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PPT – Rainfall Measurement PowerPoint presentation | free to download - id: 109efb-OWViZ

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Rainfall Measurement

- Why do we need to measure rainfall?

Agriculture what to plant in certain areas,

where and when to plant, when to harvest

Horticulture/viticulture - how and when to

irrigate

Engineers - to design structures for runoff

control i.e. storm-water drains, bridges etc.

Scientists - hydrological modelling of catchments

Things to know about rainfall

- Where to get rainfall information
- Rainfall sites in the area of interest
- Seasonal rainfall trends
- Variability of rainfall across the state and

between towns - Area specific rainfall
- Site specific rainfall
- Methods of measuring rainfall
- Where to place rainfall gauges - depends on the

site - How many gauges to place in area - depends on the

experiment - What to do with the rainfall data

Farming Scenario What information do you need?

- Seasonal rainfall
- Long term average rainfall
- Risk of drought
- Chance of rain
- Southern Oscillation Index effect on rainfall

prediction

Site Specific Rainfall Information

Wagin annual rainfall distribution

Site Specific Rainfall Information

Broome annual rainfall distribution (note

change in distribution of rainfall and change in

scale

Site Specific Rainfall Information

Rosebery West coast of Tasmania (note change in

distribution of rainfall and change in scale

Site Specific Rainfall Information

Wagin variation in rainfall

Site Specific Rainfall Information

Wagin probability of rainfall

Comparison of rainfall sites

- What if you live between Wagin and Katanning?
- Can you use their rainfall data to predict

rainfall at your farm?

What if you need to know the rainfall in a

catchment?

- Measure it yourself.
- Type of rain gauges?
- Where to put gauges?
- How many gauges?
- How do you map it?

Methods of Measuring RainfallManual

- Often have a funnel opening into a cylinder

gauge. - Come in a variety of shapes and sizes
- Calculate the rainfall (in mm) by dividing the

volume of water collected by the area of the

opening of the cup. (The gauge marking often

accounts for this).

Methods of Measuring RainfallRemote

- Tipping bucket rain gauge -The bucket tips when

precipitation of 0.2 mm, 0.5 mm, 1.0 mm has been

collected. Each tip is recorded by a data logger.

- Weather Station - Records rainfall, but also

evaporation, air pressure, air temperature, wind

speed and wind direction (so can be used to

estimate evapo-transpiration) - Radar - Ground-based radar equipment can be used

to determine how much rain is falling and where

it is the heaviest.

Weather Station

Placement of Rain Gauges

- Gauges are affected by wind pattern, eddies,

trees and the gauge itself, therefore it is

important to have the gauge located and

positioned properly.

- 1m above ground level is standard -
- all gauges in a catchment should be the same

height - 2 to 4 times the distance away from an isolated

object (such as a tree or building) or in a

forest a clearing with the radius at least the

tree height or place the gauge at canopy level

Placement of Rain Gauges

- shielded to protect gauge in windy sites
- or if obstructions are numerous they will reduce

the wind-speed, turbulence and eddies.

Raingauge with wind guards

Placement of Rain Gauges

- For sloping ground the gauge should be placed

with the opening parallel to the ground - The rainfall catch volume (mm3) is then divided

by the opening area that the rain can enter

Number and Distribution of Gauges

- Need to consider
- size of area
- prevailing storm type
- form of precipitation
- topography
- aspect
- season

Distribution of Gauges

- The distribution of gauges should not be random.

- only fixed characteristics of areas can be

sampled randomly. Random events must be sampled

by a systematic arrangement of sampling points - Practical considerations of access and exposure

mean that some pragmatism is required in

designing a network. - It is useful to locate gauges so that isohyetal

maps can be drawn. Some gauges need to be near,

or outside the catchment boundary in order to

cover the catchment completely.

Number of Gauges

- Depends on Storm type
- Cyclonic storms (large areas, low intensities)

-small number of gauges may be O.K. - Convective storms (local, intense, uneven

distribution) -denser network needed. Convective

storms may have seasonal dominance -need to

consider this as well. - Orographic rainfall due to mountains (not fronts)

-may need denser network than flatter area.

Number of Gauges

- Based ONLY on area considerations the following

tabulation has been suggested

Methods of averaging rainfall data

- Arithmetic average
- Theissen polygons
- Isohyetal method
- Although, most of these calculations are done

with computer mapping programs, it is still

useful to understand these methods.

Thiessen method for Mapping Rainfall

This involves determining the area of influence

for each station,rather than assuming a

straight-line variation. It is easier than the

isohyetal method but less accurate

- Locate all rainfall stations on a base map and

record the rainfall amount. - Connect each station by straight lines with the

several nearest stations to form a series of

triangles. - Erect perpendicular bisectors on each of these

lines and extend them to the intersect with other

bisectors, thus forming a series of irregular

polygons

Thiessen method for Mapping Rainfall

Measure the fraction of the catchment area in

each polygon (calledthe Thiessen constant),

multiply by the rainfall catch at the station

within the polygon and sum to get the catchment

average.

Isoheyetal method for Mapping Rainfall

The most basic method of representing the spatial

distribution. This is generally the most

accurate method but is also the most laborious.

- Locate all rainfall stations on a base map and

record the rainfall amount. - Draw isohyets (lines of equal rainfall) by

proportioning the distances between adjacent

gauges according to differences in catch.

Isoheyetal method for Mapping Rainfall

- Then calculate the mean precipitation for the

area corresponding to each isohyet. - Calculate the fraction of catchment area under

each isohyet, multiply by the mean precipitation

for that area and sum to get the catchment

average.

Computer Mapping Methods

- Other mapping programs such as SURFER or GIS

program ARCVIEW can be used to map rainfall at

the different measurement locations. - Maps of rainfall can be produced by using the

statistical analysis packages that come with the

program. Statistical methods include nearest

neighbour, inverse distance weighting or kriging

(which uses variogram analysis). - The statistical methods produce a grid over the

specified area of a specific size. The influence

of the actual rainfall measurement is determined

for each grid node to produce a rainfall amount

at each node.

Methods of gridding or determining missing data

- Arithmetic average
- Normal ratio method
- Inverse distance squared

Methods of determining missing data

- In the examples station X is the station with

data missing - Arithmetic average - assign it to station X
- Normal ratio method -
- get observed rainfall at surrounding stations
- These are weighted by the ratio of the normal

annual rainfall at station X and normal annual

rainfall at that station. - where Px estimation of ppt at station x, Pi

ppt at Nx normal ppt at station X and Ni is

ppt at the ith surrounding station .

Methods of determining grid data

- In the example station X is the station with data

missing - Inverse distance squared
- The closer a station is to station X the greater

the weight assigned to that station's

precipitation. - The inverse of the squared distance between a

station and station X is used as a weighting

factor in determining the rainfall at station X.

Decisions based on rainfall data?

- You should know
- where to get rainfall information
- what rainfall information means
- Therefore you can
- determine best site for planting particular

species - make decisions for the coming season based on

rainfall averages and the SOI

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