Zong-Liang Yang, Guo-Yue Niu and Robert E. Dickinson*

1
Modeling Runoff in CLM
Zong-Liang Yang, Guo-Yue Niu and Robert E.
Dickinson The University of Texas at Austin
Georgia Institute of Technology
Prepared for Hydrology Project, CCSM Land Model
Working Group Meeting March 27,
2006 www.geo.utexas.edu/climate
Introduction SIMTOP Validation SIMGM
Validation Conclusions
2
Outline

1. Introduction
2. Design of a Simple TOPMODEL-Based runoff Scheme
   (SIMTOP)
3. Validate SIMTOP against GRACE ?S
4. Development of a Simple Groundwater Model
5. Assess Model against GRACE ?S
6. Conclusions

Introduction SIMTOP Validation SIMGM
Validation Conclusions
3
Design of the Simple TOPMODEL-Based Runoff Scheme
(SIMTOP)
Surface Runoff Rs P Fmax e C f zwt p
precipitation zwt the depth to water table f
the runoff decay parameter which determines
recession curve Fmax and C topographic
parameters Subsurface Runoff Rsb Rsb,maxe f
zwt Rsb,max the maximum subsurface runoff when
the grid-mean water table is zero. It should be
related to lateral hydraulic conductivity of an
aquifer and local slopes. Rsb,max1.0x10-4 mm/s
through sensitivity experiments. SIMTOP
parameters Two calibration parameters
Rsb,max and f Two topographic parameters Fmax
and C
Introduction SIMTOP Validation SIMGM
Validation Conclusions
4
Justification of Surface Runoff Formulation and
Derivation of Topographic parameters
Relationship Between the Saturated Area and Water
Table Depth
Map of saturated areas showing expansion during a
single rainstorm. Dunne and Leopold, 1978
zwt
fsat Fmax(?) e C f zwt
fsat
? topographic wetness index derived from DEM
Introduction SIMTOP Validation SIMGM
Validation Conclusions
5
Justification of Surface Runoff Formulation and
Derivation of Topographic parameters
Topographic Wetness Index ? ln(a/tanß)
ln(a) ln(S)
The higher topographic wetness index, the wetter
the pixel
Introduction SIMTOP Validation SIMGM
Validation Conclusions
6
Justification of Surface Runoff Formulation and
Derivation of Topographic parameters
TOPMODEL (Beven and Kirkby, 1979 Sivapalan et
al., 1987) zi zm (?i
?m) / f where zi and ?i are water table depth and
topographic index at a pixel while zm and ?m are
their grid-cell (catchment) mean values. The
Saturated Fraction the Grid-Cell Fsat
Prob zi lt 0 or Prob ?i gt ?m fzm
zi, ?i
zm ?m
Lowland
highland
Introduction SIMTOP Validation SIMGM
Validation Conclusions
7
Justification of Surface Runoff Formulation and
Derivation of Topographic parameters
A 1 x 1 arid-cell in the Amazon River basin
Both Gamma and exponential functions fit for ?i gt
?m Fmax 0.45 C 0.6
Introduction SIMTOP Validation SIMGM
Validation Conclusions
8
Justification of Surface Runoff Formulation and
Derivation of Topographic parameters
A 1 x 1 arid-cell in Northern Rocky Mountain
Gamma function fails, while exponential function
works. Fmax 0.30 C 0.5
Introduction SIMTOP Validation SIMGM
Validation Conclusions
9
Justification of Surface Runoff Formulation and
Derivation of Topographic parameters
Fmax0.35 C 0.51 to 1.10
Woods and Sivapalan (2003)
Exponential function fits very well in
well-developed catchments.
The larger the catchment, the better the fitting.
C 0.6
Introduction SIMTOP Validation SIMGM
Validation Conclusions
10
Global Fmax
a Discrete Distribution
b Gamma Function
c Error of Gamma (b -- a)
Introduction SIMTOP Validation SIMGM
Validation Conclusions
11
Justification of Subsurface Runoff Formulation
TOPMODEL (Beven and Kirkby, 1979)
Rsb Rsb,max e fS where S is the deficit of
the subsurface water storage Sivapalan et al.,
(1987) and Stieglitz et al. (1997 )
Rsb K0/f e ? e f zwt It needs very
large K0, which is justified by soil surface
macropore (1000 times lager than in LSM) Chen
and Kumar (2001) Rsb a K0/f e ?
e f zwt (where aK0 is the lateral K)
Difficulties in determining a globally ? needs
very high resolution DEM (30 m or finer) to
determine slopes. Niu et al. (2005)
Rsb Rsb,max e f zwt (Rsb,max 1.0x10-4
mm/s)
Introduction SIMTOP Validation SIMGM
Validation Conclusions
12
The Exponential Relationship between Streaflow
and Water Table Depth
Groundwater level is highly correlated with
streamflow in a strong nonlinear manner and
explains 2/3 of the streamflow (Yeh and Eltahir,
2005)
Introduction SIMTOP Validation SIMGM
Validation Conclusions
13
Diagnostic Water Table Depth from Soil Moisture
Profile
?i zi
Capillary
?sat zwt
Gravity
Gravity
Soil water profile when gravity equals to
capillary force
Water profile under gravity
Koster et al. (2000)
Chen and Kumar (2001)
Introduction SIMTOP Validation SIMGM
Validation Conclusions
14
Validation Against GRACE Terrestrial Water
Storage Change Data
Introduction SIMTOP Validation SIMGM
Validation Conclusions
15
Validation Against GRACE Terrestrial Water
Storage Change Data
Introduction SIMTOP Validation SIMGM
Validation Conclusions
16
Validation Against GRACE Terrestrial Water
Storage Change Data
Introduction SIMTOP Validation SIMGM
Validation Conclusions
17
Validation Against GRACE Terrestrial Water
Storage Change Data
Introduction SIMTOP Validation SIMGM
Validation Conclusions
18
Prognostic Water Table A Simple Groundwater Model
Water storage in an unconfined Aquifer
Recharge Rate
Gravitational Drainage
Introduction SIMTOP Validation SIMGM
Validation Conclusions
19
Model Design A Simple Groundwater Model (SIMGM)
Groundwater Discharge (Baseflow or Subsurface
Runoff)
SIMTOP (Niu et al., 2005)
Properties of the Aquifer 1. Hydraulic
Conductivity 2. Specific Yield
Introduction SIMTOP Validation SIMGM
Validation Conclusions
20
Validate the Model against the Valdai Data
The model reproduces SWE, ET, runoff, and water
table depth. The water table depth has two peaks
and two valleys in one annual cycle WTD is very
sensitive to soil permeability (sand percentage,
frozen soil), Runoff, and ET parameters.
Introduction SIMTOP Validation SIMGM
Validation Conclusions
21
Validate the Model against GRDC Runoff
Good agreements between the modeled runoff and
GRDC Runoff Runoff exp(- f wtd) The modeled
water table depth ranges from 2.5m in wet regions
to 30m in arid regions.
Introduction SIMTOP Validation SIMGM
Validation Conclusions
22
Regional Averaged Runoff
Agreement between the modeled runoff and GRDC
runoff in most regions except for
mid-latitudes Surface runoff accounts for about
20 Groundwater discharge accounts for about
80 of the total runoff.
Introduction SIMTOP Validation SIMGM
Validation Conclusions
23
Validate the Model Against GRACE ?S Anomaly
The modeled ?S anomaly agrees very well with
GRACE data in river basins where ?S is not
affected by frozen soil Groundwater ?S anomaly
accounts for about 60-80 of the total ?S
anomaly The model capture the inter-annual and
inter-basin variability of the ?S anomaly .
Introduction SIMTOP Validation SIMGM
Validation Conclusions
24
Validate the model Against GRACE WTD Anomaly
GRACE ?S / 0.2 The modeled water table depth
agrees very well with GRACE data in terms of
inter-annual and inter-basin variability in river
basins where ?S is not affected by frozen soil
The uncertainty in GRACE data is mainly the
attenuation effects induced by smoothing.
Introduction SIMTOP Validation SIMGM
Validation Conclusions
25
P E, Groundwater Recharge, and Discharge
Phase lags in P E, groundwater recharge and
discharge Negative recharge in dry seasons when
P E is negative Variations of P E,
groundwater recharge, discharge are consistent
with the groundwater storage anomalies and WTD in
terms of the inter-annual and inter-basin
variability.
Introduction SIMTOP Validation SIMGM
Validation Conclusions
26
The Impacts of Groundwater Model on SM and ET
It has a large impact on bottom-layer soil
moisture, most obviously in cold regions (30
globally) It has a smaller impacts on
surface-layer soil moisture (5 globally) The
impacts on ET are mostly in arid-to-wet
transition zones, i.e., the hot spots (20 in
sensitive zones).
Introduction SIMTOP Validation SIMGM
Validation Conclusions
27
Soil Moisture Profiles in Selected Regions
It has a large impacts on the soil moisture
profile in most regions It has a relative small
impacts in arid regions because the WTD is very
deep and thus the capillary forces are weak.
Introduction SIMTOP Validation SIMGM
Validation Conclusions
28
Transpiration vs. Soil Surface Evaporation
Groundwater has a negligible impacts on
transpiration, although it greatly increases deep
soil moisture It enhanced the ground-surface
evaporation in dry seasons corresponding to the
increases in the surface-layer soil moisture.
Introduction SIMTOP Validation SIMGM
Validation Conclusions
29
Conclusions

1. We developed a simple groundwater model (SIMGM)
   for use in GCMs by representing the recharge and
   discharge processes in an unconfined aquifer,
   which is added as a single integration element.
2. It is first validated against the observed water
   table depth in a small cold-region watershed. It
   captures not only the summer valley also the
   winter valley of the observed water table.
3. On the global scale, it reproduces the GRDC
   runoff Groundwater discharge accounts for about
   80 of the total runoff.
4. The modeled ?S anomaly agrees very well with
   GRACE data in terms of inter-annual and
   inter-basin variability in most river basins.
5. Groundwater ?S anomaly accounts for about 60-80
   of the total ?S anomaly The modeled water table
   depth agrees very well with that converted from
   GRACE. The groundwater storage and WTD anomalies
   are mainly controlled by P E, or climate.
6. It produces a much wetter soil globally except
   for arid regions It produces about 4 20 more
   annual ET mainly through the enhanced ground
   surface evaporation instead of transpiration in
   humid-arid regions.

Introduction SIMTOP Validation SIMGM
Validation Conclusions