IB Geography A.2.1 Hydrograph Characteristics and Natural Influences Notes

A Storm Hydrograph Converts Rainfall into a Timed Discharge Response

1. A storm hydrograph plots river discharge against time during and after a rainfall event. Rainfall is normally shown as bars above the discharge curve so that the input and river response can be compared.
2. Base flow is the groundwater-fed discharge present before storm runoff reaches the channel.
3. Rising limb is the increase in discharge as water from the storm reaches the river. A steep rising limb indicates a rapid basin response.
4. Peak discharge is the highest flow recorded during the event. Flooding becomes more likely if the peak exceeds channel capacity.
5. Lag time is the interval between peak rainfall and peak discharge. It records how quickly water has moved through basin stores and flows.
6. Falling limb shows discharge declining as surface and soil-water transfers weaken. It is often gentler than the rising limb because stored water continues to drain into the channel.
7. Read the hydrograph from peak rainfall to peak discharge, then compare the lag time, rising limb, recession limb and base flow.

Natural Basin Characteristics Control Lag Time and Peak Discharge

1. Steep relief accelerates overland flow and throughflow, which tends to shorten lag time and raise peak discharge. Gentle slopes normally allow more time for infiltration and storage.
2. Impermeable rock and clay-rich soil restrict infiltration. Permeable rock and well-structured soil transfer more water underground, delaying the river response and supporting later base flow.
3. Dense vegetation intercepts rainfall, encourages infiltration and increases evapotranspiration. Sparse vegetation exposes the surface and can allow faster runoff.
4. A circular basin concentrates flows from tributaries at the outlet at a similar time, while an elongated basin spreads their arrival over a longer period.
5. High drainage density provides many short routes to the main channel. Low drainage density usually delays the transfer of water from slopes to the river.
6. Antecedent conditions can override other controls. Saturated, frozen or snow-covered ground may produce a rapid response even in a normally permeable or vegetated basin.

Flashy and Subdued Hydrographs Signal Different Flood Responses

1. Flashy response combines a short lag time, steep rising limb and high peak discharge. It is associated with rapid runoff and limited temporary storage.
2. Subdued response has a longer lag time, gentler limbs and lower peak discharge because more water is stored or transferred slowly.
3. The same basin can produce either response under different conditions. A small storm after a dry period may be subdued, while prolonged rainfall on saturated soil can be flashy.
4. A high peak describes the physical hazard, not total flood risk. Damage also depends on whether people, buildings and infrastructure occupy the inundated area and how vulnerable they are.

Water Balance Links Hydrograph Response to Basin Storage

1. Water-balance relationship: $$P = Q + E + \Delta S$$
2. In this relationship, P is precipitation, Q is runoff or river discharge, E is evapotranspiration and ΔS is the change in basin storage.
3. When precipitation exceeds discharge and evapotranspiration, storage increases in soil, groundwater, lakes or snow. Once stores fill, a larger share of later rainfall can reach the channel quickly.
4. When outputs exceed precipitation, storage falls and river discharge may increasingly depend on groundwater base flow.

1. A storm hydrograph examines an event over hours or days, whereas a water balance can be calculated over much longer periods. They are connected, but they do not use the same time scale automatically.