National Weather Service Training Center

Basic Hydrology Concepts

Section VII

Steps in Forecast Preparation

Lesson Objective

The objective of this lesson is to describe the steps involved in producing a hydrologic forecast for a specific point.

Hydrologic Forecast Models

Forecast models simulate the hydrology for a river basin or reach.  Consider a drainage area shown in the Alphabet River in the figure below.  The objective of a forecast model is to make a forecast for the forecast point at headwater Gage A and downstream forecast point B.

The following are given:

Drainage Basin Map


API Diagram

The hydrologic computations to make a forecast include:

· Estimate or predict flow at Gage A.

· Route that water downstream to Gage B.

· Compute the flow from the local area for Gage B.

· Add the flow from the local area for Gage B to the routed flow to compute a storm hydrograph.

The local drainage for Gage B is that area below Gage A that drains to Gage B. Local drainage may be considerable if it includes any major tributaries.

Forecast Needs

Before creating the forecast, gather the following hydrologic information for the area:

·        API coaxial relationship for Headwater Basin A

·        Unit hydrograph (UHG) for Headwater Basin A

·        Stage-discharge relationship or rating curve for Gage A

·        Routing coefficients to route the water from Gage A to Gage B

·        API coaxial relationship for the local drainage for Gage B

·        UHG for the local drainage for Gage B

·        Stage-discharge relationship or rating curve for Gage B

·        Base flow in the river at both gages A and B

Steps in Computing Forecasts

To make a forecast at Gages A and B, complete the following steps:

  1. Using the API Model and based on rainfall, either from ground truth, radar, or both, estimate runoff for the headwater basin above Gage A for a particular time period or periods.
  2. Using the unit hydrograph and based on runoff from the API model (step 1), determine the total surface runoff hydrograph from the surface runoff for each time.
  3. Calculate the Total Discharge Hydrograph; compute the ordinates of the storm hydrograph by adding the base flow to the ordinates from the UHG operation (step 2).
  4. Calculate the Discharge/Stage Conversion; convert discharge (step 3) to stage to develop a stage hydrograph for Gage A.

Note: All of the computations for a single headwater point are complete, and one can make a forecast for Gage A.  (In this exercise you are not expected to route the water to Gage B nor will you be given the parameters OR steps to route the water.)

Note: Use only steps (1 - 4) for the exercise presented here.

  1. Routing -- Route the storm hydrograph from Gage A (step 3) to Gage B.
  2. Using the API Model and based on rainfall, (either from ground truth or from radar) estimate the surface runoff for the local drainage for Gage B.
  3. Using the Unit Hydrograph and based on the runoff from the API model (step 6), determine the total surface runoff hydrograph from the surface runoff from the local drainage for Gage B.
  4. Determine the Storm Hydrograph; compute the ordinates of the storm hydrograph by adding the base flow for the local drainage for Gage B, the ordinates of the UHG operation for the local drainage for Gage B (step 7), and the routed flows (step 5).
  5. Calculate the Discharge/Stage Conversion -- Convert discharge (step 8) to stage to develop a stage hydrograph for Gage B.

This process produces forecasted hydrographs of discharge and stage at Gages A and B.  The RFC hydrologists will use their knowledge of hydrologic models, hydrologic conditions, and rainfall patterns to adjust forecast hydrographs for Gages A and B and arrive at a forecast for the sites.

River forecast models used by RFCs employ the above computational techniques.  When the hydrology warrants, RFCs will replace these simple techniques with more complicated ones.  However, the general forecast procedure remains the same.  There are some local applications programs available on personal computers to model parts of the hydrologic cycle.  These applications usually employ manual data input and run very quickly at a WFO to support flash flood and flood forecast operations.

Hydrologic Exercise

Given:
      Basin Map
      API Figure
      Rating Curve

Find:
      Stage
      Discharge
      Time to Crest

Below is a sample hydrologic forecast.  In the exercise that follows this example, you will have an opportunity to prepare a hydrologic forecast for Gage A, but will not be expected to route the water from Basin A to Basin B.  The parameters to route the water from A to B are not given.

Sample Problem

1. It is 1200 UTC on March 1, and the Alphabet River basin has received heavy rainfall over the previous 24 hours.  The average rainfall in inches for the basin is in the following table:

Headwater Drainage for A

12 – 18Z

18 – 00Z

00 – 06Z

06 – 12Z

1.00

0.50

2.00

2.50

Assume the API coaxial rainfall/runoff relationship is good for Basin A in the Alphabet River .  Assume the soil is moderately moist with an API value of 1.5 for the headwater drainage A.

Based on previous work by a hydrologist, the ordinates in cfs of the six-hour unit hydrograph for each basin are:

Unit Hydrograph for Basin A

Time (hours)

1

2

3

4

5

6

7

Flow (cfs)

200

2000

1000

400

200

100

0

Assume the following:

Challenge: Determine the forecasted stage and discharge hydrograph at Gage A.  When is gage A forecasted to crest?

The calculations for each step as described above are in the sample problem below.  The resulting forecasted discharge and stage hydrographs are given.  The forecasted crest at Gage A would be about 23 feet occurring between 1200 UTC and 1800 UTC on March 1, or the morning of March 1.

Work through the answer until you feel you know how the forecast was prepared.

Answer to Sample Problem

BASIN A

Feb 28

Mar 1

12-18Z

18-00Z

00-06Z

06-12Z

Step 1

6 hr. rain (in)

1.00

0.50

2.00

2.50

6 hr runoff (in)

0.60

0.20

1.30

2.80

Computations for discharge at Gage A

Step 2 - Discharge from surface runoff

Feb 28

Mar 1

Mar 2

Mar 3

Mar 4

12Z

18Z

00z

06z

12z

18z

00Z

06Z

12Z

18Z

00Z

06Z

12Z

18Z

00Z

RO from 12-18Z (cfs)

0

120

1200

600

240

120

60

0

RO from 18-00Z (cfs)

0

40

400

200

80

40

20

0

RO from 00-06Z (cfs)

0

260

2600

1300

520

260

130

0

RO from 06-12Z (cfs)

0

360

3600

1800

720

360

180

0

Baseflow (cfs)

50

50

50

50

50

50

50

50

50

50

50

50

50

50

50

Step 3 Total flow at A (cfs)

50

170

1290

1310

3450

5150

2470

1050

540

230

50

50

50

50

50

Step 4 Stage at Gage A (ft)

7.5

8.0

15.8

16.0

21.0

23.0

19.0

15.5

12.0

9.5

7.5

7.5

7.5

7.5

7.5

Final Summary Exercise

Following are the six-hour basin average rainfall totals for headwater drainage for Gage A.

Headwater Drainage for Basin A

28 Feb – 1 Mar

12 – 18Z

18 – 00Z

00 -06Z

06 -12Z

Flow (cfs)

2.00

2.50

0.50

0.00

Using the hydrologic parameters in Sample Problem determine the time of crest, crest stage, and flow for Gage A.

You are not expected to know how to route the water from Gage A to Gage B, nor are the parameters given to route the water.

Exercise Worksheet

BASIN A – fill in values for the shaded blocks

Feb 28

Mar 1

12-18Z

18-00Z

00-06Z

06-12Z

Step 1

6 hr. rain (in)

2.00

2.50

0.50

0.00

6 hr runoff (in)

Computations for discharge at Gage A

Step 2 - Discharge from surface runoff

Feb 28

Mar 1

Mar 2

Mar 3

Mar 4

12Z

18Z

00z

06z

12z

18z

00Z

06Z

12Z

18Z

00Z

06Z

12Z

18Z

00Z

RO from 12-18Z (cfs)

RO from 18-00Z (cfs)

RO from 00-06Z (cfs)

RO from 06-12Z (cfs)

Baseflow (cfs)

Step 3 Total flow at A (cfs)

Step 4 Stage at Gage A (ft)

Answer for Exercise Worksheet

BASIN A

    Feb 28   Mar 1  
    12-18Z 18-00Z 00-06Z 06-12Z
Step 1 6 hr. rain (in) 2.00 2.50 0.50 0.00
  6 hr run off (in) 1.30 1.80 0.20 0.00

 

Computations for discharge at Gage A

Step 2 - Discharge from surface runoff

Feb 28

Mar 1

Mar 2

Mar 3

Mar 4

12Z

18Z

00z

06z

12z

18z

00Z

06Z

12Z

18Z

00Z

06Z

12Z

18Z

00Z

RO from 12-18Z (cfs)

0

260

2600

1300

520

260

130

0

RO from 18-00Z (cfs)

0

360

3600

1800

720

360

180

0

RO from 00-06Z (cfs)

0

40

400

200

80

40

20

0

RO from 06-12Z (cfs)

0

0

0

0

0

0

0

0

Baseflow (cfs)

50

50

50

50

50

50

50

50

50

50

50

50

50

50

50

Step 3 Total flow at A (cfs)

50

310

3010

4990

2770

1230

620

270

70

50

50

50

50

50

50

Step 4 Stage at Gage A (ft)

7.5

10.0

20.1

22.8

19.6

15.8

12.3

9.8

8.0

7.5

7.5

7.5

7.5

7.5

7.5

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Updated 07/18/07