Soil Moisture Soil moisture may be a more relevant quantity for assessing the impacts of changes in the hydrological cycle on vegetation than precipitation since it incorporates the integrated effects of changes in precipitation, evaporation, and runoff throughout the year. The increases are due mainly to the increased precipitation discussed above, and the increased reaction of precipitation falling as rain in the warmer climate. At the low winter temperatures, the absolute change in potential evaporation is small, as expected from the Clausius-Clapyeron relation, so evaporation increases little even though temperature increases are a maximum in winter. Hence, the increase in soil moisture in high altitudes in winter is consistent with physical reasoning and the broad scale changes are unlikely to be model-dependent. However, it should be noted that in general the models considered here do not represent the effects of freezing on groundwater.
Most models produce a drier surface in summer in northern mid-latitudes.
In higher latitudes, earlier snowmelt leading to enhanced solar absorption and evaporation may contribute. The drying is more pronounced in regions where precipitation is reduced in summer. The changes in the hydrological cycle are likely to be sensitive to the distribution of aerosol forcing and the coupled model used. However, it is clear that aerosol effects have a strong influence on simulated regional climate change.