Worst drought of record
The worst drought of record, with respect to water supply planning, is the historical time during which the water supply reached its lowest volume. The ultimate availability of water supply depends not only on rainfall, but also stream flows and other environmental factors. In Virginia, communities must plan for an adequate safe yield, or withdrawal rate, from the water supply to withstand the worst drought of record, in the event such a condition recurs.
Virginia law requires communities to have a drought contingency and response plan. Charlottesville-Albemarle's plan recognizes the differences between long-term water conservation programs and short-term responses by the community to cut consumption in an emergency. The Rivanna Water and Sewer Authority uses computer models to evaluate stream flows and predict drought conditions. This data is used to determine when the community should declare a drought watch, warning, or emergency condition.
Charlottesville's worst drought of record
In a January 2004 report, Rivanna Water and Sewer Authority consultant Gannett Fleming analyzed climatological data from 1836 to 2003 (167 years) to calculate when the worst droughts on record occurred in Charlottesville.
- "Based on the daily simulation of the system, which is discussed in detail later in this report, it was determined that the two worst droughts of record for the RWSA Urban Area system occurred in 2002 and 1930 and lasted between 145 and 187 days (5-6 months)."
- "The drought of record occurred in 2002 and the second-worst drought occurred in 1930. Other less severe droughts occurred in 1954, 1963, 1966, and 1977."
Gannett Fleming noted that there were four other extended periods of low precipitation that may have caused droughts more severe than 2002 and 1930, all occurring prior to 1925.
At a panel discussion in February 2008, Tom Frederick, the RWSA Executive Director, said that RWSA's "records show that in mid-October of 2002 we were at 50% [of our water supply]....There was only 60 days of supply before this community completely ran out."
The 2006 community water supply plan uses 2002 as the worst drought of record and in other studies Gannett Fleming calculated the water supply system as having a safe yield of 12.8 million gallons per day (MGD) as of 2004. Because sedimentation of the South Fork Rivanna Reservoir reduces storage by about 1% annually, the safe yield is projected to drop to 8.8 MGD in 2055 if no additional water supply capacity is created. Average daily usage during 2008 was 9.57 MGD.
1976-77 was a period of very low precipitation. During the ten month period from November 1976 to August 1977 there were only 18.65 inches of rain. According to Gannett Fleming, the second worst drought occurred in 1930. However, from November 1929 to August 1930, there were 23.37 inches of rain, almost five inches more than 1976-77.
One reason 2002 was determined to be the worst drought on record is that Gannett Fleming's safe yield study takes into account more than precipitation. The study also analyzes stream flows, which are impacted by a number of other environmental factors.
- "It should be noted that the relationship between rainfall and runoff is very complex. Some of the factors affecting this relationship include rainfall intensity, spatial and temporal distribution of storm events, antecedent soil conditions, seasonal variation in evaporation and moisture demand by vegetation, land use and watershed topography. Further, the precipitation data used represents rainfall measured at one point. Spatial variability of precipitation can be considerable. Streamflow, however, represents an integration of all of the hydrologic process within a drainage area. Nevertheless, there appears to be a remarkable correlation between the worst drought periods as determined by simulating the operation of the system and the lowest 5-month moving average monthly rainfall."
In the Gannett Fleming safe yield study, "an analysis was performed consisting of plotting the 4-, 5-, 6-,7-, 8-, and 9-month moving average of monthly precipitation for the period of record from 1836 to 2003." Their conclusion was that 1976-77 was severe, but less so than 2002 and 1930. In addition, three other safe yield studies conducted between 1985 and 1997 identified a variety of dates for the historical droughts impacting different area reservoirs, and none of those studies identified 1976-77 as the worst drought on record.