Understanding hydrographs

 

The term “hydrograph” is frequently used when discussing water. What is a hydrograph, and why is it important?

As you might infer, a hydro-graph is a water graph. It is a plot of some water attribute over a period time. The most common hydrographs show flow rate over time, but others may plot depth, temperature, pH, or chemical concentration.

Hydrographs help us understand water patterns and statistics. They also allow us to forecast future conditions based on historical data. Here are three examples of common hydrographs.

Runoff hydrograph

We’ve all experienced a storm, where water falls from the sky. Some of that precipitation will collect and flow over land and into roads, gutters, and streams. This is called runoff. A runoff hydrograph describes the response of a watershed to a storm. Imagine that it starts raining. As the storm continues, more and more rain falls. Consequently, more and more water runs off. Then the storm moves on or the rain begins to stop, and the runoff decreases to nothing. This is what a runoff hydrograph (below) represents.

hg1Engineers use this information when designing systems to handle both the peak flow and total volume of stormwater that might otherwise cause flooding. Stormwater quality is also a consideration, as sediment and other contaminants transported in stormwater can impact infrastructure, human health, and the environment.

Streamflow hydrograph

While a runoff hydrograph might cover a few hours or days, a streamflow hydrograph (also called a discharge hydrograph) describes flows in a river or stream over months or years. The hydrograph below shows discharge in the Provo River during 2013.

hg2This hydrograph has the general seasonal pattern we would expect: Flows are highest in the spring when snowmelt feeds the river, and lowest in the late summer when temperatures and water demand are high. Streamflow hydrographs are used in water resources planning, hydropower evaluation, flood forecasting, environmental monitoring, and ecosystem assessment.

Wastewater hydrograph

Examining the hydrograph (below) of a residential wastewater stream reveals an intuitive diurnal pattern.

hg3Wastewater flows are lowest in the middle of the night and increase rapidly before 8 a.m. as people flush toilets, shower, and prepare for the day. Flows drop off somewhat in the afternoon and rise with the evening’s cooking and dishes. Wastewater hydrographs are important for proper design, planning, and operation of wastewater collection systems and treatment facilities.

Robert B. Sowby

Robert B. Sowby is a water resources engineer at Hansen, Allen & Luce. A graduate of BYU, MIT, and the U, he has contributed to over 100 civil, water, and environmental projects throughout North America.

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