17TH Conference on Hydrology


The interplay between vegetation evapotranspiration processes and the seasonal soil moisture evolution at the catchment scale

Reto Stöckli, ETH, Zürich, Switzerland; and P. L. Vidale

The importance of correctly including land surface processes in climate models has been increasingly recognized in the past years. Even on seasonal to interannual time scales land surface - atmosphere feedbacks can play a substantial role in determining the state of the near-surface climate. The availability of soil moisture for both runoff and evapotranspiration is dependent on biophysical processes occuring in plants and in the soil acting on a wide time-scale from minutes to years.

Euroflux site measurements in various climatic zones are used to drive three generations of LSM's (land surface models) in order to assess the level of complexity needed to represent vegetation processes at the local scale. The three models were the Bucket model (Manabe 1969), BATS 1E (Dickinson 1984) and SiB 2 (Sellers et al. 1996). Evapotranspiration and runoff processes simulated by these models range from simple one-layer soils and no-vegetation parameterizations to complex multilayer soils, including realistic photosynthesis-stomatal conductance models. The latter is driven by satellite remote sensing land surface parameters inheriting the spatiotemporal evolution of vegetation phenology. In addition a simulation with SiB 2 not only including vertical water fluxes but also lateral soil moisture transfers by downslope flow is conducted for a pre-alpine catchment in Switzerland.

Preliminary results are presented and show that - depending on the climatic environment and on the season - a realistic representation of evapotranspiration processes including seasonally and interannually-varying state of vegetation is significantly improving the representation of observed latent and sensible heat fluxes on the local scale. Moreover, the interannual evolution of soil moisture availability and runoff is strongly dependent on the chosen model complexity. For some sites, however, the hydrological cycle is simulated reasonably well even with simple land surface representations. Gravitational flow in mountaineous areas leading to a spatial water redistribution leads to differences in the seasonal soil moisture availability at catchment scale with possible non-linear effect on land surface - atmosphere feedback processes. Simulating integrated surface fluxes for a large area (e.g. in a regional or global climate model) may require to include local scale hydrological processes.

Session 3, Land-Atmosphere Interactions 2: Process Representation and Evaluation
Tuesday, 11 February 2003, 1:30 PM-5:30 PM

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