A hydrograph displays the rate of water flow, or discharge, against time at a specific point within a river, channel, or conduit. This visual representation analyzes how a body of water responds to an event, such as a rainstorm or snowmelt. The horizontal axis represents the time elapsed, while the vertical axis measures the discharge, usually in cubic meters per second (m³/s) or cubic feet per second (cfs). Hydrographs provide insights into water movement, studying the relationship between rainfall and runoff within a drainage basin.
Reading the Anatomy of a Hydrograph
The structure of a hydrograph resulting from a storm is composed of several distinct segments. The process begins with the Baseflow, which is the sustained flow of the river fed primarily by groundwater seepage before the precipitation event. This flow continues to contribute to the river even as the storm runoff starts.
Following the onset of significant rainfall, the discharge begins to increase, tracing the Rising Limb of the hydrograph. The steepness of this limb shows how quickly the river’s flow is responding to the surface runoff and prompt subsurface runoff entering the channel. The flow rate eventually reaches its maximum point, which is known as the Peak Flow or peak discharge.
A crucial measurement is the Lag Time, the duration between the point of maximum rainfall and the point of peak discharge. This delay indicates the time it takes for the water to travel across the land and through the channel network to the measurement point. After the peak, the flow begins to decrease, forming the Falling Limb or recession limb. This segment represents the withdrawal of water from the storage built up in the basin as the direct runoff diminishes.
Practical Applications in Hydrology
Analyzing the features of a hydrograph is applied to flood forecasting and risk management. The time to peak and the magnitude of the peak discharge are used to predict the potential for flooding and to issue timely warnings to communities. Studying how quickly a river responds to a storm helps gauge flood risk.
Hydrographs are fundamental to water resource management, providing data on the availability and distribution of water. The baseflow component helps assess the groundwater contribution, which is important for planning water supply systems and managing reservoirs. This information is used to estimate the water volume available for various uses across different seasons.
The data is also used in the design of hydraulic structures like bridges, culverts, dams, and levees. Engineers use the peak discharge information to ensure that infrastructure is built with sufficient capacity to safely handle extreme flow rates.
Catchment Characteristics that Influence the Shape
The characteristics of the drainage basin, or catchment area, dictate the resulting shape and timing of the hydrograph.
Catchment Size and Shape
Catchment size and shape affect how quickly water is concentrated at the outlet. Circular or compact catchments tend to have shorter lag times and sharper, higher peak flows because water arrives almost simultaneously. Conversely, elongated catchments result in a broader, lower peak flow, spreading the runoff volume over time.
Slope
The slope of the basin also plays a role; steeper slopes accelerate the speed of surface runoff, which shortens the lag time and contributes to a higher peak discharge.
Geology and Soil Type
The geology and soil type determine how much water infiltrates the ground versus running off the surface. Permeable substrates, such as sandstone or limestone, allow for greater infiltration, sustaining a higher baseflow and lengthening the lag time. Impermeable surfaces, like granite or clay, promote rapid surface runoff, leading to a steeper rising limb and a more intense peak discharge.
Land Use and Urbanization
Land use and urbanization alter the hydrograph’s shape by increasing the area covered by impervious surfaces such as roads and concrete. These surfaces reduce infiltration and increase the amount and speed of surface runoff. This results in a steep rising limb, a shorter lag time, and a higher peak flow, increasing the risk of flash flooding.