GSA Talk Preview

Figure 1: The Annual Geological Society of America conference is in the Chesapeake region this year!

The Annual Geological Society of America conference is in the Chesapeake region this year!

This is a special spooky Halloween edition…just kidding! But now that I have your attention, today’s post will feature one of our findings that I will be presenting at the Geological Society of America Meeting in Baltimore this Sunday.

If you happen to be attending the GSA meeting, my talk is at 3:55 pm November 1st in the Chesapeake session. You can also read the abstract here.

In case you are not attending, this post will feature a portion of that talk: How is the frequency of rainy days changing and what does that mean for nitrogen transport into the Patuxent River?

Historical Observations of Total Annual Precipitation

Over the past ~100 years, the near-shore Chesapeake Bay has observed more total annual precipitation. I used a 21-year moving mean to estimate the decadal rate of change, and it was significantly increasing in all three of our data sets (Fig. 1).

Fig. 1

Fig. 1: The observed historical total annual precipitation in the Chesapeake region.

The North Chesapeake has experienced the greatest change at ~17 mm of precipitation per decade. That is roughly an additional 0.65 inches of water we see every 10 years!

The South Chesapeake also had a significant decadal increase of ~5 mm (or roughly 0.2 inches). The HadEX2, which is an area-weighted average that represents the entire near-shore Chesapeake Bay region, was similar to the South Chesapeake at +7 mm per decade.

Okay, so we have observed more annual precipitation….but how we receive that additional rain matters! Lucky for us, we have climate indices that can investigate!

Why we Care?

Precipitation can be like vitamins. As a vegetarian, I know that I need iron to be healthy. But if I have too much iron, or too little iron, I could get sick. Precipitation is similar to this analogy. The environment needs rain in order to keep on functioning. But getting too little (drought!) or too much (flood!) precipitation can have some negative effects.

Fig. 2:

Fig. 2: Precipitation can cause nutrient run-off, which sometimes results in nuisance algal blooms or fish kills.

How does that apply to the Chesapeake? Well, we are receiving more precipitation, but how we receive it can matter. Are we getting fewer rainy days that just happen to all be stronger? Are we getting more rainy days that are lighter? 

We care about precipitation for many reasons, but one is nitrogen transport the Bay. It is well known that precipitation causes run-off, carrying nutrients and sediments to bodies of water. But when many people think about run-off events, they think about intense rain storm causing visible streams of water to “flow like rivers” in the street.

Here we bring to light that smaller rain storms could also be important in understanding nutrients.

Rainy Day Hindcast: Umbrellas wanted!

The very first question to address: has the annual amount of days with rainfall changed (Fig. 3)?

Fig. 3:

Fig. 3: The total number of rainy days was apportioned into three categories: Very Wet, Moderately Wet, and Mildly Wet.

Total Rainy Days: We do not have an index specifically for total rainy days, but we can still calculate it using Total Annual Precipitation and the Simple Daily Intensity Index (SDII), which is the ratio of rain amount divided by the amount of days with rain. So we can calculate total rainy days by dividing the total annual precipitation by the SDII index.

Very Wet Days: We have an index for this! R20mm is the count of days when more than 20 mm of precipitation fell!

Moderately Wet Days: This is the difference between our R10mm and R20mm indices, which determines the amount of days in which 10 to 20mm precipitation fell.

Mildly Wet Days: This is the count of days in which less than 10 mm of precipitation fell. It can be calculated as the “leftover days” from the Total Rainy Days-Very Wet Days-Moderately Wet Days.

From Fig. 3, you can see how neat this finding is! Historically speaking, the amount of rainy days has not changed. But the distribution of rain between those days has! What we see is more very wet days and less mildly wet days.

In other words, a rainy day today is likely to be stronger than it was 100 years ago!

Connection to the Environment

We plotted the three different “types” of wet days described above (and the total count of wet days) against the mean annual total nitrogen load in the Patuxent River, which is where Jug Bay is situated. Total nitrogen was significantly correlated to all four of these wet day types (Fig. 4).


Fig. 4: Linear regression of wet days and mean annual nitrogen load in the Patuxent River from 1985 to 2013.

In other words, nitrogen transport is affected by precipitation, regardless of strength!

Another detail to note, years that have a lot of total annual precipitation will have a higher nitrogen load in all three types of rainy days. Even moderately rainy days matter for nutrient movement!

Since the regressions were significant, we can use the equation of the line to estimate the mean annual nitrogen transport to the Patuxent River. For example, if we had 20 moderately wet days (the time series mean), we can estimate a mean annual nitrogen load of 134,000 lbs/yr.

Final Thoughts

Like this “vignette”? It is part of the climate paper currently in draft!

The other “vignettes” featured in this talk are the relationship between warmer-than-normal summers and SAV diebacks and a teaser for the observation than warm autumns and winter cold snaps are related to organism deaths like pelicans and spot fish!

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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