The hydrologic cycle

The amount of water on the planet has been here since the beginning of time; understanding the cycle is fundamental to using this precious resource responsibly

By Susan Williams

Graphic by Becky Buher

Enter Sally Letsinger, a research hydrogeologist at the Center for Geospatial Data Analysis (CGDA) at the Indiana Geological Survey on the IU Bloomington campus. We’re talking, she said, about the hydrologic cycle—the constant movement of water between the earth’s surface and atmosphere. Remember that the Earth’s water supply is finite. In fact, the amount of water we have now on this planet—no more, no less—has been here since the beginning of time. In what is a model recycling system, Mother Nature reuses her water over and over.

Photo by Paul Martens
Letsinger

“The hydrologic cycle is powered by the energy of the sun,” said Letsinger. “The energy initiates the cycle by evaporating water from the oceans and land surfaces. The water vapor enters the atmosphere and is then transported by air masses and condenses to form clouds. The clouds produce precipitation in the form of rain or snow. Precipitation falls and drains off of the land into streams, seeps into the ground, is taken up by vegetation or is evaporated back into the air.”

According to Letsinger, 97 percent of the water on earth resides in the oceans, but as consumers, we’re focused on the resources found in lakes, rivers, soil moisture and groundwater. Approximately 60 to 70 percent of the population of Indiana relies on groundwater.

“Groundwater is water that fills the pore space of geologic formations below the water table,” said Letsinger. “Water supplies from groundwater are extracted through water wells that tap aquifers. For groundwater to be considered an aquifer, it must be in the right kind of geologic formation to absorb, store and transmit water in sufficient quantity. Also, for water to be useful, it also must be of good quality.”

According to Letsinger, in Indiana there are two main types of aquifers—deposits of sand and gravel, and fractured rock limestone or sandstone formations. While the supply of groundwater in them is renewable through recharge by precipitation, it is a fragile resource.

“Replacing water in aquifers takes a very long time,” Letsinger explained. “The water currently stored in subsurface geologic formations is often quite old, which means that if water is extracted from aquifers at an excessive rate, normal rates of precipitation seeping into the subsurface can’t replace the amounts withdrawn from the aquifer. So, some aquifers can be depleted of their supply through the actions of humans, like the lack of conservation, some land-use decisions, and some industrial and agricultural practices.

“The other primary factor affecting water supply is climate,” Letsinger continued. “Climate impacts the kinds of vegetation in a region, which, in turn, can affect groundwater supply through soil moisture uptake and evapotranspiration. The impact of vegetation can influence groundwater levels to the extent that some water wells show a seasonal pattern of rising and falling water levels in response to the use of water by the plants.

“Climate, in times of warmer temperatures and low precipitation rates, can also cause direct evaporation from the land surface. Shallow wells are especially vulnerable.”

In the Midwest, water quantity isn’t the problem. For us, at least for now, quality is the issue.

“The water supply can be easily contaminated,” said Letsinger, whose research centers on understanding not only the physical flow of water, but also the reasons that groundwater and surface water become contaminated. “Water quality degradation is a difficult problem to solve once it occurs, especially in the subsurface.”

One of Letsinger’s projects, with other researchers in CGDA, concerns the safety of drinking water in small water supply wells. The research involves making calculations about how fast contaminants, if present, would reach a water supply well. The project should provide well owners with the information to understand and protect their own drinking water.

Another project involves numerical modeling and visualization of bacterial contaminant—in this case E. coli—transport through stream networks that discharge into Lake Michigan. Researchers hope the project will protect areas such as swimming beaches where people can potentially come in contact with contaminated water. But they also aim to identify sources and mechanisms of contamination to better focus on forecasting contaminant movement and remediation of the causes.

“In the Midwest, it is a common expectation that the water resource will always be clean and abundant,” said Letsinger. “An understanding of the hydrologic cycle, including the impacts of our actions on the quantity and quality of our water supply, is fundamental to our responsible use of this resource.”