# 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