Land Water Storage and Sea Level Rise
Massive anthropogenic transformations of Earth's surface during the 20th century have begun to impact continental-scale patterns of river runoff, ultimately affecting global sea level. Sequestration of water in reservoirs and artificial lakes diminish the outflow of water to the sea. On the other hand, streamflow increases due to groundwater mining, deforestation, and urbanization. Overall, human activities have shifted the balance toward greater storage of freshwater on land. If not for these human diversions of runoff, the current rate of sea level rise (1-2.5 mm/yr) might be even 0.9 mm/yr higher.
Between 4510 and 5330 km3 of water -- 11% to 13% of the total annual river runoff (~41,000 km3/yr) -- is presently sequestered behind large dams. More than 90% of this total reservoir capacity has been created since the 1950s. The volume of water retained on land is equivalent to a decrease in sea level of 1.3 to 1.8 mm/yr.
However, other processes increase the rate of streamflow to the oceans. These include groundwater mining (or pumping of groundwater at rates exceeding the natural recharge rate), deforestation, and urbanization. Deforestation reduces the infiltration capacity of the soil due to compaction by heavy logging, farm machinery, overgrazing and trampling by cattle, and increased soil erosion. In newly cleared areas, runoff usually increases, especially in the rainy season, leading to greater chances of flooding. Urbanization expands the area of impermeable surfaces, which impedes infiltration of rainwater, reduces evapotranspiration, and thus augments streamflow. The combined effect of these processes is to increase sea level by 0.4 to 0.9 mm/yr.
Overall, human manipulations of land hydrology withhold a volume of water of 328±162 km3/yr -- equivalent to 0.9±0.5 mm/yr -- from the sea. This net increase in terrestrial water storage represents a nearly 1% annual reduction of runoff to the ocean, but around 9% of the yearly volume of freshwater used by people (3760 km3/yr).
How might future anthropogenic changes in land water storage affect sea level? A simple extrapolation of recent trends may be unrealistic for several reasons. A major dam-building boom is underway, especially in the developing world. An additional 800 km3 water could therefore be sequestered on land, amounting to between 15% and 18% of present reservoir storage. However, this rate of dam-building is likely to slow down in coming decades as suitable sites become scarcer and public opposition to environmental impacts of dams grows. Similarly, deforestation may also decrease as forests are cleared from all but inaccessible sites and as stricter environmental regulations aim to preserve any remaining forests.
Earth-observing satellites will monitor major hydrologic mass transfers and land cover changes. This should allow us to more accurately define water storage changes and assess future environmental and climatological impacts.
- Gornitz, V. 2000. Impoundment, groundwater mining, and other hydrologic transformations: Impacts on global sea level rise. In Sea Level Rise: History and Consequences (B.C. Douglas, M.S. Kearney, and S.P. Leatherman, Eds.), p. 97-119. Academic Press.
Please address all inquiries about this research to Dr. Vivien Gornitz.