Storm hydrographs are an essential concept in IB Geography, particularly within Option A: Freshwater – Drainage Basins. A storm hydrograph is a graph that shows how a river’s discharge responds to a rainfall event over time. By analysing storm hydrographs, geographers can understand how drainage basin characteristics influence flooding and river behaviour.

A typical storm hydrograph has two main components: a rainfall graph and a discharge curve. Rainfall is usually shown as bars, while river discharge is shown as a line. The horizontal axis represents time, and the vertical axes show rainfall amount and river discharge. Together, these features illustrate how quickly and intensely a river responds to precipitation.

One of the most important features of a storm hydrograph is lag time. Lag time is the delay between peak rainfall and peak river discharge. Short lag times indicate a rapid river response and higher flood risk, while longer lag times suggest slower movement of water through the drainage basin. In IB Geography, lag time is a key indicator of how efficiently water moves through the system.

Peak discharge refers to the highest point on the discharge curve. It represents the maximum volume of water flowing in the river after a rainfall event. High peak discharge increases the likelihood of flooding, especially if the river channel cannot contain the increased flow. Factors such as intense rainfall, saturated soils, and impermeable surfaces contribute to higher peak discharge.

Another important feature is baseflow, which is the normal flow of the river before and after a storm. Baseflow is maintained by groundwater inputs and reflects long-term water storage within the drainage basin. Rivers with high groundwater storage tend to have higher baseflow and more stable discharge patterns.

Storm hydrographs are influenced by both physical and human factors. Physical factors include basin size, shape, slope, soil type, geology, and vegetation cover. Steep slopes, impermeable rock, and thin soils produce flashy hydrographs with short lag times and high peak discharge. Vegetation increases interception and infiltration, lengthening lag time and reducing flood risk.

Human activities also affect storm hydrographs. Urbanisation increases impermeable surfaces, reducing infiltration and increasing surface runoff. This creates flashy hydrographs with steep rising limbs and high peak discharge. Deforestation reduces interception and soil stability, further increasing runoff and flood risk.

In IB Geography exams, students are expected to describe, interpret, and explain storm hydrographs clearly, linking graph features to drainage basin characteristics. Accurate use of terminology such as lag time, rising limb, falling limb, and peak discharge is essential for high-mark answers.

Overall, storm hydrographs provide a valuable tool for understanding river response to rainfall and assessing flood risk within drainage basin systems.

RevisionDojo helps IB Geography students master storm hydrographs by linking theory, diagrams, and exam-style explanations into clear and confident responses.