Elementary Engineering Hydrology BY Deodhar M. J.

PRECIPITATION RS & GISc, Institute of Space Technology October 2, 2015 Hydrology and Water Resources RG744 PRECIPITATION October 2, 2015 RS & GISc, Institute of Space Technology

Precipitation Any form of solid and liquid water that falls from atmosphere to earth surface Rain, drizzle, hail, dew, snow Rain: size of drop larger than 0.5mm (Das & Saikia). Light (2.5 mm/hr), Moderate (2.5-7.5 mm/hr), Heavy (>7.5). Drizzle: sprinkler of water droplets, size 0.1-0.5mm and intensity <1mm/hr (does not appear to fall) Hail: lumps of ice of size > 8mm Dew: during night when surface cools due to outgoing radiation, condensation on ground starts Snow: composed of ice crystal Also sleet (solid grain of ice frozen near earth surface), glaze (ice coating clear n smooth) Thus, fog and mist are not precipitation but suspensions, because the water vapour does not condense sufficiently to precipitate.

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 The air close to the warm earth gets heated and rises due to its low density, cools adiabatically and clouds are made that burst into thunderstorm Convective storms are generally known as thunderstorms When accompanied by destructive winds, they are called ‘tornados’. 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 Frontal: condensation and precipitation occur at the surface of contact. The surface of the contact from where it precipitates is called ‘front’. Adiabatic: no heat is gained or lost by the system (into or out of the system) Dew point: The temperature at which air becomes saturated and produces dew. Adiabatic cooling occurs when the pressure of a substance is decreased as it does work on its surroundings. Adiabatic cooling occurs in the Earth's atmosphere and can form clouds if the air is cooled below the dew point. When the pressure applied on a parcel of air decreases, the air in the parcel is allowed to expand; as the volume increases, the temperature falls and internal energy decreases.

Orographic Precipitation Source: Hydrology By Das & Saikia

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 Cyclone: 1) Tropical cyclone called hurricane or typhoon smaller in diameter (300-1500 Km) causing high wind velocity and heavy rain. 2) Extra-tropical cyclone of larger diameter of 3,000Km causing wide spread frontal type precipitation.

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 Temporal and areal variation in rainfall is important to determine hydrologic response of a watershed.

Methods of Measurement Rain gauges Radar (Radio Detection and Ranging) Weather Satellites Radars send out short pulses of radio waves that bounce off particles in the atmosphere, and the energy is reflected back to the radar dish. A computer processes the returned signals and, through algorithms, can make conclusions about what kinds of particles it “saw,” including the directions they are moving in (Doppler effect) and the speed of their movement. Satellite: pg 86 pdf GIS and Hydro

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 Weighing bucket: Gives mass curve of rainfall Recording (weighing type, float type, tipping bucket) and non-recording gauges. Storage Rain gauge: description from Das and Saikia Tele…Containing electronic unit to transmit data to some base station at regular interval.

Symon’s

Weighing bucket type Source: Das and Saikia

Tipping Bucket Source: Hydrology and Flood Plain Analysis by Bedient.

Siphon Rain gauge Source: Das and Saikia Description form book.

Source: Hydrology Principles (Text Book) (HP)

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 Rain-gauges must be so located as to avoid exposure to wind effect or interception by trees or buildings nearby Source: Text Book HP

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) Rain gauge density or network density: average area of influence of rain gauge stations. If area is plain and rain variation is small….less number of rain gauges.

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 Where n = total number of rainfall gauges in the watershed.

Cv = variation as % of the mean.

Examples Das & Saikia Optimum number of rain gauges.

Elementary Engineering Hydrology  By Deodhar M. J

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 both in daylight and darkness, through thick clouds. Weather radars transmit a microwave beam and then "listen" for echoes that bounce back from precipitation-sized particles. Precipitation intensity can be determined by measuring the strength of the echoes received by the radar antenna.

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 Source: bug_1420069136.pdf - pg 86

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 The first weather satellite to be considered a success was TIROS-1, launched by NASA on 1 April 1960. Both geostationary and polar orbiting. These satellites provide frequent observations (even at night with thermal sensors), and the characteristics of potentially precipitating clouds and the rates of changes in cloud area and shape can be observed. From these observations, estimates of rainfall can be made that relate cloud characteristics to instantaneous rainfall rates and cumulative rainfall over time. Improved analysis of rainfall can be achieved by combining satellite and conventional gauge data. Geostationary Operational Environmental Satellite GOES

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 Predicting Watershed response to a given precipitation event requires knowledge of the average precipitation that occurs over a watershed area in a specified duration. Estimate of Point and Areal Precipitation????? Area weighted rainfall????

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???? Given period = say annual precipitation Satisfactory where gauges are uniformly distributed and individual variation from mean is not much. Not good for large areas with variable rainfall distribution.

Thiessen Polygon Method Tries to eliminate the error due to non-uniformity of rain gauge distribution Suggested by A. M. Thiessen in 1911 Weighted effect of the area in the form of polygon closest to the station. Consider 6 stations in a watershed. Does not consider effects due to elevation. A1, A2,…..A6 = polygon areas

Thiessen Polygon Method Steps: Join stations by line Draw perpendicular bisector of these lines The perpendicular bisector produces polygons called Thiessen polygons Calculate average rainfall of the area P1, P2….. =rainfall in stations 1, 2…. Polygons are the boundaries of the effected area for each station.

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 This too is an area-based weighting scheme. Contour lines of equal precipitation are estimated from the point measurements.

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. Reciprocal Distance Method

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 Source: Hydrology and Flood Plain Analysis

Arithmetic Mean Method Break in station data Requires data from at least 3 other stations (evenly distributed) close to that station Normal precipitation 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 The 3 stations are called index stations. 30 years of data is required to calculate normal annual precipitation.

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 Source: Hydrology and Flood Plain Analysis http://www.learningace.com/doc/7740793/68b0122ba59c2767b2f4f4377e0f0896/1-hydrologic-principles

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 Normal precipitation = N Precipitation = P

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 Check with another station C and if the two results differ slightly then take average of two results.

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 Annual rainfall of H and I may differ due to elevation differences or some other factors.

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 The common problem of obtaining rainfall data for the watershed of interest using point rain gauges is addressed using spatial interpolation procedures.

Spatial Interpolation Methods Nearest neighbor (Thiessen Polygon Method ) Isohyetal Triangulation Distance weighting Krigging The nearest-neighbor method simply assigns a value to the grid equaling the value of the nearest data point.

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 Powers other than 2 may be used to change the rate of decay of the weighting function.

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 The spatial correlation, expressed as the covariance between pairs of sample points and the sample points being estimated, is first computed and the weights then determined. Over the field of interest, the variogram is formed as an inverse plot of the autocorrelogram, the covariance versus distance. Variograms provide a measure of the spatial continuity of the interpolated surface.

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 http://www.prism.oregonstate.edu

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. topographic positions (terrain elements, or topographic features)

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 Hyetograph: Plot of rainfall intensity (in/hr) verses time Hydrograph: Time history of rainfall intensity. Plot of flow rate vs. time measured at a stream cross section primarily made up of overland flow. Hydrologic response of rainfall at the outlet of an area plot of the stream flow at a particular location as a function of time. Actual shape and timings of the hydrograph is determined largely by the size, shape, slope, and storage in the basin and by the intensity and duration of input rainfall.

Plotting a Hyetograph Discrete representation of rainfall hydrograph Source: Das and Saikia Area under hyetograph represents the total rainfall received in the period

Rainfall Mass Curve Plot of accumulated rainfall against time

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Elementary Engineering Hydrology  By Deodhar M. J.