Utilization of oxygen-18 and deuterium in stem flow for the identification of transpiration sources: soil water versus groundwater in sand dune terrain

Forestation and reforestation in an arid sand dune terrain raise the issue as to what extent they affect the local groundwater system: that is, the effects of transpiration versus groundwater recharge. In general, sand dunes are considered to be the most efficient zone for groundwater recharge in deserts. This is especially true for arid zones with a limited amount of rainfall where most of the precipitation infiltrates into the dunes and a high percentage of the deep percolated soil water reaches the groundwater reservoir. In the late 1940s, a tamarisk forest was planted in the sand dune desert at the north west of the Negev desert. Tamarisk roots were found within the shallow aquifer 16 m below surface as well as within the top 2 m of the sand. As δ¹⁸ and δD composition is significantly different in soil water versus groundwater, the main research objective was to determine the relative role of soil water versus shallow groundwater as sources for transpiration. The distribution of oxygen-18 and the deuterium composition in the topsoil and in groundwater were related to the spatial isotopic distribution along 15 m of double root system, stem flow and twigs of two tamarisk trees. One tree is situated within a small forest and its root length was determined by means of a 20 m borehole. The second is a single tree located on top of a sand dune, and exposure of its double root system was made by cutting into the dune to 12 m below surface. In the forested area, the total flux of transpiration was measured as stem flow using a heat pulse method. Water was extracted by azeotropic and vacuum distillation from twigs, small roots and from phloem and xylem taken from a stem core 0.5 m above the surface. Similar cores were also obtained along the main root connecting the deep and shallow roots from the exposed single tree on the dune. Best results were obtained with oxygen-18 composition as long as the xylem was separated from the phloem. Isotopic mass balance expressions were solved to elaborate on the relative water contribution from groundwater and soil water to the total rate of transpiration in winter and summer. Sets of water, δ¹⁸O and δD mass balance expressions were solved to identify the relative contribution of transpired water. Results indicate that groundwater serves as the main source of transpiration, even though a sufficient amount of water exists in the topsoil. This is perfectly correlated with the soil water budget obtained with a soil neutron emission moisture detector. The greater the soil moisture level, however, the less water is pumped from groundwater. This phenomenon is explained by the higher level of energy required to extract soil water from the high soil matrix potential compared to the energy needed to lift saturated groundwater to the surface.