# Presentation: 400356

## BEGIN

- Precipitation as the Input

## Some Huge Rainfalls

## Precipitation As Input

- Precipitation is generally “pre-processed”
- Uniform in space and time – never!
- Gages - Recording & non-recording
- Radar
- Satellite Derived
- QPF

## The Basic Process....

**Excess Precip. Model**

**Excess Precip****.**

**Excess Precip****.**

**Runoff Hydrograph**

**Runoff Hydrograph**

**Stream and/or Reservoir “Routing”**

**Downstream Hydrograph**

**Basin “Routing” UHG Methods**

- Necessary for a single basin

**Focus on Precipitation**

## From A Basin View

**Excess Precip. Model**

**Excess Precip.**

**Basin “Routing”**
**Unit Hydrograph**

**Runoff Hydrograph**

**Stream “Routing”**

- Excess precip. is uniformly distributed!

##     Precipitation

- ... primary "input" for the hydrologic cycle (or hydrologic budget).
- ... The patterns of the precipitation are affected by large scale global patterns, mesoscale patterns, "regional" patterns, and micro-climates.
- ... In addition to the quantity of precipitation, the spatial and temporal distributions of the precipitation have considerable effects on the hydrologic response.

## Precipitation

- ... In lumped models, the precipitation is input in the form of average values over the basin.  These average values are often referred to as mean aerial precipitation (MAP) values.
- ... MAP's are estimated either from 1) precipitation gage data or 2) NEXRAD precipitation fields (MAPX).

## Precipitation (cont.)

- ... If precipitation gage data is used, then the MAP's are usually calculated by a weighting scheme.
- ... a gage (or set of gages) has influence over an area and the amount of rain having been recorded at a particular gage (or set of gages) is assigned to an area.
- ... Thiessen, isohyetal, and the inverse-distance squared are some of the more popular methods.

## Precipitation Issues for the Hydrologist

- Characteristics of precipitation in or on my basin(s)!
- Quantity – How much are we getting?
- Space – Where will it fall?
- Time – When will it fall (and where)?
- Integrity of the Data – Is this data valid?

## Characteristics

- Convective, Frontal, Orographic, etc...

## Convectional Storms....

- Thunderstorms are the classic example.
- Warm moist air is rapidly lifted - making it unstable.
- As the air lifts it cools and precipitation forms.
- As the precipitation falls - it cools the air
- This is why you may feel very cool bursts of air during those hot summer days when a thunderstorm kicks up.

## Urban Areas & Thunderstorms...

- It has been reported that urban areas may contribute to the development of thunderstorms due to the presence of a heat source and the typically darker areas.

## Orographic Effects.....

- Terrain can also cause lifting - which is a major component in the precipitation mechanism.
- The mountains provide a lifting mechanism for the warm *advecting* moist air.

## Orographic effects

## Local Effects – e.g. the Great Lakes...

- Do lake effect events alter the volume of Lake Superior?

## Ice....

- Hail, Rime, Sleet, and Graupel
- Very difficult to measure
- Antifreeze or heated gages

## Snow, A Few Brief Points .....

- Snow or snowfall reaches the ground to form the *snowpack*. Snowpack is generally reported as snow depth.
- We must also consider the *snow water equivalent* or *SWE* - WHY?

- NOAA Photo Library

## SWE....

- SWE is reported as a ratio - i.e. 10:1
- Meaning 10 inches of snow equal 1 inch of water - when melted.
- We also report this as density.
- 10:1 would be a density of 10% or 0.1.
- When is the snowfall most dense and least dense.
- When is the pack most or least dense?

- NOAA Photo Library

## Measuring Snow and SWE...

- Snow gages
- Snow tubes
- Radar - VERY difficult!!  -  WHY?????

## Quantity

- Measuring the Precipitation

## Rainfall.....

- Rainfall varies in both space and time
- This is referred to as spatial and temporal variability.
- Rainfall amounts vary considerably

## Measuring Precipitation....

- Generally use rain gages
- Measure depth
- What are the problems with rain gages?
- Point coverage...
- Interference - wind, trees, etc...
- How many others can you name?
- Radar

## Standard Gage(non-recording)

## Fisher & Porter Tipping Bucket 

## Universal

## Precipitation Gage Networks

- A system of gages
- Design Issues:
- density
- location
- quality (of data)
- collection & transmission
- processing, filing, managing

## Factors Affecting Density

- Purpose of Network – Desired Quality/Precision/Accuracy
- Finances – Installation and UPKEEP!
- Nature of Precipitation – rain, rain + snow, orographic, convective, etc..
- Accessibility
- to name a few.....

## Network Densities

- Many studies
- Brakensiek et al., 1979 – Brakensiek, D. L., H. B. Osborn, and W. J. Rawls, cooridnators. 1979. Field Manual for research in Agricultural Hydrology. USDA, Agricultural Handbook, 224, 550 pp, illustrated.

## Spatial Characteristics

- Where will it fall and
- how will I use it?

## Precipitation in Models

- ... In lumped models, the precipitation is input in the form of average values over the basin.  These average values are often referred to as mean aerial precipitation (MAP) values.
- ... MAP's are estimated either from:
- 1) precipitation gage data or
- 2) NEXRAD precipitation fields (MAPX).

## Precipitation (cont.)

- ... The MAP's are usually calculated by a weighting scheme.
- ... a gage (or set of gages) has influence over an area and the amount of rain having been recorded at a particular gage (or set of gages) is assigned to an area.
- ... Thiessen, isohyetal, and the inverse-distance squared are some of the more popular methods.

## Calculating Areal Averages....

- Arithmetic
- Isohyetal
- Theissen
- Inverse Distance

## Arithmetic....

## Thiessen

**Thiessen method** **is a method for areally weighting rainfall through graphical means.**

## Isohyetal

**Isohyetal method** **is a method for areally weighting rainfall using contours of equal rainfall (isohyets).**

## Inverse-Distance Squared

- Used to compute average precipitation at any point based on nearby gages.  The weight of the nearby gages is dependant on the distance from the point to each of the nearby gages.

- Gage A

- Gage B

- Gage C

- dA

- dC

- dB

## Radar Precip. as Input

- Radar gives a good picture of where it is raining - may indicate how to adjust the Unit Hydrograph for moving and partial area storms!
- May also give good estimate of how much, BUT
- Will differ from gages in total basin average.
- Historical records are based on gages!
- This makes calibration rather difficult.

## WSR-88D

- Weather Surveillance Radars - 1988 Doppler
- 1st WSR-88D sites installed in 1991
- At the present time, there are more than 160 radars in place.
- Should optimally provide coverage for a large percentage of the United States.
- Optimally used because under many circumstances, the useful range of the radars varies considerably.

## Locations

## NEXRAD

**Nexrad** **is a method of areally weighting rainfall using satellite imaging of the intensity of the rain during a storm.**

## Temporal

- When will fall and where?

## Temporal Distributions

- Gages record data at intervals - 10 min., 15 min., 1 hour, 24 hour, etc....
- Models use the data at 1-hour, 6-hour, etc...
- Must either aggregate or disaggregate precipitation amounts....
- i.e. Combine 1 hour values into a 6-hour value...  Not a problem!
- Or... Break a 24-hour value into 6 hour values... Much more difficult!

## Temporal Disaggregation

**24-hour gage**
**3.6 inches total**

**1 hour gage with 2.2 total inches and the following distribution:**

**Distribute the 3.6 inches using the breakdown of the hourly gage**

## Intensity, Duration, & Frequency

- Intensity, duration, & frequency
- Duration - the length of time over which the rain falls.
- Intensity - the rate at which the rain falls or the amount / duration.
- Frequency - the frequency of occurrence - i.e. How rare is this storm? - We’ll get back to this.....
- General relationships:
- the greater the duration, the greater the amount
- the greater the duration, the lower the intensity
- the more frequent the storm, the the shorter the duration, and;
- the more frequent the storm, the less the intensity

## Let’s Look at at an Example

- First...
- Let’s compute the Rainfall/Runoff ratios for the Little J at Spruce Creek.

## The Situation....

## 1996 Totals

## Some Issues

- How to handle the missing data
- Which basin averaging technique to use.
- Gage Average
- Thiessen
- Isohyetal
- Inverse Distance Weighting

## Missing Data

- Filling in missing data is a major issue.
- In this case, we are filling it in space – not time.
- There are many ways to fill in this data:
- Averaging nearby stations
- Weighting (averaging is a special case)
- Isohyetal

## The Missing Data

- Averaging = 57.06 inches
- Weighting would depend on local knowledge and would require creation of historical relationships between all of the local gages.
- Isohyetal would imply that the value is closer to 62 to 63 inches – *see next slide*
- For this exercise we will use 60 inches.

## Isohyetal

## Now Lets’ Find Basin Average

- Arithmetic Averaging
- Thiessen
- Isohyetal
- IDW

## Gage Average

- I used Excel to average the gages.  The small worksheet is shown at the right ->

## Thiessen Polygons

## Thiessen Wts. (%)

## Combine % w/ Totals

- Replace w/ 60.0

## Thiessen - Final Computations

## Isohyetal Approach

## Isohyetal Areas

## Combine % and Precip. Values

## Inverse Distance Weighting

- Need coordinates of each gage
- Need coordinates of basin centroid or point of interest.
- Then Calculate gage weights:

## Measure 4 Distances

## The Computations

## In Summary

## What if this had been a 6-hour storm instead of yearly totals?

- What would we do?

## Use Thiessen Weights

- Just average each incremental contribution using the pre-calculated Thiessen weights!

## Area-Depth (amount) Relationship....

- Indeed we should get less basin average precipitation than for a single gage......

## Use this Chart

- A gage in the middle of a 200 square mile basin records 5 inches of rain in 3 hours.  Estimate the basin average rainfall:
- For 200 square miles, the basin average is ~ 80% of the gage total or 0.8 * 5 = 4 inches!

## Temporal Distributions

- Understanding *Temporal Distributions* is very important,as this greatly affects runoff timing and volumes.

## Temporal Distributions

- Precipitation is a continuous process.
- Intensities vary depending on amount and duration
- Gages record data at intervals - 10 min., 15 min., 1 hour, 24 hour, etc....
- Models may use the data at 1-hour, 6-hour, etc...
- Must either aggregate or disaggregate precipitation amounts....
- i.e. Combine 1 hour values into a 6-hour value...  Not a problem! Or... Break a 24-hour value into 6 hour values... Much more difficult!

## Understanding Intensities

## Intensities & Durations

- A 5-minute recording gage
- Recorded a storm for 40 minutes
- Calculate:
- Total Rainfall
- Cumulative Rainfall Curve
- Max. 5,10, & 30 minute intensities
- The average intensity

## The Data

## Solutions

- Total rainfall – simply sum the precipitation values: 56.16 mm or 2.21 inches

- Cumulative data is shown and plotted below:

## Solutions, cont....

- The maximum 5 minute intensity was 15.6 mm between 10-15 minutes at 187.2 mm/hr or 7.3 inches/hr. This is illustrated in the data below:

## Solutions, cont...

- The maximum 10 minute intensity was found by aggregating sequential 5-minute periods.  The maximum 10-minute intensity is illustrated below, between 10-20 minutes with 22.56 mm or 135.36 mm/hr or 5.29 inches/hr.

## Solutions, cont...

- The maximum 30 minute intensity was found by aggregating sequential 5-minute periods.  The maximum 30-minute intensity is illustrated below, between 5-35 minutes with 52.8 mm or 105.6 mm/hr or 4.125 inches/hr.

## Solutions, cont...

- The total rainfall was 56.16 mm over a duration of 40 minutes for an average intensity of 84.24 mm/hr or 3.29 inches/hr.  In summary:

## Temporal Aggregation

- Simply aggregate values to desired periods.

## The Previous 40-minute Storm

- Recombine into 10, 20, and 40 minute hyetographs.
- What are the issues here?

## The Graphs

## Temporal Disaggregation

**Basin  gage records**
**66.2 mm total**

**5-minute gage with 56.16 mm total precip. and the following distribution:**

**Distribute the 66.2 mm using the breakdown of the 5 minute gage**

## The Solution

- We made a very large assumption about the 66.2 mm total duration – what was it ?

## END

- Precipitation as the Input