Precipitation Trends across the Commonwealth of Virginia (1947 – 2016)

Water is an important resource for the Commonwealth of Virginia. Too much water increases runoff, disrupt transportation networks, and contributes to school closures. Too little water may adversely impact agricultural operations. To improve climate-related information to Virginia citizens, this study assesses means and changes in precipitation across the Commonwealth of Virginia (1947 – 2016). Using daily stationlevel precipitation data from the Global Historical Climate Network (GHCN), descriptive statistics were calculated for 43 locations in terms of total precipitation (inches decade-1), precipitation days (x>0”), and heavy precipitation days (x>1.0”). On average, locations showed an overall increase in total precipitation across the time period. The frequency of heavy rainfall events has also increased across many of the analyzed locations. Precipitation has important ramifications for agriculture, storm water management, and hazard response, and improved coordination of atmospheric-related information may be beneficial to various stakeholders across the Commonwealth. INTRODUCTION Heavy rainfall can lead to numerous hazards including flooding, landslides, and loss of life. From 1980 – 2013, 19 flood-related, billion-dollar disasters occurred in the United States (Smith and Matthews 2015). Combined, these events averaged a price tag of $4.5 billion. Hurricane Agnes, Fran, and Irene coupled with non-tropical events such as rapid snowmelt and ravine flooding highlight the concern in the Commonwealth of Virginia. Of the 64 federal disaster declarations issued by FEMA for the Commonwealth, 27 highlighted flooding. Others explicitly reference tropical systems or hurricanes (FEMA 2018). While tropical systems often impact the coastal plain, precipitation associated with hurricanes often leads to inland flooding (Rappaport 2000). Mesoscale features recently flooded parts of Cape Charles, and heavy rain associated with Tropical Storm Virginia Journal of Science Volume 70, Issue 1 & 2 Spring & Summer 2019 doi: 10.25778/3cay-z849 Note: This manuscript has been accepted for publication and is online ahead of print. It will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Virginia Journal of Science, Vol. 70, No. 1, 2019 https://digitalcommons.odu.edu/vjs/vol70/iss1 2 Michael flooded parts of southwest Virginia. Flooding and heavy precipitation events (HPE) are recognized as a major hazard across the Commonwealth. While state legislature refers to some events as so-called nuisance or sunny day flooding, flooding disrupts transportation networks, leads to cancellation of school, and plays a role in more prolonged impacts such as mold and mildew (Wong et al. 2014; Chew et al. 2006). HPE intertwine land use policy, hazard mitigation awareness, and future climate change. Recent studies have indicated shifts in the frequency and intensity of precipitation events (Lewis et al. 2018; Kunkel 2003; Wuebbles et al. 2014; Heineman 2012). In the southeast, extreme rainfall events are increasing though many stations along the Appalachian Mountains show a downward trend (USGCRP 2018). While both natural and anthropogenic forcing mechanisms are associated, it is likely that increases in atmospheric water vapor content may lead to further increases in such heavy precipitation events (Kunkel 2003). In the United States, Peterson et al. (2013) provides a comprehensive overview of weatherand climate-related extremes. While regional assessments exist (Agel et al. 2015; Sayemuzzaman et al. 2014; Boyles and Rama 2003), the existence of climatological reviews of HPE in Virginia are unknown. The Virginia Climate Office provides some information, but the data portal is based on a climatological period that may not adequately represent the most recent changes in our Virginia climate. This research assesses the climatological trends associated with precipitation across the Commonwealth of Virginia and draws attention to the need to better coordinate weather-related information that may be beneficial in emergency management operations, planning of our cities, or minimizing the consequences of future weather/climate-related impacts. DATA AND METHODS The Global Historical Climate Network (GHCN) is a database of land-based stations around the world. Subjected to detailed quality control (Menne et al. 2012), these data include a wide range of atmospheric variables such as temperature and precipitation at a variety of temporal scales (e.g. hourly; daily). Using the Climate Data Online (CDO) tool, daily precipitation values were obtained through the National Centers for Environmental Information (NCEI; 1947 – 2016). Shorter, more recent sub-periods were analyzed (1987 – 2016), but due to the heterogeneity in precipitation and limited statistical significance, these results are not shown. For this research, stations located in Virginia were selected, and only stations with at least 90% complete record were included in the analysis. In total, 43 locations met this criterion (Figure 1; Appendix A). Using SPSS 19, descriptive statistics were generated for annual precipitation totals, precipitation days, and precipitation days exceeding 1.0” (heavy precipitation days). While other studies may use a higher threshold (Pommerenk 2016) or a cumulative value (e.g. Smirov et al. 2017), precipitation characteristics were selected based upon previous thresholds defining high precipitation days (Boyles and Raman 2003). Virginia Journal of Science, Vol. 70, No. 1, 2019 https://digitalcommons.odu.edu/vjs/vol70/iss1 3 In addition to means, simple linear regression was used to calculate observed changes in precipitation and whether such changes were significant (p<0.05). The study also evaluated other thresholds (e.g. 2” days), but due to the rarity of these events and limited statistical significance, the results are not shown. RESULTS a. Means Across the Commonwealth, annual precipitation averaged 43.02” year-1 (Figure 2; Appendix B). Only three locations average more than 50” of precipitation: Wallaceton, Meadows of Dan, and Woolwine. With only 36.2”, Woodstock averaged the lowest mean annual precipitation of any station. Local topography may help explain the variability in precipitation. From 1947 – 2016, an average of 111 precipitation days occurred at each of the locations (Figure 2; Appendix B). Burkes Garden averaged the most precipitation days (140 year-1) while Clarksville the fewest (88 year-1). On average, Virginia locations experienced 10 days year-1 with precipitation exceeding 1.0” (Figure 2; Appendix B). Similar to annual precipitation statistics, Meadows of Dan and Woolwine observed the most 1” precipitation days, with both exceeding an average of 15 such days year-1. Six locations (Blacksburg, Lafayette, Pulaski, Staffordsville, Woodstock, Wytheville) averaged less than 8, one-inch precipitation days year-1. b. Changes and Trends Regression analysis indicates an upward trend in mean annual precipitation for 39 of the 43 locations (Figure 3; Appendix C). As a whole, precipitation increased in Virginia 0.57” decade-1. Of the increases, the greatest change was found in Wallaceton (1.40 inches decade-1). Several other locations observed a significant increase of more than 1.0” decade-1. In total, eight significant changes were found across the Commonwealth, all indicating an upward trend. On average, total precipitation days increased 1.69 days decade-1 across the commonwealth. Burkes Garden showed the largest shifts with nearly 10 more precipitation days per decade. Thirty six of the 43 stations analyzed showed an upward trend in total precipitation days. Seventeen were found to be significant (p<0.05). While similar results were found for heavy precipitation days, the overall frequency of these heavy events reduced the statistical significance (Figure 2; Appendix C). All locations except two indicated a shift towards more frequent heavy precipitation days. Eight of these stations showed a significant trend. On average, heavy precipitation days increased 0.29 days decade-1. With values in excess of 0.60 days decade-1, Buena Vista and Hopewell showed the largest changes. As an illustration, Figure 4 displays the annual observations and associated changes at Norfolk International Airport. Virginia Journal of Science, Vol. 70, No. 1, 2019 https://digitalcommons.odu.edu/vjs/vol70/iss1 4 DISCUSSION AND RECOMMENDATIONS Over the 70-year period (1947 – 2016), mean annual precipitation in Virginia seems to have increased. Additionally, both individual precipitation and heavy precipitation days, days exceeding 1.0”, have also increased across much of the Commonwealth. While some precipitations variations may be associated with orographic features, the changes support other findings that show similar increase in precipitation (Smirnov et al. 2017; Pommerenk 2016; USGCRP 2018; Lewis et al. 2018). Karl et al. (2009) showed at 27% increase in heavy precipitation events in the southeast United States. In North Carolina, Boyles and Raman (2003) showed increases in precipitation during the fall and winter seasons but decreases in the summertime. The 4th National Climate Assessment (Figure 19.3) showed similar increases in precipitation across the southeast region. Over the past 25 years, days with 3 inches or more of precipitation has been historically high though decreasing trends did exist along the Appalachian Mountains (USGCRP 2018). While these studies utilize different time periods and geographic domains, the general increases in frequency and intensity in rainfall is consistent with the presented findings for Virginia. Using NOAA climate divisions, Hoffman et al. (2019) recently showed spatial variability in terms of precipitation across the Commonwealth. Future studies may explore the seasonal variability in station-level precipitation changes to uncover how precipitation is changing over the course of