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United Arab Emirates

Water Evaporation Rate at Ras-Al-Khaimah, UAE

Introduction United Arab Emirates has an arid climate with less than 100 mm/yr average rainfall. The low percentage of cloudy days and the high solar radiation over the region result in high evaporation rates. The total annual potential evaporation ranges from 2.5 m in the coastal areas to more than 4.5 m inland. Estimation of evaporation losses is very important in this region in particular where water ponds or pools are used for industrial, domestic or agricultural purpose. In order to evaluate water evaporation losses in a water pond at Solar Island site of CSEM-uae, a brief study has been carried out which provides estimation of evaporation rates in the Emirate of Ras-Al-Khaimah (RAK), UAE. Methods for the Evaporation Estimates Evaporation pans provide one of the simplest, inexpensive, and most widely used methods of estimating rate of evaporation. The most useful methods for estimating evaporation from a free water surface are those which require meteorological input parameters. These methods estimate the evaporation rate from a class A evaporation pan. The data can be used for the estimation of evaporation from a large water body by applying a pan coefficient (The ratio of evaporation of water in a large water body to that measured in an evaporation pan). In this study, seven climatological methods (Table.1) were used to analyze the rate of evaporation of water in Ras-Al-Khaimah and AbuDhabi, UAE. The climatological data is collected from NASA Surface meteorology and Solar energy Tables. Table1: Data required for evaporation estimates methods

Method Kohler-Nodenson(K-N) (1955) Penman (1948) Penman-Monteith(P-M) (1965) Bulk-Aerodynamics(BA) (2003) Priestly-Taylor(P-T) (1972) Hargreaves (1975) Hamons (1961) Temperature Humidity Wind speed Radiation

The other different parametrs like declination, hour angle, day length, vapor pressure deficit, net radiation etc., were calculated based on the meteoroloical data and the monthly mean evaporation rates in RAK were estimated (Table 2). Similar calculations were performed for Abu Dhabi and the comparison of

CSEM-UAE Innovation Center LLC Al Jazeera Al Hamra Area PO Box 31208 Ras Al Khaimah, UAE T +971 (7) 244 6929 F +971 (7) 244 6951 [email protected]

United Arab Emirates

annual average estimates in both regions Ras-Al-Khaimah and Abu Dhabi is shown in Figure 1. The interpretation of the current equations shows that there is highest similarity is between P-M and P-T method estimates and Bulk Aerodymanics method gives annual average estimates close to P-M equation. The estimates based on K-N and Penman equation are high whereas the estimates based on the Hargreves and Hamons methods are very low. In the later case, low values are due to the dependency on a single parameter (temperature). Therefore it shows that the evaporation estimates are significantly affected by the meteorological parameters like Radiation, Humidity and wind speed. Table 1: Monthly mean estimates of evaporation rates in RAK


Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Annual average


11.3 13.8 12.1 13.7 17.9 18.7 17.3 17.3 17 17.2 14.4 12.2 15.2


9.2 11.5 9.7 11.12 15.0 15.8 14.9 14.6 14.0 13.9 11.5 10.0 12.6


10.7 13.1 12.2 14.8 20.6 23.0 23.4 22.1 20.2 18.3 14.4 11.7 17.1


9.7 12.5 10 11.4 15.3 15.8 13.9 14.3 14.1 15.2 12.8 10.7 13


4.8 5.76 7.1 9.9 15.1 18.0 19.8 17.6 15.0 11.1 7.5 5.5 11.5


2.3 3.0 2.5 3.1 4.2 4.0 3.2 3.2 3.4 3.9 3.2 2.9 3.2


0.8 1.0 1.1 2.3 3.7 4.2 3.5 3.7 3.2 2.1 1.8 0.9 2.3

CSEM-UAE Innovation Center LLC

Al Jazeera Al Hamra Area PO Box 31208 Ras Al Khaimah, UAE

T +971 (7) 244 6929 F +971 (7) 244 6951 [email protected]

United Arab Emirates

Penman's equations are based on the combination of energy budget and aerodynamics methods. These are fairly complex functions of temperature, humidity, wind velocity and radiation, so the results obtained could be reliable. In order to compare the theoretical values obtained from climatological data with the practical values of evaporation rate, Pan Evaporation experiments are performed at CSEM-uae. Pan evaporation is a measurement that combines the effects of several climate elements: temperature, humidity, solar radiation, and wind. An Evaporation pan is used to hold water for the determination of the quantity of evaporation at a given location by observation. Experiments are conducted continuously for a period of 3 months and based on the comparison of experimental results with estimates, Penman-Monteith (P-M) Method is found to be suitable to estimate the evaporation losses at RAK. Monthly average estimates of Evaporation rates are calculated using the Penman-Monteith(PM) equation for both RAK and Adu Dhabi regions and the results are shown in Figure.2.

Conclusion It could be concluded the evaporation estimates by the variation of temperature only are not suitable for the calculation of real evaporation losses. Meteorological factors like wind velocity, air humidity and solar radiation largely affect the evaporation estimates. The best estimates for evaporation rates at RAK, UAE are given by the Penman- Monteith(P-M) method which considers both heat and mass transfer processes by evaporation. Based on the estimates, the annual evaporation in RAK (UAE in general) is found to be around 4.5m. The maximum and minimum evaporation values calculated by these equations were recorded in January and June months respectively.

CSEM-UAE Innovation Center LLC

Al Jazeera Al Hamra Area PO Box 31208 Ras Al Khaimah, UAE

T +971 (7) 244 6929 F +971 (7) 244 6951 [email protected]


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