← Back to usersum index

WEPP Advanced Options

This page explains every control in WEPP -> WEPP Advanced Options in WEPPcloud.

Most projects should run with defaults. Use these controls when you have a clear reason (for example, calibration against observed hydrographs, known snow/frost behavior, or a post-fire recovery scenario).

Location in the UI

1. Open a run.
2. Open the WEPP control panel.
3. Expand WEPP Advanced Options.

How To Use This Page

Each section below uses the same pattern:

• What this section does: plain-language intent.
• Inputs and parameters: every field in that advanced section.
• Modeling impact: how water balance, runoff, baseflow, or erosion can change.
• When to adjust: practical guidance.
• Cautions: common mistakes and side effects.

Recommended workflow

1. Change one section at a time.
2. Re-run WEPP.
3. Compare runoff, peak flow, recession behavior, and erosion/sediment outputs.
4. Record what you changed and why.

1) WEPP UI - Hourly Seepage

What this section does

Switches WEPP from daily water-balance seepage handling to an hourly path (wepp_ui.txt present).

Inputs and parameters

Modeling impact

• Runs soil-water calculations in hourly substeps (24 per day) and then rolls seepage/drainage totals up to daily outputs.
• Percolation and seepage are handled differently than the daily path, so infiltration/percolation partition and runoff timing can shift.
• Usually most noticeable when short-duration storm timing matters.
• Can change event timing and short-term peaks, even when seasonal totals are similar.
• Initial near-surface wetness can differ from the daily path, which can affect early-event runoff and erosion response.

When to adjust

• Use when daily behavior is too coarse for the watershed response you are calibrating.
• Common in workflows using 7778-style soil behavior.

Cautions

• For 2006-era soil versions, this option may not produce a wepp_ui.txt effect.
• This is a structural simulation-path toggle, not just an output setting.
• The file acts as an on/off trigger by presence; there are no numeric fields in wepp_ui.txt.

2) Potential ET (PMET)

What this section does

Controls whether WEPP uses Penman-Monteith ET coefficients from pmetpara.txt.

Inputs and parameters

Modeling impact

• kcb primarily shifts plant transpiration demand. Higher values usually increase transpiration and reduce water left for deep drainage/baseflow.
• rawp sets how soon water stress starts (RAW = rawp x TAW). Lower values trigger stress earlier and suppress transpiration sooner in dry periods.
• ET changes alter soil moisture carryover, which feeds back into runoff generation and erosion potential in later storms.

When to adjust

• ET calibration against observed seasonal dryness/wetness.
• Cases where default ET assumptions over- or under-dry the soil profile.

Cautions

• In disturbed workflows, PMET coefficients may be generated from disturbed land/soil lookups instead of manual entries.
• In current WEPP builds, if a crop/land-cover code is missing from pmetpara.txt, the model may use the first record in that file as fallback.
• Changing ET parameters can improve one season but degrade another; always validate across multiple years/storms.

3) Frost

What this section does

Writes frost.txt to control winter freeze/thaw process parameters.

Inputs and parameters

Modeling impact

• Freeze/thaw settings strongly influence winter infiltration capacity and runoff partitioning.
• Thermal factors (ksnowf, kresf, ksoilf) affect how quickly frost forms/thaws through snow/residue/soil layers.
• wintRed directly controls whether the model redistributes water around freeze/thaw fronts, which can change thaw-season runoff pulses.
• kfactor values set lower bounds on frozen-soil hydraulic conductivity, which can strongly shift winter runoff and related erosion transport.

When to adjust

• Cold-region basins where winter runoff timing or frozen-soil response is a major calibration target.

Cautions

• Aggressive changes can create unrealistic winter hydrographs.
• Keep calibration physically defensible (snowpack, frost depth, thaw timing).

4) Snow

What this section does

Writes snow.txt to control rain/snow partition and snow density behavior.

Inputs and parameters

Modeling impact

• Threshold temperature directly shifts how much precipitation enters as rain vs snow.
• newsnw controls new-snow density and early snowpack compaction behavior.
• ssd controls when settling behavior changes in denser snowpacks.
• These settings mainly affect runoff timing/volume and therefore event-driven erosion timing.

When to adjust

• Snow-dominated or mixed rain/snow watersheds with known snowmelt timing mismatches.

Cautions

• If values are unrealistic, melt timing errors can overwhelm downstream calibration work.

5) Baseflow Processing

What this section does

Writes gwcoeff.txt to configure groundwater storage, baseflow release, and deep seepage behavior.

Inputs and parameters

Modeling impact

• Larger initial storage can increase early-period baseflow.
• Larger baseflow coefficient steepens hydrograph recession (faster groundwater release).
• Larger deep seepage coefficient removes more groundwater from channel-return pathways.
• Threshold area affects where accumulated baseflow contributes through channel network classifications (ephemeral vs perennial behavior).

When to adjust

• When simulated recession curves are too slow or too fast.
• When channel low-flow persistence is poorly matched.

Cautions

• In single-storm workflows, WEPPcloud writes gwcoeff.txt with zero initial groundwater storage and zero baseflow coefficient, so user-entered baseflow settings are effectively bypassed.
• Tune with hydrograph recession metrics, not just peak flow.

6) Channel Inputs (chan.inp)

What this section does

Controls channel hydrograph output configuration and selected channels in chan.inp.

Inputs and parameters

Modeling impact

• dtchr changes channel-routing timestep, which can change hydrograph timing/shape and numerical behavior (not only file detail).
• Smaller timestep and full hydrograph output provide richer diagnostics but larger files and longer post-processing.
• ichout and channel lists mainly control reporting scope/format, not channel physics.

When to adjust

• Troubleshooting routing behavior on specific channels.
• Producing higher-detail channel diagnostics for short windows.

Cautions

• Full hydrograph output can create very large files on long runs.
• Keep channel ID lists intentional.
• These controls apply to watershed/channel-routing runs; hillslope-only runs do not use channel output controls.

7) Channel Parameters

What this section does

Controls channel resistance and erodibility assumptions, including optional slope-dependent critical shear (tcr.txt).

Inputs and parameters

Modeling impact

• Critical shear and erodibility are first-order controls on channel detachment and sediment delivery.
• Manning roughness affects flow velocity, depth, travel time, and shear stress.
• Minimum width prevents unrealistically narrow channels from driving extreme hydraulic responses.
• Variable-critical-shear mode links channel shear resistance to channel slope using a shaped response curve.

When to adjust

• Channel sediment calibration.
• Routing/velocity/shear mismatches in channel-focused validation.

Cautions

• Changing multiple channel physics parameters at once makes calibration unstable.
• Keep chnn and chnnbr physically consistent.

8) Bedrock

What this section does

Overrides restrictive-layer hydraulic conductivity (kslast) in generated soil inputs.

Inputs and parameters

Modeling impact

• Lower kslast limits downward percolation at the restrictive layer, often increasing near-surface wetness, runoff potential, and erosion response.
• Higher kslast allows more deep percolation and can reduce quick runoff response.
• When a restrictive layer is active, this bottom-boundary conductivity can materially alter baseflow vs quickflow partitioning.

When to adjust

• Watersheds with known shallow restrictive layers or bedrock conductivity evidence.

Cautions

• This is a powerful structural soil-hydrology control; avoid using it as a generic calibration knob without field justification.

9) Clip Hillslopes

What this section does

Limits hillslope length while preserving hillslope area by increasing width.

Inputs and parameters

Modeling impact

• Shorter effective slope length often reduces hillslope transport distance and can reduce predicted erosion on very long slopes.
• Preserving area avoids changing gross contributing area while still changing hillslope geometry and hydraulic/erosion response.

When to adjust

• Extremely long abstracted hillslopes that are known to over-predict erosion.

Cautions

• Clipping is recommended when very long abstracted hillslopes are likely to overestimate soil loss.
• Multi-OFE workflows can limit applicability of this option.
• If changing the clip length does not change results, verify applied slope lengths in prep-details outputs; this control may not be wired consistently in all deployments.

10) Soil Options

What this section does

Adjusts soil profile depth constraints and initial soil saturation before runs are prepared.

Inputs and parameters

Modeling impact

• Shallower max depth reduces storage volume, which can increase runoff and erosion sensitivity.
• Larger minimum depth increases storage in shallow soils, often damping quick runoff response.
• Higher initial saturation increases early-event runoff likelihood and can increase early-event erosion.
• In hourly-seepage UI mode, initialization handling differs slightly from the daily path, so initial-saturation sensitivity can be stronger.

When to adjust

• Sensitivity analysis around storage depth and antecedent moisture assumptions.

Cautions

• If both min/max clipping are enabled, minimum depth must be <= maximum depth.
• Large initial_sat shifts can dominate first-event calibration and mask other parameter effects.

11) Phosphorus

What this section does

Writes phosphorus.txt for optional phosphorus concentration routing.

Inputs and parameters

Modeling impact

• Changes phosphorus load outputs only.
• Does not directly change runoff, infiltration, or erosion mechanics.

When to adjust

• Water-quality workflows requiring phosphorus export estimates.

Cautions

• All four values are required for phosphorus.txt to be written.
• In some regional workflows (for example Lake Tahoe), defaults can be auto-populated from module-specific data.

12) Export Configuration

What this section does

Controls post-run export products generated automatically after watershed runs complete.

Inputs and parameters

Modeling impact

• No direct impact on simulated hydrology or erosion.
• Affects post-processing time and generated deliverables.

When to adjust

• Reporting or delivery workflows that require standardized GIS artifacts.

Cautions

• Additional export products increase run-completion processing time.

13) Interchange

What this section does

Controls cleanup of raw WEPP text outputs after successful interchange conversion to parquet artifacts.

Inputs and parameters

Modeling impact

• No direct impact on hydrology or erosion simulation results.
• Changes which output artifacts remain available for debugging/inspection.

When to adjust

• Runs where storage footprint is a concern and parquet interchange outputs are the primary deliverable.

Cautions

• Keeping only converted artifacts can reduce ability to inspect raw WEPP text outputs later.

14) WEPP Exec

What this section does

Controls binary selection and whether watershed/channel execution is included.

Inputs and parameters

Modeling impact

• Binary changes can alter numerical behavior, bug fixes, or compatibility.
• Disabling watershed run prevents channel routing outputs from being generated.
• Prep-only does not produce new watershed simulation outputs by itself.
• Watershed execution controls whether channel-network routing is run after hillslope preparation.

When to adjust

• Reproducibility checks against prior runs.
• Troubleshooting version-specific behavior.
• Verifying binary lineage and compiler history using the WEPP-Forest Change Log.

Cautions

• Multi-OFE workflows require newer binaries.
• Keep binary choice consistent across scenario comparisons.

15) Revegetation Scenarios

What this section does

Applies optional post-fire cover transformation scenarios that rescale RAP cover time series before WEPP prep.

Inputs and parameters

Modeling impact

• Changes vegetation/cover trajectories used in .cov inputs.
• Cover changes feed directly into infiltration, runoff, and erosion response through canopy/residue/cover effects.
• Most influential in post-disturbance recovery simulations.

When to adjust

• Disturbance and recovery scenario analysis.
• Testing management assumptions about recovery speed and cover composition.

Cautions

• User CSV uploads are accepted by filename/type; keep file structure consistent with expected cover-transform format.
• Use physically defensible recovery assumptions and document the scenario source.

Related references

• Disturbed Land Soil Lookup Table
• WEPPcloud User Guide