Transcript Precipitation - WordPress.com

Hydrology and Water Resources
RG744
October 2, 2013
RS & GISc, Institute of Space Technology
2
Precipitation
• Any form of solid and liquid water that falls from atmosphere to earth
surface
• Rain,
• drizzle,
• hail,
• dew,
• snow
Types of Precipitation
• The precipitation types can be categorized as
• Frontal Precipitation
• The formation of precipitation due to the convergence of two air
masses with contrasting temperature and densities
• Convective Precipitation
• Upward movement of warm air with low density than its surrounding
air rises till it reaches to a level where it has the temperature of its
environment. The temperature contrast causes convection and cools
adiabatically and clouds are made that burst into thunderstorm
• Convective storms are generally known as thunderstorms
• Orographic Precipitation
• Uplift of an air mass because of a topographic obstruction.
• Uplift also causes the cooling of the air mass. If enough cooling occurs
condensation can occur and form into orographic precipitation
Orographic Precipitation
• Cyclonic Precipitation
• Air mass that converges into a low pressure area moves up
due to difference in pressure and winds blow spirally
inward
• Precipitation due to Turbulent Ascent
• Condensation of air mass that moves up due to increased
turbulence and friction of earth surface after its travel over
ocean
Measurement of Precipitation
• Total amount (depth) over some time period
(hourly, daily, monthly, seasonally, or annually)
• Problems:
• Many records are not available for rural areas
• Discontinuous or short term
Methods of Measurement
• Rain gauges
• Radar (Radio Detection and Ranging)
• Weather Satellites
Rain Gauges
• Symon’s (non-recording) Type
• Weighing Bucket Type (mass curve)
• Tipping Bucket
• Siphon or Floating Gauge
• Storage Rain Gauge (for remote areas)
• Telemetring Rain Gauge (inaccessible places)
• Automatic Radio Reporting Gauge
Symon’s
Weighing bucket type
Tipping Bucket
Siphon Rain gauge
Selection of Rain Gauge Site?
• Open space
• Least obstructions
• Distance between gauge and the nearest object should
be at least twice the height of the object
• On leveled ground (slopes are not recommended)
• In hilly areas if level ground is not available, place at top
of the hill
• Shield from high wind
• If fence is provided then it should not be less than
twice its height
Adequacy of Rainfall Station
• A well distributed network is essential
• Statistical Analysis
• To give necessary average rainfall with certain % Error
• Number of stations required for a given area
• How to estimate? (Reference: Hydrology by Das &
Saikia)
World Meteorological Organization (WMO)
Recommendations
• Flat regions of temperate, Mediterranean and tropical
zones
• Ideal – 1 station for 600-900 sq.km.
• Acceptable – 1 station for 900-3,000 sq.km.
• Mountainous regions of temperate, Mediterranean and
tropical zones
• Ideal – 1 station for 100-250 sq.km.
• Acceptable – 1 station for 250-1,000 sq.km.
• Arid and polar zones
• Ideal – 1 station for 1,500-10,000 sq.km. Depending on the feasibility.
• 10% of rain gauge stations should be equipped with self recording rain gauges
Bureau of Indian Standards (BIS) Recommendations
• In plains
• 1 station for every 520sq.km.
• In regions with average elevation of 1,000m or
above from mean sea level
• 1 station per 260-390 sq.km.
• In hilly areas with heavy rainfall
• 1 station for every 130 sq.km.
Estimation of Optimum number of Rain Gauges
Examples
Radar Measurement
• Weather radar detects, measure and locates precipitation
• Usually used to supplement gauge data
• Radar signals reflected by rain
• Determine magnitude and areal distribution
• Sending microwave signals and listening for return signals
• Timings of return signals give range of an object
• The amount of energy reflected back to the radar is
proportional to the precipitation intensity
Weather satellites
• Used to monitor weather and climate of the Earth
• Can provide useful information on rainfall
distribution over large areas and inaccessible
regions
• Direct measurement of rainfall from satellites is not
feasible because the presence of clouds
Weather satellites
• Provide information only about the cloud tops rather than
cloud bases or interiors
Examples
• Polar orbiting
• NOAA-N series
• Geostationary
• GOES (operated by the United States National
Environmental Satellite)
• GMS (by the Japan Meteorological Agency), and
• Meteosat
Analysis of Precipitation
• Mean Rainfall
• Estimating Missing data
• Double Mass Analysis
• Frequency Analysis
• Depth Area Duration Curve
• Rainfall Hyetographs
• Intensity Duration Frequency Curve
• Frequency (IDF) Curves
Calculation of Mean Watershed
Precipitation
• Arithmetic Method
• Thiessen Polygon Method
• Isohyetal Method
Arithmetic Method
• Simplest Method
• Where:
• Pm = Average Rainfall in the catchment
• n = number of rain gauge stations
• P1, P2, ….. Pn= Recorded rain at station 1, 2, ….n respectively
in a given period
• Limitations????
Thiessen Polygon Method
• Tries to eliminate the error due to non-uniformity of rain
gauge distribution
• Suggested by A. M. Thiessen in 1911
A1, A2,…..A6 = polygon areas
• Thiessen Polygon Method Steps:
1. Join stations by line
2. Draw perpendicular bisector of these lines
3. The perpendicular bisector produces polygons called
Thiessen polygons
4. Calculate average rainfall of the area
5. P1, P2….. =rainfall in stations 1, 2….
Thiessen’s Weights
Isohyetal Method
• More accurate than other methods
• Location and precipitation are plotted
• Contours of equal precipitation called Isohyets are
drawn
• Calculate area between successive isohyets
• The equivalent uniform depth of precipitation
between isohyets is assumed to be equal to the
median value of two isohyets
Adjustment for Missing Data
• If rain gauge data at 1 or 2 stations is missing
• Interpolation in the estimation of average rainfall
• Data from neighboring stations is used
• ‘Normal Rainfall’ used as a standard for comparison
• ‘Normal Rainfall’ is the average value of rainfall at a
particular date, month or year over a specified 30-year
period.
Estimating Point Rainfall at a given
Location
• Interpolation
• From recorded values at surrounding sites
• Arithmetic Mean Method
• Inverse distance square
• Normal Ratio Method
• Station Year Method
• Double Mass Curve
Arithmetic Mean Method
• Break in station data
• Requires data from at least 3 other stations (evenly
distributed) close to that station
• Normal preciptitation at other stations should be within 10%
of precipitation at that station
• PA, PB, PC = Precipitation at nearby stations
• Px = Estimated precipitation of the missing station
Inverse Distance
• From recorded values at surrounding sites
• Based on weighted average of surrounding values
• The weights are reciprocals of the sum of square of
distances D, measured from the point of interest
Normal Ratio Method
• When normal annual precipitation of nearby
stations ( NA, NB, NC, ..) differs more than 10% of
that of the station (Nx) with missing data
• N= Normal Precipitation
Station Year Method
• Record of 2 or more independent stations are combined
• Area of these stations should be climatologically same
• Missing record at certain station in particular year is found
by ratio of the average
PA2000/PA1999 = PB2000/PB1999
Double Mass Analysis
• For checking the consistency of a station against one or more
nearby stations
• Consider a station E collecting data for 45 years
• For some reasons, the catch of the station is affected
• There are other stations H and I with same storm patterns
though their annual rainfall differ
• Check for a consistent correlation between the averages of H
and I and that of E in early years
• Plot the accumulated annual rainfall at E against the
accumulated average annual rainfall at H and I
• Correct the existing rainfall catch at E when the relationship
changes against the previous relationship
Double Mass Curve
Rain-Gauge Data Spatial Interpolation
• When interpolation methods are appropriate?
• when an attribute measured at sample points is a
spatially continuous field variable
• Involves estimating the rainfall values at
unmeasured points
Spatial Interpolation Methods
• Nearest neighbor (Thiessen Polygon Method )
• Isohyetal
• Triangulation
• Distance weighting
• Krigging
Triangulation
• Joining of adjacent data points by a line to form a lattice of
triangles (TIN)
• Values at any intermediate point on the surface can be
computed through trigonometry
Distance weighting
• Moving-average procedure using points within a
specified zone of influence
• Example: Inverse distance weighting
• weights are inversely proportional to the square of the
distance between the point of interest and each of the
data points
Kriging
• Also based on a weighted sum of the points within
a zone of influence
• weights in kriging are determined from a set of n
simultaneous linear equations, where n is the
number of points used for the estimation
• Based on spatial correlation
• points closer together tend to be strongly
correlated, whereas those far apart lack
correlation
PRISM?
• Parameter-elevation Regressions on Independent Slopes
Model
• Expert system that uses point data and a digital elevation
model (DEM) to generate gridded estimates of climate
parameters
• Developed to overcome the deficiencies of standard spatial
interpolation methods, where orographic effects strongly
influence weather patterns
• PRISM has been used to map temperature, snowfall,
weather generator statistics, and others
• Uses DEM
PRISM (conti..)
• For each DEM grid cell, PRISM develops a weighted
precipitation/elevation (P/E) regression function from
nearby stations, and predicts precipitation at the cell’s DEM
elevation with this function
• In the regression, greater weight is given to stations with
locations, elevations, and topographic positionings similar to
that of the grid cell
• Whenever possible, PRISM calculates a prediction interval
for the estimate, which is an approximation of the
uncertainty involved.
Hyetograph, Mass Curve and Hydrograph
• Hyetograph is the graphical plot of the rainfall
plotted against time
• Mass Curve of rainfall is a plot of accumulated
rainfall against time
• Hydrograph is a plot of discharge in the channel vs.
time (cubic feet per second)
• Hydrograph after separating the base flow is called
Direct Runoff Hydrograph
Plotting a Hyetograph
Area under hyetograph represents the total rainfall received
in the period
Rainfall Mass Curve
Plot of accumulated rainfall against time
END
Solution: Class Exercise