[1] The Community Land Model version 3 (CLM3) is the land component of the Community Climate System Model (CCSM). CLM3 has energy and water biases resulting from deficiencies in some of its canopy and soil parameterizations related to hydrological processes. Recent research by the community that utilizes CLM3 and the family of CCSM models has indicated several promising approaches to alleviating these biases. This paper describes the implementation of a selected set of these parameterizations and their effects on the simulated hydrological cycle. The modifications consist of surface data sets based on Moderate Resolution Imaging Spectroradiometer products, new parameterizations for canopy integration, canopy interception, frozen soil, soil water availability, and soil evaporation, a TOPMODEL-based model for surface and subsurface runoff, a groundwater model for determining water table depth, and the introduction of a factor to simulate nitrogen limitation on plant productivity. The results from a set of offline simulations were compared with observed data for runoff, river discharge, soil moisture, and total water storage to assess the performance of the new model (referred to as CLM3.5). CLM3.5 exhibits significant improvements in its partitioning of global evapotranspiration (ET) which result in wetter soils, less plant water stress, increased transpiration and photosynthesis, and an improved annual cycle of total water storage. Phase and amplitude of the runoff annual cycle is generally improved. Dramatic improvements in vegetation biogeography result when CLM3.5 is coupled to a dynamic global vegetation model. Lower than observed soil moisture variability in the rooting zone is noted as a remaining deficiency.

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Improvements to the Community Land Model and their impact on the hydrological cycle

Measurement of soil water content and electrical conductivity by time domain reflectometry: a review

The energy balance components were measured throughout most of 1994 in and above a southern boreal aspen (Populus tremuloides Michx.) forest (53.629°N 106.200°W) with a hazelnut (Corylus cornuta Marsh.) understory as part of the Boreal Ecosystem-Atmosphere Study. The turbulent fluxes were measured at both levels using the eddy-covariance technique. After rejection of suspect data due to instationarity or inhomogeneity, occasional erratic behavior in turbulent fluxes and lack of energy balance closure led to a recalculation of the fluxes of sensible and latent heat using their ratio and the available energy. The seasonal development in leaf area was reflected in a strong seasonal pattern of the energy balance. Leaf growth began during the third week of May with a maximum forest leaf area index of 5.6 m 2 m -2 reached by mid-July. During the full-leaf period, aspen and hazelnut accounted for approximately 40 and 60% of the forest leaf area, respectively. Sensible heat was the dominant consumer of forest net radiation during the preleaf period, while latent heat accounted for the majority of forest net radiation during the leafed period. Hazelnut transpiration accounted for 25% of the forest transpiration during the summer months. During the full-leaf period (June 1 to September 7) daytime dry-canopy mean aspen and hazelnut canopy conductances were 330 mmol m -2 s -1 (8.4 mm s -1 ) (70% of the total forest conductance) and 113 mmol m -2 s -1 (2.9 mm s -1 ) (24% of the total forest conductance), respectively. Maximum aspen and hazelnut canopy conductances were 1200 mmol m -2 s -1 (30 mm s -1 ) and 910 mmol m -2 s -1 (23 mm s -1 ), respectively, and maximum stomatal conductances were 490 mmol m -2 s -1 (12.5 mm s -1 ) and 280 mmol m -2 s -1 (7 mm s -1 ), aspen and hazelnut, respectively. Both species showed a decrease in canopy conductance as the saturation deficit increased and both showed an increase in canopy conductance as the photosynthetic active radiation increased. There was a linear relationship between forest leaf area index and forest canopy conductance. The timing, duration, and maximum leaf area of this deciduous boreal forest was found to be an important control on transpiration at both levels of the canopy. The full-leaf hazelnut daytime mean Priestley and Taylor [1972] α coefficient of 1.22 indicated transpiration was largely energy controlled and the quantity of energy received at the hazelnut surface was a function of aspen leaf area. The full-leaf aspen daytime mean α of 0.91 indicated some stomatal control on transpiration, with a directly proportional relationship between forest leaf area and forest canopy conductance, varying α during much of the season through a range very sensitive to regional scale transpiration and surface-convective boundary laver feedbacks.

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Energy balance and canopy conductance of a boreal aspen forest: Partitioning overstory and understory components

Time-Domain Reflectometry (TDR) and Its Application to Irrigation Scheduling*

The three-layer snow model is coupled to the global catchment-based Land Surface Model (LSM) of the NASA Seasonal to Interannual Prediction Project (NSIPP) project, and the combined models are used to simulate the growth and ablation of snow cover over the North American continent for the period 1987-1988. The various snow processes included in the three-layer model, such as snow melting and re-freezing, dynamic changes in snow density, and snow insulating properties, are shown (through a comparison with the corresponding simulation using a much simpler snow model) to lead to an improved simulation of ground thermodynamics on the continental scale.

/pdf/the-impact-of-detailed-snow-physics-on-the-simulation-of-55kzisd99v.pdf

The Impact of Detailed Snow Physics on the Simulation of Snow Cover and Subsurface Thermodynamics at Continental Scales

The results of a field test of time‐domain reflectometry (TDR) to measure apparent liquid soil water contents and to locate the unfrozen‐frozen interface during thawing conditions is presented. The apparent liquid water content was observed in the fall and through a late winter thaw on two sand sites, one with a natural snow cover and the other with snow removed throughout the winter. Temperatures were monitored at intervals throughout the profile. The results indicate that TDR provides a method for monitoring apparent liquid water content andfreeze‐thaw processes.

Measurement of soil water contents and frozen soil depth during a thaw using time‐domain reflectometry

A physically‐based numerical model was developed to estimate the temporal course of the surface energy flux densities and the soil temperatures in dry and wet bare soils. Aerodynamic heat, vapour and momentum transfer theory was used to calculate the sensible and latent heat flux densities at the surface under diabatic and adiabatic conditions. A finite‐difference solution of the differential equation describing one‐dimensional heat transfer was used to calculate the surface soil heat flux density and soil profile temperatures. The surface temperature was determined iteratively by the simultaneous solution of equations describing radiative, heat and momentum transfer at the surface. The model was tested with measurements from energy balance studies conducted on a dry, sandy soil and a wet, silt loam soil, and was found to predict accurately the surface energy fluxes and soil temperatures over three‐day periods under conditions of potential and negligible evaporation. The sensitivity of the model ...

Modelling surface energy fluxes and temperatures in dry and wet bare soils

Surface soil wetness determines whether evaporation occurs at the potential rate or is limited by soil water supply. Many land surface models calculate evaporation by parametrizing the relative humidity at the soil surface (α method) or the soil water diffusion resistance (β method). The relationships of α and β to the average moisture content of soil surface layer thicknesses ranging from 0.5 to 10 cm were examined using Bowen ratio/energy balance measurements of evaporation from a bare loam/silt‐loam soil at two adjacent sites, one of which was culti‐packed while the other was disc‐harrowed. It was found that the relationships were sensitive to the surface layer thickness and tillage treatment and became better defined with larger thicknesses. Simulations of evaporation by CLASS (Canadian Land Surface Scheme) using Philip's relationship (currently used in CLASS to determine α with a 10‐cm thick surface soil layer) and our proposed relationships of α and β were compared with measurements made du...

https://www.tandfonline.com/doi/pdf/10.1080/07055900.2000.9649638

Testing the α and β methods of estimating evaporation from bare and vegetated surfaces in class

The ability of the Canadian Land Surface Scheme (CLASS) to simulate energy and moisture fluxes over tundra surfaces is tested using three dataseis collected at alpine sites in southern Alberta and British Columbia, Canada. Initial runs of the model indicate that the ground heat flux tends to be overestimated and the latent heat flux underestimated on average. With the incorporation of minor modifications to the surface thermal conductivity, the vegetation rooting depth and the calculation of the surface soil moisture, the mean bias errors in the latent and ground heat fluxes are reduced to more acceptable levels. Despite the fact that the current version of CLASS does not explicitly take into account the effects of spatial heterogeneity at the sites, the model is found to perform reasonably well with these modifications. It is recommended that the next version of CLASS incorporate a mosaic approach which will allow further subdivision of the modelling area, and that a set of algorithms specific t...

Application of the Canadian land surface scheme (class) to the simulation of energy and water fluxes over alpine tundra

The Canadian Land Surface Scheme (CLASS), a land surface parametrization scheme for use in large scale climate models, was assessed using the observed surface climate data from a full canopy crop in southern Ontario during a 10 day drying cycle period. Half‐hourly and daily modelled radiation and energy balance components and other surface climate parameters were compared to field observations. Different simulations using CLASS were employed and each contained different degrees of realism in model initialization. In general, CLASS simulations of radiation and energy balance components and surface climate parameters were good. Further, only modest improvements in model performance were achieved using more stringent initializations when compared to CLASS default specifications. It was also demonstrated that routine meteorological observations from an Atmospheric Environment Service network station could be used as meteorological input to CLASS without a loss in model output quality. From the resear...

W.G. Bailey

Papers

Measurement of soil water contents and frozen soil depth during a thaw using time‐domain reflectometry

Modelling surface energy fluxes and temperatures in dry and wet bare soils

Testing the α and β methods of estimating evaporation from bare and vegetated surfaces in class

Application of the Canadian land surface scheme (class) to the simulation of energy and water fluxes over alpine tundra

Application of the Canadian land surface scheme to a full canopy crop during a drying cycle