Frequency Analysis (Low Flow Conditions)

[waterguy]

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 ?

 

[n3rei]

I would try using a spread sheet program to analyze the data. This could offer some insight into what is actually going on. Plot curves, etc. and from those results you could fine tune the analyzing of this type of data. Look at the how the rainfall data available from the weather service is formatted for daily rainfall and substitute actual stream gauge data to come up with an average 7 day low flow and associated 10 year variable.

 

[RDWCE]

WELL THIS HAS BEEN UP AWHILE SO IM SURE YOU HAVE FIGURED OUT THE ANSWER TOTHIS PROBLEM. HOWERVER IF YOU HAVE FURTHER QUESTIONS, A GOOD REFERENCE WOULD BE HYDROLOGY AND HYDRAULIC SYSTEMS, BY RAM S. GUPTA. THERE IS A SECTION TO PREFORMING LOW FLOOD FREQUENCY CURVE ANALYSIS BY THE EMPIRICAL AND ANALYTICAL METHODS.