Early Growing Season Precipitation and Groundwater Trends for 2019
The United States Army Corps of Engineers (USACE) 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 nontidal 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 2019 monitoring season. This is accomplished by review of precipitation and soil moisture trends done by the National Weather Service (NWS) and National Aeronautics and Space Administration (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 that are located in the Coastal Plain. A detailed review of the precipitation and well monitoring data is discussed below.
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 in the majority of cases are based on the presence of observable field indicators of hydrophytic vegetation, hydric soils, and wetland hydrology, according to the procedures in the Corps’ 1987 Wetlands Delineation Manual and the applicable Regional Supplements (Atlantic and Gulf Coastal Plain Region or Eastern Mountains and Piedmont). These three characteristics are the best available evidence that an area has performed in the past and continues to perform, the functions associated with wetland ecosystems.
The 1987 manual and regional supplements address “Atypical situations” and “Problem Areas” (e.g., “Wetlands that periodically lack indicators of wetland hydrology”). Mineral flat wetlands are an example of naturally occurring wetland types that may lack primary hydrology indicators and/or have only one secondary indicator during portions of the growing season. Occasionally, property owners or their agents may elect to install and monitor shallow groundwater wells for the late winter and spring seasons to gather groundwater level data. Groundwater monitoring is addressed in Technical Standard for Water-Table Monitoring of Potential Wetland Sites, ERDC-TN-WRAP-05-2 (WRAP-05-2). The technical standard is intended for use in atypical and problem situations.
Any monitoring wells used to facilitate wetland hydrology determinations should be installed and monitored in accordance with the guidelines in WRAP-05-2.
There is no requirement that well data be submitted to obtain confirmation of a 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 after evaluation of precipitation trends prior to and during the monitoring period.
Before we will consider well data for a specified site, we require the 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.
Properly collected groundwater monitoring well data will be considered on a case-by-case basis in light of the information contained in this notice.
2019 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). Although we analyze all months after leaf drop, this timeframe is generally at least three months prior to the beginning of the growing season each year.
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. Their more recent data (7-14 day range) indicate normal to slightly drier conditions. The longer term data (30-60-90-180 day range) is on the wetter side for most of the state, although some Hampton Roads
U.S. Drought Monitor:
The U.S. Drought Monitor doesn’t identify any abnormally dry or drought conditions across the Commonwealth as of March 05, 2019.
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). The three maps attached include a shallow groundwater drought indicator, root zone (top 1 m of soil) moisture, and surface (top 2 cm) soil moisture. All maps depict wetter conditions for most of the state as of February 25, 2019, with some exceptions, including southeastern Virginia, which is generally drier.
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:
Precipitation data collected at four NRCS WETS stations located across southeastern Virginia (Norfolk, Lake Kilby [Suffolk], Langley [Hampton], & Williamsburg) was utilized for this evaluation. Monthly precipitation totals for the three full months preceding this PN (December-February) for southeastern Virginia have varied between normal & above-normal ranges. The 3-month precipitation totals for Norfolk, Suffolk, and Williamsburg all are above the 70th percentile mark; Hampton was within normal range (see attached). From leaf-fall (i.e., the start of groundwater recharge) to early February precipitation trends were generally within the normal range, but precipitation since mid- February pushed levels above the 70th percentile for much of the region.
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 (≥0% 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.
Norfolk District Wetland Reference Sites:
Lastly, we have collected and analyzed multiple years of groundwater levels from several wetland reference sites located in southeastern Virginia. These hydrographs indicate that groundwater levels generally rose above -12 inches around the second week of December this year. Many of the wells remained at or above -12 inches until February 10-11. Following the rain events of mid to late February, groundwater levels at the majority of wells continue to remain at or above -12 inches. 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, or the onset of the dry season.
Growing Season
Beginning and ending dates of the growing season are needed in the event groundwater level 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, and Hampton) from approximately November 2018 to February 2019 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 February 8, 2019. 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 instruments