7.2.2.4 Coupling of Precipitation Intensities to Leaf Water – An Issue Involving both Temporal and Spatial Scales

The bulk of the water exchanged with the atmosphere is stored in the soil until taken up by plant roots, typically weeks later. However, the rapidity of evaporation of the near-surface stores allows plant uptake and evaporation to be of comparable importance for surface water and energy balances. (Dickinson et al., 2003, conclude that feedbacks between surface moisture and precipitation may act differently on different time scales). Evaporation from the fast reservoirs acts primarily as a surface energy removal mechanism. Leaves initially intercept much of the precipitation over vegetation, and a significant fraction of this leaf water re-evaporates in an hour or less. This loss reduces the amount of water stored in the soil for use by plants. Its magnitude depends inversely on the intensity of the precipitation, which can be larger at smaller temporal and spatial scales. Modelling results can be wrong either through neglect of or through exaggeration of the magnitude of the fast time-scale moisture stores.

Leaf water evaporation may have little effect on the determination of monthly evapotranspiration (e.g., as found in the analysis of Desborough, 1999) but may still produce important changes in temperature and precipitation. Pitman et al. (2004), in a coupled study with land configurations of different complexity, were unable to find any impacts on atmospheric variability, but Bagnoud et al. (2005) found that precipitation and temperature extremes were affected. Some studies that change the intensity of precipitation find a very large impact from leaf water. For example, Wang and Eltahir (2000) studied the effect of including more realistic precipitation intensity compared to the uniform intensity of a climate model. Hahmann (2003) used another model to study this effect. Figure 7.1 compares their tropical results (Wang and Eltahir over equatorial Africa and Hahmann over equatorial Amazon). The model of Wang and Eltahir shows that more realistic precipitation greatly increases runoff whereas Hahmann shows that it reduces runoff. It has not been determined whether these contradictory results are more a consequence of model differences or of differences between the climates of the two continents, as Hahmann suggests.

Figure 7.1. Rainfall, runoff and evapotranspiration derived from climate simulation results of Hahmann (H; 2004) and Wang and Eltahir (W; 2000). Hahmann’s results are for the Amazon centred on the equator, and Wang and Eltahir’s for Africa at the equator. Both studies examined the differences between ‘uniform’ precipitation over a model grid square and ‘variable’ precipitation (added to about 10% of the grid square). Large differences are seen between the two cases in the two studies: a large reduction in precipitation is seen in the Hahmann variable case relative to the uniform case, whereas an increase is seen for the Wang and Eltahir variable case. The differences are even greater for runoff: Hahmann’s uniform case runoff is three times as large as the variable case, whereas Wang and Eltahir have almost no runoff for their uniform case.