SWATGenXTwo estuaries, one method — a controlled contrast
Tampa Bay is nitrogen-limited; Charlotte Harbor is phosphorus-driven. They sit side by side on the same Gulf coast, over the same karst Floridan aquifer system, and the same Bone Valley phosphate district touches both drainages. That makes the pair a natural controlled experiment for nonpoint-source pollution: one source region, one climate, one geology — two opposite nutrient regimes.
This fleet is the instrument for that assessment. Each estuary gets independent tributaries (Tampa Bay: the integrated bay watershed + the Manatee; Charlotte Harbor: the Peace + the Myakka), all built with the identical automated method, so cross-basin differences are signal, not modeling artifact. The water-quality assessment itself is the work these models enable — it is planned, not claimed: what you see today is the honest starting point, uncalibrated flow and groundwater baselines, published before any tuning.
Platform-generated, expert-finished. Every model here started as a standard SWATGenX build — the same automated pipeline anyone can order a watershed from. They were then finished by hand where it counts: per-aquifer target heads harvested from SWFWMD and USGS monitoring records, Florida-specific hydrostratigraphy (surficial aquifer / Hawthorn confining unit / Upper Floridan), and — for the Peace, in progress now — regional multi-gauge calibration. Automation gives the scale; expert curation gives the finish. A coupled watershed–aquifer fleet of this scale and finish is not available anywhere else.
Tampa Bay — integrated watershed model
Tampa Bay is Florida’s largest open-water estuary, and the modern benchmark for nutrient recovery: it is nitrogen-limited, so what the watershed exports — stormwater, wastewater, fertilizer, atmospheric deposition, and events like the 2021 Piney Point release — decides whether its seagrass gains hold.
This is the integrated model of the bay’s tributary drainage: 6,562 km² across 70 sub-watersheds, from the Hillsborough, Alafia, and Little Manatee headwaters (the Bone Valley phosphate district reaches into the Alafia’s upper basin) down to the estuary.
It is the hero model of the fleet — the largest domain, and the densest groundwater control of any Florida build we have made: 940 harvested head-control stations constrain its three-layer MODFLOW 6 companion.
| Groundwater validation (uncalibrated) | Wells (n) | NSE | RMSE (m) |
|---|---|---|---|
| Layer 1 · Surficial aquifer | 284 | 0.737 | 4.77 |
| Layer 3 · Upper Floridan | 203 | 0.965 | 1.55 |
| All layers | 489 | 0.839 | 3.77 |
Uncalibrated, steady-state, validated against per-aquifer median heads harvested from SWFWMD and USGS monitoring wells (2014–2022). The Upper Floridan fit (NSE 0.965, RMSE 1.55 m, n = 203) is the strongest of the fleet in absolute well count.
| Streamflow, out of the box (monthly NSE) | Drainage (km²) | Daily NSE | Monthly NSE | Rating |
|---|---|---|---|---|
| Alafia River at Lithia (02301500) | 868 | 0.46 | 0.84 | Very Good |
| Braden River at Lakewood Ranch (02300033) | 66 | 0.42 | 0.82 | Very Good |
| Bullfrog Creek near Wimauma (02300700) | 75 | 0.53 | 0.82 | Very Good |
| Manatee River at SR 64 (02299950) | 169 | 0.35 | 0.78 | Good |
Across all 58 gauges with observations: 19 rate Satisfactory or better on monthly NSE (9 Good+, 4 Very Good) — uncalibrated. 26 of the 58 sit on channels whose modeled drainage area is more than ±30–40% off the USGS value — an automated gauge‑to‑channel assignment artifact at 500 m, not a hydrology error; on the 32 correctly assigned gauges the tally is 11 Satisfactory+ / 7 Good+ / 4 Very Good. Daily scores run well below monthly (flashy Florida flow on default parameters) — that gap is the calibration headroom.
Peace River
The Peace River drains the heart of Bone Valley — the phosphate district that has supplied much of U.S. fertilizer phosphorus for over a century — across karstic Upper Floridan terrain to Charlotte Harbor, a phosphorus-driven estuary.
That legacy makes it the natural anchor for a phosphorus story: mining-altered headwaters, reclaimed lands, and a spring-fed, strongly gaining river whose groundwater half matters as much as its surface half.
It is the furthest along in the fleet: its streamflow calibration (our regional multi-memory PSO) is running on AWS right now, and its groundwater model was the first Florida build validated against harvested aquifer heads.
| Groundwater validation (uncalibrated) | Wells (n) | NSE | RMSE (m) |
|---|---|---|---|
| Layer 1 · Surficial aquifer | 81 | 0.34 | 9.7 |
| Layer 3 · Upper Floridan | 58 | 0.97 | 1.27 |
| All layers | 144 | 0.59 | 7.5 |
The Upper Floridan — the aquifer that matters for spring-fed baseflow — fits at NSE 0.97 / RMSE 1.27 m before any calibration. The shallow-aquifer miss is diagnostic, not noise: the placeholder uniform recharge under-waters the Polk ridge, exactly what feeding SWAT+ recharge into the model is expected to fix.
Manatee River
The Manatee River enters Tampa Bay from the south and carries the Lake Manatee reservoir — the drinking-water supply for three counties — so its nutrient and flow regime is a water-supply question as much as an estuary question.
It is the cleanest groundwater fit in the fleet: 520 harvested head stations in and around the basin — the densest control per square kilometre of any of the four — pin its aquifer model to an all-layer NSE of 0.95 across 318 evaluated wells, uncalibrated.
Together with the integrated Tampa Bay model it forms the nitrogen-limited half of the two-estuary comparison.
| Groundwater validation (uncalibrated) | Wells (n) | NSE | RMSE (m) |
|---|---|---|---|
| Layer 1 · Surficial aquifer | 217 | 0.95 | 1.54 |
| Layer 3 · Upper Floridan | 101 | 0.94 | 1.85 |
| All layers | 318 | 0.95 | 1.64 |
The best uncalibrated groundwater fit of the four Florida builds — a direct payoff of dense public monitoring: 520 in-window head stations krige tightly and leave the model little room to be wrong.
Myakka River
The Myakka — one of Florida’s two designated Wild and Scenic rivers — drains wet-prairie and flatwoods ranchland through Myakka River State Park to Charlotte Harbor.
In the study design it is the control lens: a second, independent tributary to the same phosphorus-driven estuary as the Peace, but without the Peace’s mining footprint — if a phosphorus signal is Bone Valley legacy rather than regional background, these two basins should disagree in a specific, testable way.
Its Upper Floridan model fits harvested heads at NSE 0.88 with an RMSE under a metre, uncalibrated.
| Groundwater validation (uncalibrated) | Wells (n) | NSE | RMSE (m) |
|---|---|---|---|
| Layer 1 · Surficial aquifer | 16 | 0.66 | 4.88 |
| Layer 3 · Upper Floridan | 21 | 0.88 | 0.88 |
| All layers | 39 | 0.68 | 3.40 |
Sparser public well control than its siblings (67 stations), yet the Upper Floridan still validates at NSE 0.88 / RMSE 0.88 m. Its shallow aquifer shows the same uniform-recharge bias signature as the Peace — the shared, known next fix.
The fleet at a glance
| Model | Estuary | Area (km²) | HRUs | Channels | Head stations | Upper Floridan NSE / RMSE (n) | All-layer NSE (n) |
|---|---|---|---|---|---|---|---|
| Tampa Bay (integrated) | Tampa Bay | 6,562 | 77,636 | 17,253 | 940 | 0.965 / 1.55 m (203) | 0.839 (489) |
| Peace River | Charlotte Harbor | ≈5,980 | 57,998 | 8,181 | 297 | 0.97 / 1.27 m (58) | 0.59 (144) |
| Manatee River | Tampa Bay | ≈1,690 | 32,380 | 2,750 | 520 | 0.94 / 1.85 m (101) | 0.95 (318) |
| Myakka River | Charlotte Harbor | ≈1,560 | 29,069 | 5,655 | 67 | 0.88 / 0.88 m (21) | 0.68 (39) |
All groundwater statistics are uncalibrated, steady-state MODFLOW 6 heads scored post-hoc against per-aquifer median observed heads (SWFWMD + USGS monitoring wells, 2014–2022 window, m NAVD88); every solve converged with 0.00% mass-balance discrepancy. Areas marked ≈ are the summed modeled HRU areas; the Tampa Bay figure is the delineated 70-HUC12 drainage.
How these were made — and what they are not
- Automated build. Each watershed came off the standard SWATGenX pipeline (NHDPlus HR delineation, national soils/land-use/climate), then ran a 10-year simulation (2015–2024) with the SWAT+ Check water-balance audit — no manual GIS work.
- Groundwater from harvested data. The MODFLOW 6 companions are standalone, steady-state models built by the same automated pipeline: three Florida layers (surficial / Hawthorn-intermediate / Upper Floridan) from lithology logs, boundary heads kriged from 1,800+ monitoring stations we harvested from SWFWMD and USGS records, and validation scored against those same wells post-hoc.
- One-way coupling in production. SWAT+ percolation drives MODFLOW 6 recharge. These four builds currently use a uniform recharge placeholder awaiting each basin’s SWAT+ recharge field — that swap is the next groundwater step, and it is expected to fix the known shallow-aquifer low bias. The daily two-way research coupling is a separate, single-basin validated capability — see how the coupling works.
- Uncalibrated means uncalibrated. Every number on this page is an out-of-the-box baseline. That is deliberate: it shows what the automation alone delivers, and it makes the calibration gains measurable when they land here.