Hydrology Education: River Forecasting

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

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 precipitation used in river forecasts in summer, 2 days of future precipitation 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.

More than just precipitation!

Even with accurate estimates of how much precipitation will occur during a storm event, determining how high a river may rise is complicated. How quickly the precipitation falls, where in the basin the precipitation falls, the type of precipitation, and the amount of moisture in the soil all can greatly affect the eventual river crest.

Graphic depicting river forecast scenarios

Soil Conditions

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. Small basins often rise more quickly than large basins, and due to their size may have more uncertain rainfall forecasts.

Small Basins vs. Big Basins

River forecasts are very dependent on precipitation information, including forecasted precipitation. Because of this, smaller drainage basins can have more uncertain river forecasts.

Think of river basins like targets. It is often difficult to narrow down the exact location of future rainfall. For large basins, there can be error in precipitation forecasts but the precipitation may still occur within the basin. For smaller basins, that same error could mean the precipitation falls on another river basin entirely.

Graphic depicting large basins and small basins as targets.