Drought Flow, A State-Wide Analysis
C. J. VELZ, Professor of Public Health Engineering
and
Chairman of The Department of Environmental Health
and
JOHN J. GANNON, Associate Professor of Public Health Engineering
University of Michigan
Ann Arbor, Michigan
Two factors prompted the undertaking of a statewide analysis of drought
flow severity of Michigan streams: first, the increasing competition for use of
the limited water resources of the streams of the state, and second, the pressing need to establish a "floor" of minimum flow available as a guide to
rational development and use.
A comprehensive statewide approach was particularly appropriate, because the stream gaging program in the state of Michigan had a late start,
coverage is limited, and few continuous records extend prior to 1930. Therefore, an effort was made at total coverage with detailed statistical analyses
for the 81 stream gaging stations with ten or more years of continuous record,
and with correlation of the 74 stations with short term records less than ten
yrs to nearby long-term stations. An overall view of drought flow characteristics in the state is thus available as an aid to professional judgment in
estimating drought flow to be expected along the course of streams at locations other than that at gaging stations.
The studies were made in cooperation with the Water Resources Commission of the state of Michigan and published under the title "Drought Flow
Characteristics of Michigan Streams, " (1) detailed in five sections: Part I --
Definition of Drought Flow Characteristics of Streams, an explanation of the
methods of analysis employed; Part II - Drainage Basin Reports, the presentation of findings by major drainage basins of the state; Part III - Precipitation
Analyses; Part IV -- Temperature Analyses; and Part V -- Analyses of Great
Lake Levels.
The state was divided into major watersheds (Figure 1) and results of the
drought flow studies are presented as Drainage Basin Reports. Each included;
basin description; stream gages with long and short term records and stream
flow regulation; geology; summary of drought flow characteristics with yield
variability ratio data; and estimates of drought flow.
Appended to each basin report are the complete tabulations of recorded
drought flows for each year for each stream gage in the basin, and the
probability plots.
STATISTICAL CHARACTER OF STREAM FLOW
At a given time and place in nature's balance undisturbed by man, a
stabilization of environment is effected through a complex interrelation of
water, land, and vegetative cover. Stability of environment is relative, depending upon the time period. In terms of geologic time there are unmistakable signs of shifts, trends, or pulsations in stability. However, within recorded
current time,   the tendency in climate is not known.   There is no reason to
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