Illinois Streamflow Assessment Model: Definitions, Illinois State Water Survey

Illinois Streamflow Assessment Model

Definitions of Each Streamflow Statistic

Average Flow Values
Description: The average annual flow estimated by ILSAM is the expected average flow over a long-term base period, typically in excess of 50 years. The average monthly flow values cover the same period, but represent only the portion of the daily flow values in that period that occur within each respective calendar month.
Statistics: Average Annual Flow (Qmean) and Average Monthly Flows
Annual Flow-Duration Values
Description: These values relate the streamflow amount to the percentage probability that the daily flow will be exceeded over a long-term base period without regard to the sequence of flow events. The 2 percent flow (Q2), for example, is the daily streamflow rate that is exceeded on exactly 2 percent of the days. The 1 percent flow (Q1) is necessarily a higher flow rate because it is exceeded less frequently.
Statistics: Q1, Q2, Q5, Q10, Q15, Q25, Q40, Q50, Q60, Q75, Q85, Q90, Q95, Q98, and Q99.
Monthly Flow-Duration Values
Description: Monthly flow duration values are defined in the same manner as the annual flow-duration values, except that they are determined using only those daily discharges that fall within a certain month of the year.
Statistics for each calendar month: Q2, Q10, Q25, Q50, Q75, Q90, and Q98.
Low Flows
Description: Each low-flow parameter is defined by a duration in consecutive days and a recurrence interval in years. A 7-day low flow for a given year is the lowest average flow that occurred within a 7-day consecutive period during that year. The 7-day, 10-year low flow is the 7-day low flow that occurs on average only once in 10 years. A 2-year low flow is the value expected to occur during an "average" year.
Low-Flow Durations: 1, 7, 15, 31, 61, and 91 days
Recurrence Intervals: 2, 10, 25, and 50 years
Drought Flows
Description: Drought flows are similar to low flows, except that the duration of the period is longer and is defined in months instead of days, and the average low flows are developed from average monthly streamflow values. Drought flows are not computed as an annual series, because a drought period typically encompasses multiple years.
Drought Flow Durations: 6, 9, 12, 18, 30, and 54 months
Recurrence Intervals: 10, 25, and 50 years

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