The Climate and the Human ActivitiesThe Climate and the Human Activities

Natural Variations of the Water CycleNatural Variations of the Water Cycle

Water Crisis in the 21Water Crisis in the 21stst Century Century

The Human Impacts on the Water Cycle:The Human Impacts on the Water Cycle: Land Use ChangesLand Use Changes Green House Gas Emission andGreen House Gas Emission and Possible Changes in Extreme EventsPossible Changes in Extreme Events

Large UncertaintiesLarge Uncertainties

An Intl. Water Cycle Research InitiativeAn Intl. Water Cycle Research Initiative1

The Physical Basis for the Intensified The Physical Basis for the Intensified Water Cycle when the Surface warmsWater Cycle when the Surface warmsIf temperature increases by only one degree Centigrade If temperature increases by only one degree Centigrade (°C) air can hold (°C) air can hold

6 % more water vapour (at 20°C) 6 % more water vapour (at 20°C)

8% more water vapour (at 0°C)8% more water vapour (at 0°C)

20% more water vapour (at –80°C)20% more water vapour (at –80°C)

at saturation (Clausius-Clapeyron Equation)at saturation (Clausius-Clapeyron Equation)

Rain rate increases on average with Rain rate increases on average with increasing surface temperaturesincreasing surface temperatures

Flash floods become more frequent Flash floods become more frequent when the surface warmswhen the surface warms

Flooding in mid-latitudes in winter Flooding in mid-latitudes in winter and spring becomes more frequentand spring becomes more frequent

ConsequencesConsequences

H.Grassl, 20022

An ExampleAn Example Starting Point: Starting Point:

Flow across a mountain range at higher temperatures (Flow across a mountain range at higher temperatures (+1°C+1°C)) same relative humiditysame relative humidity

Questions: Questions: How much more rain will fall on the windward side? How much more rain will fall on the windward side? ((+6 +6 + x+ x) % more than without this temperature increase, ) % more than without this temperature increase, whereby x is at least 2% and whereby x is at least 2% and depends on the heightdepends on the height if the if the mountain range.mountain range.

How much more will the river discharge? How much more will the river discharge?

((+6+6 + x + y+ x + y) % more, whereby y ) % more, whereby y depends on the topography that determines the snowfall area lifted by 150 m.

Result: Result: About About 10% increase in run-off per °C that often will lead to that often will lead to

serious flooding.serious flooding. H.Grassl, 2002 3

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

Virtual Temperature

Height

Cloud

Virtual Temperature

Cloud

Height

Dry Texas Wet Texas

Reduces east-west pressure gradient,less rainfall extracted from jet stream.

Demonstration that upwind soil moisture changes the rateat which atmospheric water vapor becomes precipitation

(e.g. Betts et al.; Viterbo et al.)

The predicted strength of floods in central Mississippiin 1993 was influenced by soil moisture status in Texas. 5

Sun

Interception Evaporation

Albedo

WindTranspiration

Surface Runoff Evaporation

PercolationInfiltration

Water

Precipitation

LeafDrip Stem

Improved methods for calibrating the many parameters inmodern, complex models of vegetation and snow

Atmosphere

Soil

RainfallSnowfall

Melting/Refreezing

Snow compaction

Infiltration

Runoff

Albedo

Hea

t con

duction

sT1T

2T

3T

Snow

Sublimation Evaporation

Wind

Long

wav

era

diation

Atmosphere

Soil

RainfallSnowfall

Melting/Refreezing

Snow compaction

Infiltration

Runoff

Albedo

Hea

t con

duction

sT1T

2T

3T

Snow

Sublimation Evaporation

Wind

Long

wav

era

diation

6