Watershed and Morphometry Lab

Overview

Morphometric analysis of drainage basins is among the most common subjects of theses and reports in Iraqi geology, geography and water resources, and the first step of every flash-flood, water-harvesting and dam-site study. It usually needs a desktop GIS, a downloaded elevation model and many manual steps. Here the whole chain runs in the browser from one tap on the map.

The tap is moved to the strongest stream nearby, and the analysed window grows by itself until the whole basin is inside it, coarsening the grid for large basins so that the work stays within a phone’s memory.

What you enter · What you get

How it works in the field

Method and equations

Accuracy and error limits

• Area and boundary are as good as the elevation model: with the 30 m model a basin of a few square kilometres or more is usually within 2–5 % of a map-based delineation. Drainage density, stream frequency and stream numbers depend directly on the chosen threshold area and the grid size, so compare basins only when both are the same. The flood peak of the curve-number method is commonly within ±30–50 % of gauged floods in ungauged basins.
• What is not covered: flat alluvial plains, marshes and irrigated land, where water follows canals, drains and embankments rather than the terrain; karst, where water leaves the basin underground; small basins below about 20 cells; channel losses into the bed of dry valleys, which reduce floods in deserts; and dams or reservoirs inside the basin. Large basins are analysed on a coarser grid, which shortens streams and smooths relief.
• To reduce the error: place the outlet on the valley itself and check the snapped point, use the finest detail for small basins, compare the boundary with a topographic map or satellite image, fix the threshold so that first-order streams match the blue lines of the map, take the design rainfall from the nearest station’s frequency analysis, and calibrate the curve number with any observed flood.

Who uses it

Standards and references

1. Horton, R.E. (1945). Erosional development of streams and their drainage basins. Geological Society of America Bulletin, 56(3), 275–370.
2. Strahler, A.N. (1952). Hypsometric (area-altitude) analysis of erosional topography. Geological Society of America Bulletin, 63(11), 1117–1142.
3. Strahler, A.N. (1964). Quantitative geomorphology of drainage basins and channel networks. In Chow, V.T. (ed.), Handbook of Applied Hydrology, Section 4. McGraw-Hill.
4. Schumm, S.A. (1956). Evolution of drainage systems and slopes in badlands at Perth Amboy, New Jersey. Geological Society of America Bulletin, 67(5), 597–646.
5. Miller, V.C. (1953). A quantitative geomorphic study of drainage basin characteristics in the Clinch Mountain area. Technical Report 3, Columbia University.
6. Melton, M.A. (1965). The geomorphic and paleoclimatic significance of alluvial deposits in southern Arizona. Journal of Geology, 73(1), 1–38.
7. Pike, R.J. & Wilson, S.E. (1971). Elevation-relief ratio, hypsometric integral, and geomorphic area-altitude analysis. Geological Society of America Bulletin, 82(4), 1079–1084.
8. Barnes, R., Lehman, C. & Mulla, D. (2014). Priority-flood: An optimal depression-filling and watershed-labeling algorithm for digital elevation models. Computers & Geosciences, 62, 117–127.
9. O’Callaghan, J.F. & Mark, D.M. (1984). The extraction of drainage networks from digital elevation data. Computer Vision, Graphics, and Image Processing, 28(3), 323–344.
10. Kirpich, Z.P. (1940). Time of concentration of small agricultural watersheds. Civil Engineering, 10(6), 362.
11. USDA NRCS (2004–2010). National Engineering Handbook, Part 630 Hydrology, Chapters 9, 10, 15 and 16.
12. Wilford, D.J., Sakals, M.E., Innes, J.L., Sidle, R.C. & Bergerud, W.A. (2004). Recognition of debris flow, debris flood and flood hazard through watershed morphometrics. Landslides, 1(1), 61–66.