# Hydrograph A **hydrograph** is a graph of a river's flow, or discharge, against time at one point, usually a stream gauge. After a storm it rises to a peak and recedes; its shape records how a catchment turns rain into streamflow. Paving a catchment makes more of the rain run off and delivers it sooner, so urban streams have higher, earlier and more short-lived flood peaks — a "flashier" hydrograph.[^epa-urban][^usgs-runoff] Hydrographs are the everyday tool of [[Hydrology|hydrology]] for flood forecasting and for designing bridges, culverts and [[Run-of-the-river_hydroelectricity|hydroelectric]] plants. <div class="microsim-player"> <iframe src="https://wikitube-3d-microsims.netlify.app/Hydrograph.html" width="100%" height="620" frameborder="0" loading="lazy" sandbox="allow-scripts allow-same-origin" allow="accelerometer; gyroscope" title="Hydrograph — p5.js microsim"></iframe> </div> *Microsim (THY-110): drop a three-hour storm on a catchment and pave more of it; the flood peak rises and arrives sooner. ILLUSTRATIVE: a two-reservoir catchment model, not calibrated to a real basin.* Videos: [16:9](https://wikitube-3d-microsims.netlify.app/media/Hydrograph_16x9.mp4) · [9:16](https://wikitube-3d-microsims.netlify.app/media/Hydrograph_9x16.mp4) ## Terminology The rising limb climbs from the start of runoff to the peak discharge; the recession (falling) limb follows as storage drains. The lag time is the delay between the centre of the rainfall and the peak. Baseflow is the steady flow fed mainly by groundwater, which carries a river between storms.[^theis-water] ## Types Storm hydrographs show a single event over hours or days; annual hydrographs show the seasonal cycle of a river, such as a spring snowmelt peak; flow-duration curves reorder the record to show how often each flow is exceeded. ### Urbanization Impervious surfaces — parking lots, roads and roofs — reduce infiltration and accelerate runoff, producing greater volumes of runoff that reach streams faster; urban floods rise quickly, peak higher and fall back toward baseflow sooner than rural ones.[^usgs-runoff][^epa-urban] ## Baseflow separation To study storm runoff, the baseflow contributed by groundwater is separated from the quickflow contributed by the storm, usually by drawing a line beneath the storm peak from the start of the rise to a point on the recession. ## Unit hydrograph The unit hydrograph, introduced by Leroy Sherman, is the hydrograph that one unit of effective rainfall, falling evenly over a catchment in a given time, would produce at its outlet. Treating the catchment as linear, the response to any storm is the sum of unit hydrographs scaled and shifted for each interval of rain — the convolution the microsim computes. ## Subsurface hydrology hydrograph In groundwater studies, a hydrograph records water level in a well over time, showing recharge after rain or snowmelt and decline under pumping. ## Raster hydrograph A raster hydrograph shows many years of daily flow as a grid of coloured pixels, with days along one axis and years along the other, so that seasonal patterns and long-term changes stand out. ## Lag-1 hydrograph A lag-1 hydrograph plots each day's flow against the previous day's, separating rising, falling and steady flows. ## See also - [[Infiltration_(hydrology)]] - [[Drainage_basin]] - [[Run-of-the-river_hydroelectricity]] - [[Hydrology]] ## References [^epa-urban]: U.S. Environmental Protection Agency, CADDIS. "Urbanization – Hydrology." https://www.epa.gov/caddis/urbanization-hydrology [^usgs-runoff]: U.S. Geological Survey, Water Science School. "Surface Runoff and the Water Cycle." https://www.usgs.gov/water-science-school/science/surface-runoff-and-water-cycle [^theis-water]: Theis, Tom; Tomkin, Jonathan, eds. (2015). *Sustainability: A Comprehensive Foundation*, ch. 7, module "Water Cycle and Fresh Water Supply." https://open.umn.edu/opentextbooks/textbooks/sustainability-a-comprehensive-foundation ### Portal Books - Theis, Tom; Tomkin, Jonathan, eds. (2015). *Sustainability: A Comprehensive Foundation* — [OTL record](https://open.umn.edu/opentextbooks/textbooks/sustainability-a-comprehensive-foundation); on the [[PORTAL_Energy]] shelf. ## External links - [Surface Runoff and the Water Cycle](https://www.usgs.gov/water-science-school/science/surface-runoff-and-water-cycle), USGS Water Science School ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Hydrograph) : [Wikitube](https://en.wikitube.io/wiki/Hydrograph) · pinned revision [1372760032](https://en.wikipedia.org/w/index.php?oldid=1372760032) · 2026-09-10 ## Previous hub tags Hubs: `Life_Physics`, `Systems`. Portals: [[PORTAL_Thury_Hydrodynamics_Apex_Spine]], [[PORTAL_Stock_and_flow]]. --- *Thury station wave · 2026-09-10 · drafted · microsim THY-110 (p5.js) · parent [[Hydrology]].*