Sensitivity of an ecological model to soil moisture simulations from two different hydrological models

Although advanced land surface schemes have beendeveloped in the past decade, many biosphere models stilluse the simple bucket model, partly due to its efficiencywhen it is coupled with an CGCM model. In this paper,we use a sophisticated land surface model, the Simulatorfor Hydrology and Energy Exchange at the Land Surface(SHEELS), including an explicit vegetation canopy and itsphysiological control on evapotranspiration and multiple,interactive subsurface soil layers. It is found that this modelhas potential for improving the carbon cycling descriptionof a widely used biosphere model, the Carnegie-Ames-Stanford approach (CASA), especially for multiple seasonalintegrations.Verifying with observations from Oklahoma AtmosphericSurface-layer Instrumentation System (OASIS) stations, weshow that a bucket model as implemented in the CASAproduces good simulations of the seasonal cycle of soil moisturecontent, but only for the upper 15-cm soil depth, nomatter how it is initialized. This is partly due to its inabilityto include vegetation characteristics other than a fixed wiltingpoint. Although only approximate, the soil depth towhich CASA simulates storage of below-ground carbon isassumed to be about 30 cm depth, significantly deeper thanthe 15 cm depth. The bucket model cannot utilize the soilprofile measurements that have recently been made widelyavailable.A major finding of this study is that carbon fluxes aresensitive to the soil moisture simulations, especially the soil water content of the upper 30cm. The SHEELS exhibitspotential for simulating soil moisture, and hence the totalsoil water amount, accurately at every level. For the NetPrimary Production (NPP) parameter, the differences betweentwo hydrological schemes occur primarily duringthe growing seasons, when the land surface processes aremore important for climate. However, soil microbial respirationis found to be sensitive all year round to soil moisturesimulations at our 7 selected Oklahoma Mesonet stations.These suggest that for future implementing of interactiverepresentation of soil carbon in CGCMs, the accompanyinghydrological scheme should not be over-simplified

Available energy and energy balance closure at four coniferous forest sites across Europe

The available energy (AE), driving the turbulent fluxes of sensible heat and latent heat at the earth surface, was estimated at four partly complex coniferous forest sites across Europe (Tharandt, Germany; Ritten/Renon, Italy; Wetzstein, Germany; Norunda, Sweden). Existing data of net radiation were used as well as storage change rates calculated from temperature and humidity measurements to finally calculate the AE of all forest sites with uncertainty bounds. Data of the advection experiments MORE II (Tharandt) and ADVEX (Renon, Wetzstein, Norunda) served as the main basis. On-site data for referencing and cross-checking of the available energy were limited. Applied cross checks for net radiation (modelling, referencing to nearby stations and ratio of net radiation to global radiation) did not reveal relevant uncertainties. Heat storage of sensible heat J (H), latent heat J (E), heat storage of biomass J (veg) and heat storage due to photosynthesis J (C) were of minor importance during day but of some importance during night, where J (veg) turned out to be the most important one. Comparisons of calculated storage terms (J (E), J (H)) at different towers of one site showed good agreement indicating that storage change calculated at a single point is representative for the whole canopy at sites with moderate heterogeneity. The uncertainty in AE was assessed on the basis of literature values and the results of the applied cross checks for net radiation. The absolute mean uncertainty of AE was estimated to be between 41 and 52 W m(-2) (10-11 W m(-2) for the sum of the storage terms J and soil heat flux G) during mid-day (approximately 12% of AE). At night, the absolute mean uncertainty of AE varied from 20 to about 30 W m(-2) (approximately 6 W m(-2) for J plus G) resulting in large relative uncertainties as AE itself is small. An inspection of the energy balance showed an improvement of closure when storage terms were included and that the imbalance cannot be attributed to the uncertainties in AE alone