Applications and limitations of hydrograph theory

Hydrograph theory (unit hydrograph, S-curve, synthetic UH) relates the shape of the runoff hydrograph to rainfall excess and the basin characteristics.

Applications

1. Flood estimation and design floods: a design storm (with its rainfall excess) is applied to the UH to get the design flood hydrograph for dams, spillways, bridges and culverts.
2. Flood forecasting and warning: the hydrograph of a river can be predicted from the rain that has fallen, giving the peak and its timing.
3. Extension of flow records: where rain data are available but stream-flow data are short, flows can be generated from rainfall.
4. Reservoir and flood routing studies: the inflow hydrograph for routing through reservoirs and channels is obtained from the UH.
5. Ungauged basins: with synthetic unit hydrographs (Snyder, SCS) the flood can be found for catchments without records.
6. Study of basin response: comparison of UH of different basins, or before and after land-use change (urbanisation, deforestation), indicates the change in runoff behaviour.

Limitations

1. Linearity and time invariance: the theory assumes direct runoff is proportional to rainfall excess and that the UH does not change with time. In real basins both are only approximately true, especially for very large floods.
2. Uniform rain assumption: rain is assumed uniform over the basin and constant in intensity during the storm; in actual storms the spatial and temporal variation affects the hydrograph.
3. Basin size: it is reliable for basins about 25 km² to 5000 km². For very small basins rain distribution varies too much, and for very large basins channel storage and non-uniform rain make routing necessary.
4. Base flow separation and loss estimation are subjective, and errors in φ-index or base flow directly change the UH.
5. Not applicable where there is large storage (lakes, reservoirs), snowmelt, or heavy channel overflow, or where the basin changes (new dams, urbanisation) after the UH was derived.
6. Needs data: the UH requires a good rainfall–runoff record of isolated, uniform storms, which may not exist in many hilly Nepal catchments.