Frequency Analysis (Low Flow Conditions)
Frequency Analysis (Low Flow Conditions)
(OP)
Does anybody know of any good resources on the net or have any personel experience with low flow analysis (ie. 7Q20 flow etc...) ?
We are involved in a project that requires the analysis of time series data (daily stream flow data) and the determination of the 7Q10 low flows (lowest average 7 day flow with a 10 year recurrence) as well as some other low flow combinations. During some intial analysis I compared my numbers with those of a sample analysis from a different project (see below) and noticed that my flow values were increasing, rather then decreasing as shown in the sample data.
Low Flow Sample Analysis Results
7Q2 = 0.059 m^3/s
7Q5 = 0.039 m^3/s
7Q10 = 0.032 m^3/s
7Q20 = 0.027 m^3/s
I am more used to dealing with peak flow data where the flow increases as the return period increases. Based on the sample data provided above, am I correct in assuming that the situation is reversed for the case of low flows ? For example, in peak flow analysis, a 20 year return period flow has a 5% (1/20) probability of being exceeded. If I am doing a low flow analysis, am I looking for that flow rate that has a 95% (100 - 5) probability of being exceeded then ?
I notice that if one were to do a manual peak flow analysis the flow data is arranged in descending (high to low) order, while in a low flow analysis it is in ascending order (low to high). I suspect that the program that is being used to do the frequency analysis assumes that it is performing a peak flow analysis and arranging the data as such.
Any thoughts ?
We are involved in a project that requires the analysis of time series data (daily stream flow data) and the determination of the 7Q10 low flows (lowest average 7 day flow with a 10 year recurrence) as well as some other low flow combinations. During some intial analysis I compared my numbers with those of a sample analysis from a different project (see below) and noticed that my flow values were increasing, rather then decreasing as shown in the sample data.
Low Flow Sample Analysis Results
7Q2 = 0.059 m^3/s
7Q5 = 0.039 m^3/s
7Q10 = 0.032 m^3/s
7Q20 = 0.027 m^3/s
I am more used to dealing with peak flow data where the flow increases as the return period increases. Based on the sample data provided above, am I correct in assuming that the situation is reversed for the case of low flows ? For example, in peak flow analysis, a 20 year return period flow has a 5% (1/20) probability of being exceeded. If I am doing a low flow analysis, am I looking for that flow rate that has a 95% (100 - 5) probability of being exceeded then ?
I notice that if one were to do a manual peak flow analysis the flow data is arranged in descending (high to low) order, while in a low flow analysis it is in ascending order (low to high). I suspect that the program that is being used to do the frequency analysis assumes that it is performing a peak flow analysis and arranging the data as such.
Any thoughts ?





RE: Frequency Analysis (Low Flow Conditions)
RE: Frequency Analysis (Low Flow Conditions)