Hydrographs: A Tool for Illustrating Stormwater Flow

Precipitation from rain or snow has three possible pathways after it falls to earth. It can land on something — trees or other vegetation — and remain stuck until it evaporates. It can soak into the soil and get taken up by plant roots or become groundwater. Or it can drain off the surface of the land until it reaches a ditch, stream, or other body of water. Precipitation that follows this last pathway is called stormwater runoff.

Stormwater is not a new phenomenon, but the percentage of precipitation that becomes stormwater has increased ever since Pennsylvania was developed by European settlers. Commonwealth communities are covered by buildings, roadways, compacted soils, and other hard surfaces that don't absorb water like Pennsylvania’s old-growth forests did several centuries ago. The people who manage water resources today — most often local municipalities — use a tool called a hydrograph to visualize how stormwater changes the flow of water in manmade ditches or natural waterways (streams and rivers).

Hydrographs illustrate how water flow increases or decreases over time at a specific location.  Figure 1 shows the hydrograph for a spot along a fictional stream that receives runoff from a 10-square-mile watershed that contains a mix of rural, suburban, and urban development. Stream flow is shown in light blue. It was measured over a 24-hour period that followed two weeks without rain. The vertical scale shows the amount of water the stream carried in cubic feet per second (cfs), and the horizontal scale shows time. According to this hydrograph, the stream carried a steady 11cfs over 24 hours. Since the watershed has been dry for two weeks, we can safely reason that the flow illustrated by the hydrograph comes from groundwater springs. Groundwater provides a stable baseflow for most streams in Pennsylvania.

Figure 2 shows a hydrograph for the same stream location over the next 24 hours. Near the beginning of this 24-hour period, an intense thunderstorm started at 2:00 AM and lasted for 30 minutes. The storm drenched the watershed with one inch of precipitation. Some of the rain we can assume soaked into the dry ground, but the hydrograph shows that much of the rain became stormwater and drained into the creek.  According to the hydrograph, it only took around 60 minutes until the runoff from the thunderstorm, shown in dark blue, started raising water levels in the stream. Levels increased rapidly from only 10cfs at 3:00 AM, to 140cfs at 7:00 AM when the peak stormwater flush came roaring by. Water levels then decreased back to baseflow over the next 5 hours.

The flash torrent of stormwater shown in Figure 2 is a common occurrence in streams draining developed areas where buildings, roads, compacted soils, and other hard surfaces have the potential to create large amounts of runoff quickly during rainstorms. When water managers see a peaky hydrograph, they know the cause is very likely stormwater runoff from a developed watershed. Installing water management practices in the drainage area can flatten and stretch out the stormwater peak by reducing stormwater volume and lengthening the time it takes for stormwater to drain off developed areas.

The hydrograph in Figure 3 shows the same stream location, weather conditions, and 24-hour duration as Figure 2, except several BMPs were recently installed in the 10-square-mile watershed. The practices included several structures designed to capture and infiltrate runoff from residential areas, and a 1000-foot-long vegetated stormwater swale created to slow stormwater discharge from a strip mall and hospital complex.

Comparing the hydrographs in Figures 2 and 3, it is clear that the BMPs changed stream flow. The stormwater peak in Figure 3 happened later, was not as large (60cfs vs 140cfs), and tapered off more gradually. Stream flow still increased, but not as much as it did before the BMPs were installed, and the stormwater took longer to drain into the stream. In other words, the 30 minute rain storm still caused a surge of runoff to enter the stream, but the flow and intensity were lower. By capturing and slowing the flow of stormwater, the BMPs reduced stream bank erosion, protected fish habitat, and minimized flooding in downstream communities. 

While the three scenarios are fictional, their hydrographs accurately represent how water flows through streams and stormwater ditches in Pennsylvania.  It is not hard to create basic hydrographs for real streams when flow data is available from the USGS National Water Dashboard or another source. 

Hydrographs can also be used to model how stream flow changes when factors such as precipitation or land use are modified. This process requires more data, but it is another way they are used by water managers to reduce Pennsylvania's growing stormwater risks.