VIC: Model Overview

Ted BohnUW Surface Water Hydrology

SeminarNovember 15, 2012

Outline VIC Overview VIC Web Site How to Run VIC How to Run the Routing Model Future Development Conclusions

VIC Model OverviewWhat is VIC? Large-scale hydrologic

model (Liang et al, 1994) Simulates water & energy

storages and fluxes Inputs: daily or sub-daily

meteorological drivers Precipitation Air temperature Wind speed etc

Model time step length = daily or sub-daily

VIC Model Overview

VIC is intended forLarge regions (> 100,100

km2)Monthly flows and land-

atmosphere fluxesCoupling with GCMs

VIC Model OverviewVIC Makes Some Approximations:

Land surface = grid of large (>1km), flat, uniform cells Equations derived for small, uniform cases are applied to very large

heterogeneous areas Sub-grid heterogeneity is handled via statistical distributions

Elevation Land cover

Water can only enter a grid cell via the atmosphere Non-channel flow between grid cells is ignored MOST runoff reaches local channel network before reaching grid cell

boundary Once water reaches the channel network, it is assumed to stay in the

channel (it cannot flow back into the soil) Grid cells are simulated independently of each other Routing of stream flow is performed separately from the land surface

simulation, using a separate model (Lohmann et al., 1996 and 1998)

VIC Model FeaturesValidity of these assumptions?

1. Uniform grid cells/small-scale equations Depends on:

Linearity of the processes (does average land surface produce average results?)

Statistical independence of processes (no correlations/interactions)

2. Runoff goes to local channels/ doesn’t come back/independence of grid cells

Depends on: Grid cell size Channel network density

Rule of thumb: Grid cell size > spacing between channels (or big enough to contain typical hill or mountain)

Groundwater flow must be small

VIC Model FeaturesMeteorological Inputs VIC can accept any combination of daily or sub-

daily met. variables At minimum, VIC requires daily Prcp, Tmin,

Tmax, and Wind Speed VIC uses MTCLIM algorithms to compute daily (and

hourly) incoming shortwave radiation and daily humidity

VIC uses Prata algorithm to compute daily incoming longwave algorithm

VIC uses cubic spline to interpolate diurnal cycle of T From diurnal T, computes sub-daily values of other

forcings Prcp and Wind held constant throughout day

VIC Model FeaturesLand Cover “Tiles”

Each type of vegetation gets 1 tile

Optional lake/wetland tile Spatial distribution ignored

Each tile is homogeneous Trees have same height, LAI,

etc Each tile modeled separately Fluxes and storages from the

tiles are averaged together (weighted by area fraction) to give grid-cell average for output

VIC Model FeaturesEvapotranspiration (ET) From canopy storage From soil via transpiration OR bare soil evap

If veg present, no bare soil evap If overstory, no understory evap

6 types of Potential Evap (PET) Open water Wet soil Natural Veg with no soil moisture limitation (but other

resistances intact) Natural Veg with no environmental limitations Short reference crop Tall reference crop

VIC Model FeaturesPenman-Monteith formulation

= f(radiation, humidity, wind, surface roughness, architectural resistance, stomatal resistance)

Architectural Resistance Shape of canopy; impedance to turbulent transport = f(LAI, canopy structure)

Stomatal Resistance Stomata: pores in leaves for transport of H2O, CO2 Plants actively change stomatal opening to optimize photosynthesis and retain

moisture = f(radiation, temperature, humidity, soil moisture, LAI)

Evap of canopy storage (dew on leaves) = Penman-Monteith with stomatal resistance set to 0

Transpiration = Penman-Monteith

Bare soil evap = Penman-Monteith with architectural and stomatal resistance set to 0 Only applied to saturated soil fraction

VIC Model FeaturesSoil Typically 3 soil layers Infiltration into the top-most layers

controlled by variable infiltration capacity (VIC) parameterization

Top-most layers can lose moisture to evapotranspiration

Gravity-driven flow from upper layers to lower layers

Q = f(soil moisture, Ksat) ARNO baseflow formulation for

drainage from bottom layer Optional organic content in soil

parameter file Optional water table position

computed via Van Genuchten curve

VIC Model FeaturesSoil T profile 1. QUICK_FLUX approximation

(QUICK_FLUX TRUE in global parameter file)

Assumes exponential decay towards “deep temperature” with depth

2. Finite element scheme (QUICK_FLUX FALSE in global parameter

file) Heat diffusion equation Anywhere from 3 to 50 “nodes”

Hydro soil layer average T can be output (as of VIC 4.1.2)

Optional soil freeze/thaw: FROZEN_SOIL TRUE in global parameter file

Finite element + Frozen Soil is computationally expensive

Only recommended where soil freezing is important process

VIC Model Features

Soil T heterogeneity Option: SPATIAL_FROST (user_def.h) Linear (uniform) distribution of soil T around mean Allows some moisture movement in soil when avg T below

freezing

VIC Model FeaturesSnow 2-layer formulation Precip, Sublimation and melt Albedo decay Energy balance Need diurnal radiation to get

snowmelt rate right SNOW_STEP in global parameter

file Partial snow cover can also be

considered (affects melt rate) (SPATIAL_SNOW TRUE in

user_def.h)

VIC Model FeaturesElevation Bands Snow pack size depends non-linearly on

altitude, via precipitation and temperature In temperate mountainous regions, with

seasonally-varying snow line, using the average elevation, temperature, and precipitation over the entire grid cell gives wrong answer

How it works: Subdivide grid cell into arbitrary number of

elevation bands Within each band, meteorologic forcings

are lapsed from grid cell average elevation to band's elevation

Geographic locations or configurations of elevation bands are not considered; VIC lumps all areas of same elevation range into 1 band

Fluxes and storages from the bands are averaged together (weighted by area fraction) to give grid-cell average for writing to output files

Specify in global parameter file Less necessary when running VIC at high

resolution (1/16 degree)

VIC Model FeaturesDynamic Lake/Wetland Model (Bowling, 2002, 2009)•Multi-layer lake model of Hostetler et al. 2000

•Energy-balance model•Mixing, radiation attenuation, variable ice cover

•Dynamic lake area (taken from topography) allows seasonal inundation of adjacent wetlands•1 composite lake per cell•Lakes can (as of VIC 4.1.2) receive input from upstream channel network:

•Requires running upstream cells first•One option: run entire basin without lakes, then route flows, then simulate lake as single grid cell•Note: lake parameters (bathymetry, wfrac, mindepth, etc) not available on global scale; so far calibrating case-by-case

VIC Model FeaturesRiver Routing Routing of stream flow is performed

separately from the land surface simulation, using a separate model, typically the routing model of Lohmann, et al. (1996; 1998)

Each grid cell is represented by a node in the channel network

The total runoff and baseflow from each grid cell is first convolved with a unit hydrograph representing the distribution of travel times of water from its points of origin to the channel network

Then, each grid cell's input into the channel network is routed through the channel using linearized St. Venant's equations

A lot of slop here: we typically don’t calibrate the unit hydrograph, channel velocities, etc. Errors are small when looking at monthly flows from large basins

Outline VIC Overview VIC Web Site How to Run VIC How to Run the Routing Model Future Development Conclusions

VIC Web Sitewww.hydro.washington.edu/Lettenmaier/Models/VIC/

Outline VIC Overview VIC Web Site How to Run VIC How to Run the Routing Model Future Development Conclusions

How to Run VIC1. Download source code2. Compile (via “make”)3. Prepare input files4. Run VIC

vicNl –g global_parameter_file >& log.file.txt

Other Usage: To see version info:

vicNl –v To see compile-time option settings:

vicNl –o

Global Parameter FileGlobal Parameter File = Master Input File Names/locations of all other input/output files Contents/formatting of input forcing and output

files You can make whatever output files you want

Settings of most simulation options Start/stop dates Model time step Water Balance v. Full Energy mode Etc

Options and their allowed values are listed in vicNl_def.h

Other Parameter Files ASCII text 1+ line(s) per grid cell Formats described on VIC web site Optionally, soil parameters can be

specified in Arc/INFO grid format

Met. Forcing Files ASCII or binary Can be traditional units (e.g. mm) or ALMA convention (e.g.

m3/sec) ALMA = standard for atmospheric models

You MUST describe file contents in the global parameter file Variables Start date Time step Measurement height

Usually 1 file per grid cell Optionally a 2nd file

Can have different variables, time step, start date than first file New input forcing variable, CHANNEL_IN, is only used for input

into a lake Can be the routed runoff of upstream cells

State Files Can be ASCII or Binary Initial state file has same format as final

state file Initial state file can be the final state file

from a previous VIC run

Output Files Can be ASCII or Binary Units can be traditional or ALMA convention 1+ file(s) per grid cell You can either:

Specify the files’ names and contents in the global parameter file

Or omit this information and have VIC create default output files

You can either: Specify the output time step (n-hourly or daily) Or omit this and the output time step will = model time

step

VIC Simulation Mode Settings2 types of settings: Run-time

Most options are of this type Set in global parameter file Examples of most available in global.param.sample

Compile-time Only a few options Set in user_def.h

Values in user_def.h may be misleading, if it has been edited since the last “make”

To see their current values, typevicNl –o

To change these: Edit user_def.h Make clean Make

vicNl_def.h Describes Most Model Options Input Forcing Variables/***** Forcing Variable Types *****/

#define N_FORCING_TYPES 24

#define AIR_TEMP 0 /* air temperature per time step [C] (ALMA_INPUT: [K]) */

#define ALBEDO 1 /* surface albedo [fraction] */

#define CHANNEL_IN 2 /* incoming channel flow [m3] (ALMA_INPUT: [m3/s]) */

Output Forcing Variables/***** Output Variable Types *****/

#define N_OUTVAR_TYPES 160

// Water Balance Terms - state variables

#define OUT_ASAT 0 /* Saturated Area Fraction */

#define OUT_LAKE_AREA_FRAC 1 /* lake surface area as fraction of the grid cell area [fraction] */

#define OUT_LAKE_DEPTH 2 /* lake depth (distance between surface and deepest point) [m] */

vicNl_def.h Describes Most Model Options Simulation Modes: options_structtypedef struct {

// simulation modes

int AboveTreelineVeg; /* Default veg type to use above treeline;

Negative number indicates bare soil. */

char AERO_RESIST_CANSNOW; /* "AR_406" = multiply aerodynamic resistance

…

} option_struct

Global.param.sample######################################################################## VIC Model Parameters - 4.1.x######################################################################## $Id: global.param.sample,v 5.7.2.28 2012/04/15 05:39:10 vicadmin Exp $######################################################################## Simulation Parameters#######################################################################NLAYER 3 # number of soil layersNODES 10 # number of soil thermal nodesTIME_STEP 3 # model time step in hours (set to 24 if FULL_ENERGY = FALSE, set to <

24 if FULL_ENERGY = TRUE)SNOW_STEP 3 # time step in hours for which to solve the snow model (should =

TIME_STEP if TIME_STEP < 24)STARTYEAR 2000 # year model simulation startsSTARTMONTH 01 # month model simulation startsSTARTDAY 01 # day model simulation startsSTARTHOUR 00 # hour model simulation startsENDYEAR 2000 # year model simulation endsENDMONTH 12 # month model simulation endsENDDAY 31 # day model simulation endsFULL_ENERGY TRUE # TRUE = calculate full energy balance; FALSE = compute water balance

onlyFROZEN_SOIL TRUE # TRUE = calculate frozen soils

######################################################################## Soil Temperature Parameters

•Example global parameter file•Included in code distribution•To make global parameter file for your project, copy this file and edit it•Contains most options, set to default values

Outline VIC Overview VIC Web Site How to Run VIC How to Run the Routing Model Future Development Conclusions

How to Run the Routing Model1. Download source code2. Compile (via “make”)3. Prepare input files4. Run rout

rout input_file >& log.file.txt

Input Files“Input” File = Master Input File Names/locations of all other input/output files Settings of simulation options

Start/stop dates Etc

Unit Hydrograph File Distribution of overland travel times for a pulse of runoff to reach

the channel (impulse response) ASCII text Format:time(h) flow_fraction

Station Location File ASCII text List row,column coordinates of station(s) for which you want a

hydrograph

Input FilesFlow Direction File ASCII Arc/INFO Numeric code indicates pixel’s flow

directionFlow Fraction File ASCII Arc/INFO Fraction of pixel’s runoff that enters

channel network Usually = fraction of pixel that’s in the

basin

Input FilesOther Optional ASCII Arc/INFO Files:

(You can either specify a filename or just a single value that will be used in all pixels)

Flow Velocity File Diffusivity File

Input FilesRunoff Files ASCII Text (or NetCDF, in surface water

monitor) ASCII: require the following format

Year Month Day (data) (data) Runoff Baseflow Extra columns at the end of the line are

allowed (and ignored) VIC’s default output “fluxes” files have this

format Daily runoff and baseflow will be added

together

Output FilesDaily, Monthly, Annual flow filesUnits depend on settings in main input file

Outline VIC Overview VIC Web Site How to Run VIC How to Run the Routing Model Future Development Conclusions

Future DevelopmentPossible additions: NetCDF Input/Output Complete Carbon Cycle (port from my work) Lakebottom heat flux (port from my work) Distrib water table (port from my work) Methane? More user-friendly lake model Multiple soil types per cell Replace (veg tile, snow band) array with

“patches”, each with own soil, veg, elevation, etc characteristics

Flow routing within VIC Suggestions?

Large-Scale Modeling: ChallengesHow to Validate Model

Results? Large-scale simulations vs. point measurements

Soil moisture extremely heterogeneous One point is NOT representative of entire grid cell

Streamflow is our friend! Aggregates the runoff of the entire basin above the

gauge How would we do this with other hydrologic

variables? Satellites can measure things visible at surface

Problem: lateral extent (e.g. snow cover) not equal to total storage (e.g. snow depth)

Conclusions Goals of large-scale models not the same as

hillslope-scale models Different assumptions & approximations Large-scale behavior may look different from

small-scale Non-linearities Interactions

Fine-scale variations, where important, represented by statistical distributions

Validation of large-scale models requires large-scale observations, special techniques

Thank You More information at VIC web site:

www.hydro.washington.edu/Lettenmaier/Models/VIC/

Large-Scale Modeling: OverviewWhat is a model? (Idealized) Representation of

some aspect of natural world Aid to understanding the real

world (Usually) Simpler than the real

world Captures most important

features of the system Less important features may

be omitted or approximated

Large-Scale Modeling: OverviewTypes of Models: Numerical (computer) Analytical (equations)

Penman-Monteith Transpiration

Newton’s Gravity Conceptual (need not be

mathematical) Role model Fashion model Model airplane Etc

VIC = Large-Scale Land Surface Hydrology Model

Captures important features of large hydrologic systems Sub-continental to global systems >100,000 km2

Coarse resolution (why?) Spatial: 1/16- to 2-degree (5-200 km) Temporal: generally analyze outputs at monthly scales

Inputs Meteorology Soil, vegetation, topography

Outputs Water & energy storages and fluxes (snowpack, streamflow,

soil moisture, etc)

Large-Scale Modeling: OverviewWhy use a large-scale hydrologic model?

Hydrologic impacts of large-scale climate variations/change El Nino, Pacific Decadal Oscillation, etc Global Warming(These all have signatures that vary over large spatial scales)

Land surface component of global climate model

Estimating continental/global: water budgets land-atmosphere energy

fluxes Allocation of international

water resources Drinking water Agriculture Hydropower Environmental

Large-Scale Modeling: OverviewWhy Coarse Resolution? Computational Efficiency Large-scale questions don’t require fine

resolution Uncertainties at fine resolution

Observations Model Equations

Large-Scale Modeling: OverviewPrimary Analysis Needs: Availability of water resources

River discharge Soil moisture Snow pack & Reservoir storage

Large areas Historical and projected Seasonal averages Interannual variability & trends

Generally, coarse outputs (monthly data, at 1/8-degree or larger resolution), are sufficient

Room for slop…

Large-Scale Modeling: OverviewWhat are the largest sources of uncertainty in hydrologic modeling?

Observations ΔS = P – ET – R

R: Moderately well-observed (at surface) at large scale (but not everywhere) P: Well-sampled only in populated affluent areas ET: Very poorly sampled S (Snow, Soil Moisture, Lakes/Reservoirs): Very poorly sampled

Meteorologic inputs are a substantial component of uncertainty

Model equations Equations from “pure” lab settings applied to large, heterogeneous areas

Usually can’t predict “effective” parameters of these equations a priori Must calibrate them by comparing model outputs to observations

Model behaviors generally not sufficiently constrained by observations Errors in S can be compensated by errors in ET

Aggregating these over larger scales reduces errors (at the expense of precision)

Large-Scale Modeling: OverviewTherefore, large-scale models may use

different equations from what you might expect on the hillslope scale…