## Snyder Unit Hydrograph (1938)

**t**

_{l}=C_{t}(LL_{c})^{0.3}where,

t

_{l}=basin lag time (hr)

C

_{t}=coefficient for representating differences in types and locations of streams, generally ranging from 1.8 to 2.2.(although several other values outside this range are documented).Steeper slopes tend to have lower values.

L=length of the main channel from the outlet to the watershed divide. (Note here that the length is upto the watershed divide and not where the stream begins)- mile.

L

_{c}=length of the main channel from the outlet to the center of gravity of the basin. (mile)

The peak discharge of the unit hydrograph is given as:

**Q**Where,

_{p}(cfs)=640C_{p}A/t_{l}A=drainage area in square miles.

C

_{p}=storage coefficient from 0.4 to 0.8, can be obtained from standard texts (inversely related to C

_{t}).

The time base of the unit hydrograph is (days):

**t**

_{b}=3 + (t_{l}/8),Note:After plotting unit hydrograph, you should always check if the area under the unit hydrograph is 1.0 and the time base should be adjusted, if necessary, until the area under the unit hydrograph is 1.0.

The duration of excess rainfall, is (hr):

**D = t**

_{l}/5.5.If the unit hydrograph is desired for duration other than above, use following relation for adjusting lag time:

**t**

_{la}=t_{l}+0.25(D_{R}-D)Use this adjusted lag time in equation for computing peak flow, Q

_{p}.

The other ordinates useful for shaping hydrograph are the widths at 50% and 75% of the peak flow values:

**W**

_{50}=770(Q_{p}/A)^{-1.08}**W**

_{75}=440(Q_{p}/A)^{-1.08}**Following are the steps that you may find useful for constructing snyder UH:**

1. Compute the lag time t

_{l}for given C

_{t}and C

_{p}.

2. Compute the peak flow rate Q

_{p}.

3. Find time base t

_{b}. For small watersheds, time base can be estimated by multiplying time to the peak by 3 to 5.

4. Find duration of rainfall D.

5. Use W

_{50}and W

_{75}ordinates for shaping the hydrograph at 50% and 75% of the peak flows. W

_{50}and W

_{75}are located at 1/3 before Q

_{p}and 2/3 after Q

_{p}.

6. Ensure that the area under the constructed unit hydrograph is 1.0.