Estimation of Evaporation in Sabkhas on the Arabian Peninsula

Stephan Schulz1, Marcel Horovitz1, Randolf Rausch2, Maximilian Köhne1, Christian Siebert1, Ralf Merz1, Nils Michelsen3, Mohammed Al-Saud4, Christoph Schüth3
1 Helmholtz–Centre for Environmental Research UFZ
2 GIZ International Services and TU Darmstadt, Institute of Applied Geosciences
3 TU Darmstadt, Institute of Applied Geosciences
4 Ministry of Water & Electricity, Kingdom of Saudi Arabia

O 10.5 in Hydrogeologie arider Gebiete

30.05.2014, 18:40-19:00, H17, NW II

Sabkhas are salt flats with a shallow saline water table. Generally, it is possible to distinguish between two types of sabkha - coastal and inland. On the Arabian Peninsula, coastal sabkhas are located along the coastline of the Arabian Gulf. Inland sabkhas predominantly occur in the eastern part of the Rub’ Al Khali desert (Schott et al. 2008). Generally, sabkha soil matrix consists of siliciclastic sediments and evaporates (e.g. gypsum and anhydrite nodules). Capillary rise occurs from depths of up to 2 m, depending on soil texture (Sultan et al. 2008). Sabkhas are important natural sinks of the Upper Mega Aquifer System (Al-Saafin 1996). Unfortunately, assumptions widely vary about the amount of groundwater evaporation taking place in sabkhas. Shehata and Lotfi (1993) estimated the annual evaporation in Sabkhas to be 120 mm; in contrast, GDC (1980) assumed more than 400 mm/a. Besides, sources of the evaporating water in coastal Sabkhas have not yet been adequately investigated. It is not exactly known whether the saline water table is supplied only by ascending groundwater or whether it is supplied by seawater intrusion up to a certain distance from the coast. This leads to the conclusion that additional investigations are needed for a more reliable estimation of groundwater evaporation in sabkhas.

In this study, sabkha evaporation will be derived from a column experiment. Four soil cores (height:  50 cm, diameter: 16 cm) from different places (Al-Budu, Yabrin, sabkha Matti and an inter-dune sabkha in the Rub’ Al Khali desert) are taken with specially developed sampling equipment. During the column experiment it is possible to vary different boundary conditions, such as temperature and humidity of the surface, and water level. For a reliable estimation of evaporation, it is necessary to transfer results from the column experiment to field conditions. This makes an on-site monitoring of these parameters essential. For this purpose, a 3.5 m deep well was installed in the Al-Budu sabkha and equipped with groundwater data logger. Additionally, a temperature and humidity data logger was installed at the surface.  First results from this experiment indicate a lower evaporation rate (less than 100 mm/a) than previously assumed.

For the quantification of sabkha evaporation as a major sink of the groundwater resources, it is important to determine the sources of the evaporating water especially in coastal sabkhas. Therefore, sabkha water was sampled at several locations with increasing distance to the sea. Reference samples were taken in the Arabian Gulf and in various wells. These samples are analysed for chloride-bromide ratio, potassium-bromide ratio and delta 34S. Here, the clear picture resulted that coastal sabkhas are strongly influenced by seawater intrusion up to a few kilometres from the coast – those parts cannot be seen as a sink term of the groundwater system.

Al-Saafin, A.K. (1996): The Characterization of Sabkhas in the Eastern Parts of Saudi Arabia and its Implications for Engineering. Dissertation, Department of Engineering – Queen Mary and Westfield College, University of London

GDC (1980): Hydrology. Groundwater Development Consultants International. Report Vol. 3, Annex C In: Umm Er Radhuma Study

Schott, U., Faber, P.,  Jensen, H., Rausch, R.  (2008): Groundwater Evaporation from Sabkhas in the Northern Rub’ Al Khali Desert – Identification and Characterization of Active Sabkha Areas. – FH-DGG-Tagung 2008. Schriftenreihe der Deutschen Geol. Gesellschaft für Geowissenschaften, Heft 57, S. 170; Hannover.

Shehata, W., Lotfi, H. (1993): Preconstruction Solution for Groundwater Rise in Sabkha. Bulletin of the International Association of Engineering Geology 47: 145-150

Sultan, M., Sturchio, N., Al-Sefry, S., Milewski, A., Becker, R., Nasr, I., Sagintayeval, Z. (2008): Geochemical, Isotopic, and Remote Sensing Constraints on the Origin and Evolution of the Rub Al Khali Aquifer System, Arabian Peninsula. Journal of Hydrology 356: 70-83


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Letzte Änderung 08.10.2013