Drought Conditions in Maine, 1999-2002: A Historical Perspective

Abstract

Hydrologic drought can be defined as reduced streamflow, declining ground-water levels, and (or) reductions in lake or reservoir levels. Monthly precipitation totals, annual 7-day low-flow surface-water recurrence intervals, and month-end ground-water levels from drought years 1999-2002 show that 19992002 was the driest period of hydrologic drought in more than 50 years of record in Maine. Record lows were set in all three data sets at select locations in central Maine in April 1999, and in September 2001 and 2002. Although streamflows recovered to normal levels during 2000, ground-water levels in central Maine indicate that the drought carried over through 2000 into 2001 and 2002 in some locations. In 2001, annual 7-day low flows with greater than 100-year recurrence intervals were recorded in central Maine and low flows with up to 75-year recurrence intervals were recorded in coastal areas. In 2002, annual 7-day low flows with greater than 100-year recurrence intervals were recorded at 4 of 14 stations analyzed statewide, placing it as the driest single year of hydrologic drought on record. Month-end groundwater levels at one location in central Maine indicate that the recent hydrologic drought years were the most severe in more than 50 years in that region. The period from 1947 to 1950 may have been the only comparable period of drought to the 1999-2002 period, in Maine. The 1960s drought, although extreme in the far northern and far southern regions of the State, was most exceptional for its duration from 1963 to 1969. INTRODUCTION Drought is among the most complex and least understood of all natural hazards, affecting more people than any other natural hazard (American Meteorological Society, 1997). Although drought typically is not considered a problem in the humid northeastern United States, it is a normal, recurring feature in all climatic regimes. Drought is a temporary aberration, relative to some long-term (tens of years) average condition, as opposed to aridity, which is a permanent feature of some regional climates (American Meteorological Society, 1997). Many questions still remain concerning the physical mechanisms responsible for the onset, persistence, and spatial extent of regional hydrologic drought in the northeast because of hydrologic variability and the inherent complexity of hydrologic systems (Bradbury and others, 2002). Dry conditions were present in Maine from 1999 to 2002, with a severe drought in 2001-2002. Most U.S.Geological Survey (USGS) monitoring wells, and many streamflow-gaging stations, set record lows during this period. An estimated 7 percent, or approximately 17,000 private wells in Maine went dry in the 9 months prior to April 2002 (Maine Emergency Management Agency, 2002). Wells in central Maine were the most likely to have low water levels. Thirtyfive public water supplies, including eight large community systems, were affected severely (Andrews Tolman, Maine Drinking Water Program, written communication, 2003). Most major surface-water reservoirs released water at levels below their regulatory minimum flows, instream flows for aquatic life were reduced, and critical summer irrigation was limited. Farmers in Maine lost more than 32 million dollars in crops in 2001 and 2002, with some wild blueberry growers recording crop losses of 80 to 100 percent according to a Maine Department of Agriculture water-use survey to which 28 percent of Maine farmers responded (Maine Agricultural Water Management Advisory Committee, 2003). The effects of past droughts nationwide have been exacerbated by the absence of preparedness plans (American Meteorological Society, 1997). An integral part of any preparedness plan would include meteorologic and hydrologic thresholds based on historical Abstract 1 droughts in the region. During 1999-2002, waterresource professionals, farmers, business owners, and others who were concerned with instream flows, storage, or ground-water levels lacked the quantitative historical information necessary to compare the severity of the 1999 to 2002 drought to historical droughts, and to assess the potential of drought to stress water resources. Because droughts will occur in Maine in the future, water-resource professionals will benefit from documentation and analysis of the hydrologic conditions experienced from 1999 to 2002 in Maine. In particular, emergency management workers and public-water suppliers in Maine will benefit from this information by better understanding the complexity of how droughts move through the hydrologic system, and thus, be better able to anticipate drought effects. Purpose and Scope The purpose of this report is to document the relative regional and historical severity of the dry hydrologic conditions experienced from 1999 to 2002, and to provide information regarding the occurrence and persistence of droughts in Maine. This report includes a comparison of 1999-2002 daily mean streamflows, month-end ground-water levels, and total monthly precipitation values to historical statistics at select stations. The interaction among precipitation, surface water, and ground water, the annual 7-day surface-water low-flow recurrence intervals for this drought period and for historical droughts, and a comparison of month-end ground-water levels to ground-water statistics for historical droughts also are examined. Drought Definition Droughts can be measured or defined on the basis of a wide variety of parameters including precipitation deficits, streamflows, ground-water levels, soil moisture, and economic impacts. The relation among the intensity, duration, and spatial and temporal extent of these parameters defines many different types of events, all of which may be considered droughts. For example, a growing season with no rain in the northern region of the State would be characterized differently than a statewide, multi-year period with below-average precipitation, but both could be considered droughts. Although drought can be defined strictly as a percentage of normal precipitation, it is more often defined as a period of moisture deficit sufficient to have some adverse effect on the social or economic activity of a region (Changnon, 1980; Paulson and others, 1991). The integration of multiple definitions into a combined measure of drought has been problematic in the past for natural resource managers. The many definitions of drought make it difficult to declare the beginning or end of a drought or assess its severity during a drought period. The American Meteorological Society groups drought into four types including climatologic drought, agricultural drought, hydrologic drought, and socioeconomic drought (American Meteorological Society, 1997). Climatologic drought often is defined by a threshold precipitation deficit or a ratio of actual precipitation to normal precipitation. Agricultural drought links climatologic drought to agricultural effects and is largely the result of a deficit of soil moisture. Hydrologic drought is defined as reduced streamflow, declining ground-water levels, and (or) reductions in lake or storage levels. Socioeconomic drought associates the supply and demand of some economic good with the elements of climatologic, agricultural, and (or) hydrologic drought (American Meteorological Society, 1997). These types of drought usually take place simultaneously; however, hydrologic droughts typically are out of phase with or lag climatologic and agricultural droughts. Meteorologic elements, such as temperature, wind, and relative humidity, can aggravate the severity and the effects of drought (American Meteorological Society, 1997). Although various aspects of all four drought types defined here occurred in Maine from 1999 to 2002, this report primarily documents the characteristics of the hydrologic drought and how the climatologic drought contributed to it. Previous Studies There is very little documentation of historical droughts in Maine. In 1991, the USGS defined multiyear historical droughts in each state of the United States, and calculated their recurrence intervals in a National Water Summary on floods and droughts (Paulson and others, 1991). Drought periods identified for the Maine Water Summary are listed in table 1 (Maloney and Bartlett, 1991): 2 Drought Conditions in Maine, 1999-2002: A Historical Perspective Table 1. Low-flow recurrence intervals for historical droughts in Maine from 1938 to 1988, as identified by Maloney and Bartlett in the National Water Summary, 1991 [Low-flow recurrence interval, the average interval of time within which streamflow will be less than a particular value; >, greater than] The recurrence intervals for the droughts in table 1 were calculated on the basis of the cumulative departure from mean monthly streamflows. This method is described in the introduction to State summaries of floods and droughts in the National Water Summary (Jordan and Jennings, 1991). The intensity of the drought was taken into account, but the duration was not; droughts that varied in duration from 1 year to 6 years were ranked on the same scale. This method may be appropriate for identifying multi-year periods of drought, or assigning recurrence intervals to total water deficits in systems that depend on reservoirs such as in the western United States, but it is not appropriate for eastern States except in the most approximate terms. This is because drought in the eastern United States may depend on the timing of precipitation as much as on the total amount of precipitation. Resource managers in the northeastern United States often use national indices to assess regional conditions despite that these indices may be more appropriate for nationwide water-status monitoring (Skaggs, 1975). Often, the national indices fail to give those affected by the drought or those who make policy a local basis for evaluation and action (Russell, and others, 1970). Even different user groups within a region may experien