BULLETIN NO. 43 1955 STATE OF ILLINOIS WILLIAM G. STRATTON, Governor 1952-1955 ILLINOIS DROUGHT with special reference to IMPOUNDING RESERVOIR DESIGN BY H. E. Hudson, Jr. and W. J. Roberts DEPARTMENT OF REGISTRATION AND EDUCATION VERA M. BINKS, Director STATE WATER SURVEY DIVISION A. M. BUSWELl, Chief [Printed by the authority of the State of Illinois] CONTENTS Page SUMMARY 1 INTRODUCTION 1 Development of Drought Situation 1 Collection of Data . 2 Scope of Report 2 Acknowledgments 2 HYDROLOGIC ASPECTS OF WATER SUPPLY 3 Sources in Use 3 Normal Rainfall in Illinois 8 Normal Runoff in Illinois 8 Seasonal Variances in Hydrologic Cycle 10 CAUSES OF WATER SHORTAGE 11 Onset of Shortages 11 Increases in Water Demand 11 General 11 Water Demand in Drought Area 12 National Trends 12 Deficiencies in Hydrologic Data 13 Municipal Finance Problems 14 Insufficient Records 14 Experience of Communities Affected 14 Relation of Source to Incidence of Shortages 14 Source Capability versus Water Demand 15 Summary of Causes of Shortages 15 EXTENT AND INTENSITY OF THE DROUGHT 16 Rainfall Conditions 16 Rainfall Deficiency Patterns 17 Runoff Conditions 18 Periods of Lowest Streamflow 19 Drought Frequency Studies 21 Period of Record 22 Computing Sliding Totals 22 Plotting Positions 24 Graphic Determination of Frequency and Duration of Low Flows 25 Frequency of Periods of Low Rainfall 31 Recurrence Interval of 1952-55 Drought 32 ESTIMATING IMPOUNDING-RESERVOIR STORAGE 35 Introduction 35 Review of Methods for Calculating Storage 35 Suggested Design Criteria 37 Reservoir Sedimentation 39 Evaporative Losses 41 Gross Evaporation 41 Area Exposed to Evaporation 43 Net Evaporative Rates 45 Net Yield of Reservoirs 45 Limitation of Method 46 i SUMMARY Normal annual rainfall in Illinois varies from 34 inches in the northern part of the State to 46 A large part of central and southern Illinois inches in the extreme southern tip. experienced a severe drought beginning early in 1952. By June 1955 the drought situation had Average annual runoff in Illinois displays a greatly improved. Water shortages occurred in marked geographic variation within the State. Run- the fall of 1952 as a result of rainfall deficiencies. off varies from less than 8 inches in the west and The situation was partially relieved by above- northeast portions of the State to more than 15 normal rainfall during the first three months of inches in the area south of the Shawnee Hills. 1953. Rainfall deficiencies began to grow again These results are indicated by the analysis of 25 after April 1953. By September 1953 it was ap- of the 165 gaging stations in the state. parent that the effect of the 1952 rainfall defi- ciency had not been fully overcome. The situation Quantitative indications can be obtained of move- grew worse and by January 1954 there were 41 ment of water in the hydrologic cycle by comparison Illinois communities with serious water problems. of rainfall, runoff, transpiration and evaporation, and water levels in shallow wells. From April 1 In January 1954 a program was set up by the until mid-September vegetative and evaporative Water Survey to report the drought situation month- demands exceed the water available. This differ- ly. Data were obtained on water supplies through ence is made up either by water taken from the visits, correspondence and telephone communica- soil or by the reduction of transpiration and evap- tions with communities affected by the drought. oration. From mid-September until the end of This program was continued until January 1955. March, in the average year, water returns to stor- By June 1955, all drought-connected water short- age in the ground. From the behavior of ground- ages had ended. Figure 50 shows locations of water hydrographs for shallow wells which commonly places mentioned; show no particular recovery before the end of November, it is judged that the excess water avail- able during September-November largely goes to make up deficiencies in capillary moisture. The Sources of Water Supply groundwater level recovers between December 1 and April 1. To appreciate the drought, it is necessary to have an understanding of the normal water supply Vegetative and evaporative demands are so great situations in Illinois. Public water supplies are that below-normal rainfall through July and August most frequently obtained from groundwater sources leads to recession of groundwater levels, reduction in the northern part of Illinois. In the southern of soil moisture, and delay in recovery of stream- part of the State glacial deposits are much thinner flow. Such a situation occurred during the summer and their effect upon the storage and travel of periods of 1952-55 drought. water underground less important. The ground- water contribution to the low flow of streams is Municipal Water Supply Difficulties consequently lower. Total groundwater pumpage for municipal sup- Municipal water supplies depending upon surface plies in Illinois increased from 40 million gallons water are most common in the southern portion of daily in 1900 to about 106 mgd in 1940 and to 135 the State where groundwater is generally more mgd by 1948. difficult to develop. Cities in southern Illinois lo- cated on major streams take water directly from Of the 41 communities that faced possible water them. Other cities generally provide an impounding shortages in January 1954 all but 8 experienced reservoir on a smaller stream, or a side-channel population increases for the period 1930 to 1950. reservoir into which water is pumped from a main In 24 instances the increases were less than 20 stream. per cent. The highest population increase was 57 per cent. Municipal pumpage in 24 of these com- In the drought region of the state 202 public munities increased by more than 100 per cent from water supplies obtain water from groundwater 1938 to 1953. In three cases pumpage increased sources. Ninety-eight systems in the region uti- 600 per cent or more. lize surface water. Deficiencies in hydrologic data appear to have The majority of the impounding reservoirs in had an important role in the failures of public the drought region have been constructed since water supply sources. At the time of design of 1920. Eighty per cent of them were built prior most of the reservoirs now in use there were few to 1940. Ninety per cent of the reservoirs utilize streamflow data available. drainage areas of less than 20 square miles. Me- dian runoff required to fill these reservoirs is Twenty-two of the 202 public water supplies in 3.3 inches. the drought region relying on groundwater sources ii. experienced shortages. Of the 98 systems in the (ending June 1955) appears to have been a severity region that utilize surface water, 53 suffered short- that would occur once in approximately 100 years. ages. Fifty-eight of the surface water systems in Rainfall data indicate the drought to have been of the region rely on impounding reservoirs, and 40 a severity that would occur once in 60 to 100 of these suffered shortages. years. The most representative value available for evaluating the drought as a single event was During the drought it became apparent that grave the median recurrence interval. On this basis the water shortages occurred most frequently with im- drought of 1952-55 appears to have been a severity pounding reservoir supplies. This report therefore that would occur once in 83 years. emphasizes hydrologic information directly applica- ble to design and appraisal of impounding reservoir Estimating Reservoir Storage Requirements sources of supply. The report reviews the methods available for Of the 75 Illinois communities suffering water calculating the amount of storage necessary for an shortages during the drought, 38 experienced dif- adequate surface water supply. In general, these ficulty because developed source capacity was in- methods do not make possible the calculation of sufficient to meet demands, 28 had trouble because the amount of storage required to meet droughts of of unexpectedly large increases in demand, and 9 various durations and frequencies. The available encountered difficulty because of declining capacity methods do not provide a direct approach to the of facilities. evaluation of evaporation losses from reservoirs. Most of the early reservoirs in Illinois were de- Extent and Intensity of the Drought signed without allowance for loss of storage due to reservoir silting. The area covered by the drought was generally A new procedure, utilizing the streamflow fre- within the southern half of the State although it quency determinations discussed above is proposed extended northward into Hancock and McDonough for the determination of impounding reservoir ca- Counties and north of Champaign County at times. pacity. The procedure may be divided into two parts: (1) determination of storage required for Maximum rainfall deficiency in the drought re- various draft rates and drought frequencies, and gion of the State for the period April to December (2) evaluation of evaporative and sedimentation 1952 amounted to about five inches. Deficiencies losses. For the study of draft rates, a weighing in the drought region range from 10 to 15 inches of low-flows of various frequencies and durations during 1953 and from 5 to 10 inches during 1954. leads to the determination of the most critical These deficiencies are depicted in map form in storage requirement and drought duration for any the report. given draft rate. These values have been consoli- dated into a single graph which enables rapid de- Maps of runoff during the drought are presented. termination of the storage required at a given site These are based on streamflow records of the for any drought frequency and draft rate in southern U. S. Geological Survey at 15 stations in the drought Illinois. region. The patterns of runoff were generally similar to those of precipitation.