Ok, so maybe I’ll have some issues with copyright infringement here. If the name of the post changes, then you’ll know. Otherwise, no, this is not a new attempt at the Anthony Burgess novel nor is it an attempt to rewrite the script of the Kubrick movie. Alas, this is a tale of flooding and soil moisture. Yes, perhaps a little less dramatic than some setting in a futuristic dystopia. Anyway, I’ll get to why I titled the post as I did eventually. So, let’s get to it.
At the end of November and early in December, many locations in the Tennessee Valley received a fairly substantial rainfall. The first image below shows the rainfall total for a 5-day period ending at 12Z on 2 December 2015.
This image (Image 1) is centered on northern Alabama and surrounding locations. Notice that amounts around 2.5 to 4 inches were common across much of the area.
So, let’s see what transpired during this event, and how soil moisture (per the SPoRT LIS) and streamflows responded during this more flood-prone time of the year. The next image below shows the SPoRT LIS 0-200 cm Relative Soil Moisture (RSM) on the morning (12Z) of November 30th. Values around 55% were common after the initial bout of rainfall (~1.5-2 inches) on the morning of the 30th. Local use of the data at the NWS Office in Huntsville indicates that the risk for areal and stream flooding increases significantly once 0-200 cm RSM values surpass 55% coincident with >2.00 inches of rainfall. As can be seen in the image below, values had reached 55% after the first round of rainfall. All that was needed was an additional 2 or more inches of rainfall in about two days or less to put some streams at risk of flooding.
The area did receive the additional ~2 inches of rainfall. In fact, take a look at the rainfall that occurred over subsequent days, from 12Z on Nov 30th through 12Z on December 1st, in the next two images below. Again, we’re focusing on northern Alabama here (sorry about the multiple scales and domains used in the series of images).
Additional rainfall totals in our basins of interest was about 2-3 inches. So, how did rivers and streams in basins with >55% 0-200 RSM respond after the 2nd batch of heavy rainfall? Well, several streams in these basins reached minor flood stage. I won’t show all of the stream hydrographs here, but will show a representative case: the hydrograph from the Paint Rock River as measured at Woodville, AL (Image 5).
In the hydrograph above, notice the sharp rise that took place on the 30th, eventually leading to the rise above minor flood stage on the evening of December 1st. Forecasters here who are used to using the SPoRT LIS data in this manner had relatively high confidence in at least a minor flooding situation along this river and perhaps a couple of other rivers in the area. As noted the the Area Forecast Discussion from the HUN office during the afternoon of November 30th, “LIS DATA OUTPUT FROM NASA SPORT WAS SHOWING SOIL MOISTURES OF AROUND 50 TO 60 PERCENT, WHICH COUPLED WITH AN EXPECTED ADDITIONAL 2 INCHES OF RAINFALL SHOULD LEAD TO A MAJORITY OF THE RAIN OCCURRING AS RUNOFF AND LEAD(ing) TO MINOR FLOODING…”
So, the use of these data for this purpose at NWS HUN has become routine, and important rainfall and soil moisture thresholds are now fairly well established. Until a more robust and objective statistical study is conducted, involving inputs of soil moisture and rainfall and outputs of stream rises, we’ll continue to rely on these crude thresholds…which suffice in most cases for our purposes. But, the river rises in some of our basins here, given these inputs of rainfall and soil moisture has become like clockwork, leading to increased confidence in eventual river stages, and importantly, when flood stage is likely to be reached (the thin orange line on the graph). Thus, “Like Clockwork Above Orange”. Ok, maybe that’s pretty corny, but it’ll have to work for now.