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Ecosystem modeling: challenges in water-carbon cycling in cold region numerical simulation

Numerical simulation of water and carbon cycling in cold region usually encounter some unique challenges which must be taken into consideration in climate change study.

First, the presence of snow, glacier and permafrost has direct influence on the water cycling. Snow hydrology has been long studied and I try to avoid discussing it here. Glacier dynamics have also drawn a lot of attentions in recent decades due to the warming climate.
Permafrost also plays an important role, which has not yet been fully understood and investigated. Permafrost and its upper soil layer (Active Layer) have seasonal dynamics in water/ice content and temperature. As temperature continues to increase, the thawing permafrost also releases the ice to the surface layer, which means the active layer thickness is expected to increase in the near future.
Other than that, unlike that in temperate regions, permafrost acts as a confining layer between surface water and groundwater, which means the groundwater and surface water interactions are expected to change dramatically as well.

To date, our research community often focus on one or a few of these factors and ignore/simplified the other factors. For example, surface hydrology community usually focus on snow dynamics and river discharge. Groundwater investigations also usually fail to consider surface/subsurface water. None of these researches have captured the big picture so far.

Carbon cycling is always coupled with water cycling. The changing water cycling alone already means that carbon cycling is expected to change. There are already evidences from remote sensing that the whole Arctic is getting "greener", which may be a result of increasing water availability for vegetation growth.
In cold regions, soil stores a huge mount of carbon. And this carbon maybe released through hydrology network or atmosphere. For example, when permafrost degradation starts, the carbon which is stored in permafrost previously can join the general carbon cycling and be transported in forms of greenhouse gases (carbon dioxide) or dissolved organic carbon (DOC).

Similar to the water cycling, these factors are also not well understood so far. Even though lots of researches claim that they are using coupled water-carbon approach to investigate these dynamics, most of them are over-simplified and can only be used to improve our understandings of  the roles these factors play in cold region climate change responses. That is also why we see lots of time new researches found that the future situations might be worse than previous thought. A fully integrated approach must be used to consider these critical factors if we want to estimate the current and future scenarios.

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