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Zong-Liang Yang Guo-Yue Niu

Simulation of Runoff in CAM2/CLM2. Zong-Liang Yang Guo-Yue Niu. Department of Geological Sciences University of Texas at Austin. Introduction (Bonan et al., 2002, JCL):. Improvement.

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Zong-Liang Yang Guo-Yue Niu

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  1. Simulation of Runoff in CAM2/CLM2 Zong-Liang Yang Guo-Yue Niu Department of Geological Sciences University of Texas at Austin

  2. Introduction (Bonan et al., 2002, JCL): Improvement Compared to LSM1, CLM2 greatly improved the annual cycle of runoff, especially in arctic and boreal regions Degradation Excessive canopy evaporation (interception loss) in CLM2 Low soil moisture in the Amazon basin Hypothesis Inclusion of realistic canopy interception and runoff schemes will further improve hydrological simulations

  3. Experiments with CAM2/CLM2 Interception Default: precipitation occurs over the entire grid cell Experiment 1: assuming subgrid-scale precipitation is given by a pdf of an exponential form, modifying canopy interception and throughfall (Wang) (Interception Run) Runoff Default: TOPMODEL concept, but surface runoff and baseflow are based on BATS; super-saturated water discarded as baseflow Experiment 2: Experiment 1 + more closely following TOPMODEL but improved schemes for water table, saturated area, surface runoff and baseflow; super-saturated water recharged to unsaturated soil layers (Simplified Topmodel Run) Experiment 3: Experiment 2 + explicit use of global topographic (wetness) indices (Full Topmodel Run)

  4. Water Table Depth Topography-based Runoff Scheme Runoff production mechanism • Surface runoff • Saturation excess • Infiltration excess • Subsurface runoff • Topographic control • Bottom drainage • “Over-saturated” water recharged into upper unsaturated layers Saturation Excess Infiltration Excess Topography Bottom Super-saturation

  5. Observed Seasonal Variations of Saturated Area Seasonal variation in pre-storm saturated area [from Dunne and Leopold, 1978]

  6. Interception versus Default

  7. Improvement index Relative Improvement of the Interception Run over the Default Run Compared to Observations Runoff Degrade Improve

  8. Simplified Topmodel versus Default

  9. Relative Improvement of the Simplified Topmodel Run over the Default Run Compared to Observations Runoff Degrade Improve

  10. Regional Average

  11. The Amazon Basin

  12. Full Topmodel versus Simplified Topmodel

  13. Mean Topographic Index at T42 Derived from 1km topography, but adjusted with 100 m data Directly affecting baseflow

  14. Wolock and McCabe (2000)

  15. Gamma CDF(%) Topographic index Cumulative  Distribution Function and Saturated Fraction 8.5 + 2.0*0.0 = 8.5 ~ 40% 8.5 + 2.0*1.0 = 10.5 ~ 20%

  16. Potential Saturated Fraction (water table depth = 0) Directly affecting surface runoff

  17. Relative Improvement of the Full Topmodel Run over the Simplified Topmodel Run Compared to Observations Runoff Degrade Improve

  18. Water table depth and saturated fraction in July Simplified TOPmodel TOPmodel

  19. Regional Average

  20. Conclusions The interception and runoff processes are intimately coupled, and their schemes should be changed together to ensure the improvement in the hydrological simulations. The simplified TOPMODEL runoff scheme and the subgrid interception scheme proposed here, when used together, result in better simulations of runoff. With the above schemes, the water budgets show favorable changes in the Amazon basin. Canopy evaporation is reduced, runoff is increased, and soil is wetter. The full version of TOPMODEL shows marginal improvement in the tropical regions but more work is needed to increase the accuracy of the global topographic index.

  21. snow Soil water Ground water Lake Ocean Saturated Hydraulic Conductivity Exponential decay rate Soil Depth Implementing TOPMODEL into Community Land Model Evapotranspiration Supply Limited ET PDF of Topographic Index Demand Limited ET Saturated Zone Infiltration Mean Water Table Depth Runoff = Surface runoff + Baseflow

  22. Sensitivity to ‘anisotropic factor’ resulted from TOP2 Sivapalan et al. (1987) Introduced by Chen and Kumar (2001) to account for the differences of the saturated hydraulic conductivities in the lateral and vertical directions. x=2.5 obtains the best fit with observed streamflow. This value coincided with the work of Zhang and Montgomery (1994) . This parameter may be interpreted as a compensating agent to reduce the error caused from using a coarse-resolution DEM

  23. TOPMODEL • Runoff subgrid topographysubgrid soil moisture • Inadequate treatment in GCMs • Our goal is implement TOPMODEL into NCAR CLM2/CAM2

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