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Why how much water runs down the rivers

Humans are increasingly altering the amount of water that runs from the land to the sea or inland waters. Calculations with a global vegetation and hydrology model indicate that precipitation had the largest impact on global river discharge over the 20th century.

Regionally, however, discharge varied according to factors such as land use change and irrigation practices, temperature, and the concentration of the greenhouse gas carbon dioxide (CO2), researchers from the Potsdam Institute for Climate Impact Research (PIK) report in the journal "Geophysical Research Letters". The impact of these mainly anthropogenically driven factors on discharge and the availability of water for human use is expected to grow in the future.

"How much the subtle increase in discharge during the 20th century is due to the rising CO2 concentration is the subject of scientific debate," says Dieter Gerten, lead author of the study. His team studied the effects of changes in climate, CO2 concentration, land cover and land use on river discharge using the dynamic global vegetation model LPJmL ("Lund-Potsdam-Jena managed Land" model). "Modelling shows that an increase in global precipitation was the driving force for the increase in river discharge", says the geographer and hydrologist.

The researchers used data from the Climatic Research Unit (CRU) at the University of East Anglia in Norwich, UK, a standard data set used in global modelling, as input to the vegetation model for simulating river discharge patterns. The simulations show large changes in the amounts of discharge during the last century in many regions of the world. In line with observations, river discharge decreased in North and West Africa, Central and Eastern Europe and parts of South Asia. It increased in parts of Siberia, as well as North and South America.

Global river discharge normally amounts to 35,000 to 40,000 cubic kilometres annually. According to the CRU climate data it increased by 7.7 percent during the last century, the researchers report. Approximately 95,000 to 110,000 cubic kilometres of precipitation fall on land surface each year. However, because regional amounts of precipitation and trends vary between different data sets, and other data do not indicate a clear global trend, it remains unclear whether there is actually an increase in global river discharge.

Following precipitation, land use had the largest impact on river discharge. During the last century, humans have increased global discharge by 1.7 percent, especially by deforestation. While irrigation caused significant regional decreases in discharge, its global effect on river discharge was negligible.

Over the last century, global warming decreased river discharge by 0.9 percent. This trend, due basically to increased evapotranspiration, was strongest at high latitudes and in parts of Central Asia. The global temperature signature has become increasingly evident in recent decades, the researchers write. Calculations based on three IPCC scenarios indicate that this trend will continue and that global warming alone could reduce global river discharge by six percent by the end of the 21st century.

Theoretically, the rise in the concentration of CO2 could reinforce this trend. The greenhouse gas could have a fertilizing effect that leads to an expansion of vegetation cover. On a regional level, more plants would take more water from the soil and thus decrease river discharge. Globally, however, the fertilization effect is negligible. Another direct effect of increased CO2 concentration has increased discharge by more than one percent between 1901 and 2002: under a higher concentration of CO2 plants need to open their stomata less in order to take up enough CO2 for growth. Therefore, evapotranspiration decreases and they take less water from the ground.

"The net effect of the rising CO2 concentration in the atmosphere could increase global river discharge by a further five percent by 2100," says Dieter Gerten. Globally, this would probably compensate the negative impacts of temperature. However, temperature and CO2 effects would not necessarily affect the same regions. The researchers are planning further studies to investigate possible developments of future water availability and demand worldwide.

"Currently, our model is the only one that can take the effects of all these factors into account," says Wolfgang Lucht, chair of the PIK research domain "Climate Impacts & Vulnerabilities". To achieve this, knowledge about hydrology needs to be combined with knowledge about vegetation dynamics. "The calculations indicate that human activities are having an increasing impact on the Earth's water balance", says Lucht. To be able to make more precise projections of future water availability, more methods of measurement and more data are needed. The researchers therefore call for a stop to the current deconstruction of the global meteorological network.

Gerten, D., S. Rost, W. von Bloh, and W. Lucht (2008), Causes of change in 20th century global river discharge, Geophysical Research Letters, 35, L20405, doi:10.1029/2008GL035258.
For further information please contact the PIK press office:
Phone: +49 331 288 2507

Patrick Eickemeier | idw
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