Wetland Wednesday
February 19, 2020
Antecedent Precipitation Tool
The 2020 Navigable Waters Protection Rule (NWPR) requires the use of an Antecedent Precipitation Tool (APT) to calculate wet, dry and typical rainfall years. This calculation involves extensive data crunching from both NOAA and NRCS. An example would include daily NOAA observations for a local weather station and NRCS growing season measurements also from a local observation center. This would include 30-years or more of recorded data on a daily basis (Roughly 11,000 days) with multiple parameters adding up to over 60,000 data points per site.
Next, we apply some statistics. The generally accepted estimation for a wet, dry or typical year is 30 percent from the calculated year. It is possible to do some predications based upon this so that the user will know what type of year they are working in.
The USACOE has announced that they will not do wetland delineations based upon wet or dry years. They will only issue jurisdictional determinations based upon typical rainfall years. This is immediately problematic for two reasons. First, without the ability to predict what type of year is currently being experienced, the wetland scientist does not know if the field data and delineation will be accepted. Second, this could result in planning and construction delays due to the lack of regulatory concurrence in wet or dry years. A permit applicant would have to wait until a normal year was being experienced before the wetland delineation could be done. This is unmanageable for multiple reasons.
The USEPA recently announced that there is an effort to develop an Antecedent Precipitation Tool. However, it is unknown how long this will take and what regulatory compliance procedures they will need to undergo to release such a tool.
Fortunately, an Antecedent Precipitation Tool already exists and is part of a water budget modeling program that has been in use for years. WetBud is a water budget modeling software package that includes an APT. NOAA weather data and NRCS WETS data are combined to calculate wet, dry and normal years and the rainfall amounts associated with these years. The rainfall totals can be extrapolated to include the 30% variance. This provides the wetland scientist with the data needed to determine the “typical” status of the site.
WetBud can be a challenge to master. At issue is the pre-set weather data is limited to Virginia. Other locations can be configured around the globe. It just takes a bit of training to set it up. We currently include WetBud configuration as part of our Water Budget workshop. However, we have found that students need the hands in-person workshop to get past several tweaks needed to set up WetBud. Keep an eye on our APT workshop schedule for upcoming dates and locations.