Precipitation Rabi H. Mohtar

Transcription

1 Precipitation Rabi H. Mohtar The objectives of this module are to present and analyze: 1) Precipitation forms, characteristics, and measurements 2) Intensity, Duration, Frequency (IDF) curves and rainfall patterns 3) Analytical methods for determining rainfall averages in a watershed with more than one station: arithmetic average, Thiessen, and Isohyetal methods 4) Derive rainfall distribution for a site of interest. Atmosphere water holding capacity: Types of precipitation: 1) Frontal 2) Convection 3) Orographic A storm is characterized by: 1. Depth 2. Duration 3. Distribution (amount vs. time) Why precipitation data is needed? How and who collects it? Rules Observations Longer storms have larger depths, while shorter storms have greater intensity

2 Storm area relationship Distribution Storm eye Intensity Area higher peak diffused localized less intense less volume more volume Area Number of gages guide per watershed area: 40 acres acres 3 5 mi mi mi Types of rain gages: Tube on a post 8 funnel strip chart recorder

3 data logger with pressure transducer tipping bucket with data logger Method for determining rainfall within a certain return period: 1. Sort all the recorded data with decreasing order. Record should not be less than ½ max return period. 2. determine the probability of occurrence (Fa): 100( 2n-1) Fa= 2y n=rank y=total number of events 100 Fa = return period 3. Plot precipitation amounts against probability of occurrence on log probability paper. 4. Fit a straight line between data. 5. Do interpolation and extrapolation to find the points of interest. Probability of a given return period to be exceeded in a given number of years: P( T, n ) 1 = 1 1 T n T=return period, n=years to be considered Probability that a T-year return period will occur at least once during n years: 1 P ( 500,500) = =0.6 ( ) P 500,1 = or 0.2% 500 Hydrologic cycle and its relationship to stormwater management: + precipitation + abstraction More detailed water budget important in single-storm for a short time + runoff - evaportranspiration Less detailed water budget important in long-term continuous streamflow - soil water movement

4 Continuity Equation: I - O = s Input output = change in storage + precipitation (streamflow + deep seepage + ET) = (soil water + ground water + ponds + lakes + reservoirs + channel storage + surface storage = detention = interception) Simplifications based on the particular use of the equation: Event-based hydrology use Continuous hydrology use Recording rainfall is: 1) 24 non-recording gage 2) Time-intensity gage Both are needed and will be used for different purposes. Mean Area Precipitation: 1. Arithmatic mean 2. Thiesseu polygon 3. Isohyetal method _ R = W R / W i i i _ R = average watershed rainfall W i = weighting factor R i = the rainfall amount 1) For arithmetic mean method W i =1 least accurate R _ = ) For Thiessen method W i =area of thiessen polygon, W i do not change with the storm _ R = ) The isohyetal method W i =area of adjacent lines R i = average depth associated with W i (av. of the two enclosed isohyetal lines). The W i and R i have to be determined for each storm. R _ = 2.69

5 IDF and DDF curves Intensity Duration Frequency Depth Duration Frequency D=iT Depth = intensity * duration ( ) x i=kf / T+b n F=frequency K, b, x, n are constants found by regression Rainfall DDF data are derived from point rainfall information. When applied to an area it should be multiplied by a reduction factor. Reason for the shape: - Short intense storms cover small areas - IDF data are maximum values observed at a point

6 Rainfall Charts: Probable Maximum Precipitation: These values are very conservative and used when extreme caution is to be practiced. They are 4-6 times of the value used for 100-year storms. Rainfall time distribution Needed for hydrograph determination Rainfall hyetograph or hyetograph historical storms Synthetic storms are emphasized here

7 SCS Rainfall Pattern For durations less than 24-hour the steepest part of the curve is used. D D P 12+t- P 12- PD D D P 12+ P P(t) 2 2 = (Equation 3.7 in textbook) 1) Find 2) Find P P PD=(24 storm) P24 P24 t f P 12- D Use table ti Find P 12+ D 2 Use table 3.4 3) Find P (t) a function of t t starts at 0 until the end of the storm 4) Fill the table time 0 P D M 12+ t- 2 increment as needed P (t) end of storm use table 3.4 desired value