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Point Precipitation Measurement, Areal Estimates
And Relationships To Hydrologic Modeling
Precipitation Measurement
Liquid precipitation is traditionally measured using various types
of rain gages such as the non-recording cylindrical container type or the
recording weighing type, float type and tipping-bucket type. All of the
above gages measure precipitation at a point. Another method of measuring
precipitation is through the use of radar. The actual measurement taken
by the radar is of backscattered power of the echo returns. This return
power is used to calculate a reflectivity factor, Z. The Z reflectivity
factor is then used in an equation referred to as a "Z-R relationship"
to determine rainfall rate. The ABRFC produces NWS WSR-88D 1-hour radar
precipitation estimates for our area of responsibility on an approximate
4 by 4 km square grid resolution.
Hydrologic Requirement for Areal Estimates of Precipitation
The river forecasting models in use by the NWS require areal estimates of
precipitation. This is because the models simulate the rainfall-runoff process
on a drainage catchment area basis. Large river basins are sub-divided into
these smaller catchments or runoff zones. Runoff from excess precipitation
in each zone is used to calculate streamflow which is then used to produce
river stage forecasts.
Estimation of Areal Precipitation
A single point precipitation measurement is quite often not representative
of the volume of precipitation falling over a given catchment area. A dense
network of point measurements and/or radar estimates can provide a better
representation of the true volume over a given area. A network of precipitation
measurements can be converted to areal estimates using any of a number of
techniques (click here for summary) which include
the following:
1) Arithmetic Mean - This technique calculates
areal precipitation using the arithmetic mean of all the point or areal
measurements considered in the analysis.
2) Isohyetal Analysis - This is a graphical technique
which involves drawing estimated lines of equal rainfall over an area based
on point measurements. The magnitude and extent of the resultant rainfall
areas of coverage are then considered versus the area in question in order
to estimate the areal precipitation value.
3) Thiessen Polygon - This is another graphical
technique which calculates station weights based on the relative areas of
each measurement station in the Thiessen polygon network. The individual
weights are multiplied by the station observation and the values are summed
to obtain the areal average precipitation.
4) Distance Weighting/Gridded - This is another
station weighting technique. A grid of point estimates is made based on
a distance weighting scheme. Each observed point value is given a unique
weight for each grid point based on the distance from the grid point in
question. The grid point precipitation value is calculated based on the
sum of the individual station weight multiplied by observed station value.
Once the grid points have all been estimated they are summed and the sum
is divided by the number of grid points to obtain the areal average precipitation.
5) MAPX - This is a NWS-specific gridded technique.
Areal runoff zone precipitation estimates are made using the 4 x 4 km WSR-88D
1-hourly gridded precipitation estimates. The arithmetic mean calculation
technique is used to average the grid point estimates.
6) Index Stations - In some areas of the country (primarily mountainous
areas), pre-determined station weights based on climatology are used to
compute basin average precipitation.
Areal Precipitation Terminology As Used In ABRFC Hydrologic Modeling
MAP - Mean Areal Precipitation - Areal runoff zone precipitation estimate
normally based on point precipitation observations. The distance weighting
calculation technique is used. MAP is used as input to the river forecast
model on a routine basis.
MAPX - Radar Based Mean Areal Precipitation - Areal runoff zone precipitation
estimate based on the 4 x 4 km WSR-88D 1-hourly gridded precipitation estimates.
The arithmetic mean calculation technique is used to average the grid point
estimates. MAPX is used as input to the river forecast model on a routine
basis.
FMAP - Future Mean Areal Precipitation - Future or forecast areal runoff
zone precipitation estimate. The Weather Forecast Offices (WFOs) develop
precipitation forecasts based on input from sources which may include meteorlogical
model output, national guidance products, local forecast procedures and
individual forecaster experience. After weather analysis is complete, the
WFO forecaster uses a computer program to draw isohyets of forecast precipitation
and then the program performs an automated isohyetal analysis calculation
technique to convert to areal estimates. The forecast precipitation information
is generated for four 6-hour periods. The area of coverage is that of each
WFOs area of responsibility. The ABRFC Hydrometeorological Analysis and
Support (HAS) function mosaicks the input from the WFOs so as to cover the
entire ABRFC area of responsibility. The HAS function also coordinates any
required changes in the individual WFO QPF information. FMAP is used as
input to the river forecast model on a routine basis.
Various mean areal precipitation computation techniques yield the following
results.
1.63" = Distance Weighting
1.90" = Isohyetal Analysis
2.00" = MAPX (WSR 88-D estimate)
2.03" = Thiessen Polygon Weighting
2.21" = Arithmetic Mean of Point Values
( click any method for an example )
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