Analysing Flood Hydrographs
When tackling hydrograph questions, don't just describe what you see - explain why the pattern exists using those key factors. Always include specific timings (like "lag time of 3 hours from 14:00 to 17:00") and measurements suchas"peakrainfallof15mmbetween13:00−14:00".
Link the hydrograph shape to probable causes. A short lag time suggests impermeable surfaces, steep slopes, or limited vegetation. A long lag time indicates permeable geology, gentle relief, or dense forest cover.
Use the BSSORV factors to build your explanations: Basin size, Surface type, Soil type, Organisation (drainage density), Relief, and Vegetation. Each factor affects the speed and volume of water reaching the river differently.
Remember that real drainage basins usually show combinations of these factors, creating unique response patterns. Urban basins might have impermeable surfaces but also storm drains (high drainage density), whilst rural basins might have gentle slopes but thin soils over impermeable rock.
Exam Success: Always support descriptions with specific data from the graph and clear explanations using geographical processes and factors.