Precipitation and Streamflow Connections

The Susquehanna River. Credit

The beautiful Susquehanna River. Credit: Wikipedia

Last week, we took a look at streamflow changes in the Susquehanna, Potomac, and James Rivers. Today, we’ll look at precipitation connections to that streamflow.


At the heart of this study are the 26 climate extreme indices that were calculated for the Chesapeake Bay National Estuarine Research Reserves of Maryland and Virginia. Besides just being informative, these indices were calculated to provide insights into environmental issues currently facing Chesapeake Bay.

Ten of these indices are based on precipitation. So can we find a relationship between these precipitation indices and streamflow? It may seem like an obvious yes, but we first must use some statistics in order for this “obvious” connection to be defensible.

Connections? You bet!

I used a simple linear regression to plot mean annual streamflow against five annual precipitation-based climate indices. You can see from Table 1 that most were statistically correlated.

Table 1:

Table 1: The slopes and the coefficient of determination (R2) of streamflow versus annual precipitation indices for the Susquehanna, Potomac, and James Rivers. N.S. means not significant.

R10mm and R20mm are the annual count of days when at least 10 mm (~0.4 inches) and 20mm (~0.8 inches) of precipitation fell in a day. Each river had a significant correlation. In other words, years with a high frequency of very wet days were also years with a high annual streamflow. That makes sense!

Figure 2:

Figure 2: Linear regression of streamflow versus the R10mm index.

Figure 3:

Figure 3: Linear regression of stream versus total annual precipitation.

Additionally, total annual precipitation (PRCPTOT) was also significantly related to streamflow. This also makes sense, but is really helpful information. For example, you can use the equation of the line to estimate annual streamflow if you happen to know the annual precipitation (which is easy to get in the SWMP data!). This relationship also suggests that if total annual precipitation increases in the next few decades, streamflow into Chesapeake Bay (along with the nutrients and sediments it carries) could also increase.

The last index duo is R95p and R99p, which is the annual precipitation that exceeds the 95th and 99th percentile. It is the “extra” rain in a rainy year. Interestingly, R99p was not correlated the streamflow in the Potomac or James Rivers. A year with a high R99p can also be related to a single event, such as a big rainstorm or hurricane. So the mean annual streamflow may not have a solid relationship in year that gets a ton of rain in a short period.

Lastly, it is interesting to note that the Susquehanna River, in general, had weak (although still significant) relationships compared to the Potomac and James. Our precipitation index is calculated around the Chesapeake Bay, thus outside of the Susquehanna watershed which extends more northward in parts of Pennsylvania and New York. It is actually really cool that such a strong relationship is still there!

Final Thoughts

Precipitation changes will likely affect the streamflow volumes into Chesapeake Bay. That is one of the many reasons why understanding past and future climate changes are important when investigating ecosystems and environmental issues (like water quality) in any body of water. More to come!

Kari Pohl

About Kari Pohl

I am a post-doctoral researcher at NOAA and the University of Maryland (Center for Environmental Science at Horn Point Laboratory). My work investigates how climate variability and extremes affect the diverse ecosystems in Chesapeake Bay. I received a Ph.D. in oceanography from the University of Rhode Island (2014) and received a B.S. in Environmental Science and a B.A. in Chemistry from Roger Williams University (2009). When I am not busy being a scientist, my hobbies include running, watching (and often yelling at) the Boston Bruins, and taking photos of my cat.
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