The Imperial Valley Water Authority was established in 1989 by referendum. It comprises all of Mason County and four townships in Tazewell County. It operates under the Water Authorities Act of 1951. The act gives the trustees various powers, one of which is 'To reasonably regulate the use of water and during any period of actual or threatened shortage, to establish limits upon or priorities as to the use of water.'
The year 2000 saw dramatic drops in the water levels in most of the 13 observation wells in Mason and Tazewell counties where rainfall was below average for the year. However, despite below average rainfall in 2000, data collected by the Imperial Valley Water Authority suggest that the number of gallons of water pumped for irrigation was down too, probably due to the timing of the rainfall.
The long-term annual rainfall measured in the Imperial Valley Water Authority's Rain Gauge Network over the past seven years has been 35 to 37 inches. For the first 11 months of 2000 precipitation levels fell below that amount with an average of 27.60 inches among the 20 sites.
Interestingly, 8 to 12 inches of that amount came in the last ten days of June and the first five days of July, representing nearly 40% of the year's total.
The Imperial Valley Water Authority also has a network of observation wells which have been checked monthly since May 1995. These wells have shown a steady decline since the July-August period of 1998. It appears the shortfall on precipitation translated into lower water levels. Throughout most of 2000 the water in the wells was 2 to 3 1/2 feet lower than the same period
Not surprisingly, the only deviation was the June-July period, which was the only time that rainfall was dramatically above average.
Using a formula based on power consumption a way has been determined to estimate the amount of groundwater used by agricultural irrigation. For the irrigation season from June to September it is estimated that 36.2 billion gallons were used in 2000 for irrigation. This compares to 45.7 billion gallons in 1999, 35.9 billion gallons in 1998, and 45.3 billion gallons in 1997.
Generally, July and August see the greatest average irrigation withdrawals. Although 2000 had below average rainfall, there was also below average irrigation use. What rainfall there was came at the right time because only 60% of the total irrigation was used during July-August, as compared to 70% in 1999, 74% in 1998 and 87% in 1997. Despite the shortfall in rain, more water was pumped in five of the past six years than was pumped in 2000, even though there are more systems in use today than ever before.
As we enter the new millennium, observers look at the historical data to seek out expected trends for the future. Groundwater data going back to 1930 shows that in the first part of each decade, the water table has been above the average depth below land surface, and the last half of each decade has been below the average depth below land surface, with the exception of the 1950s when the water levels remained below average form 1949 to 1961.
If the pattern holds, it could be expected that this coming year should begin a time of average to above average rainfall for a few years.
(The Imperial Valley Water Authority board has contracted service with the Illinois State Water Survey to set up and maintain a network of rain gauge and observation sites. It is the strategy of the board to continue to gather information and work with the State Water Survey to obtain analysis and understanding of the information gathered.)