Lag Times and Peak Coefficients for Rural Watersheds in Kansas
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Lag Times and Peak Coefficients for Rural Watersheds in Kansas

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      Research Paper
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      Kansas
    • Abstract:
      Lag time is an essential input to the most common synthetic unit-hydrograph models. The lag time for an ungaged stream must be estimated from the physical characteristics of the stream and its watershed. In this study, a lag-time formula for small rural watersheds in Kansas was developed from gaging data. The database consisted of approximately a decade of 15-minute-interval rainfall and streamflow data for 19 rural watersheds with drainage areas from 2 sq km to 36 sq km. Lag times for 200 significant events were determined and the average lag time for each watershed was estimated. The average lag time was related to the physical characteristics of the stream and watershed by stepwise multiple regression. The recommended formula for the lag times of small rural watersheds in Kansas is "T sub lag = 0.086 times (L/square root of S) to the 0.64 power" for T sub lag in hours, L in km and S in m/m. The variable L is the total length of the main channel, extended to the drainage divide. The variable S is the elevation difference between two points on the channel, located 10% and 85% of the channel length from the outlet, divided by the length of channel between the two points (0.75 L). This formula has a standard error of estimate of approximately 24%. It is applicable to watersheds with drainage areas up to 50 sq km. The peak coefficients for the unit hydrographs of the gaged watersheds range from 0.46 to 0.77, with a mean value of 0.62 and a standard deviation of 0.10. The peak coefficient is not correlated significantly with any of the watershed characteristics. A peak coefficient of 0.62 is recommended as input to the Snyder unit hydrograph model for ungaged rural watersheds.
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