Precipitation Impact

Intense Rainfall Analysis
Daily Precipitation Analysis

Intense Rainfall Analysis

The %Score of dissolved oxygen (DO) for the entire Anacostia in 2015 decreased from 58 to 54 though the precipitation pattern in terms of intense rainfall events was in favor of DO. There are many factors that influence water quality as discussed below. Thus, the relationship between the precipitation pattern and %Score of DO is quite complex.  Also, AWS found that discussing only intense rainfall events may not be appropriate to explain the %Score of DO because rainfall events actually brings oxygen rich water to the river.  Rain drops are exposed to air.  Thus, rainwater is rich in DO.  Moderate precipitation that will not cause Combined Sewer Overflow will bring large amount of dissolved oxygen to the river improving %Score of DO.  From the point of view, AWS added Daily Precipitation Analysis section on this page next to this section.

Because the District has a Combined Sewer Overflow (CSO) system that carries raw sewage and rainwater in the same pipe, the mixture will be discharged when it rains approximately more than 0.5 inches. After an intense rainfall, AWS has observed from monitoring that DO values become very low for approximately 2 weeks. An example graph is inserted below.

Year 2009 2010 2011 2012 2013 2014 2015
Number of days that had intense (equal to or greater than 1 inch) precipitation in the previous 14 days 104 48 55 58 93 103 60
%Score of DO 72 70 68 48 41 58 54

It turned out that we had intense rainfall events in 2009, 2013, and 2014 frequently from April through October, the time duration from which we use the DO data for our assessment. However, we had high %Score in 2009. Also we had low %Scores when we had less intense rainfall events in 2012 and 2015. It seems that intense rainfall events alone do not explain %Score of DO very well.

The graphs for each year are shown below (click on graphs for full size). The shaded areas represent the days that had intense rainfall events in14 days prior to those day.

In 2009, one more day was influenced negatively by intense rainfall than in 2014, however, DO values were better than those in 2014 (i.e., %Score (72) in 2009 was higher than that (58) in 2014). This is probably because the intensity of rainfall was stronger in 2014 than in 2009 though the frequency was similar. Also, the tide might have influenced DO values. If there is an intense rainfall when the tide is coming in, the CSO discharge will be carried to upstream portion of the Anacostia, thus, the negative influence will remain longer. There are many factors that influence DO values.The weather pattern is definitely one of the most influential factors.

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Daily Precipitation Analysis

Since the intense rainfall analysis cannot explain %Scores of DO very well, AWS started Daily Precipitation Analysis from 2017 State of the River Report (data year is 2015). We have counted number of days over various amount of precipitation. The result was summarized in the table and graph below.

%Score of DO and Number of Days over Various Precipitation*

Year 2009 2010 2011 2012 2013 2014 2015
# of days over >0.5 inches 25 13 18 14 16 14 21
# of days over >0.6 inches 17 12 15 8 13 14 17
# of days over >0.7 inches 16 7 14 7 13 13 13
# of days over >0.8 inches 13 6 12 7 12 10 10
# of days over >0.9 inches 10 6 9 6 10 9 8
# of days over >1.0 inches 9 5 8 5 10 8 6
%Score of DO 72 70 68 48 41 58 54

*Daily precipitation data from 4/1 through 10/31 was used.

The above graph can explain the %Score better. Generally when we have many smaller amount of precipitation events (blue thin line, # of days over >0.5 inches), the %Score is higher. When we have higher number of intense rainfall events (orange line, # of days over >1.0 inches), the %Score is generally worse.

In 2015 the number of smaller precipitation events was high but the number of larger precipitation events were also relatively high. This may account of the lower %Score of DO.

The pattern in 2015 and 2011 is similar in the above graph. However, in 2011 intense rainfall events occurred around the same time period from 7/15 to 7/29 and from 8/15 to 9/23 leaving other time period free from intense rainfall events. This might have allowed having the high %Score in 2011.

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