Hydrographs and Runoff Prediction

Hydrograph

•rate of runoff vs. time for a given point

•measures Stream discharge

lag to peak time

• Time difference between the center of mass of rainfall and the peak rainfall

• lag= rain, peak= the streams response

• The hydrograph will peak at the end of the rainfall

  • Then a rapid decline

  • Recessional limb shows the arrival of the remaining water on surface

Stream Response

• amount of stream flow per unit of rainfall

Stream response varies depending on

• Distance from rainfall

  • Headwater vs. downstream

  • watershed size

  • watershed properties( landuse, geology, soils, etc.)

Headwaters

where the watershed starts

• lag time is shorter here

• peak higher at the edges

• less sinuous

• coarser materials

Downstream

• lag length

• rising limb takes longer time to rise and less water

• more curvy

• finer material

Why look at a short period of time

• show immediate changes

• more detail

• but harder to make generalizations

Why look at a longer period of time such as one yr

• can observe seasonality changes

• get a range of stream flows

why look at longer longer period ( 10yrs)

• create data sets

• can see seasonality trends

More data gets you 

more generalizations

flashy stream in a hydrograph

• up and down very rapidly in precipitation

Urban areas runoff

• flashy

• quick response

Forested runoff

• peak is not as high but longer

• recessional peak gradually recedes

The more infiltration opportunities the more

muted the response will be

Less infiltration means

more flashy

Type 0 hydrograph

no response, all rain absorbed by soil

Type 1: hydrograph

• All rain infiltrates and reemerges as baseflow

Type 2: hydrograph

• almost all water runs off, no change in baseflow

Type 3: hydrograph

overland flow and groundwater recharge (increased baseflow)

When finding Surface Runoff it is important to know at least one of the following

• Depth or volume of runoff

• Peak runoff rate

• Storm hydrograph

Curve Number Method

• A way to predict runoff depth and volume

Most common method in the US

• Combines the amount of precipitation, losses to infiltration, and surface depression storage

• it is a function of the ability of soil to infiltrate water, land use, and antecedent moisture conditions

  • Determined from the hydrologic soil group and land use

Hydrologic Soil Group A

high infiltration rates even when thoroughly wetted; sands

Hydrologic Soil Group B

moderate infiltration rates when thoroughly wetted.

Hydrologic Soil Group C

sandy clay loam, low infiltraion rate

Hydrologic Soil Group D

clay, low very low infiltration

Curve Number Method formula

Q = (P − 0.2S)² / (P + 0.8S)

what does Q represent in the CN method 

runoff (excess rainfall) in inches

what does P represent in the CN method 

rainfall depth in inches

what does S represent in the CN method 

storage parameter