correction factor. Scientists worked actively to try to estimate evaporation from climatic data. Many formulae were produced. Most of them fitted the humid zones. Even the best known Penman formula has a problem with its wind term. Monteith and Unsworth worked out the wind term based on the physics of evaporation. Then they developed a formula for estimating reference evapotranspiration (ET o ), which is nowadays the basis for estimating crop water requirements. The data needed to use the resulting Penman-Monteith formula for ET o are available at only a limited number of stations in Sudan. Whereas Piche evaporation and relative humidity data, as measured in a Stevenson screen, are available in many more stations. The objective of this paper is to develop a relationship between Penman-Monteith based ET o and Stevenson screened Piche evaporation. An exponential relationship was established (R 2 = 66%) between the ratio of ET o and Piche evaporation and relative humidity using the most recent climatic normals of provided by the Sudanese Meteorological Authority. The validation using Wadmedani station datasets for two consecutive years (2010 and 2011) resulted in acceptable root mean square errors of 0.2 mm and 0.5 mm, respectively. Based on the mean bias error (MBE) indicator, the validation over -predicted ET o resulted in acceptable values of 0.55 and 0.60 mm, respectively. Introduction Evaporation measurement and calculation are very important for estimating water losses from ponds, canals, lakes and reservoir (Sadek et al., 1997). * Corresponding Author. Tel.: address: shams_id@yahoo.com canals, lakes and reservoir (Sadek et al., 1997). Reference crop evapotranspiration has become the corner stone for estimating crop water requirements when used with appropriate crop coefficients (Hargreaves and Allen, 2003). Many formulae, mostly empirical, were given for es- 70

2 timating evaporation from some climatic data. Thornthwaite used temperature as the main climatic element. Penman (1948) used four climatic elements: temperature, vapor pressure, sunshine and wind speed. His first version has two terms: the energy term and the wind term. The energy term was derived using equations of sensible heat, latent heat and net radiation. However, the wind term was empirically derived using evaporation from a sunken pan at Rothhamstead, UK. Being a humid area the evaporation from the pan gives a good measure of evaporation from an open water surface. The Penman formula failed to give a good estimate of evaporation in dry areas due to advected energy which was not accounted for in the energy balance, i.e. energy term. Later the formula was modified by Monteith and Unsworth (1973), who managed to derive the wind term based on the physics of evaporation. The modified formula became the well known Penman-Monteith formula (Monteith and Unsworth, 1990). In 1990, FAO called a team of 16 experts from all over the world led by Monteith. Their assignment was to recommend a formula to be used for estimating reference evapotranspiration. The team compared the Penman-Monteith formula to 19 formulae e.g. Priestley and Taylor, Busingervan Bavel, Thornwaite, Hargreaves and Turc. The Penman-Monteith was found to be the best performing method to estimate reference evapotranspiration (Kassam and Smith, 2001; Allen et al., 1998), especially for semi arid environments (DehghaniSanij et al., 2004). This is because of its solid physics basis. In addition, it was well-tested and documented by several tests (Landeras et al., 2008; Droogers and Allen, 2002). Evaporation pans (Class A pan) and Piche tube evaporimeters inside the standard Stevenson Screen were used at meteorological stations. Trials were made to find correction factors for these instruments to estimate evaporation. Class A pan needs careful handling: design of Pan (127 cm diameter and 25 cm height), grey color to reduce absorption and reflection from the sides of the Pan, installed on wooden logs to allow free passage of air underneath the Pan and to site the Pan away from obstacles, e.g. trees, buildings. Operation and maintenance are very important for the accuracy of measurement: changing the water daily to avoid debris and sometimes algae floating on the surface of the Pan and the Pan has to be well maintained so that it does not rust and leak. Personal observation in some Arab and African countries showed that these and other precautions were not observed and thus render evaporation measurements unreliable. The Piche tube, on the other hand, was much easier to maintain properly, but also here precautions were necessary in its use (Stigter and Uiso, 1981; Stigter et al., 1995). With a proper correction factor it gives a good estimate of evaporation. Adam (1973) has related Penman evaporation to Piche evaporation for the various climatic zones of Sudan. In principle an unshaded green Piche blotting paper in the open might be expected to measure reference evapotranspiration better. The Piche in the screen does not receive the net radiation that a crop receives or loses, and its wind reactiondiffers from air movement in the screen. The method with the 71

3 screened Piche was chosen, because these were the data available in the Sudan. This choice may be defended because it is physically based on the assumption that the temperature in the screen follows more or less net radiation and may therefore be considered as a proxy, while the air movement inthe screen is somehow related to the air movement outside the screen (Stigter et al., 1995). It should be remembered that physically a shaded Piche outside the screen was proven to be well related to the wind term in the Penman equation (Stigter et al., 1984; Ibrahim et al., 1989) and was therefore able to even serve as an auxiliary anemometer in complex agricultural spaces (Kainkwa and Stigter, 2000). So the physics of the outdoor but shaded Piche evaporimeter was well understood from the beginning (Uiso and Stigter, 1980; Stigter et al., 1984). But this could only be applied outside a Stevenson screen (Stigter et al., 1995). With the available data in Sudan only the statistical approach could be applied as highlighted further below. In this paper an attempt was made to relate the ratio of reference evapotranspiration (ET ) calculated using the Penman-Monteith formula to Piche evaporation measured inside the Stevenson screen. The Penman-Monteith formula to estimate ET o, however, requires availability of a large and reliable weather datasets. For instance, the reliable data of wind speed, relative humidity and solar radiation data is very limited in remote areas of developing countries such as Sudan (Droogers and Allen, 2002; Hargreaves and Allen, 2003; Pereira and Pruitt, 2004). Providing a reasonably reliable method for estimating ET o will thus improve water management in general and field water management in particular. Materials and methods Data collection Data needed to compute ET o requires measurements of maximum and minimum temperature, relative humidity, wind speed at 2 m height and sunshine duration in the given latitude and time of the year. Data of Piche evaporation were made available by the Sudan Meteorological Authority (SMA) for 26 stations together with their locations and altitudes (Table 1). The collected data was the most recent climatic normals ( ). Methodology The monthly ET o was calculated following the Penman-Monteith formula (equation 1) as mentioned in Allen et al. (1998) using CROPWAT version 8.0. Then, monthly ratios of the estimated ET o and Piche readings were obtained. The relationship was set using Excel sheets. (1) Where, ET 0 is the reference evapotranspiration (mm/day), R n is the net radiation at the crop surface (MJm -2 day -1 ), G is the soil heat flux density (MJm -2 day -1 ), T is the air temperature ( o C), u 2 is the wind speed at 2 m height (m/s), e s is the saturation vapor pressure (kpa), e a is the actual vapor pressure (kpa), (e s e a ) is the saturation vapor pressure deficit (kpa), is the slope of vapor pressure curve (kpa/ºc), and γ is the psychrometric constant (kpa/ ºC). 72

4 Table 1 Studied stations Station Climatic zone Lat.( o N) Lon.( o E) Alt. (m, amsl) Port Sudan Red Sea Wadihalafa Desert Dongola Desert Abuhamad Desert Karima Desert Atbra Desert Hudeiba Semi-desert Shendi Semi-desert Shambat Semi-desert Ed Duem Dry Wadmedani Dry Kassala Dry Halfaelgadida Dry El-Fasher Dry Kosti Semi-dry Sennar Semi-dry El-obied Semi-dry Al-nuhud Semi-dry El-Geneiana Semi-humid El-Gedarif Semi-humid Abu Naama Semi-humid Eddamazine Semi-humid Babanusa Semi-humid Kadugli Semi-humid The relationship was statistically tested using the determination coefficient (R 2 ). The relationship was validated on the basis of the root mean square error (RMSE) and the mean bias error (MBE) (Droogers and Allen 2002; Landeras et al., 2008). (2) (3) ET o /piche = 0.34 e (RH%) (4) 73

5 Where, ETmodel stands for the ET o based on the developed exponential relationship; ETPM stands for the ET o on the basis of Penman-Monteith formula and RH is the relative humidity. Results and discussion Figure (1) shows the temporal distribution of the obtained ET o for the tested stations using the climatic normals. Recently, automatic weather stations were introduced in the Sudan. Such introduction could improve the data quality. However, care should be taken in calibration process, especially when newer instruments are introduced. Allen (1996) stated the utmost need for such calibration. He also described, in details, the recommended procedures, and guidelines for assessing integrity, quality and reasonableness of measured weather data and instruments calibration and validation. Figure (2) shows the developed relationship. It is obvious that the relationship is exponential one. This confirms the previous relationship found by Adam (1986) in Ghana where the relationship between the ratio of ET o to Piche and relative humidity was found to be exponential. The validation using Wadmedani stations datasets for two consecutive years (2010 and 2011, Fig. 3) resulted in RMSE values of 0.2 mm and 0.5 mm/day, respectively. The validated exponential equation over-predicted ET o values since the MBE were positive (0.55 and 0.60 mm, respectively), contrasting the reported under-estimation of the Penman-Monteith formula for dry areas (Allen et al., 1998). The estimated annual average daily Penman- Fig. 1 Monthly distribution of Penman-Monteithbased ET o for the period Fig. 2 Statistical relationships between evapotranspiration ET o and screened Piche evaporation where Ratio is indicative of (ET o /Piche) Monteith ET o (6.6 and 6.9 mm/day, respectively) were lower than the climatic normal of Wadmedani station (7.5 mm/day) by 14% and 8%, respectively. Thus, the developed equation may be described asvery good in estimating the monthly and annualaverage daily ET o. The class A pan does not give a consistent and reliable measurement. This is because of advection in dry areas and the difficulties encountered to keep a standard pan. The Piche Tube in the Screen appea- 74

6 Fig. 3 The developed relationship validation results: (a) for the year 2010 and (b) for 2011, Wadmedani station. Data are average monthly evapotranspiration in mm/day. rs a better instrument to approximate evapotranspiration in the Sudan, the necessary precautions notwithstanding (Stigter et al., 1995). Thus, this Piche evaporation could be safely used to estimate the reference evaporatranspiration (ET o ) using the following exponential relationship: ET o /piche = 0.34 e Acknowledgements (RH%) The authors would like to thank the Sudanese Meteorological Authority for providing the climatic data. Many thanks to Prof. C.J. Stigter for his valuable comments. References Adam, H.S. (1973). A note on estimated evaporation using Penman formula. Sudanese Meteorological Authroity. Pamphlet No. 8. Khartoum, Sudan Adam, H.S. (1986). Agrometeorologist inception report, FAO regional office in Accra and FAO Headquarter, Rome, Italy. Allen, R., Pereira, L., Raes, D., Smith, M. (1998).Crop Evapotranspiration. Guidelines for Computing Crop Water Requirements. FAO Irrigation and Drainage Paper (56). Rome, Italy. Allen, R.G. (1996). Assessing integrity of weather data for reference evapotranspiration estimation. Irrigation and Drainage Engineering 122, DehghaniSanij, H., Yamamoto, T., Rasiah, V. (2004). Assessment of evapotranspiration estimation models for use in semi arid environments. Agricultural Water Management 64, Droogers, P., Allen, R., (2002). Estimating reference evapotranspiration under inaccurate data conditions. Irrigation and Drainage Systems 16, Hargreaves, G., Allen, R. (2003). History and evaluation of Hargreaves evapotranspiration equation. Irrigation and Drainage Engineering 129, Ibrahim, A., Stigter, C., Adeeb, A., Adam, A., Jansen, A. (1989). Development and validation of a shaded Piche Evaporimeter for the tropics to replace humidity and wind speed data in the aerodynamic term of the Penman equation. In: Proceedings of the Fourth Technical Conference on Instruments and Methods of Observation (TECIMO IV). Instruments and Methods of Observation Report No. 35, WMO/TD No. 303, Geneva, Kainkwa, R., Stigter, C. (2000). Measuring wind gradie- 75

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