Weather and Climate Information
Illinois temperature and precipitation data included in these monthly IWCS reports are from measurements taken at selected sites within the Cooperative Observer Network of the National Weather Service (NWS), an agency of the National Oceaninc and Atmospheric Administration (NOAA), U.S. Department of Commerce (USDOC). Near real-time data from this network are received by the Midwestern Regional Climate Center (MRCC) at ISWS. These data are provisional. Complete, quality controlled data are recieved at MRCC from the National Climatic Data Center (NCDC), NOAA, USDOC about three months after the data are collected.
The Illinois Climate Network (ICN) is a 19-station array of automated weather stations scattered across Illinois, operated and maintained by ISWS staff. The network provides enhanced temporal weather observations on numerous weather and climate variables including: temperature, precipitaion, relative humidity, barometric pressure, wind speed, wind direction, and solar radiation. Values of potential evapotranspiration and dew point temperatures are computed and added to the data set. Direct observations of other weather-related variables are also taken, including soil temperatures and soil moisture at various depths, and continuous observations of the shallow water table level at the station.
Sites are located primarily at university agricultural experimental farms and community colleges across the state. Most sensors are polled automatically every 10 seconds and are averaged by hour and day. Hourly and daily extremes and times of occurrence for some variables are recorded as well. All data are downloaded to an ISWS computer once a day. The daily temperature and precipitation data are added to the MRCC records.
ICN data provide valuable information on extreme and usual weather events, as well as short and long-term trends in climate data, which may have future direct impacts on other water resources of Illinois.
Extended climate outlooks are produced on a monthy and seasonal basis by the Climate Prediction Center (CPC), NOAA, USDOC based on extensive sources of timely climate information. "CPC's products are operational predictions of climate variability, real-time monitoring of climate and the required data bases, and assessments of the origins of major climate anomalies. The products cover time scales from a week to seasons, extending into the future as far as technically feasible, and cover the land, the ocean, and the atmosphere, extending into the stratosphere." (Source: CPC). The outlooks for Illinois are extracted from the CPC records and are provided in this report as a service to other readers.Soil Moisture Information
Soil moisture monitoring is performed at 19 sites across Illinois by ISWS staff at sites mostly co-located at the ICN stations. Historic data were collected manually from site visits twice a month during the growing season (March to October) and once a month during the remainder of the year. This continues at some sites; others have been converted to automatic readings in connection with the ICN data stream. The information aids in pinpointing areas and extents of unusual soil moisture levels, their impacts on Illinois agriculture, and their potential insight on pending trends in other water resources of the state. These data become especially important during prolonged periods of precipitation drought.Surface Water Information
River and stream discharge and stage data are obtained from gaging stations operated by the U.S. Geological Survey (USGS). or the U.S. Army Corps of Engineers (USACE). The USGS gaging station network is supported in part by the IDNR Office of Water Resources, ISWS, and the USACE. Provisional discharge data are obtained from the USGS. Stage observations are reported at 14 locations along the Illinois, Mississippi, and Ohio Rivers. River stage is the observed water surface level at the gage location relative to the gage's own datum, or zero mark. The elevation of gage zero for each site is set arbitrarily, and varies from station to station. Stage is not equivalent to the depth to the river bottom. Flood stage is a specified stage value, typically describing the approximate level at which the river goes out of its banks at or near the gage location. Flood stage designations vary from station to station, and apply locally. Stage may be converted to an elevation (relative to mean sea level) by adding the stage value to the gage datum elevation, as reported in a common vertical datum (e.g. National Geodetic Vertical Datum 1929).
Streamflow data are obtained for 26 Illinois stations along selected streams and rivers from long-term monthly means published by the USGS. Monthly median flow is determined by ranking the monthly mean flows for the same month for each year of record, and selecting the middle value to determine a 50% chance exceedence value. The current month's flow condition at each station is classified from much above normal to much below normal by a comparison of the mean flow for the current month relative to the monthly mean flow statistics during the period of record for the same month. With very few exceptions, the median flow for a particular month will be less than the mean flow. The current month's flow as a percent of the median in nearly every case will be higher than the percent of mean.
Reservoir levels are obtained from a network of cooperating reservoir operators who are contacted each month by ISWS staff. Most of these reservoirs serve their local communities as public water supply reservoirs. The ISWS began collecting month-end water surface elevations at reservoirs in 1983 with the number of reporting stations increasing over time to a present total of 39 reservoirs. The current month's average month-end water surface level for each reservoir is the arithmetic average of the month-end levels for the period of record. Trends in these data give indications of potential water deficit problems during periods of extended drought conditions.Ground-Water Information
Shallow ground-water well monitoring is conducted at 19 locations in Illinois sited in rural areas to measure water table conditions under natural conditions remote from pumping stations. Wells are monitored with on-site recording devices and are downloaded during monthly site visits. The data obtained enable ISWS staff to assess short and long-term trends in water table levels to enhance our understanding of the impacts and extents of phenomena such as droughts and floods in Illinois. In particular, these data become important to monitor the lingering effects of periods of deficit precipitation on the shallow groundwater resources of the state.
Data from two small, long-term regional precipitation networks in Illinois are included within the Illinois Water and Climate Summary as addenda. The Cook County Precipitation Network is a 25-site weighing bucket raingage array operated by the ISWS for the USACE since 1989. The network is located in the Lake Michigan and Des Plaines River watersheds of Cook County to provide accurate precipitation measurements for modeling storm runoff, a crucial parameter used to compute the amount of water diverted from Lake Michigan.
The Imperial Valley Precipitation Network is a 20-site weighing-bucket raingage array operated by the ISWS for the Imperial Valley Water Authority since 1992. The network is located in Mason and Tazewell Counties of Illinois within the most heavily irrigated region of the state. Soils here are thick sand and gravel deposits associated with the confluence of two major ancient river valleys, the Mississippi and the Mahomet-Teays. The precipitation data helps to determine the rate of groundwater drawdown in dry periods and during the growing season, and the rate at which the aquifier subsequently recharges.