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INTERACTIVE GIS LAB // 01

Watershed Delineation & Flow Routing

A client-side digital elevation model (DEM) demonstrating deterministic 8-neighbor (D8) steepest descent routing and flow accumulation. Select an analysis layer, click any cell to set a pour point, and watch the watershed delineate in real time.

LAYER:
DRAINAGE DIVIDE: RIDGE LINE SEPARATING WEST (46.1%) & EAST (53.9%) BASINS
4104354504654784924984854704554404304454604403853954154384524704824684454184053984124354153553453754054204454604464153723653603754053853202903303653854154354203803253203153353703502802352753202553804053903402702752652903303102401951802352203403703552952252152182452852651951551301852052953303152501851751651952352151501159014017524528527020514513012014518516511080686012519023522016010892809513511875524436781351801681187562505585754530≈0≈02882125115784832≈0≈048422818≈0≈0≈0457568422814≈0≈0≈022▲ DRAINAGE DIVIDE ▲
Digital Elevation Model (DEM)Hypsometric Tinting & Water Surfaces
0 m (Sea level / coastal bay & receiving lakes)498 m (Drainage divide summit ridge)

Displays surface elevation in meters above sea level, with coastal bay and terminal lake water bodies at sea level datum (0 m).

Map Features & Delineation
Selected Pour Point (Edge)Outlet where runoff is collected and gauged
Downstream Flow PathSimulated route of water to edge
Delineated Catchment BasinAll upstream cells draining here
≈
Water Surface Pixels (Sea / Bay, 0 m)Receiving coastal water bodies at 0 m
Drainage Divide LineRidge boundary dividing runoff between catchments
HYDROLOGY FIELD NOTESInspected Pixel: [12, 4]

Behind the Math: How Discharges & Travel Times Are Computed

The hydrograph curves and peak flows here aren't black-box estimates—they run on classic analytical water resources equations. Follow the steps below to verify how any cell's values can be derived by hand.

1
Peak Discharge via the Rational Method (Qp)
Qp = (C · I · A) / 3.6

The Rational Method is the established engineering standard for peak runoff estimation in small catchments. For this model, our design parameters are:

C = 0.45
Runoff Coefficient

Mountainous, semi-rocky terrain where 45% of rainfall turns directly into surface runoff.

I = 35 mm/sa
Design Rainfall Intensity

Critical design storm intensity corresponding to a 10-year recurrence interval (T = 10 yr).

A = Cell Count × 1.0 km²
Contributing Drainage Area

Each DEM raster cell measures 1000m × 1000m (1.0 km²), making total area equal to cell count.

Why the 4.375 multiplier? Substituting our constants into the formula: (0.45 × 35) / 3.6 = 4.375
The 3.6 divisor is the exact SI unit conversion factor: 1 mm/h × 1 km² = (10-3 m / 3600 s) × 106 m² = 1 / 3.6 m³/s.
Qp = 4.375 × A