HYDROGRAPHS Water Budget Rainfall intensity i precipitation Infiltration

HYDROGRAPHS

Water Budget • Rainfall intensity (i) – precipitation • Infiltration ( ) – losses into the ground • Transpiration (T) – losses on trees • Rainfall excess )R) – runoff (rainfall – losses)

Hydrograph • Also called Direct Runoff Hydrograph (DRH) • Stream flows with respect to time • When would the flow in a stream change? • Response of catchment to a rainfall

Unit Hydrographs • Area under hydrograph is runoff volume [Trapezoidal method] • Divide runoff volume by the watershed area is runoff depth • Divide runoff hydrograph by runoff depth to get unit hydrograph • Unit hydrograph (UH) is hydrograph for 1 cm excess

Trapezoidal Method • Divide the area into small trapezoids and get an approximate area h= f(x)

Unit Hydrograph Properties • Depth of runoff = 1 cm or 1 inch (depends on the units of rainfall measurement) • Each hydrograph is for specific duration • Duration can be changed using S-hydrograph method • Runoff volume is always same regardless of the duration • Catchment area is constant • Runoff depth is constant

TRANSFORMATION OF UH TO DRH • Rainfall information is available as hyetograph • Subtract losses to get rainfall excess • Duration of rainfall should match the duration of the unit hydrograph • For example: a 30 min UH can only be used with rainfall information taken every 30 mins

TRANSFORMATION OF UH TO DRH • Let us assume that the Rainfall Excess for the any given Duration (Di) is Ri where “i” is the rainfall interval • For 1 st duration (D 1), multiply the co-ordinates of UH by the R 1 to get UH 1 • For 2 nd duration (D 2), multiply the co-ordinates of UH by R 2 to get UH 2 and so on… • To get the DRH, add UH 1 + UH 2 (lagged 1 duration) + UH 2 (lagged 2 durations) + UH 3 (lagged 3 durations)+ ……. .

EXAMPLE PROBLEM 50, 00 90 minute Unit Hydrograph Rainfall Hyetograph 0 -90 Rainfall excess (cm) (Ri) (D 1) 2. 75 (R 1) 90 -180 (D 2) 3. 25 (R 2) 180 -270 (D 3) 0. 75 (R 3) Flow (m 3/s) Time (mins) 40, 00 30, 00 20, 00 10, 00 0 90 180 270 360 450 540 630 720 810 900 990 1080 1170 1260 Time (mins)

For the first duration D 1 Rainfall starts at time = 0 120, 00 1 st duration 0 -90 minutes 110, 00 100, 00 90, 00 0 -90 mins = 2. 75 cm excess Flow (m 3/s) 80, 00 70, 00 60, 00 50, 00 40, 00 UH = 1 cm excess 30, 00 20, 00 10, 00 0 90 180 270 360 450 540 630 720 Time (mins) 810 900 990 1080 1170 1260

For the second duration D 2 Rainfall starts at time = 90 140, 00 130, 00 2 nd duration 90 -180 minutes 120, 00 110, 00 100, 00 Flow (m 3/s) 90 -180 mins = 3. 25 cm excess 90, 00 80, 00 70, 00 60, 00 50, 00 40, 00 UH = 1 cm excess 30, 00 20, 00 10, 00 0 90 180 270 360 450 540 630 720 810 900 990 1080 1170 1260 1350 Time (mins)

For the third duration D 3 Rainfall starts at time = 180 50 3 nd duration 180 -270 minutes 180 -270 mins = 0. 75 cm excess UH = 1 cm excess Flow (m 3/s) 40 30 20 10 0 0 90 180 270 360 450 540 630 720 810 900 990 1080 1170 1260 1350 1440 Time (mins)

For the third duration D 3 140, 00 Rainfall Hyetograph Combined Response 0 -270 minutes 130, 00 120, 00 0 -90 90 -180 110, 00 Rainfall excess (cm) (Ri) 100, 00 1 cm excess 90, 00 2. 75 3. 25 Flow (m 3/s) Time (mins) 2. 75 cm excess 80, 00 3. 25 cm excess 70, 00 0. 75 cm excess 60, 00 50, 00 40, 00 30, 00 180 -270 0. 75 20, 00 10, 00 0 90 180 270 360 450 540 630 720 810 900 990 1080 1170 1260 1350 1440 Time (mins)

300, 00 Direct Runoff Hydrograph 250, 00 1 cm excess Flow (m 3/s) 200, 00 2. 75 cm excess 3. 25 cm excess 150, 00 0. 75 cm excess DRH 100, 00 50, 00 0 90 180 270 360 450 540 630 720 810 900 990 1080 1170 1260 1350 1440 Time (mins) DRH is obtained by adding the hydrographs for 2. 75 cm excess, 3. 25 cm excess and 0. 75 cm excess