Physically-based Distributed
Hydrologic Modeling
Civil and Environmental Engineering Dept.
Hydrology and Water Resources
Goal of Phys.-based Distrib. Hydrologic Modeling
To date we have learned about:
 Key forcings at land surface (precipitation/net radiation)
 Physical processes at surface/subsurface (infiltration, soil moisture
redistribution, evapotranspiration, groundwater flow, runoff, etc.)
Goal: Develop physically-based model of hydrologic response across a
watershed by tying together various processes across landscape.
In this context “Distributed” refers to variables being spatially-distributed
in space.
So we aim to explicitly model how the hydrologic states/fluxes evolve in
space and time throughout the watershed.
Note: Because of complexity/nonlinearity of processes this modeling is
necessarily done numerically (i.e. by building appropriate computer
models coupling together hydrologic processes)
Civil and Environmental Engineering Dept.
Hydrology and Water Resources
Representation of Dist. Hydrologic “Units” in Space
Numerical simulations of catchment hydrologic processes require a method for
representing a basin. Methods can be categorized as lumped versus distributed
modeling where the physical processes are solved for each discrete unit.
Basin-Averaged Models
(e.g. HEC-HMS)
Raster (Grid) Models
(e.g. MIKE SHE)
Triangular Irregular
Network Models
(e.g. tRIBS)
Will focus on this model as an example
Civil and Environmental Engineering Dept.
Hydrology and Water Resources
tRIBS Distributed Model
TIN-based Real-time Integrated Basin Simulator (tRIBS) is a fullydistributed model of coupled hydrologic processes (Ivanov et al,
Vivoni et al.)
Model Processes
• Coupled vadose and saturated
zones with dynamic water table.
• Moisture infiltration waves.
Radiation
• Soil moisture redistribution.
• Topography-driven lateral fluxes
in vadose and groundwater.
• Radiation and energy balance.
• Evaporation and Transpiration.
• Hydrologic and hydraulic routing.
Key point: You now know about all of these
processes; a distributed model simply ties
them all together.
Civil and Environmental Engineering Dept.
Hydrology and Water Resources
Process Representation: Surface Processes
Land-Atmosphere Interactions
Vegetation
Coupled Energy and Hydrology
Processes on Complex Terrain
• Radiation: Incoming short-wave
and long-wave, outgoing long-wave
radiation (including effects of terrain).
• Vegetation: Canopy interception,
drainage, throughfall and transpiration
using vegetation functional type.
Soil
3D Complex
Topography
Radiation Balance
Rn  Rs (1  )  Rl  Rl
Aquifer
Surface Energy Balance
Rn  LE  H  G
Civil and Environmental Engineering Dept.
• Energy Balance: Net radiation,
ground heat, sensible heat and latent
heat fluxes.
• Evapotranspiration: Soil-moisture
controlled bare soil evaporation and
canopy transpiration in root zone.
• Unsaturated Zone Dynamics: Soil
moisture balance, infiltration,
redistribution
Hydrology and Water Resources
Process Representation: Subsurface Processes
Uses a simplified 2D unconfined aquifer model which allows
moisture recharge in shallow aquifer to be redistributed.
Shallow Groundwater
Variable, dynamic
water table field
(plan view)
• Space/time variable groundwater
table position.
• Single and multiple direction GW
flow to downstream neighbors.
• Coupled to unsaturated zone to
enable moisture mass balance
(recharge).
• Bounded by a uniform or spatiallyvariable bedrock surface
(impermeable bottom boundary).
head gradients drive flow
Civil and Environmental Engineering Dept.
Hydrology and Water Resources
Process Representation: Unsat.-Sat. Dynamics
Runoff is generated via multiple mechanisms depending on the
interactions of infiltration fronts and the water table.
Runoff Generation
Example: Model output for saturationexcess runoff occurrence
• Interaction of rainfall, infiltration capacity,
actual infiltration and lateral flows lead to
various runoff types.
• Various runoff types occur at the same
time in different basin parts.
• Various runoff types can occur in single
element as a function of state.
• Infiltration-excess (Hortonian) Runoff.
• Saturation-excess (Dunne) Runoff.
• Perched Subsurface Runoff.
• Groundwater Runoff.
Civil and Environmental Engineering Dept.
Hydrology and Water Resources
Atmospheric Forcing
Primary reason for using distributed models is to take advantage of
new distributed atmospheric forcing datasets (e.g. precipitation,
radiation, etc).
NEXRAD MOSAIC PRECIP.
SATELLITE ESTIMATES OF:
LONGWAVE RAD.
SHORTWAVE RAD.
Civil and Environmental Engineering Dept.
Hydrology and Water Resources
tRIBS Model Output
tRIBS provides output at the scale of each individual node in the basin, for
channel nodes along the network, and as maps of distributed variables (at a
point in time or integrated over time).
• Time Series of Node Behavior: Unsaturated and Saturated Node
Dynamics, Hydrologic and Energy Fluxes and State Variables.
• Basin Outlet and Interior Channel Nodes: Runoff Depth,
Discharge, Stream Velocity, Partitioned Hydrographs.
• Dynamic Distributed Maps: Groundwater dynamics, Surface Runoff
Generation Mechanisms, Soil Moisture, Evapotranspiration, Rainfall,
Interception, Unsaturated Zone Dynamics, Energy and Radiation.
• Integrated Distributed Maps: Percent Runoff Mechanisms,
Saturation Occurrence, Evaporation Fraction, Soil Moisture.
• Time Series of Basin Averaged Properties: Rainfall, Saturated
Area, Evapotranspiration, Soil Moisture.
NOTE: Provides much more information than a lumped model!
Civil and Environmental Engineering Dept.
Hydrology and Water Resources
Illustrative Example: Peacheater Creek
Two-year precipitation
record
Civil and Environmental Engineering Dept.
Hydrology and Water Resources
Parameter Definitions for Basin
(silt
loam)
(mixed
forest)
(everg.
forest)
(decid.
forest)
(silty
clay)
(crops)
(clay/
urban)
(urban)
Note: Spatially varying inputs in soil/vegetation -- impacts spatial
variability in hydrologic response
Civil and Environmental Engineering Dept.
Hydrology and Water Resources
Streamflow Response (Storm at ~Hour 11800)
Civil and Environmental Engineering Dept.
Hydrology and Water Resources
Groundwater: Before/after
Civil and Environmental Engineering Dept.
Hydrology and Water Resources
Soil Moisture: Before/after
Civil and Environmental Engineering Dept.
Hydrology and Water Resources
Surface Energy Balance
Civil and Environmental Engineering Dept.
Hydrology and Water Resources
Summary
• Distributed hydrologic modeling provides an integrated framework
for taking into account hydrologic processes occurring within the
basin (surface energy balance, flow partitioning, etc.)
– Allows for not only simulating design flows/flood forecasts (i.e.
as done using UH-method), but for things like assessing spatial
response to inputs, hydrologic impacts resulting from
urbanization of watersheds, assessing climatology of hydrologic
states, etc.
• Takes advantage of many new distributed forcing/parameter
databases obtained via remote sensing (lumped models do not
take advantage of spatially distributed inputs)
• Is computationally demanding (e.g. compared to UH) and
therefore whether it should be used is largely application
dependent
Civil and Environmental Engineering Dept.
Hydrology and Water Resources