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Wetland Hydrology Determinations for the 2018 Monitoring Season

Posted: 4/13/2018

Expiration date:


The Norfolk District believes it beneficial to the public to provide seasonal Public Notices (PNs) regarding preceding precipitation conditions for shallow groundwater well monitoring associated with non-tidal wetland determinations. The purpose of this PN is to inform the public of relevant parts of our process, and our interpretation and findings regarding current hydrologic conditions for the 2018 monitoring season. This is accomplished by review of precipitation and soil moisture trends done by the National Weather Service (NWS) and NASA respectively, comparison of current precipitation to long-term data from several local weather stations, and comparison of recent groundwater levels to previous years of groundwater level data from local reference wetland locations. 

This PN focuses on southeastern Virginia (Hampton Roads) because of the large percentage of non-tidal wetlands in the Coastal Plain. A detailed review of the precipitation and well monitoring data is discussed below. In summary, many sources of precipitation data indicate drier-than-normal climatic conditions in southeastern Virginia. In addition, our groundwater monitoring data indicate a shorter than typical wetland hydro-period this season, although many monitoring wells did met the wetland hydrology requirements of the Technical Standard mentioned below. Given these conditions, jurisdictional determinations should be based on current Corps guidance and evaluation of field indicators of vegetation, soils, and wetland hydrology. However, properly collected groundwater monitoring well data will be considered on a case-by-case basis in light of the information contained in this notice.  

Background 

Wetlands are defined as "those areas that are inundated or saturated by surface or groundwater at a frequency and duration sufficient to support, and that under normal circumstances do support, a prevalence of vegetation typically adapted for life in saturated soil conditions" (33 CFR 328.3(b)(emphasis added). Wetland determinations typically entail observation of field indicators of wetland vegetation, hydric (wetland) soils, and wetland hydrology.  

Occasionally, property owners or their agents may elect to install and monitor shallow groundwater wells for the late winter and spring seasons to gather data about the levels and duration of groundwater (i.e. saturated soil conditions) for particular areas to attempt to clarify the limits of wetlands. There is no requirement that well data be submitted to obtain confirmations of wetland delineation. However, the Corps does consider groundwater well data in its determinations if such data are collected in accordance with proper well installation and monitoring standards, and during periods consistent with "normal circumstances" prior to and during the monitoring period.   

Any monitoring wells used to facilitate wetland hydrology determinations should be installed and monitored in accordance with the guidelines in Technical Standard for Water-Table Monitoring of Potential Wetland Sites, ERDC-TN-WRAP-05-2. 

Before we will consider well data for a specified site, we require submittal and approval of a well monitoring plan, which includes a review of the location and installation of the monitoring wells.   In addition, during the monitoring season (typically February through April), the Corps' staff must be allowed periodic access to the particular sites and wells without any prior notice to provide proper quality assurance. 

When reviewing shallow groundwater well data in order to determine whether normal circumstances for wetland hydrology are present, we consider the amount and distribution of precipitation prior to the start of the growing season (after leaf drop in the fall) and during the early growing season. The U.S. Department of Agriculture’s Natural Resource Conservation Service (NRCS) National Water and Climate Center calculates normal precipitation ranges for each month (defined as between the 30th and 70th percentiles of monthly precipitation totals) for NOAA’s NWS stations throughout the United States (see NRCS WETS tables).

                                                                                                                    2018 Precipitation 

Some methods to evaluate precipitation trends for wetland delineation purposes are described in Accessing and Using Meteorological Data to Evaluate Wetland Hydrology, ERDC TR-WRAP-00-1.  Short-term water-table monitoring data (i.e., <10 years) must be evaluated with consideration of the amount and distribution of precipitation that fell prior to the beginning of the growing season (but after leaf drop in the fall). 

NWS Precipitation Analysis:

Precipitation trends (e.g., percent of normal) for different time periods for the entire Commonwealth are available from NWS’s Advanced Hydrologic Prediction Service. This analysis depicts a consistent trend of drier-than-normal conditions for southeastern Virginia for the majority of time periods from 7 days to 180 days prior to 06 April 2018 (see attached), and the driest areas correspond well with the “Abnormally Dry” mapping by the U.S. Drought Monitor as of 03 April 2018. The Drought Monitor has consistently mapped the south side as drier than typical since at least 07 November 2017 (see attached). 

NASA’s Gravity Recovery and Climate Experiment (GRACE) data:

NASA’s GRACE satellites were launched in 2002 and collect data about water on and just beneath the land surface by detecting small changes in Earth’s gravity field. Using a computer model, NASA generates maps of near-surface water based on the GRACE data and other meteorological data (see Groundwater and soil moisture conditions from GRACE Data Assimilation). In agreement with the evaluation of recent precipitation trends by the NWS the maps of surface (top 2 cm) soil moisture, root zone (top 1 m of soil) moisture, and shallow groundwater drought indicator all show consistent drier conditions for southeastern Virginia from 12 February through 09 April 2018 (see attached). NASA’s website states:

The drought indicators describe current wet or dry conditions, expressed as a percentile showing the probability of occurrence within the period of record from 1948 to the present, with lower values (warm colors) meaning dryer [sic] than normal, and higher values (blues) meaning wetter than normal”.

 NRCS WETS Tables:

This analysis is based on evaluation of monthly precipitation data for four WETS stations in southeastern Virginia (Norfolk, Lake Kilby, Langley, & Williamsburg). Although we analyze all months after leaf drop, this timeframe is generally at least 3 months prior to the beginning of the growing season each year. Monthly precipitation totals for the 3 months  preceding this PN (January-March) for southeastern Virginia have been within normal ranges; but the 3-month precipitation totals for Suffolk, Norfolk, and Hampton all are closer to the 30th percentile mark (particularly Suffolk; see attached). Since leaf-fall (i.e., the start of groundwater recharge), precipitation trends have generally been at or below the 30th percentile for November, December, and February. While January precipitation totals were within the normal range (above normal for Langley and Williamsburg), the majority of the precipitation was delivered in one or two events. In general, March precipitation was within the normal ranges. Precipitation totals for most of the months since leaf-fall being below or near the lower end of normal (i.e., 30th percentile) correspond well with the other, independent analyses described above and they all tell a consistent story of drier-than-normal climatic conditions in southeastern Virginia. 

It’s important to note that the NRCS WETS Tables evaluation described above is based on monthly precipitation totals, so the distribution of precipitation events within a particular month are not factored in even though it can affect water loss rates (e.g., runoff, infiltration) and observed groundwater levels.  Additionally, there are limitations in making predictions about wetland hydrology by simply evaluating trends of monthly precipitation totals.  For example, this evaluation does not factor in air temperatures which can influence the timing of flowering/leaf-out and evapotranspiration rates, which in turn can affect seasonal water table levels. Also, direct precipitation is not the primary water source for all wetlands; different wetland classes have different primary sources of sustaining hydrology. These differences highlight the importance of having multiple years of hydrology observations, and why the Corps’ technical standard for wetland hydrology includes a minimum frequency interval of 5 years in 10 (≥50% probability). Comparison of hydrographs over several years from the same site can help identify when precipitation alone may not be an accurate surrogate for assessing overall climatic conditions that may influence wetland hydrology.

Dry wells in a drier-than-typical precipitation period and wet wells in a wetter-than-typical precipitation period are of limited value in making definitive determinations regarding wetland hydrology.

NAO Reference Sites:

Lastly, we have reviewed water levels in several shallow groundwater reference well sites in southeastern Virginia which have been monitored for several years. In comparison to previous years where groundwater levels were monitored during more typical precipitation periods, water levels at all sites this year wet up later than usual (i.e., water levels did not come up to within -12 inches until late January to mid-March this year v. early December to mid-January in years with more typical precipitation).  While groundwater levels rose above -12 inches for most monitoring wells by mid-March, the overall groundwater levels trended lower than usual throughout the winter and into the early growing season. 

In summary, overall lower than normal groundwater levels and shorter duration of seasonal high water levels were observed in the NAO reference wells this season. Representative hydrographs from several of these reference sites are enclosed. 

Regardless of precipitation conditions, we will continue to make wetland determinations based on the field indicators of vegetation, soils, and wetland hydrology described in the Regional Supplement to the Corps of Engineers Wetland Delineation Manual:  Atlantic and Gulf Coastal Plain Region (version 2.0), the Regional Supplement to the Corps of Engineers Wetland Delineation Manual: Eastern Mountains and Piedmont Region (version 2.0), including use of Chapter 5 when appropriate, and portions of the Corps of Engineers Wetlands Delineation Manual (1987) that were not replaced by the regional supplements.  Chapter 5 (Difficult Wetland Situations) of both Regional Supplements applicable in Virginia contain a section addressing “Wetlands that periodically lack indicators of wetland hydrology”. This section describes a number of approaches that can be used to determine whether wetland hydrology is present on sites where indicators of hydrophytic vegetation and hydric soils are present, but hydrology indicators may be lacking due to factors including periods with below-normal rainfall conditions.  Considering the current and antecedent precipitation trends, use of this portion Chapter 5 may be appropriate. 

Growing Season 

Beginning and ending dates of the growing season are needed in the event water-table monitoring data must be analyzed for wetland hydrology determinations. The Regional Supplements state that the growing season has begun and is ongoing in a given year when two or more different non-evergreen vascular plant species growing on the site or surrounding areas exhibit certain indicators of biological activity, or when soil temperature measured at the -12-in. (-30-cm) depth is ≥ 41 °F (5 °C).  Soil temperature and plant activity are both surrogates for estimating soil microbial and plant root activity which produces anaerobic conditions in the soil that are characteristic of wetlands. 

Based on soil temperature data collected at several reference sites in southeastern Virginia over the past 8 years, soil temperatures typically remain above 41 °F for most of the year. A graph of soil temperature data collected at reference sites in southeastern Virginia (Virginia Beach, Chesapeake, Isle of Wight County, Hampton, and Gloucester) from approximately November 2017 to March 2018 is attached. The data collected indicate soil temperatures remained above 41 °F for the entire monitoring period for all reference sites. 

Several of the indicators of plant biological activity signaling the start of the growing season as described in the Regional Supplements were observed broadly across southeastern Virginia as early as the second full week of February 2018.  We consider this to be the general start date of the growing season in southeastern Virginia. However, we will give consideration to any site-specific data submitted that supports a different start date. 

This PN does not relieve those that have constructed wetland mitigation projects from monitoring hydrologic conditions.  Monitoring should be conducted in accordance with the associated permit, approved plan, and mitigation banking instrument.

groundwater hydrology precipitation Southeastern Virginia wetlands