2002 Marked the Beginning of the End of the Invisible Drought

by Todd Giddings, Ph.D., P.G.

The Invisible Drought was the cumulative decline in the ground-water resources of the Spring Creek Watershed that began in the summer of 1998 and is shown by the dashed arrow on the hydrograph.  The vertical lines are January 1^st of each year, almost in the middle of each winter, and for Well CE 686 (blue line), the winter declines in the water-table levels are apparent during the Invisible Drought period.  During 2002 the Spring Creek Watershed received 10% above normal precipitation, and this marked the beginning of the end of the Invisible Drought.  The above-normal rainfall began in May 2002, and the dotted arrow on the hydrograph shows the rising water-table level trend in Well CE 686 that continued through mid 2003.  By the end of 2002, 19 inches of snowfall had occurred in last winter and the snow pack had started to accumulate.

So how did the 10% above-normal precipitation in 2002 and the snow pack end the Invisible Drought?  The answer is the timing of the precipitation.  During the winter months of November, December, January, February, and March, plants are dormant and their consumption of water is negligible.  Evaporation is also minimal due to the cold temperatures, limited sunshine, and the low sun angle.  During the spring, summer, and early fall, plants consume water and discharge it into the atmosphere as water vapor by a process called transpiration.  Evapotranspiration is the combined movement of water back into the atmosphere by both evaporation and transpiration, and this process moves approximately two-thirds of our annual precipitation back into the atmosphere each year.

            The hydrograph of Well CE 686 (located two miles southwest of State College) shows that almost 40 feet of water-table level rise occurred during last winter, and this refutes the myth that ground-water recharge can’t occur through ground frost.  We were fortunate that last winter’s snow pack melted rather slowly, which allowed most of the melt water to become recharge and not runoff.  The timing of the precipitation in 2002, more than the total amount, contributed to the beginning of the end of the Invisible Drought.

            Well CE 118, shown by the red hydrograph line, is located 5 miles west-southwest of State College on Game Lands 176 in the Barrens.  This monitoring well is located in the headwaters recharge area of Big Spring, and the ground water under the Barrens flows 12 miles northeast to discharge from Big Spring.  The response of water-table levels is very subdued in Well CE 118 due to the overlying sandy soil and the high ground-water storage capacity of the Gatesburg Formation bedrock.  The sandy natural forest floor conditions and the complete lack of urbanization in this headwaters area promote ground-water recharge to this important ground-water resources area of the Spring Creek Watershed.  The slight upturn in the red hydrograph line of Well CE 118 also indicates the beginning of the end of the Invisible Drought in this area of the watershed.

    You can view a real-time hydrograph of Monitoring Well CE 686 on the US Geological Survey Web site by clicking this link.

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Last modified: July 21, 2008.