CHAPTER 2 Flashcards
That part of atmospheric moisture that falls on the earth surface
PRECIPITATION
what are the 10 types of precipitation?
- rain
- ice crystals
- sleet
- freezing rain
- snow
- snow grains
- graupel
- hail
- drizzle
- freezing fog
Water droplets in clouds are formed by nucleation of vapor on aerosols, then go through many condensation evaporation cycles as they circulate in the cloud, until they aggregate into large enough drops to fall through the cloud base.
SUBLIMATION NUCLEI
a process of artificially nucleating clouds to induce precipitation
CLOUD SEEDING
is a common nucleating agent and is spread from aircraft in which a silver iodide solution is evaporated with a propane flame to produce particles.
SILVER IODINE
___________ and ___________are known to co-exist in the atmosphere at sub-freezing temperatures.
ICE CRYSTALS and WATER DROPLETS
_________ of water droplets as a result of their coming in contact through the action on them of air movement and
gravitational pull.
COALESENCE
air is drawn upwards by convective action, such as in the center of a thunderstorm cell
CONVECTIVE PRECIPITATION
where warm air is lifted over cooler air by frontal passage
FRONTAL PRECIPITATION
which an air mass rises to pass over a mountain range
OROGRAPHIC PRECIPITATION
what are the 4 types of precipitation?
- CONVECTIVE PRECIPITATION
- FRONTAL PRECIPITATION
- OROGRAPHIC PRECIPTATION
- CYCLONIC PRECIPITATION
the simplest method of determining areal average rainfall which involves averaging the rainfall depths recorded at a number of gages
ARITHMETIC AVERAGE
This method is satisfactory if the gages are uniformly distributed over the area and the individual gage measurements do not vary greatly about the mean.
ARITHMETIC AVERAGE
Assumes that at any point in the watershed the rainfall is the same as that at the nearest gage so the depth recorded at a given gage is applied
out to a distance halfway to the next station in any direction.
THIESSEN METHOD
The relative weights for each gage are determined from the corresponding areas of application in a Thiessen polygon network, the boundaries of the polygons being formed by the ____________.
PERPENDICULAR BISECTORS OF THE LINES JOINING ADJACENT GAGES.
- Composed of isohyets which are lines of
equal rainfall magnitude - Most accurate and more flexible than
Thiessen method - Harder to construct for complex storms
with dense network of gages
ISOHYETAL METHOD
The gaging networks in the Philippines
- Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA) weather stations and facilities (more or less 500 in total)
- PAGASA Automated Weather Stations (AWS)
- National Irrigation Administration
- Bureau of Forest Development and the
- Bureau of Plant Industry
- State colleges and Universities
- Private entities
3 types of estimation of Areal Precipitation
a. ARITHMETIC AVERAGE
b. THIESSAN METHOD
c. ISOHYETAL METHOD
2 types of rainfall adjustments
A. Estimation of Missing Rainfall Data
B. Adjustment of Rainfall due to Changes in Locations
Complete measured precipitation data are important to many problems in hydrologic analysis and design but there are missing values
ESTIMATION OF MISSING RAINFALL DATA
type of Estimation of Missing Rainfall Data that is preferrable for relatively flat areas
STATION-AVERAGE METHOD
- A consistent record is one where the characteristics of the record have not changed with time
- An inconsistent record may result from any one of a number of events
ADJUSTMENT OF RAINFALL DUE TO CHANGES IN LOCATION
is the method that is used to check for an
inconsistency in a gaged record.
DOUBLE-MASS CURVE ANALYSIS
is a graph of the cumulative catch at the rain gage of interest versus the cumulative catch of one or more gages in the regions that have been subjected to similar hydrometeorological occurrences and are known to be consistent
DOUBLE-MASS CURVE
If a double- mass curve has a constant slope, the record is ________.
CONSISTENT
If a double –mass curve has not a constant slope, the record is ________________.
NOT CONSISTENT AND NEED TO BE ADJUSTED
what are the 4 characteristics of a rainstorm?
- Intensity (mm/hr)
- Duration (min, hours, days)
- Areal extent (m2, ha): area over which it is distributed
- Frequency: once in 5, 10, 15, 20, 25, 50 or 100 years
- graph showing the depth or intensity of
rainfall with respect to time - useful in determining the maximum
intensities of rainfall during a particular
storm as is required in land drainage and
design of culverts.
HYETOGRAPH
3 basic terms in Statistical Hydrology
- Time period of interest (N)
- Probability of occurrence (P)
- Recurrence interval or return period (T)
probability that a T-year storm may occur
in any series of N years.
PROBABILITY OF EXCEEDANCE
- Enables the hydrologists to develop hydrologic systems that consider worst-case scenarios of rainfall intensity and duration during a given interval of time
- It is often used by entering with the duration and frequency to find
the intensity. Or - It is used to find the frequency for a measured storm event. The
predicted frequency is determined by finding the intersection of the
lines defined by the measured intensity and the storm duration.
IDF CURVE (Intensity vs. Duration vs. Frequency)
Refers to the probabilistic analysis of hydrologic processes.
HYDROLOGIC FREQUENCY ANALYSIS
probability that a T-year storm may
NOT occur in any series of N years
PROBBAILITY OF NON-EXCEEDANCE
any hydrologic phenomenon which undergoes continuous changes with time.
HYDROLOGIC PROCESS
Categories of Hydrologic Process
- DETERMINISTIC
- PROBABILISTIC
- STOCHASTIC
- is one in which a definite law of certainty exists
- it is one where the chance element is ignored as in the case of flood routing
- Commonly used for man-made canals channels
DETERMINISTIC PROCESS
- is one governed by chance
phenomena, that is, there are
so many causes at work that
the influence of one cannot be
readily identified - Time-series independent
PROBABILISTIC PROCESS
- means that the sequence of occurrence of events or the time series is considered in its analysis
- Time-series dependent
STOCHASTIC PROCESS
what is the objective of Hydrologic Frequency Analysis?
to relate the magnitude of events to their frequency of occurrence through probability distribution, that is, a frequency curve
steps in Frequency Analysis
- Selection of Data
- Determination of Statistical Parameters
- Fitting of Data
- Testing the Significance of Results
types of Probability Density Function
- Normal density function
- Log-normal function with two parameters
- Gamma density function with two parameters
- Kappa density functions with two parameters
all data above a given threshold value
PARTIAL DURATION SERIES
only one data point per year, usually the maximum
ANNUAL SERIES
a hydrologic event is not influenced by another event
INDEPENDENCE
events under the same type of conditions or statistical properties are not changing
HOMOGENITY
Fitting of data to known statistical distributions
- Normal distribution
- Log-normal distribution
- Log-Pearson Type III distribution
- other applicable density functions
