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Hydrology is a complicated science overlapping with many other sciences such as meteorology, geology, agronomy, and geography. Accurate river forecasts can't be made without accurate weather forecasts, for example. Learn more about the basics of hydrology such as the water cycle, monitoring current conditions, forecasting, and human impacts on hydrology, below.

The Water Cycle

Graphic showing the water cycle

Water moves around our planet in multiple ways. This is called the water cycle. The water cycle shapes the land by transporting materials and is essential to most life on Earth. Particularly important to hydrologists are the water cycle components precipitation, infiltration, and runoff.


Water that falls from the atmosphere to the Earth’s surface. Includes rain and snow.


Precipitation that soaks into the soil.


Precipitation that does not soak into the soil but instead moves on the Earth’s surface toward streams.


Evaporation refers to water that changes from a liquid to a gas and moves from the Earth's surface back into the atmosphere. Transpiration refers to water that is pulled out of the soil and released into the atmosphere by plants.

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Monitoring Current Conditions

Picture of a weather station and a river gauge

Keeping track of the different components of the water cycle is important to hydrology. For example, monitoring current conditions helps scientists better understand where flooding or drought may occur.

Rain gauges may be used to keep track of precipitation and stream gauges may be used to keep track of how much water is in rivers. Other observations may include information about soil moisture, drought, snow cover, and river ice cover.

River Gauges

River gauges are used to monitor how much water is in streams. Typical measurements include stage and discharge.

Stage: The water level above a reference elevation, called a datum. This is not the same as depth!

Discharge: The rate of water moving past a location.

Precipitation Gauges/Observers

Observations of precipitation can come from automated gauges which report by satellite or modem and also manual observations. Automated gauges are particularly helpful for determining the  

Talk about how people can assist with observations of rainfall and flooding with CoCoRaHS, mPING, and the ice spotter program.

Soil Moisture

Changes in soil moisture affect how precipitation behaves when it reaches the ground. Generally, higher soil moisture increases the potential for water from precipitation to reach streams & rivers. Soil moisture can be measured or modeled. Knowledge of soil moisture content tends to improve river forecasts.


Drought typically refers to deficiency in precipitation, relative to the local area's climate, which persists over a period of time such that it causes a water shortage. This water shortage may impact streams & rivers, local water supplies, and agriculture. Monitoring drought may include observations of precipitation over a longer period of time as well as information about impacts to agriculture and water supply.

Snow Cover

Snow and ice accumulation does not impact river conditions right away but is instead stored at the soil's surface. During warmer periods, snow cover melts, which may cause river rises similar to that caused by rainfall. Snow cover is modeled to help improve river forecasts.

River Ice

River ice may have very complicated effects on the behavior of a river. For example, flowing ice may become stuck, or jam, on the upstream side of bridges, near sharp river bends, or other locations where the river changes abruptly. Ice jams may cause flooding in local areas. The rapid freeze-up of smaller streams and rivers can also cause a sudden, temporary drop in water level downstream, a situation sometimes referred to as "ice bite."

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A hydrograph is a way of displaying water level information over time. A hydrograph plot may display stage, streamflow, and sometimes both. Hydrographs can be a helpful way to show water level observations and forecasts visually on a single graphic. The NWS provides hydrographs of water level forecasts on the Advanced Hydrologic Prediction Service (AHPS).

Learn more about how to use NWS hydrographs by viewing the example below (left) with the explanations of each hydrograph part (right).

  • AHPS_example_1.png
  • AHPS_example_2.png

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River Forecasting

Graphic depicting river forecast scenarios

Forecasting river levels depends on many factors, such as precipitation, soil moisture, snow and ice cover, and the characteristics of the river basin.

Forecasts for rivers and streams in the NWS Chicago area are provided by the North Central River Forecast Center. Forecasts are used to decide whether to issue flood watches and warnings, and are also available on the Advanced Hydrologic Prediction Center (AHPS) web page.


Precipitation is one of the biggest factors that goes into river forecasting. Important questions that must be considered include:
How much precipitation has fallen? How much more precipitation may fall? Where is the precipitation within the basin?

Past/Observed Precipitation

Observed precipitation, also known as QPE, comes from a combination of multiple sources. Sources include rain gauge networks (both official and volunteer) as well as radar estimates. Precipitation estimates are reviewed for accuracy on an hourly basis.

Future/Forecast Precipitation

Forecast precipitation, also known as QPF, comes from weather forecasts. Picking the right amount of time to look into the future is actually quite tricky - it's a balancing act between forecast accuracy and providing extra advanced notice for flooding. Precipitation forecasts, just like any forecast, can be uncertain. Forecasts become more uncertain the further into the future you look. River forecasts in the NWS Chicago area use 1 day of forecasted precipitation in the warm season (April through September) and 2 days of forecasted precipitation in the cool season (October through March). This is because precipitation patterns in the warm season tend to be more scattered and variable, while precipitation patterns in the cool season tend to be less variable.

Graphic indicating 1 day of future rainfall used in river forecasts in summer, 2 days of future rainfall used in river forecasts in winter

Snow Cover

Precipitation that falls in frozen form becomes stored on the soil's surface until melted by warmer temperatures. A significant storage of water in the snow cover at the end of winter can lead to large water level rises due to warm spring temperatures, even if little additional precipitation occurs.

Soil Conditions

Image showing rainfall soaking into soil

Once the amount of total precipitation expected to fall on a basin (past + future) is estimated, the next step is to estimate how much precipitation will soak into the soil. Depending on soil type, there is a maximum rate at which precipitation can soak in. A higher intensity would mean runoff. If the soil near the surface becomes saturated, however, almost all liquid precipitation and any snow melt will become runoff.

River forecast models use estimates of soil moisture near the surface to help determine how much precipitation and snow melt will soak in to the soil, and how much will runoff toward streams and rivers.

Basin Characteristics

Other basin characteristics also impact how quickly runoff reaches rivers and streams, and how quickly rivers and streams will rise. These characteristics include basin shape, basin land cover, basin terrain and slope. River basins in steep, mountains areas as well as urban areas typically see faster rises in water level compared to rural, flatter areas. Dense vegetation may block some precipitation, reducing how much reaches the the soil's surface.

Image showing rainfall and vegetation

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Flood Products

If river forecasts suggest that flooding may occur, The National Weather Service will issue various products depending on the threat.


A hydrologic outlook is issued to provide notice of noteworthy rises on area rivers, but when the details are uncertain. River flooding and ice jams may be possible but there is low confidence in the timing, location, or extent.
Keep informed.


A flood watch is issued to provide notice that flooding is probable, but not yet certain. Many of the ingredients that lead to flooding are likely to occur in and near the indicated area, but there is not yet high confidence in the timing, location, or extent.
Prepare now.


A flood warning is issued to provide notice that flooding is very likely or already occurring. Confidence is high that flooding will occur within the area and time specified. Flooding may be life-threatening and damaging to property.
Take Action!


A flood advisory is issued to provide notice that flooding is very likely or already occurring, but flooding will be generally minor and a nuisance. Risk to life and property is expected to be very low when persons use caution to avoid particularly flood-prone areas. If rainfall is still occurring, a flood advisory may be upgraded to a flood warning.
Be aware.


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Flood Safety

Graphic showing flood risk to cars and people. Turn around don't drown when encountering flooded roadways.

It is important to act quickly during flooding to protect life and property. Although most flood injuries and fatalities occur in vehicles, the threat from flood waters should not be under-estimated in any situation.

  • 12 inches of moving water can float a car
  • 6 inches of moving water can knock a person off their feet
  • When encountering flooded roadways, find an alternate route - turn around don't drown!


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Climate and Climate Changes

Weather vs. Climate


Weather and climate are tied together, but not the same! Weather is what one expects at a given location at a given time. Climate is the long-term average weather condition. Temperatures can average in the 50s in March (climate), but on any given day be 20F or 80F (weather). This also applies to precipitation.
Changes have been observed on almost all area rivers over the last several decades. Some of these changes are due to climate change and how it has impacted the water cycle. Humans may also impact hydrology in other ways, such as large scale changes to land cover. Converting prairie and forest land into urban areas, for example, reduces how much precipitation can be soaked up by soil and increases runoff.

Rainfall Trends

Over the last several decades, average weather patterns across area river basins have changed. These climate changes have included not just increases in average temperature, but also increases in average rainfall. For example, in the northeast Illinois climate region, average annual precipitation was about 34 inches (liquid equivalent) back in the 1950s but climbed to 40 inches in the 2010s. An increase in average precipitation has been observed across the entire area, about 10-25%.

Graph of rainfall trend in the northeast Illinois climate division Graph of rainfall trends in climate divisions covering all river basins

This increase in precipitation does not mean that precipitation in any given year will always be above average - precipitation will continue to vary from year to year. These changes do mean, however, that the chance of a very dry year has decreased, and the chance of a very wet year has increased. This increase in precipitation has implications for the water cycle of area river basins.


Streamflow Trends

As with weather, there are typical ranges for water level at a given location throughout the year. In general, the long-term average water level is controlled by the water cycle – especially the precipitation and evapo-transpiration within a river basin.

Changes in precipitation have led to significant changes in runoff into area rivers, although not all area rivers are changing at the same rate. For example, the average mflow in the Rock River (measured at Joslin, IL) has increased from near 5500 cfs in the 1950s to near 11000 cfs in the 2010s, a 100% increase. Flow in the Kankakee River (measured at Wilmington, IL) has only increased by about 40% over the same period.

Graph of streamflow trends in the Rock River Graph of streamflow trends in the Kankakee River

Due to this increase in streamflow being passed down area rivers, average water levels have increased. This means that the chance of reaching flood stage in any given year has increased for most areas. Due to a combination of urbanization and increases in precipitation, river basins in the Chicago Metropolitan Area have seen some of the largest increases in streamflow.

Changes to the Water Cycle

How can a small increase in precipitation (10-20%) cause such a large increase in streamflow (50-100%)? It is related to the complicated way that climate changes are impacted the water cycle. The various parts of the water cycle are changing - or not changing - in different ways and at different speeds.

In the Rock River Basin, for example (below), precipitation has increased from about 32.5 inches a year in the 1950s to just over 39.0 inches per year in the 2010s, while evapotranspiration has stayed close to the same at around 24.5 inches per year. This has caused runoff to increase from about 8.0 inches per year in the 1950s to about 14.5 inches per year in the 2010s. The 20% increase in precipitation has caused runoff to almost double (more than an 80% increase).

Water Cycle Changes: Rock River Basin
In the Rock River Basin (upstream of the outlet near the Quad Cities), precipitation has increased about 6.5 inches per year since the 1950s, while evapotranspiration (evaporation + transpiration) has stayed about the same. This has resulted in a significant increase in runoff.
Precipitation 32.5 inches 39.0 inches
24.5 inches 24.5 inches

Runoff 8.0 inches 14.5 inches
Runoff is directly comparable to streamflow (and average river levels) in the water cycle. This change has been confirmed by measurements of streamflow taken in the Rock River (near the basin outlet), which have also nearly doubled over the same time period.


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