Groundwater in Tibet – invisible and restless

Abstract:

Changes in groundwater flow rate and storage at high elevation and mountainous regions such as the Tibet Plateau remain difficult to decipher and often are the last terms in water balance equations to be deduced from surface water components. How does the groundwater dynamics change under a warming climate and contribute to the water cycle at high elevation and mountainous regions?

In the shallow subsurface, groundwater can be dictated by the dynamics of permafrost that extensively underlies the Tibet Plateau and its thickness varies from meters to more than one hundred meters. Modeling of coupled groundwater flow and heat transport has been used to examine the response of heat and groundwater to permafrost degradation under a changing climate. When warmer surface temperature propagates to subsurface, groundwater invigorates in active layers and supra-permafrost aquifers due to permafrost thawing enhanced hydraulic permeability and new liquid water liberated from thawing. Heat is transported not only by conduction but also advection due to mobilized groundwater, forming a feedback between groundwater flow and heat transport. Groundwater discharge to surface waters increases and significant groundwater flow could last for months from early summer into early winter.

At the regional scale, groundwater flow is largely driven by topography and sustained by recharge at high elevations. Such regional-scale groundwater flow is generally at much smaller but involves larger travel distances of hundreds of kilometers and greater depths of kilometers. Faulted and fractured rocks and unfrozen taliks below river valleys promote hydrologic connections between deeper groundwater and surface waters. For an average topographic gradient of 0.002 sloping from the inner Tibet Plateau to the east, the rate of groundwater flow can be on the order of 10^-9 to 10^-7 m/s, which contributes to sustaining rivers’ baseflow.

It should be noted that increased groundwater flow from permafrost thawing or regional deep groundwater flow can only be sustained if there were water at high elevation sources available to replenish the groundwater system. Future efforts need to integrate water source dynamics and coupled heat and groundwater models to assess climate impacts on hydrologic processes and treat groundwater and surface water as one source.