- Evapotranspiration (1) (remove)
- Modeling the exchange of energy and matter within and above a spruce forest with the higher-order closure model ACASA (2010)
- Multilayer SVAT-models that contain an advanced turbulence scheme are necessary for the detailed simulation of all relevant exchange processes above and within a forest canopy. The Advanced Canopy-Atmosphere-Soil Algorithm (ACASA) model incorporates such an advanced turbulence scheme, the third-order turbulence closure. This study presents the application of the ACASA model for a spruce forest at the Waldstein-Weidenbrunnen site in the Fichtelgebirge (Germany). The comprehensive micrometeorological and plant physiological measurements performed during the EGER project (ExchanGE processes in mountainous Regions) provided the necessary data base for this purpose, particularly eddy-covariance and sap flux measurements at several heights within the canopy. Thorough model tests were a main focus of this study and led to an improvement of the investigated model. This included both the exploration of the sensitivity and predictive uncertainty of the modeled fluxes and the analysis and correction of model errors that were encountered while working with the model. Furthermore, the ability of the ACASA model to reproduce measured quantities within and above the forest canopy was assessed, with an emphasis on the vertical structure of evapotranspiration and its components. To study the sensitivity and predictive uncertainty of the ACASA model, the Generalized Likelihood Uncertainty Estimation (GLUE) methodology was employed for two five day fair weather periods. Here, the sensitivity of the sensible heat flux, the latent heat flux and the net ecosystem exchange above the forest canopy was assessed. This analysis allowed the identification of influential parameters for the three fluxes. The fluxes were strongly sensitive to only a few parameters while the problem of equifinality was revealed for many parameters. Equifinality is a common problem for complex process-based SVAT-models. The calculated uncertainty bounds showed the ability of the ACASA model to well reproduce the fluxes for two periods with different meteorological conditions. Furthermore, the results of the GLUE analysis indicated weaknesses in the model structure concerning the soil respiration calculations. The latest ACASA version includes multiple improvements in comparison to older model versions which were introduced after a comparison of modeled within- and above-canopy fluxes and turbulence statistics with measurements. The former version of the ACASA model did not explicitly close the energy balance. Rather, an error was included in the model output. This modeled error, however, did not agree with the measured residual at our site and was shown to reach substantial magnitudes depending on the value of the leaf area index. Thus, a method to ensure a closed energy balance for all layers in ACASA was introduced. Measured third-order velocity statistics were largely underestimated by the former ACASA version, which required correcting the calculation algorithms for the third-order moments in the latest ACASA version. Comparisons of third- and second-order velocity statistics showed that simulations of the latest ACASA version were improved but only partly reproduced measurements. Sap flux and eddy-covariance measurements at several heights within the profile provided estimates of all components of evapotranspiration of the forest and its vertical distribution. Canopy transpiration of the stand measured with the eddy-covariance technique delivered larger estimates than measured with the sap flux technique. Possible reasons for this mismatch are discussed, such as a contribution of evaporation from intercepted water that was still present at the beginning of the study period and differences between the eddy-covariance footprint and the area to scale up sap flux measurements. The modeled evapotranspiration components by ACASA compared well to these measurements when taking the uncertainties of these measurements into account. Also, modeled in-canopy profiles of canopy (evapo-) transpiration agreed well with measurements, with a better agreement of mean profiles for daytime, a partly and completely coupled canopy than for nighttime and a decoupled canopy. Largest contributions to canopy (evapo-) transpiration stem from the upper half of the canopy at daytime, whereas during nighttime, the contribution shifted towards lower parts of the canopy. Additionally, model simulations of the 3D model STANDFLUX were included in this study. This study revealed that the ACASA model is a powerful tool to simulate in detail a large range of the relevant exchange processes within and above a spruce forest site. At the same time existing weaknesses in the model code were identified that should be improved in future ACASA versions.