Cape Geography Unit1: The Storm Hydrograph

The video introduces the storm hydrograph, which illustrates river discharge over time in response to rainfall within a drainage basin. It defines discharge as the amount of precipitation reaching the channel via runoff (overland flow, throughflow, and base flow), after accounting for evapotranspiration and storage changes. The discharge equation is compared to the water balance equation, highlighting that discharge is synonymous with runoff in this context.

Water reaches the river channel through several pathways, varying in speed: direct precipitation, quick overland flow, slower throughflow (water infiltrating and flowing sideways), and the slowest base flow (water percolating deeper then flowing laterally). The channel's shape and roughness influence the water's velocity.

A storm hydrograph graphically represents river discharge variation over time. Key elements include the rising limb (water level increase), falling/recession limb (water level decrease), lag time (time between peak rainfall and peak discharge), base flow (groundwater contribution), storm flow (overland and throughflow), bankfull discharge (channel capacity), and peak discharge (maximum water in the river). A shorter lag time and a steeper rising limb increase the likelihood of flooding.

Before rainfall, discharge is low. As rain begins, discharge slowly rises due to direct precipitation, then rapidly increases with overland flow and throughflow. If discharge exceeds bankfull capacity, flooding occurs. After rainfall, water recedes, albeit slowly, as base flow continues to contribute.

Climatic factors significantly affect hydrograph shape. Low temperatures can freeze soil, making it impermeable, increasing overland flow, and shortening lag time. High temperatures increase evapotranspiration. High-intensity or long-duration rainfall reduces infiltration, leading to more overland flow. Snowfall stores water, but rapid snowmelt over frozen ground causes fast runoff, leading to a shorter lag time and steeper rising limb.

Basin size affects lag time: smaller basins have shorter lag times and steeper rising limbs, while larger basins have longer lag times. Circular basins lead to faster water concentration than long, narrow ones. Steeper land relief results in faster water flow, a shorter lag time, and a steeper rising limb, whereas gentle slopes allow more infiltration and a longer lag time.

Geology and soil properties also play a role. Permeable rocks and soils (like sand) encourage infiltration, leading to longer lag times and gentler rising limbs. Impermeable rocks and soils (like clay) increase overland flow, resulting in shorter lag times and steeper rising limbs.

Vegetation reduces discharge by intercepting precipitation, increasing evapotranspiration, and absorbing water through roots. Greater vegetation leads to a longer lag time and a gentler rising limb. Urbanization, with impermeable surfaces like concrete and tarmac, causes steeper rising limbs and shorter lag times, increasing flood risk.

The video concludes with an example analysis of a storm hydrograph diagram, identifying lag time, peak discharge, and differentiating between hydrographs for urban areas (short lag time, steep rising limb, high peak discharge) and wooded rural areas (longer lag time, gentler rising limb, lower peak discharge) based on the factors discussed.