Reading a Catchment's Land-Use Change from Its Hydrograph
A 12 km² catchment in the Western Ghats foothills has a stream gauge at its outlet and a rain gauge on the ridge. Mean annual rainfall is 2,400 mm, of which about 60% falls in June–August. Between 2015 and 2022 the catchment changed: forest cover fell from 80% to 55% of the area, replaced by rubber and other plantations, and the plantation tracks were graded and compacted. The gauging records show the change clearly: annual runoff rose from 1,100 mm to 1,500 mm; peak discharge in a storm of similar rainfall rose from 18 m³/s to 41 m³/s; and the March baseflow fell from 0.9 m³/s to 0.3 m³/s. Sediment trapped behind a small check dam also increased. A local newspaper attributed the 2022 flash flood to 'unusually heavy rain', and the panchayat has asked the class to comment on that explanation.
1. Calculate the runoff coefficient for 2015 and 2022, and state what the change shows about the catchment's water balance.
Read the worked answer
Runoff coefficient = annual runoff ÷ annual rainfall.
- 2015: 1,100 ÷ 2,400 = 0.46
- 2022: 1,500 ÷ 2,400 = 0.63
Because annual rainfall is used as the same 2,400 mm for both years, the coefficient isolates the change in the catchment's behaviour from any change in rainfall. A rise of about 0.17 means roughly 400 mm more water leaves as streamflow each year - that water is no longer going into soil moisture, deep percolation and groundwater storage.
The newspaper's explanation therefore does not fit the evidence. If heavy rain alone were responsible, the flood peak would rise, but annual runoff and baseflow would not shift in opposite directions. The coefficient shows a partitioning change: a larger share of rainfall is being converted to quick surface flow, and a smaller share to storage that is later released. That is a land-cover signal, not a rainfall signal. The rainfall figure should still be quoted (a storm of similar magnitude was recorded in both periods), but it is the denominator, not the cause.
2. Explain the mechanism that produces a higher peak and a lower dry-season baseflow at the same time, using infiltration, interception and slope.
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The two symptoms are one process: water is being delivered to the stream faster and stored less.
Interception and canopy: forest canopy and litter intercept rainfall and slow its arrival at the soil; a rubber/plantation cover with cleaner ground and graded tracks intercepts and delays less.
Infiltration: forest soils are porous, root-rich and burrowed, with high infiltration capacity. Plantation tracks are graded and compacted, and compaction can cut infiltration capacity by an order of magnitude. Rain that used to enter the soil now runs off.
Slope and connectivity: the new tracks and drains form continuous flow paths downslope to the stream, so water that falls in a storm arrives within hours rather than days. Convergent tracks deliver it to the same point - hence the jump from 18 to 41 m³/s.
Storage: because less water infiltrates, the soil and weathered zone - the catchment's sponge - is not refilled. Through the dry months there is less stored water to release slowly, so baseflow falls from 0.9 to 0.3 m³/s. The increased sediment is consistent with this: faster overland flow on bare, disturbed surfaces detaches and carries soil.
So a higher peak and lower baseflow are the same 'flashier' response, and both are predicted by reduced infiltration plus better connectivity.
3. Recommend two watershed-management measures and state exactly how each would show up in the hydrograph if it worked.
Read the worked answer
1. Contour bunding, trenching and gully plugging on the plantation slopes. These interrupt the downslope flow paths and give water time to infiltrate; trenches act as small storages. Expected hydrograph signature: peak discharge falls and the rising limb becomes less steep, while a small rise in dry-season baseflow appears as infiltration recharges the weathered zone. If only the peak falls but baseflow stays flat, the structures are storing water that still evaporates rather than infiltrating - a useful diagnostic.
2. Protection and regeneration of the remaining forest, with reforestation of riparian strips. Riparian and forest cover provides interception and high-infiltration soil, and holds streambanks against sediment loss. Expected signature: a slower rising limb and lower sediment yield; baseflow improvement will lag by several years, so the class should not expect a same-season result, and should say so rather than claim success early.
Both should be monitored against the same gauges and the same storm magnitude, and the class should recommend that the panchayat record rainfall alongside discharge, because a single gauge cannot separate a land-use effect from a rainfall effect. The framing for the panchayat is that these measures treat the cause behind the flood peak, whereas dredging or embankment work downstream only treats the symptom.