Hydrology, catchments, rainfall and runoff

Define the catchment and its water account

A surface catchment is the area draining toward a chosen outlet, bounded by topographic divides. Groundwater paths or built transfers may cross that surface boundary. State whether the account includes only the surface system or also subsurface stores and imported water.

Over a specified interval, change in storage equals inputs minus outputs. Inputs can include rain, snowmelt and imported water; outputs can include streamflow, evapotranspiration, withdrawals and groundwater transfers. Infiltration moves water into the ground and may later contribute to streamflow. It is not necessarily a permanent loss from the larger water system.

Volume and peak describe different parts of the event

A rainfall depth times catchment area gives a volume only when that depth represents the area. A hyetograph plots rainfall intensity through time. A hydrograph plots outlet discharge Q through time; area beneath it gives volume after consistent time conversion. A peak is a rate, not the whole event volume.

Soil moisture before the storm, infiltration capacity, impervious surfaces, vegetation, slope, drainage paths and temporary storage affect response. Intensity and spatial distribution matter as well as total depth. Two storms with equal total rain can therefore produce different peaks and timings. A short intense burst may be important for one small drainage system while prolonged rain dominates a larger basin.

Separate storm-related runoff from any prior baseflow when making an event comparison. Define the start and end of the integration window, retain observations and explain interpolations. A hydrograph still elevated at the end of the window has not necessarily released all of the event water.

Use models to connect evidence rather than replace it

A rainfall–runoff model simplifies spatial paths, infiltration and storage. Its parameters and input resolution must suit the question. Matching one observed peak does not establish the correct volume, timing or behaviour under a wetter starting state. Evaluate relevant independent events and plausible changes in land cover or climate.

Catchment management can alter runoff, sediment and pollutant transport together. Moving water away faster from one location may increase a downstream peak if timing aligns unfavourably. Compare the receiving system and event sequence before calling a local reduction a basin-wide benefit.

Integrate an original event before judging its peak

Assume representative rainfall 10 mm during the first hour and 20 mm during the second over a 2.00 km² catchment. Total depth .030 m over 2,000,000 m² gives 60,000 m³. These are constructed inputs, not gauge observations.

Supply a direct-runoff hydrograph with Q=0,4,6,2,0 m³/s at hours 0,1,2,3,4, linear between points, with no baseflow. Four trapezoids have areas 2,5,4,1 in units (m³/s)·h. Multiplying their sum 12 by 3600 s/h gives 43,200 m³. Peak is 6 m³/s at hour 2.

Runoff depth is 43,200/2,000,000=.0216 m, or 21.6 mm; event runoff ratio is .72. The remaining 16,800 m³ is a combined residual of stores and other transfers under this window, not a uniquely determined infiltration measurement. This one constructed ratio is not a universal catchment coefficient.

Try the reasoning

1. Can the same total rain produce a different flood peak?

   Show the answer and reasoning

   Yes. Intensity, spatial pattern, previous wetness, paths and storage can change peak and timing.

2. Why multiply hydrograph area in these graph units by 3600?

   Show the answer and reasoning

   Discharge is per second but horizontal time is hours; the conversion yields cubic metres.

3. Does the residual determine infiltration by itself?

   Show the answer and reasoning

   No. Other outputs, changed stores, baseflow separation and measurement/model uncertainty also contribute.

What to carry forward

Connect area, depth, time and storage before interpreting a catchment’s peak or runoff ratio.