Your stormwater permit most likely requires you to include information about weather conditions from when you take your sample. We find many people don't know where they can get rainfall data, so here we discuss some free resources to help you find that information.
We’re always being asked by those with NPDES stormwater discharge permits, "where can I get information on rainfall data for the storm event I took my sample from?" Usually, this information is necessary for reporting on Discharge Monitoring Report (DMR) forms, or is required to be recorded as part of the sampling record for permit compliance.
Where to find this information is important, since without it you risk not being in compliance, or having to submit incomplete DMR reports, or even face a fine from your permitting agency. Luckily, there are some places where you can find this information fairly easily, and use it for your stormwater reporting needs and to keep yourself in compliance.
Finding Rainfall Data for Your Stormwater Permit
Now of course, rather than seeking out rainfall data, you could generate your own. You can purchase a rain gauge and keep your own records. These devices range from simple inexpensive plastic tubes to complex and expensive weather stations. This will certainly work, and might just in fact produce the best information, since all data is going to be generated right where you need it from – your site. Now, there’s even some fairly sophisticated automatic rainfall monitoring devices that do it all for you, including lots of data logging. If you have sophisticated needs, you might want to consider a sophisticated solution.
But for the rest of us, maybe more simple means of obtaining rainfall data might work. Here are some on-line options we use a lot (and in fact are the only kind we use anymore), and recommend to our clients:
- The Weather Channel - A great place to get information on when a storm event should begin in your area, how long it might last, and how much rain to expect (usually in inches, some very useful information). There is historic rainfall data here, but it’s not super easy to find it and use it to get the type of information you’ll likely need for a DMR. We use it mostly for watching upcoming weather, and for obtaining recent data (within the past month or less).
- Weather Underground - I like to think of this site as a network of weather geeks across the country who report data. A lot of data! This site has good historic information, and while it can be a bit tricky to navigate to historic data, there’s a lot here. Our general go-to place for historic rain event data.
- Other Places - I hate to say it like this, but anywhere else you can find it. NOAA records, university websites, state climatic office sites, newspaper sites, airport websites, whatever else you’re lucky enough to find. There’s a lot of information out there, sometimes too much. Wade through this stuff to see if you can find something useful.
Finally, the best source of rainfall data is you yourself, even if you don’t have a gauging station or weather station. Most NPDES permits allow good, solid estimates of data such as when it began raining, how much it rained, how much rainfall was there at the time of sampling, when it last rained, etc. Notice what I said? Estimates! You can really become good at this by listening to a good source of weather information (like the Weather Channel), and then just using your judgment to come up with some pretty good estimates. In most cases, this will suffice.
You don’t usually need an expensive weather station or rainfall gauging system for NPDES stormwater permit compliance. Like all other things, you need to first read your permit’s requirements carefully, and do what’s needed to stay in compliance. But you can likely make do with some good information, and some good estimates based on your own observations.