Snyder’s Unit Hydrograph

To Plot snyder’s unit hydrograph we need value of basin lag (tpr), time

base(tb) , width of hydrograph at 50% discharge(W50) , width of

hydrograph at 75% discharge(W75)


The peak time is distance from centroid of effective rainfall to peak

of the discharge.

\LARGE t_p=5.5t_r \\ else \\ t_p=t_{pr}+\frac{t_r-t_R}{4}  \\ t_p=C_1C_t(LL_c)^{0.3}

where,

tp = Peaking time in standard condition

tpr = Basin lag in general condition

C1=0.75

Ct=Watershed factor

\LARGE q_p=\frac{C_2C_p}{t_P}

where,

qp is peak discharge per unit catchment area

Cp=Watershed factor

C2=2.75

\LARGE q_{pr}=\frac{q_pt_p}{t_{pr}}

C3=5.56

The width of hydrograph at 50% of peak discharge and 75% of peak discharge is given as:

\LARGE W_{50}=2.14q_{pr}^{-1.08}
\LARGE W_{75}=1.22q_{pr}^{-1.08}

The time base of hydrograph is:

\LARGE t_b=\frac{C_3}{q_{pr}}

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