Ohio State University Extension Fact Sheet

Ohio State University Extension Fact Sheet

Food, Agricultural and Biological Engineering

590 Woody Hayes Dr., Columbus, Ohio 43210

Ohio's Hydrologic Cycle


Larry C. Brown

Water is Important!

Recent estimates indicate that Ohioans use approximately 11,700 million gallons of water per day (MGD) for various purposes. Over 9,000 MGD of the 11,700 MGD is for thermo-electric use. This water is supplied by our state's abundant water resources, which include surface- and ground-water supplies (see Table 1). Surface-water resources, such as ponds, lakes, reservoirs, streams and rivers, supply water to about 55 percent of the state's population. These resources include 43,900 miles of streams and 2,200 lakes. The remaining 45 percent of the population is served by ground water, which is extracted from water-bearing geologic formations beneath the Earth's surface. These formations, called aquifers, are of two types in Ohio: unconsolidated deposits and sedimentary bedrock, which are confined or unconfined.

Table 1. Water Facts for Ohio 1
Water ResourceEstimated Percent of the Total Population Served Estimated Percent TotalsEstimated Water Use (MGD)2
From Public Supply Systems From Rural Water Self-Supplied Systems
Surface Water54.40.354.7932.7
Ground Water29.116.245.3534.8
1. Abstracted from USGS, 1984; 1985; 1990; 1993
2. Million gallons per day

Water use can be separated into the following supply categories: public, rural (domestic and livestock), industrial, and irrigation. As indicated in Table 2, about 70 percent of the public water supply in Ohio is furnished by surface water, while 98 percent of the rural domestic supply is furnished by ground water. Surface and ground water both play important roles in supplying water for domestic purposes in Ohio.

Table 2. Categories of Water Use in Ohio1.
Surface Water2 Ground Water
Category of Use Percent of Use Supplied by Surface Water Percent of Total Surface Water Use2Percent of Use Supplied by Ground Water Percent of Total Ground Water Use3
1. Abstracted from USGS, 1984; 1985; 1990; 1993
2. Offstream use only.
3. Percentages may not add up to 100 percent because of independent rounding errors.

Where does all of this water come from and where does it go? The answers to these questions are important not only for establishing a reliable water source but also for developing an awareness of how human activities can influence the quantity and quality of Ohio's water resources. This publication provides an overview of the hydrologic cycle as it relates to Ohio.

What is the Hydrologic Cycle?

The Earth holds more than 300 million cubic miles of water beneath the surface, on the surface, and in the atmosphere. This vast amount of water is in constant motion in a complex cycle known as the hydrologic cycle.

The hydrologic cycle, illustrated in Figure 1, describes the pathways that water travels as it circulates throughout the world by various processes. The visible components of this cycle are precipitation and runoff; however, other components, such as evaporation, infiltration, transpiration, percolation, ground-water recharge, interflow, and ground-water discharge are equally important.

Figure 1. The Hydrologic Cycle. (Modified from What is Groundwater?, 1988)

Components of the Hydrologic Cycle


Water that evaporates from the Earth is stored temporarily, in the form of water vapor in the atmosphere, before returning to the Earth's surface in one of several forms of precipitation: rain, snow, sleet, or hail. The amount of average precipitation in Ohio averages 38 inches statewide, and ranges from 42 inches on the southern border to about 32 to 34 inches along most of the northern border. Because of winds blowing over Lake Erie, the average annual precipitation reaches 44 inches in parts of northeastern Ohio, with a substantial proportion in the form of snow. Precipitation is a natural phenomenon that humans can do very little to control.


Through the process of evaporation, water moves back to the atmosphere in the form of vapor. While in the atmosphere, this water is in the form of vapor and small water droplets that form clouds. As the atmosphere becomes saturated, water is released back to Earth as some form of precipitation. Some of the precipitation can evaporate before it reaches the ground. Precipitation reaching the ground can evaporate from bare soil surfaces, plant surfaces, the surface of ponds, lakes, and streams, and just about any water surface. Wind, solar radiation, and heat can greatly increase the evaporation rate, whereas a high water vapor percentage in the air (high relative humidity) can decrease the potential for evaporation.


Infiltration is the entry of water into the soil. The amount of water that infiltrates the ground varies widely from place to place. The rate at which water infiltrates depends on soil properties such as moisture content, texture, density, organic matter content, permeability and porosity. Permeability is a measure of how fast water flows through certain soils or rock layers. Infiltration and permeability are greater in porous materials, such as sands, gravels, or fractured rock, than in clay soils or solid rock. Porosity is a measure of the amount of open space in soil or rock which may contain water; this water is part of the soil storage.

Conditions at the soil's surface also influence infiltration. For example, a compacted soil surface or frozen soil conditions restrict the movement of water into the soil profile. Vegetation can play a prominent role in infiltration. The surface soil layer in a forest or a pasture will generally have a far greater infiltration capacity than will a paved parking area or a compacted soil surface at a construction site. Topography, slope, and the roughness of the surface also affect infiltration, as do human activities in urban and agricultural areas where alteration of soil properties and surface conditions have taken place.


Water can take several paths after it enters the soil. Some water becomes part of the soil storage. This water is not stationary; it is under the pull of gravity and moves downward at a rate that depends on various soil properties, such as permeability and porosity. While in storage near the surface, some of this water is used by plants and eventually returned to the atmosphere as water vapor. The process by which plants release water vapor to the atmosphere is called transpiration. This water vapor is a natural byproduct of photosynthesis.


Because of the difficulty in separating the processes of evaporation and plant transpiration, we usually view these two processes as one process called evapotranspiration. This term then includes both the water that evaporates from soil and plant surfaces, and the water that moves out of the soil profile by plant transpiration. A large proportion of the water that enters the soil is returned to the atmosphere through evapotranspiration.

Percolation and Ground-Water Recharge

Another path that water can take after it enters at the soil surface is that of percolation. Percolation is water moving downward through the soil profile under the pull of gravity after it enters the soil. Water that moves downward through the soil, below the plant root zone, towards the underlying geologic formation is called deep percolation. For the most part, deep percolation is beyond the reach of plant roots and this water replenishes the ground-water supply. The process of replenishing or refilling the ground-water supply by deep percolation is called ground-water recharge.


As water percolates, some of it may reach a layer of soil or rock material that restricts downward movement. Restrictive layers can be formed naturally (clay pan or solid bedrock) or as a result of human activities. Once water reaches a restrictive layer, it may move laterally along this layer and eventually discharge to a surface-water body, such as a stream or lake. The lateral movement of water is called interflow.

Ground-Water Discharge

Ground water can flow into or discharge to a surface-water body such as a lake or river. This process creates a baseflow for the surface-water body and is an important connection between ground and surface waters. In Ohio, many aquifers discharge to surface-water bodies.


Once the precipitation rate exceeds the infiltration rate of the soil, depressions on the soil surface begin to fill. These surface depressions are called surface storage. When surface storage is filled, and if precipitation continues to exceed infiltration, water begins to move down-slope as overland flow or in defined channels. This process is called surface runoff, or simply runoff. A large percentage of runoff water reaches stream channels. Runoff can also occur when the soil is saturated (soil storage is filled). In this case, all the voids, cracks, crevices of the soil profile are filled with water and the excess begins to flow over the soil surface.

Adding It Up

As stated earlier, the average annual precipitation in Ohio is 38 inches. Of these 38 inches, about 10 inches become runoff, which moves immediately to surface-water bodies. Two inches are retained at or near the ground surface and evaporate back into the atmosphere in a relatively short period of time.

Twenty-six of the 38 total inches enter the soil surface through infiltration. Twenty of these 26 inches go into soil storage and later are returned to the atmosphere by the combined processes of evaporation and transpiration (evapotranspiration). The remaining 6 inches recharge the ground-water supply. Two of these 6 inches eventually move to springs, lakes, or streams as ground-water discharge. The remaining 4 inches either return to the atmosphere by evapotranspiration or are withdrawn to supply water needs.

These numbers are averages for Ohio. Values for particular locations will differ according to local conditions.

The Hydrologic Cycle and Water Quality

As water constantly moves from the Earth to the atmosphere and back to the Earth, the constituents dissolved in it and transported by it are modified as a result of natural processes and human activities. Chemicals and particles in dust, smoke and smog in the atmosphere eventually fall back to the Earth with precipitation. Water moving across the soil surface as runoff can detach soil particles and transport them to a stream or lake. Runoff from lawns, pastures, and agricultural fields can also carry dissolved nutrients and pesticides. Certain chemicals attach to soil particles and also are transported to receiving waters. Runoff from roadways and parking lots wash grit and metal particles directly into storm sewers and streams.

Water that percolates to the underlying aquifer can be polluted by the leaching of chemicals, nutrients and/or organic wastes from the land surface or from materials buried in landfills. Aquifers close to the surface or in porous, unconsolidated strata (sands and gravels) can be very vulnerable to pollution. Deep aquifers are also vulnerable, especially if connected to the surface by fissures or sinkholes in underlying formations as in limestone rock areas. Certainly, surface and underground conditions differ all across Ohio. However, human activities in any part of the state can have a dramatic impact on the quality of our surface- and ground-water resources.


Although Ohio is a water-rich state, we must continue to be concerned about the protection and proper use of our valuable water resources. Many human activities (urban, rural, agricultural, and industrial) have an influence on the quantity and quality of water. In order to make wise decisions about the proper protection and use of these resources, we must have a good understanding of the basic processes of the hydrologic cycle through which water continually circulates from the Earth's surface to the atmosphere and back to the Earth.

This publication presents an overview of the hydrologic cycle as it relates to Ohio. Basic terminology was used in this publication to describe the components of the hydrologic cycle. The publication Ground- and Surface-Water Terminology (AEX 460) provides a listing of generally accepted definitions of many water resource terms. For more information on this or other water resources topics, refer to the publications listed below, or contact your County Extension Office, Soil and Water Conservation District Office, the Ohio Department of Natural Resources, Division of Water (Fountain Square, Columbus, OH 43224), or the U.S. Geological Survey (Ohio District, 975 W. Third Ave., Columbus, OH 43212).

Additional sources of water resources information may be available from your county Extension office. Extension agents in many of Ohio's 88 counties have prepared fact sheets in cooperation with ODNR, Ohio EPA, USGS, and the USDA-Soil Conservation Service. For those counties that are complete, there are two fact sheets specific to the county: the first is an overview of the county's water resources; the second is specific to the county's ground water. Not all counties have completed these fact sheets. Contact your county office of Ohio State University Extension for more information.


Estimated Use of Water in the United States in 1990. 1993. W. B. Solley, R. R. Pierce, and H. A. Perlman. U. S. Geological Survey Circular 1081.

Estimated Water Use in Ohio, 1990. Livestock, Animal Specialities, and Irrigation Data. 1993. R. J. Veley. U. S. Geological Survey Open- File Report 93-646.

Estimated Water Use in Ohio, 1990. Mining Data. 1993. R. J. Veley. U. S. Geological Survey Open-File Report 93-453.

Estimated Water Use in Ohio, 1990. Public-Supply Data. 1993. R. J. Veley. U. S. Geological Survey Open-File Report 93-72.

Estimated Water Use in Ohio, 1990. Thermoelectric PowerData. 1993. R. J. Veley. U. S. Geological Survey Open-File Report 93-645.

Gazetteer of Ohio Streams. 1960. Report No. 12, Ohio Water Plan Inventory. ODNR Division of Water.

Ground- and Surface-Water Terminology. 1994. L. C. Brown and L. P. Black. AEX 460. Ohio State University Extension.

Hydrologic Atlas for Ohio: Average Annual Precipitation, Temperature, Streamflow, and Water Loss for the 50-Year Period 1931-1988. 1990. L. J. Hartstine. Water Inventory Report No. 28. ODNR Division of Water.

Inventory of Ohio's Lakes. 1980. Ohio Water Inventory Report No. 26. ODNR Division of Water.

Ohio Ground-Water Quality. USGS National Water Summary- Ohio. 1986. U.S. Geological Survey Water-Supply Paper 2325.

Ohio Ground-Water Resources. USGS National Water Summary- Ohio. 1984. U.S. Geological Survey Water-Supply Paper 2275.

Ohio Surface-Water Resources. USGS National Water Summary- Ohio. 1985. U.S. Geological Survey Water-Supply Paper 2300.

Soil and Water Conservation Engineering. 1993. G. O. Schwab, D. D. Fangmeier, W. J. Elliot, and R. K. Frevert. Wiley and Sons, NY.

What is Groundwater? 1988. L. S. Raymond. Bulletin No. l. New York State Water Resources Institute, Center for Environmental Research, Cornell University.


This publication was originally produced through the Water Resources Educational Materials Project funded by the Innovative Grant Program of Ohio State University Extension. Project Team: Larry C. Brown (Project Leader), Ron Overmyer (Sandusky County), John Hixson (Union County), Gary Wilson (Hancock County), Marcus Dresbach (Northwest District), Glen Arnold (Putnam County), Jay Johnson (School of Natural Resources), and Robert Roth (School of Natural Resources), The Ohio State University, and Leonard Black and Margo Fulmer (ODNR, Division of Water).

The author thanks Kim Wintringham (Associate Editor, Section of Communications and Technology) for editorial and graphic production.

All educational programs conducted by Ohio State University Extension are available to clientele on a nondiscriminatory basis without regard to race, color, creed, religion, sexual orientation, national origin, gender, age, disability or Vietnam-era veteran status.

Keith L. Smith, Associate Vice President for Ag. Adm. and Director, OSU Extension.

TDD No. 800-589-8292 (Ohio only) or 614-292-6181

| Ohioline | Search | Fact Sheets | Bulletins |