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Ash Transport (WATAR)

Use the Wildfire Ash Transport And Risk (WATAR) control when you need post-fire ash movement estimates that build on an existing WEPPcloud fire run. In WEPPcloud, this is a hillslope-and-watershed ash screening workflow that uses the run's hydrologic context from WEPP and then applies ash-specific transport assumptions on top of it.

What This Is For

Use Ash Transport to answer questions such as:

  • How much ash is likely to move off burned hillslopes?
  • Which parts of the watershed contribute the most transported ash?
  • How do ash loads change through time after a fire?
  • How sensitive are results to different ash-calibration choices?
  • Which scenarios suggest greater contaminant-loading concern after wildfire?

This workflow is for post-fire screening and comparison. It is not a replacement for field sampling, drinking-water compliance testing, or a full water-quality fate model.

What You Actually See In The UI

The control card is titled Wildfire Ash Transport And Risk (WATAR). The visible run controls are:

UI control What the user sees Why it matters
Fire day for ash model (month/day) Fire timing input Anchors when the ash model starts relative to the post-fire simulation period
Ash depth mode Specify Depth, Specify Load, Upload Maps Chooses whether you enter ash as depths, loads, or raster inputs
Initial ash depth – moderate/low severity and Initial ash depth – high severity Only shown in Specify Depth mode Directly sets the starting ash depth by severity class
Initial ash load – high severity and Initial ash load – moderate/low severity Only shown in Specify Load mode Sets ash as surface load instead of depth
Load map (tonne/ha) Only shown in Upload Maps mode Required raster input for gridded ash loads
Ash type map (optional) Only shown in Upload Maps mode Optional raster where 0 = none, 1 = black ash, 2 = white ash
Field-measured ash bulk density – low/moderate severity and Field-measured ash bulk density – high severity Always visible Used directly in load-based setup and as ash-property context
Run Model Main action button Starts the ash run or, in batch/base-project contexts, saves ash settings for later processing

Under Advanced options, the user also sees:

  • Run wind transport
  • Ash model
  • Transport mode when Ash model = Watanabe2025
  • white-ash and black-ash property fields such as bulk density, particle density, decomposition factor, roughness limit, and model-specific transport parameters

What Each Ash depth mode Means

Specify Depth

Use this when you already want to enter ash thickness directly in millimeters.

The visible fields are:

  • Initial ash depth – moderate/low severity
  • Initial ash depth – high severity

This is the simplest setup path when you have defensible depth assumptions by severity class.

Specify Load

Use this when your initial ash information is better expressed as surface mass rather than thickness.

The visible fields are:

  • Initial ash load – high severity
  • Initial ash load – moderate/low severity
  • Field-measured ash bulk density – low/moderate severity
  • Field-measured ash bulk density – high severity

In the current rq-engine route, WEPPcloud converts the entered loads to ash depth before enqueueing the run. That means bulk density is not just metadata in this mode; it changes the starting ash depth the model actually uses.

Upload Maps

Use this when the ash distribution is spatially variable enough that one value for the whole burned area is too coarse.

The visible fields are:

  • Load map (tonne/ha) required
  • Ash type map (optional)

The upload help is explicit that raster uploads should remain under 100 MB and retain source projection. In the current route, Upload Maps fails if the load map is missing. The ash-type map is optional.

Practical caution

The top-level setup fields are labeled by burn-severity grouping, while many advanced fields are labeled by white ash and black ash. Treat those as one connected parameterization, not as unrelated inputs. If you are calibrating carefully, keep your severity-based setup assumptions and your white/black ash property assumptions internally consistent.

Available Models

In Advanced options, the Ash model dropdown currently shows:

  • Srivastava2023
  • Watanabe2025

If you choose Watanabe2025, a second dropdown appears: Transport mode, with:

  • Dynamic
  • Static

Both model families use the same watershed, climate, burn-severity, and WEPP runoff context. The difference is how they convert ash availability and runoff conditions into transported ash.

UI option Additional UI option Main assumption Mode-specific fields the user sees
Srivastava2023 None Simpler depletion-style ash transport formulation Initial erodibility (white ash), Initial erodibility (black ash), Final erodibility (white ash), Final erodibility (black ash)
Watanabe2025 + Dynamic Transport mode = Dynamic The UI text says Dynamic formulation with K(x) and states transport capacity responds to shear stress, organic matter, and ash availability Organic matter (white ash, fraction), Organic matter (black ash, fraction)
Watanabe2025 + Static Transport mode = Static The UI text says Static exponential formulation (A, B) and states transport follows a fixed exponential decay controlled by initial capacity A and depletion coefficient B Initial transport capacity (white ash, t ha^-1 mm^-1), Initial transport capacity (black ash, t ha^-1 mm^-1), Depletion coefficient (white ash, mm^-1), Depletion coefficient (black ash, mm^-1)

All three model choices still share a large common set of ash-property fields, including:

  • Initial bulk density (white ash) and Initial bulk density (black ash)
  • Final bulk density (white ash) and Final bulk density (black ash)
  • Bulk density factor (white ash) and Bulk density factor (black ash)
  • Particle density (white ash) and Particle density (black ash)
  • Ash decomposition factor (white ash, 1/day) and Ash decomposition factor (black ash, 1/day)
  • Roughness limit (white ash) and Roughness limit (black ash)

The control description above the form also states that ash depths below the roughness limit are treated as non-transportable.

How The Model Choices Differ

Srivastava2023 is the simpler baseline option. It keeps the older depletion-style structure and exposes explicit erodibility terms.

Watanabe2025 with Dynamic is the more process-sensitive option. The visible UI description says transport capacity responds to shear stress, organic matter, and ash availability. Use it when you want the newer calibration family and the more dynamic transport-capacity formulation.

Watanabe2025 with Static stays in the Watanabe family but switches to the fixed A, B style transport-capacity decline shown in the UI text. Use it when you want the newer family without the dynamic K(x) formulation.

If you compare Srivastava2023, Watanabe2025 Dynamic, and Watanabe2025 Static on the same watershed, treat the differences as model-structure uncertainty. A difference across those runs means the answer is sensitive to the transport assumptions, not that one option is automatically correct by itself.

What The Button Actually Does

Run Model

Clicking Run Model submits a multipart form request to:

  • POST /rq-engine/api/runs/<runid>/<config>/run-ash

The rq-engine route requires:

  • ash_depth_mode
  • depth fields when ash_depth_mode = 1
  • load and bulk-density fields when ash_depth_mode = 0
  • a Load map (tonne/ha) upload when ash_depth_mode = 2

If validation passes, WEPPcloud updates the Ash controller state, stores uploads when present, and either:

  • enqueues run_ash_rq for a normal run, or
  • returns Set ash inputs for batch processing when the run is in batch/base-project context

Run wind transport

This checkbox is not only submitted on the next full run. In the current controller, changing it posts immediately to:

  • /runs/<runid>/<config>/tasks/set_ash_wind_transport/

Use it when you want the ash workflow to include wind-driven removal in addition to water-driven transport. If you are comparing scenarios, keep this setting consistent unless wind transport itself is the thing you are testing.

What You See In Results

After a successful run, the ash run-summary surface shows:

  • View Watershed Ash Transport Model Results
  • Contaminant and Reservoir Loading Analysis

If the watershed has no burned hillslopes, the run summary instead says:

  • Watershed does not have burned hillslopes.

These are report surfaces, not setup controls. The watershed and contaminant reports are where the user interprets results after the background job completes.

Interpreting Results

The most useful outputs are usually:

  • hillslope patterns showing likely ash source areas,
  • daily or annual transport totals,
  • cumulative transported ash over the modeled period,
  • burn-severity summaries,
  • contaminant-loading summaries when those outputs are configured,
  • wind-transport totals when Run wind transport is enabled

High ash transport usually reflects some combination of:

  • more starting ash available,
  • stronger runoff forcing from the WEPP side,
  • model settings that keep ash transportable for longer,
  • less ash being trapped below the roughness limit

If values are low, that can mean either ash supply was limited or the selected transport assumptions made ash less available for movement.

Assumptions And Limits

  • Ash Transport is a post-fire comparison workflow that depends on the existing WEPP hydrologic context. If the WEPP fire run is weak, the ash results will be weak too.
  • Ash model and, for Watanabe2025, Transport mode materially affect results. They can change both total transport and source-area ranking.
  • Specify Load is not just another way to enter the same thing. The route converts load and bulk density into the starting depth used for the run.
  • Upload Maps requires a load map. The ash-type map is optional.
  • Run wind transport changes the process assumptions. Compare like with like if your goal is scenario ranking rather than process sensitivity.
  • The UI mixes severity-based setup labels with white/black ash property labels. Use care when mapping your field assumptions into the visible controls.
  • Contaminant outputs indicate modeled ash-associated loading potential, not confirmed concentration at a real sampling point.