Aihui Wang, Kaiyuan Li, and Dennis P. Lettenmaier

1. University of Washington Surface runoff 9c m ~ 10cm Surface runoff Not fixed Base flow 3.43 m Base flow 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 Comparison of model simulated streamflow with observation over two large river basins Monthly averaged snow water equivalent (SWE) over a grid cell (43.4N, -110.2W) 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. 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 CLM-VIC simulated ET better than CLM. 2. Soil hydrology schemedescriptions • 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. • 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). CLM3 VIC • 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. • 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. b. Hapex-Mobilhy ~ 20cm Arkansas-Red River basin ~ 1.5m • 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…). • 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. • 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. • 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. • 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. ;;;;; Colorado River Basin • 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 • 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. • 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. 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. 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. Colorado River Basin • 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. • 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. References 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