By W. J. ROBERTS
Practically all Illinois public water supply difficulties encountered during recent years have been blamed either directly or indirectly on the drought. This has been a convenient reason, for few people agree on the definition of the drought. A drought is generally thought of as a period of little or no rainfall, but engineers will probably want to qualify that definition with additions, such as: "a period of low streamflow; a time of law water levels in wells and water supply reservoirs; a time when water supply demands far exceeded the yield." This last thought reflects an important phase of a changing economy, for our growing cities are placing an increasing burden on our water supply facilities far in excess of their designed demands.
No one yet knows much about the physical aspects of persistent weather features that are prominent at times of drought. Some believe that long-term circulation changes are self-evolving, or that they are somehow connected with sun spots. All we know at present are the general circulation patterns that favor area droughts.
Water shortages creep up quietly on communities for many years; droughts generally accentuate them. Often the occurrence of a water shortage has been delayed by normal or above-normal rainfall even though the causes have been growing in strength, ever ready to assert themselves at the first opportunity. The chief reasons for most water shortages can be attributed to greater than anticipated population increases, decreases in well capacity, sediment accumulations in reservoirs, and increased water requirements, both domestic and industrial.
Census figures show that most Illinois municipalities have had increases in population from 1940 through 1950. Chicago, Springfield and East St. Louis have registered increases in population from 7 to 9 per cent. Champaign-Urbana has had a 66 per cent increase in population, while Carbondale, another university town, has had a 28 per cent increase. Effingham, a manufacturing town, has shown a 12 per cent increase but some coal mining towns, such as West Frankfort and Johnston City, show decreases in population varying from 8 to 17 per cent. It is interesting to note that these last two municipalities did not experience the severe water shortages that other expanding towns suffered.
Shallow well water supplies, that are locally recharged, can warn communities of their growing inadequacies. Normal rainfall generally provides sufficient recharge for such wells to meet usual pumpage demands. During dry periods lack of recharge permits well water levels to decline and less water is pumped. This warns the community in the form of periods of low pressure in the distribution lines on peak-load days. During protracted dry periods these distribution problems show up more frequently, and the customers are gradually impressed with the inadequacy of their well supply. On the other hand, surface water sources are designed to meet periods of low streamflow. When streamflow is normal or above normal the source can supply many times the demand. In fact it would be possible for municipal use to double or triple in wet years without any strain on the source. The strain would show only on the distribution end of the system.
Natural laws dictate the beginning of destruction of a reservoir as soon as it is constructed. The muddy water which formerly ran unhindered through the valley floor is blocked. The still water is unable to support the sediment it carried in transit and this material falls to the floor of the lake and accumulates.
During the height of the drought in 1954, the Illinois Water Survey made eight special silt surveys of municipal reservoirs in the drought area of central and southern Illinois. Loss in storage capacity due to siltation at some of these lakes was highly significant.
Silt accumulation in the municipal reservoir at
* Reprinted, with permission, from The Illinois Engineer.
ILLINOIS MUNICIPAL REVIEW—THE VOICE OF ILLINOIS MUNICIPALITIES 89
Bunker Hill, which was built in 1937, had reduced the reservoir capacity by nearly 55 per cent. As the original capacity of the lake was only 31 million gallons, the loss of 17 million gallons of storage is serious at this town. At the Carlinville reservoir, which was built in 1939, a 16 per cent loss of capacity due to siltation reduced the reservoir capacity from 559 million gallons to 467 million gallons.
The Johnston City reservoir, built in 1923, showed nearly a 20 per cent loss of storage by 1954. The younger reservoirs showed losses of storage of generally less than 10 per cent.
Actually the Illinois experiences during the recent drought indicated that loss of storage due to reservoir silting was the principal cause of shortage in only five surface water-using communities. It was a contributing cause in many others.
Both domestic and industrial water requirements have increased greatly since the end of World War II. Many southern Illinois communities are experiencing demands that are four to six times those that occurred in the period from 1930 to 1943. This growth has been due to extension of water service to a larger proportion of the population, as well as increases in population. Increases in demand caused by new water-using devices have not yet taken place to a material extent in the smaller communities. As sales of automatic washing machines, garbage grinders, dish-washing machines, and air conditioners grow, further increases in demand will take place.
The majority of the 300 Illinois cities located in the drought area of 1952-55 did not experience serious water problems. One could therefore conclude that present-time engineering practice usually provides for withstanding droughts no more serious than the recent one. However, 41 municipalities encountered water shortage troubles which were associated with management problems or insufficient design data.
Most Illinois communities in the recent drought zone were fortunate in having management that planned ahead and thus prevented water shortages. In a few instances needed waterworks construction may have been delayed for lack of public approval. Some water departments suffer because of inability to attract adequately qualified operating personnel on a permanent basis. Then there are deficiencies involving avoidable increases in water use such as leaks in water mains and lack of water meters. In several cases water rates have not kept pace with the increased cost of maintenance. The problems are part of the broader one of cities and villages trying to operate on inadequate revenues.
The 1953-55 drought has shown that some water systems failed to meet the demands of extreme dry weather conditions due to insufficient design data. Among the factors that may have caused failures are: the rate of reservoir siltation; lack of adequate precipitation records; absence of streamflow history; or inadequate evaporation information. Hydrologic data increase in usefulness with time. It takes many years of continuous observations to develop good precipitation and streamflow records. The Illinois State Water Survey and similar organizations are constantly adding to the storehouse of hydrologic knowledge and making the data available for engineering use and planning.
Mention has already been made of the Illinois Water Survey's nationally-recognized program for obtaining information on reservoir silting. Until recently, reservoirs were constructed without full regard to soil and water relationships. Little was done to hold the topsoil on the land instead of letting it accumulate in reservoirs. The County Soil Conservation Districts are at work on this job, but in the meantime the designer must use existing reservoir survey data to keep the rate of sedimentation to a minimum.
Rainfall data deficiencies result from lack of
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SOME CAUSES OF WATER SUPPLY SHORTAGES
sufficient measurements and from difficulties in processing the data already collected. The Illinois Water Survey's modern punch-card method of recording and processing weather information is making available precipitation and other meteorological data in ready form for analysis.
The recent drought emphasized the need for more Streamflow information. As rainfall deficiencies increased, rainfall data became less important than Streamflow in evaluating the severity of water shortages. There were many instances of rains over watersheds in the drought zone that produced no runoff into the reservoirs. The dry ground absorbed all the moisture, leaving little or none for runoff. Such rains may have aided agriculture but they added nothing to the reservoir capacity except the rain that fell directly on the water surface. The program for obtaining Streamflow data and the establishment of cooperative arrangements between the state and cities that have surface water reservoirs is desirable and is being expanded to overcome this deficiency.
During years of normal temperature and precipitation, the loss of moisture from water surfaces in a large part of Illinois exceeds 35 inches annually. Water evaporated from a reservoir in extended dry periods may exceed many times the municipal pumpage. A reduction in evaporation loss would be equivalent to increasing the storage. Preliminary data from one Illinois Water Survey project suggests that evaporation may be retarded 30 per cent.
In many instances it is difficult to attribute the cause of a water shortage to a single factor. Often several factors collectively create difficulties. One fact stands out: our modern economy cannot tolerate water shortages. In order to meet the squeeze between water supply and demand considerably greater attention must be given to the causes and cures of water supply shortages.
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