More on Precipitation

Picture6This past week, Jenny Allen and I have been at the Coastal and Estuarine Research Federation (CERF) conference in Portland, Oregon. We presented a poster on Monday, highlighting a few aspects on this work.

For today’s post, I am continuing with one of our ecosystem vignettes which demonstrates the connections between total annual precipitation, the type of wet day, and the mean total nitrogen loading into the Patuxent.

Type of Wet Day versus Total Annual Precipitation

Just to refresh your memory, we have broken total annual precipitation into three different types of wet days: Very Wet Days (>20mm precipitation), Moderately Wet Days (10 to 20mm), and Mildly Wet Days (<10 to 1mm).


Fig. 1: Total annual precipitation in the North Chesapeake versus the total amount of rainy days.

As one would expect, the amount of precipitation we receive in a year is significantly correlated to each type of wet day. In other words, if we have a wet year, we can expect more wet days. That makes sense!

Since each type of wet day is correlated the total annual precipitation, we can now estimate how many wet days we had using the equation of the line. (y=mx+b)

For a simple example, if we had a year in which 1000 mm of precipitation fell, we can use the linear regression from Fig. 1 to estimate that we had ~88.2 days in which precipitation fell. And of those wet days, we could also estimate that 14.8 days were very wet, 19.1 were mildly wet, and 54.3 were mildly wet.

This relationship can also give insights into the fractions of each type of rainy day: 16.8% were very wet, 21.6% were moderately wet, and 61.6% were mildly wet. Also, very wet days had the best correlation with total annual precipitation (R2=0.70) and mildly wet days had the weakest (R2=0.07), although it was still significant.

Type of Wet Day versus Total Nitrogen

Fig. 2:

Fig. 2: Northern Chesapeake wet days versus mean annual total nitrogen load in the Patuxent River.

In Fig. 2, we switched out total annual precipitation with the mean total nitrogen load to the Patuxent River. Again, each type of wet day was significantly correlated to the annual nitrogen load in this small tributary.

This linear relationship is neat since we can use it to estimate the mean annual nitrogen load if we happen to know how many wet days we had or the total annual precipitation. For example, using the 14.8 very wet days we derived above, we could predict that ~73,500 lbs of total nitrogen was transported to the Patuxent.

What is interesting to keep in mind for the future, the Chesapeake Bay Watershed Agreement is actively trying to reduce the amount of nutrients entering Chesapeake Bay to help improve water quality. This data suggests that very wet years are likely to still transport high quantities of nitrogen, which is important to understand for the best management practices (BMPs) to be implemented.

Fig. 3:

Fig. 3: Correlation between the North Chesapeake total annual precipitation with the mean annual nitrogen load in the Patuxent River.

Also, just to cover my bases, I also regressed total annual precipitation with mean annual nitrogen! That was significant as well!

Once we hit the new year, I will be able to use the total annual precipitation from 2015 to “test” these relationships! Stay tuned!

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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