Water’s journey

Clouds form as air is pushed upward and cools, and rain and snow start to fall. More heavy water than light water rains out, making heavy water increasingly rare in clouds further inland, and also in the snow that falls there. On the ground, there is more heavy water near the coast than further inland and in the mountains.

Water on the ground and in the soil and groundwater

During the cold season, precipitation often falls as snow (number 3). If the soil temperature is above 0 °C, the snow melts when it reaches the ground and the water infiltrates into deeper soil layers. However, if the soil temperature is below 0 °C, or if large amounts of snow accumulate, the snow remains on the ground and builds up throughout the winter. It usually begins to melt in spring, around April.

When the soil is frozen, meltwater cannot infiltrate and instead flows directly into nearby streams and lakes. If the soil is not frozen, the meltwater infiltrates into the soil and recharges the groundwater. When the soil and groundwater are already saturated, incoming meltwater either pushes out “older” water stored in the soil and groundwater (e.g., autumn rainfall) which then flows into streams and lakes or flows directly over the land into streams and lakes itself.

It is hard to measure these processes using traditional snow, soil and ground water measurements. Instead, water isotopes complement traditional measurements and allow us to distinguish between snow, recent snowmelt, and older soil water from previous seasons. This helps us better understand the hydrological processes occurring in the landscape.

Water in the streams and rivers

If we look beyond small soil and groundwater processes (number 4), we see that the landscape is not uniform. Snow cover is uneven: some areas have deep snow with a lot of water stored in it, while other places have only thin snow, small patches, or no snow at all. At the same time, temperatures can rise and rain may fall on top of the snow or directly onto bare ground.

The landscape itself is also very diverse. It includes bare rock, forests, wetlands, agricultural land, and urban or industrial areas. Each of these areas reacts differently to snowmelt or rainfall. As a result, the water from each part of the landscape contributes to rivers in different ways and at different times.

Traditional measurements can tell us how much snow there was, how much rain fell, or how much water flows in the river. However, because water is stored and released differently across the landscape, these measurements alone cannot clearly show which source of water is contributing to the river at a given time.

Water isotopes provide a solution. They help us distinguish between water coming from different parts of the landscape and from different moments in time (for example, recent rainfall versus older stored water). This information allows us to build better hydrological models that represent natural processes more realistically. With improved models, we can make more accurate forecasts of snowmelt and extreme rainfall events that may lead to flooding.

Winds and the sun drive water out of the ocean and into the air. Evaporation is particularly strong when cold air from the sea ice or land gets swept over the warmer ocean waters.  Heavy water lags behind in the ocean, making it more rare in the air.

Groundwater may follow several different paths, seeping into waterways, lakes and oceans or into aquifers. Some water in lakes, rivers and streams is carried back to the oceans, or is evaporated back to the atmosphere.

When water reaches the ground, it can evaporate back into the atmosphere from the land surface, and from the surface of tree leaves and other vegetation.

Storms form where warm and cold air meets. Winds become stronger, and sweep together moist air from near and far. Eventually, some storms meet the coast of Scandinavia. Heavy and light water changes little while being transported, keeping an imprint of its origin.