Droughts

Mild droughts are common in Michigan, but severe droughts are infrequent and generally of short duration (Nurnberger, 1980). The normally even distribution of precipitation and moderate humidity are helpful in meeting the large demand for moisture by crops. Dry weather can last as long as several weeks. Rain-free periods generally do not destroy an entire crop but can result in slowed growth or decreased yields.
A drought that is only temporarily eased and then resumes may not seem to be severe meteorologically. From a hydrologic view, however, drought-easing precipitation may not be sufficient to replenish soil moisture, percolate to the water table, and eventually return streamflow to normal. Thus, if a drought is considered to continue until streamflow returns to normal, a hydrologic drought may include more than one meteorological drought.
The maps in figure 4 show the severity of the State's five historically most extreme droughts and the areas that were affected. The hydrographs show annual departures from long-term-average streamflow for six of the gaging stations used in the drought analysis. Drought recurrence intervals are calculated on the basis of the magnitude of cumulative streamflow deficiencies. Droughts are easily recognized in the 1930's, 1940's, 1950's, 1960's, 1970's, and, most recently, from 1986 to present (1989), but there is no evidence that droughts have a cyclic pattern in Michigan (Fred V. Nurnberger, Michigan Department of Agriculture, oral commun., 1988).
A combination of several meteorological droughts starting in the 1930's led to the most severe hydrologic drought, both in magnitude and duration, in Michigan's history. The drought had a recurrence interval that was greater than 25 years for the entire State but was most severe in the Lower Peninsula. The maximum recurrence interval at any locality was about 70 years.
During the summer and fall of 1930, precipitation at many locations was less than 30 percent of normal. Statewide, the total precipitation for the year was about 9 inches less than normal, and the lack of precipitation caused streamflow to decrease rapidly. Soil was reported to be unusually dry and hard. Winter precipitation only temporarily relieved the drought, and subsoil moisture remained abnormally dry. In the summer of 1931, many crops were stunted, and many wells were dry. In the summer of 1932, crops again were affected by the dry conditions, but not to the same extent as in 1931. Precipitation was normal during the winter of 1932 but returned to less than normal in 1933. During the growing seasons in 1930, 1934, and 1936, precipitation was about 5 inches less than normal. Because of the severity of the 1930-37 drought, 41 counties were recognized by the Federal Drought Relief Administration as needing assistance. Numerous deaths were attributed to extreme heat in July 1936. The 1930-32 and 1933-37 dry periods cannot easily be distinguished on the basis of streamflow. For this reason, the drought is considered herein to span the interval 1930-37. As indicated by the annual-departure graphs for gaging stations having record during the drought (fig. 4, sites 3-5), streamflow was substantially less than normal in most of those years.
During 1947-50, a drought developed in the Upper Peninsula and the northern one-half of the Lower Peninsula. In the springs of 1947 and 1948, much of the southern Lower Peninsula experienced wetter than normal conditions, which helped to avert drought in that area. Deficient streamflow in the drought-stricken part of the State is readily apparent in the annual-departure graphs for the Middle Branch Ontonagon River near Paulding, the Manistique River near Manistique, and the Muskegon River at Evart (fig. 4, sites 1, 2, and 4). The calculated recurrence intervals of streamflow deficiencies measured during the drought ranged from 5 to 20 years in the Lower Peninsula to as much as 45 years in the western Upper Peninsula.
The 1947-50 drought was characterized by greater than normal temperatures, particularly during the summer of 1947. The drought was moderate over much of the State, and precipitation was severely deficient only in the western Upper Peninsula. Crops in general were not damaged, but as a result of the dry conditions in October 1947, numerous forest fires destroyed thousands of acres of timber in northern Michigan.
During 1952-56, the southern one-half of the Lower Peninsula experienced a drought at the same time streams in the northern Lower Peninsula and the Upper Peninsula (fig. 4, sites 1, 2, and 4) had greater than normal flow. The areas most severely affected by drought were the Clinton River, River Rouge (Detroit basin), and Kalamazoo River basins. In south-central Michigan, precipitation was about 9 inches less than normal during the summer of 1953, and 1955 marked the end of 4 consecutive years of greater than normal temperatures. In drought-stricken areas, recurrence intervals for this drought ranged from about 5 to 25 years.
The drought of 1955-59 affected primarily the area that had not been affected by the 1952-56 drought. However, two areas in southern Michigan had dry conditions in both 1952-56 and 1955-59. In parts of the Clinton (fig. 4, site 5) and Flint River basins, streamflow was less than normal beginning in 1957. In the St. Joseph, Kalamazoo, and Grand River (fig. 4, site 3) basins, streamflow was less than normal beginning in middle to late 1956. Recurrence intervals for this drought ranged from 15 to 35 years in the Lower Peninsula and from 15 to 45 years in the Upper Peninsula.