8.3 Surface Water

Stable isotopes in surface water bodies may vary spatially and temporally, and capturing this variation is important to generate a meaningful average, as well as to understand seasonality, stratification, inflow sources and other hydrologically important variations. From a hydrogeological perspective, the isotopic variations in a surface water body may help determine the degree or nature of surface water-groundwater interaction.

Temporal changes in surface water isotope composition are not as sudden as with precipitation, which changes by the minute, but it is equally important to capture them because these isotopic changes are often associated with large changes in flow. A flood may only last a few hours in a small catchment, but may be a substantial percentage of the mean annual runoff. This is particularly true for arid environments where watercourses may not flow much during most of the year such that the mean annual runoff may occur in a single event. In more temperate catchments where the mean annual runoff is the result of many storms throughout the year, temporal changes are important because seasonal, and other changes, in weather or land use affect flow dynamics and water quality. Increasingly erratic and stormy weather is predicted due to global climate change and this will require a preparedness to capture such variations in order to obtain representative samples (IPCC, 2014).

The extent of spatial variation of isotopes in surface water bodies depends largely on the water body type. Rivers tend to be fairly well mixed, so the spatial variations are restricted to upstream and downstream as influenced by inputs and outputs. Lakes are more complex because they may be stratified and may vary temporally between net inflow or net outflow. Reservoirs (man-made) have similar properties to lakes. Wetlands are even more complex in that they may be seasonal or episodic, having no surface water in dry periods that may last months to years. The role of groundwater is usually very important in wetlands.

The degree to which a study needs to capture the temporal and spatial variations in surface water is dependent on the goals of the study. Researchers must be aware that having only one sample from a single location is not as reliable as having multiple samples. The value of one sample for representing the larger system, depends on the system.