Storm Hydrographs By Sofia Gastelu What is it

Storm Hydrographs By: Sofia Gastelu

What is it • A storm hydrograph is a way of showing how the discharge of a river can change over time in response to a rainfall event

• It is meausred in cumecs • Lag time represents the difference between peak rainfall and peak discharge • The rising limb represents the rapid increase resulting from rainfall • The peak discharge occurs when the river reaches its highest level • The falling limb is when the discharge decreases and the level falls • Base flow is the normal flow of the river What is it

Factors affecting a river’s discharge Land use • Surfaces in urban areas are impermeable, water runs into drains, gathers speed and joins rainwater from other drains then spilling into the river • In rural areas ploughing up and down hillsides creates channels which allow rainwater to reach rivers faster increasing discharge Rock and soil type • Permeable rock and soils absorb water easier so surface run-off is rare • Impermeable rock and soils are closely packed, rainwater can’t infiltrate so water reaches the river quickly

Rainfall • The type + amount of rainfall affects the river’s discharge • Heavy continual rain means more water flowing into the river and antecedent rainfall is rain that has already happened. The ground is saturated so further rain will then flow as surface run-off. Relief • Rainwater in steep slopes is likely to run straight over the surface before it can infiltrate • In gentle slopes, infiltration is more likely to happen Weather conditions • Hot dry weather will bake the soil so when it rains the water can’t soak in, so it will run off the surface straight into the river • High temperatures increase evaporation rates from water surfaces reducing discharge • Long periods of extreme cold weather lead to frozen ground so the water can’t soak in