PDSI

In the Spring of 2016, I was updating a North Dakota water resource presentation, which contained a Palmer Hydrological Drought Index (PHDI) chart from April of 2013. I was replacing an older NOAA PHDI chart (May 2005 to October 2013) with an updated NOAA PHDI chart (May 2005 to March 2016). These charts are simple screen captures from the NOAA web site. Then, something caught my eye. I noticed that the summer of 2011 was wetter in the 2016 dataset than in the 2013 dataset, and the summer of 2013 was drier in the 2016 dataset than in the 2013 dataset. I scaled both charts on the Y-Axis to check my first visual impression. The results of this comparison are shown below in Figure 1. This comparison showed that there were several years where the newer 2016 dataset was different from the older 2013 dataset, both wetter and drier. Obviously, NOAA had  modified the historic data.

Figure #1

This prompted me to look more closely at previous Palmer Drought Index data I have collected over the years. In 2006 I wrote a report using the Palmer Drought Severity Index (PDSI) and had downloaded the PDSI data from the NOAA website for the time frame of January 1977 to September, 2006. This 2006 dataset was for all nine NOAA climate divisions in North Dakota. Figure 2 shows the North Dakota climate divisions, which follow county boundaries.

Figure 2


Using my historical 2006 PDSI data as the time frame reference, I downloaded (in May 2016) the PDSI dataset from the NOAA website, see Figure 3. I then plotted the two dataset downloads (2006 & 2016) as bar charts for each climate division and also plotted the differences between the two datasets for each climate division. These charts (Figures 4 to 12) provide a nice visual comparison for how NOAA has modified the historical PDSI data, in comparison to the newer 2016 NOAA dataset.

Figure 3

To provide for a better visual comparison of the difference for all nine climate divisions, I created scatter plots of the “gray differences bar chart” and calculated the regression line for each of the charts. I then scaled the charts so that all nine of the climate divisions fit on a single page. From the scatter plots and regression you can see that seven of the climate divisions have had their historical data adjusted to produce a “drier” trend line, while two have their historical data adjusted to produce a “wetter” trend line. Also included are histograms of the “differences” data for each climate division.

I discussed this analysis with my hydrologist colleagues and none of them were aware of the differences between the older PDSI data and the newer PDSI data. They agreed with my analysis, that the PDSI data had been changed. We then contacted the regional NOAA office for an explanation, and they were unaware of any changes made to the PDSI data. The regional NOAA office then contacted the national NOAA office. The national office said they had made changes to the algorithm used to calculate the PDSI. They also stated that the changes made to the PDSI algorithim had been peer reviewed.


Shown below are the charts from my anlysis of the PDSI data.