Tracer Methods

Tracer methods can provide valuable information on the water transport in soils. In WATERAGRI, the isotopic composition of precipitation water is used as an environmental tracer. The isotopic composition of oxygen and hydrogen isotope in precipitation water varies throughout the year, which depends on factors such as temperature and water vapor sources. These changes have a pronounced sinuosity with a minimum and maximum value in winter and summer in the climates covered by the project (i.e. Boreal, Continental and Pannonian).

This isotopic signal (δ18O and δ2H) can also be followed and observed in soil water in the vertical direction, as precipitation water pushes water from previous precipitation events downwards. Depending on the soil type, climatic characteristics and sampling timing, the seasonal distribution of isotopes might still be observable in vertical soil profiles. It is, therefore, possible to track the movement of water and quantify the water flux in the soil. Soil water flux below the root zone basically equals the potential groundwater recharge, which is essential for renewing groundwater resources. By means of a water balance, other inputs and outputs, such as evapotranspiration, can be estimated.

In order to use this simple method, a soil core is extracted and divided into 5 – 10 cm subsections. Soil samples are stored in sealable, inflatable and leak-tight bags, the isotopic composition of the water is analyzed, and the water content is determined. The isotopic signal of soil water can be compared with that of precipitation water to identify winter or summer peaks, considering the accumulated precipitation and isotopic composition. The volume of water per m² in that interval has to be related to the elapsed time (yr) to obtain the water flux or potential groundwater recharge. One of the main advantages of this method is the possibility to quantify water fluxes in fields that are difficult to access or have little data available by carrying out a single sampling campaign.