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Integration of the VIC model soil hydrology scheme into CLM
Aihui Wang, Kaiyuan Li, and Dennis P. Lettenmaier
Department of Civil and Environmental Engineering, University of Washington
5. Model evaluations
1. Introduction
Land surface models predict energy and moisture fluxes at the land surface for coupled land-atmosphere
models. Of the various processes parameterized in such models, representation of soil hydrology is
particularly important since it affects both the surface water budget, and the surface energy budget through
the evaporation feedback. In order to improve its land surface hydrology representation, a new soil
hydrology scheme for use in the NCAR Community Land Model (CLM) is proposed. The new scheme
incorporates the fundamental principles and concepts of the three-layer Variable Infiltration Capacity (VIC)
soil moisture generation scheme, as well as its surface runoff and base flow schemes. The modified version of
CLM makes direct use of VIC soil parameters that have been developed for off-line regional, continental,
and global simulations. The performance of the new model and CLM are evaluated through a comparison
with observations. Overall, the results show that the new model better reproduces observed soil hydrological
variability, in particular, the seasonal evolution and amplitude of soil moisture as compared to CLM. The
river basin simulations show that in the new model, total runoff is generally less than in CLM, and better
agrees with observations. Due to the interaction of runoff and soil moisture, the simulation of total
evapotranspiration (or latent heat) is also improved in the new model.
2. Soil hydrology scheme descriptions
CLM3
VIC
9c m
Surface runoff
~ 10cm
Surface runoff
~ 20cm
Not fixed
Base flow
3.43 m
University of Washington
Base flow
~ 1.5m
CLM
• Surface runoff scheme is based on
TopModel and BATs (Beven and Kirkby,
1979 , Dickinson et al. 1993),and is
contributed by soil water from the top
3 layers.
• Baseflow is from the soil water and
lateral runoff in 6-9 layers and the 10th
layer drainage.
Monthly averaged snow water equivalent (SWE)
over a grid cell (43.4N, -110.2W)
Comparison of model simulated streamflow with observation over two
large river basins
Comparison of modeled water and energy fluxes at tower flux sites
a. Abracos
• CLM-VIC captures the magnitude of soil moisture
over the wet seasons, although the simulated soil is
slightly wetter than observed in the dry season,
while the soil in CLM3 is too dry, and the evolution
of soil moisture has much less seasonal variation
than the observations.
• CLM-VIC simulated ET is closer to observed
whereas CLM3 in general underestimates ET. The
timing of base flow in CLM-VIC is consistent with
the timing of the maximum soil moisture storage,
whereas in CLM3 it is lagged by about two months.
• The net radiation is well simulated in both models.
CLM-VIC does a better job in the simulation of
laten heat than the CLM, which also reflects CLMVIC simulated ET better than CLM.
• The RMSEs and relative biases from CLM-VIC are general smaller than that from
standard CLM, which confirm the improved performance of the new model for the larger
drainage areas.
• CLM-VIC modeled soil moisture is in overall
agreement with observation of both seasonal variation
and amplitude, beside from slight underestimates in the
wet season, while the soil in CLM is too dry and soil
moisture variations are too small.
b. Hapex-Mobilhy
Arkansas-Red River basin
VIC
• Surface runoff is controlled by the
infiltration capacity formulation and
its water comes from the top two layers
soil water.
• Baseflow follows a nonlinear curve
whose source is the third soil layer
(Liang et al. 1994).
• Both models simulate net radiation well, but
overestimate the sensible heat and underestimate latent
heat. CLM-VIC simulated latent heat is better than in
CLM, which corresponds to better simulation of ET in
CLM-VIC.
• Note, the poor simulation of soil heat fluxes could be
the result of mischaracterization of the physics of soil
heat in the model or the different representations of the
measured and modeled soil heat.
3. Methodology
• Implement VIC soil moisture generation scheme, as well as its surface runoff and base flow scheme
into CLM3; remain all other processes (e.g., vegetation related process, soil thermal process)
unchanged.
• The CLM3 10-soil layer is divided into 3 layers which match the VIC layer depths, and all VIC soil
parameters are read into CLM (soil hydraulic conductivity, base flow parameters, etc…).
• CLM tends largely overestimates runoff. The simulated streamflow by CLM-VIC is closer
observation than in CLM, even though the simulated streamflow peck is still somewhat higher
than the observed.
• CLM-VIC performs better in some downstream areas such as Little Rock and Shreveport.
c. Valdai
Summary
• The Variable Infiltration Capacity model (VIC) soil hydrology scheme has been
incorporated into the NCAR Community Land Model (CLM3). The new model
makes direct use of VIC soil parameters that have been developed for off-line
regional, continental, and global simulations. The new model was evaluated using
streamflow data from two large river basins, as well as the surface flux over tower
observations.
Colorado River Basin
• Compute soil moisture, runoff, and base flow in the VIC scheme. The simulated soil moisture were
compared with flux tower measurement. The simulated runoff pluses base flow was routed to the
location of naturalized flow records using a routing algorithm, and then compared with observation.
• The new model improves the soil hydrology representation and in turn surface
moisture and energy fluxes, especially in the reproduction of streamflow and soil
moisture.
• The variation and amplitude of soil moisture in the new model are generally in
closer agreement with observations.
4. Data descriptions and experiment designs
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
• Two large river basins
Red-Arkansas River Basin
• Three fluxe towers
a. Abracos (10.1S, 61.9W) — a low latitude pasture clearing
in the Amazon rain forest site
b. Hapex-mobilhy (43.7N, 0.1W) — a mid-latitude
agricultural site
c. Valdai (57.6N, 33.1W) — a high-latitude grassland site
;;;;;
•
Similar to other sites, CLM-VIC simulated soil moisture has
more variation and is closer to observations than simulated
by CLM3.
•
The CLM-VIC simulated ET is in closer agreement with the
observations than that from CLM.
•
Colorado River Basin
Over two river basins, meteorology forcing data are from North
American Land Data Assimilation System (NLDAS) with a resolution of
1/8 (Maurer et al. 2002), soil and vegetation parameters are also from
Maurer et al. Using the Lohmann et al. (1998) routing algorithm at a daily
time step. The simulated runoff and bas flow were routed onto the gauge
location and compared with observed naturalized streamflow.
Over the flux tower sites, the forcings and model setup follows the PILPS
project. The simulated surface soil hydrological variables and fluxes data
are compared with the available observation
The CLM3 with the VIC soil hydrology scheme is referred as CLM-VIC, and
the standard CLM3 is referred as CLM.
Both model-simulated snow depths are in general
agreement with observations, although the maximum snow
depth is overestimated in some years by both models and the
snow melts too early in spring in both models.
• The new model produced streamflows that are generally smaller than those from
CLM3 and in closer agreement with observations. Due to the interaction of runoff
and soil moisture, the simulation of total evapotranspiration (or latent heat) is also
improved in the new model.
Acknowledgements:
The research reported herein was supported by the U.S. Department of Energy
under DOE Agreement Number DE-FG02-04ER63873 to the University of
Washington.
• CLM-VIC performs reasonably well in reproducing observed streamflow, while CLM3
overestimates seasonal peak stream-flow during the entire comparison period for all stations.
• CLM-VIC performance is quite similar to the VIC model in its reproduction of streamflow.
References
• The runoff in the Colorado basin is mainly contributed by snowmelt water. A consistent
phase shift exists in the CLM simulations, that is, the runoff peck appears about one month
early in CLMs’ simulation. The reason is most likely a bias toward early snow melt in CLM
relative to VIC. The below figure compares snow water equivalent (SWE) from both CLM
and VIC over a grid cell (43.4N, -110.2W) in the upper portion of the basin, where most
snowmelt originates.
Wang A., K. Li, and D.P. Lettenmaier (2007), Integration of the Variable Infiltration Capacity (VIC) model soil hydrology scheme into the Community
Land Model (CLM), J. Geophys. Res. (in review).
Liang, X., D. P. Lettenmaier, E. F. Wood, and S. J. Burges (1994), A simple hydrologically based model of land surface water and energy fluxes for
general circulation models. J. Geophys. Res., 99, 14,415-14,428.
Oleson K. W., and coauthors (2004), Technical description of the community land model (CLM), NCAR Technical Note (NCAR/TN-461+STR), 174pp