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Rivers run from faraway
Water and land governance need to consider effects of distant land-use change, because local land-use decisions are not as local as we have always assumed
- Study highlights the importance terrestrial moisture recycling (TMR) in local and distant water governance structures
- TMR incorporates evaporation and precipitation, and is known to operate regionally and continentally
- Authors use hydrological models and land use data to understand TMR’s role
While sometimes it’s good to go with the flow, in other cases it should raise questions. When thinking about actual rivers, modifications, be it by humans or nature, are usually attributed to local land use changes. For example, if a dam is constructed, it would change the course of flow of the entire river.
Teleconnections, atmospheric anomalies that occur both over time and long distances, account for a number of observed land-atmospheric changes around the world. El Niño-Southern Oscillation is one of the more well-known teleconnections, but studies have also connected other long-distance phenomenon. For example, local land-uses changes have been connected to rainfall modifications in faraway areas, and this has had ripple effects on altering river flow.
While river flow is known to be influenced by distant rainfall patterns, the process and to what extent has not been well understood. A recent study by centre researchers Lan Wang-Erlandsson, Ingo Fetzer, Line Gordon, and colleagues, looked at how far away land use changes can actually impact distant rivers’ flow, particularly through examining the moisture recycling processes around the globe.
“Perhaps the most important take-home message for now is simply that local land-use decisions are not as local as we have always assumed, and decision makers need to work together across borders under the unavoidable numeral uncertainties.
Lan Wang Erlandsson, lead author
Terrestrial moisture recycling
Wang-Erlandsson explains that "Regional patterns of land-use change and moisture recycling are important to consider in explaining runoff change, integrating land and water management, and informing water governance."
To understand this better, the authors examine terrestrial moisture recycling (TMR), a process in which water is evaporated and precipitated, and operates over large distances on a regional to continent-wide scale. They use TMR as a way to further detangle how evaporation and precipitation influence river flow from a distance, something that previous studies had not yet considered.
Using a combination of hydrological models and land use data, the authors found that, "globally aggregated river flow change as a response to land-use change can be almost halved by taking TMR into account," explains Ingo Fetzer. "TMR overall dampens direct increase in mean annual river flows from deforestation, and do so by slowing down the water cycle and decreasing precipitation."
Line Gordon adds, “However, these effects vary widely by regions. While the TMR effects are negligible in some basins, remote land-use change effects can exceed the impact on river flow from local land-use change in other basins and even propose new transboundary relationships."
New water governance
Currently, water governance tends to focus on local water and land interactions within a basin, such as the Integrated River Basin Management framework. However, this does not account for any consideration of atmospheric influence.
The authors suggest that atmospheric moisture recycling might need to be considered in governance structures – potentially by extending the management area to encompass the entire region that provides the local precipitation (i.e., the precipitationshed (link to Pay Keys’ work)) as well as the region that receives the local evaporation (i.e., the evaporationshed). It is also important to understand how each basin is influenced.
As Wang-Erlandsson explains, “Land-use change impacts river flow through TMR in different ways depending on how precipitationshed, river basin, and evaporationshed are aligned. For example, where an evaporationshed has a limited overlap with river basin boundaries, local reforestation or deforestation impacts on the mean annual quantity of river flows will likely be exported to other neighboring basins.”
Not as local as first thought
Setting up precise moisture recycling governance structures for considering moisture recycling can be challenging though, given that land-use change can have non-linear effects on circulation patterns, which was not accounted for in this study. They are difficult to accurately predict even using highly complex climate models. Global warming also increases the difficulty to define appropriate management boundaries for TMR.
“Atmospheric circulation and moisture recycling patterns – including the shape and intensity of the precipitation- and evaporationsheds – are changing, as we saw with the air flow stalling that caused massive heatwaves in the Northern Hemisphere this summer,” says Wang-Erlandsson.
"Perhaps the most important take-home message for now is simply that local land-use decisions are not as local as we have always assumed, and decision makers need to work together across borders under the unavoidable numeral uncertainties."
Understanding the interactions between TMR, land-use change, and river flow highlights how important it is to look beyond local water governance and resource management. As the authors conclude, "consideration of atmospheric processes is essential for understanding river flow modifications and managing water resources in a rapidly changing and tele-coupled world, facing increasing pressure on both land and water."
In other words, how rivers run can come from faraway.
To determine how moisture recycling interacts with land-use change and river flow, the authors looked to hydrological and moisture tracking models. The authors first used the global hydrological model Simple Terrestrial Evaporation to Atmosphere Model (STEAM) to simulate evaporation and river flow change from land cover and use change without consideration of terrestrial moisture recycling (TMR) effects. To capture the TMR effect, the authors then used STEAM evaporation together with other publicly available meteorological data to iteratively feed the moisture tracking model Water Accounting Model-2 (WAM-2layers) until the simulated water cycle was in equilibrium. The authors finally isolated the TMR effect by calculating the hydrological difference globally and by river basin between the STEAM-only and the coupled STEAM-WAM-2layers run. To identify the responsibility for land-use change impacts on river flows by nation states, the authors post-processed the model simulated data.
Wang-Erlandsson, L., Fetzer, I., Keys, P.W., van der Ent, R.J., Savenije, H.H. and Gordon, L.J., 2017. Remote land use impacts on river flows through atmospheric teleconnections. Hydrol. Earth Syst. Sci. Discuss., https://doi.org/10.5194/hess-2017-494, in review.
Lan Wang Erlandsson's work focuses on the the large-scale interactions between land, water, and climate, and their implications for resilience
Ingo Fetzer has a strong interest in whole system processes and investigates the interaction of multiple stressors across scales - global to local - and their interactions on systems' stabilities.
Line Gordon is the director of the Stockholm Resilience Centre. As a researcher, Gordon is particularly interested in how intentional and unintentional actions in one place can influence systemic change elsewhere.
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