The Climate and the Human Activities The Climate and the Human Activities Natural Variations of the Water Cycle Natural Variations of the Water Cycle Water Crisis in the 21 st Century Water Crisis in the 21 st Century The Human Impacts on the Water Cycle: The Human Impacts on the Water Cycle: Land Use Changes Land Use Changes Green House Gas Emission and Green House Gas Emission and Possible Changes in Extreme Events Possible Changes in Extreme Events Large Uncertainties Large Uncertainties An Intl. Water Cycle Research Initiative An Intl. Water Cycle Research Initiative 1

The Physical Basis for the Intensified Water Cycle when the Surface warms If temperature increases by only one degree Centigrade (°C) air can hold  6 % more water vapour (at 20°C)  8% more water vapour (at 0°C)  20% more water vapour (at –80°C) at saturation (Clausius-Clapeyron Equation) Rain rate increases on average with increasing surface temperatures Rain rate increases on average with increasing surface temperatures Flash floods become more frequent when the surface warms Flash floods become more frequent when the surface warms Flooding in mid-latitudes in winter and spring becomes more frequent Flooding in mid-latitudes in winter and spring becomes more frequent Consequences H.Grassl,

An Example Starting Point: Starting Point:  Flow across a mountain range at higher temperatures (+1°C)  same relative humidity Questions: Questions: How much more rain will fall on the windward side? (+6 + x) % more than without this temperature increase, whereby x is at least 2% and depends on the height if the mountain range. How much more will the river discharge? How much more will the river discharge? (+6 + x + y) % more, whereby y (+6 + x + y) % more, whereby y depends on the topography that determines the snowfall area lifted by 150 m. Result: Result: About that often will lead to serious flooding. About 10% increase in run-off per °C that often will lead to serious flooding. H.Grassl,

Predicted Changes in Water Cycle With very high confidence: Increase of the global average of precipitation Changes in the timing and regional patterns of precipitation Increase of the ratio of rain to snow and the flood frequencies in mountainous and higher latitude areas With high confidence Increase in annual average runoff in the high latitudes More severe water-quality problems With medium confidence Changes in flood frequencies with regional variety Changes in drought frequencies with regional variety With low confidence Projections for specific regions due to different model results Increase of the frequency and intensity of the heaviest precipitation events with a little agreement on detail regional changes due to the mismatch between the resolution of models and the extreme event scales. (U.S. Department of the Interior, 2000) 4

Reduces east-west pressure gradient, less rainfall extracted from jet stream. Demonstration that upwind soil moisture changes the rate at which atmospheric water vapor becomes precipitation (e.g. Betts et al.; Viterbo et al.) The predicted strength of floods in central Mississippi in 1993 was influenced by soil moisture status in Texas. 5

Improved methods for calibrating the many parameters in modern, complex models of vegetation and snow 6