3.-precipitation.pdf

Precipitation - Water that reaches the ground as rainfall or snowfall -

Evaporation from ocean surfaces is the chief source of moisture for precipitation

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Orographic barriers often exert more influence on climate if a region than nearness to a moisture source does.

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Single strongest variable driving hydrologic processes

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Formed by water vapor in the atmosphere

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As air cools its ability to ‘hold’ water decreases and some turns to liquid or ice (snow)

Forms of Precipitation: 1. Drizzle. This is a finest mist with drops just a little larger than heavy fog, about 0.1 – 0.5 mm (0.004 – 0.02 in). 2. Rain. Rain drops average about 1 mm, but may range from 0.5 to 5 mm (0.02 – 0.2 in) in diameter. 3. Ice Pellets. Also called sleet, that occurs when raindrops freeze as they fall through air where the temperature is below 0oC (32oF). Ice pellets are transparent spherical grains of ice, usually with diameter of less than 5mm (0.2 in). 4. Snow. Snow is precipitation that reaches the ground in the form of ice crystals. The size of snowflakes may vary from a few millimeters to several centimeters. 5. Hail. Hailstones, which may range from 5 to 10 cm (0.2 – 0.4 in) or more in diameter, are rounded lumps of ice that fall during thunderstorms.

Types of Precipitation: 1. Cyclonic / Frontal Precipitation – results when the leading edge of a warm, moist air (warm front) meets a cool and dry air mass (cold front).

Frontal Precipitation

Convective Precipitation – caused by upward movement of air which is warmer than its surroundings; this precipitation is generally showery nature with rapid changes of intensities.

Convective Precipitation

Orographic Precipitation – caused the air masses which strike the mountain barriers and rise up, causing condensation and precipitation.

Orographic Precipitation

Rainfall Characteristics The characteristics of rainfall are the amount, the intensity, the duration,

.

the frequency or return period, and the seasonal distribution

The amount is of course important to the overall hydrologic cycle and replenishment of the soil water, and the amount is an accumulation or product of the intensity times the duration. For example, the amount may be the same for a high intensity short duration rainfall as it is for a low intensity long duration rainfall

However, the intensity and duration can have a large influence on whether the rainfall infiltrates or becomes surface runoff. Higher rainfall intensity produces larger size raindrops which have more impact energy, and thus higher intensity storms can damage delicate vegetation and bare soil. High intensity storms can literally displace soil particles, causing soil crusting or starting the soil erosion process. High intensity storms may also overwhelm the soils ability to infiltrate the rainfall at the same rate, causing infiltration-excess runoff.

The duration refers to the length of time rainfall occurs. A high intensity rainfall for a short duration may affect tender seedlings, but it will not likely have much effect on soil erosion and runoff. Rainfall of longer duration can significantly affect infiltration, runoff, and soil erosion processes.

The frequency, or more specifically, the return period refers to how often rainfall occurs at a particular amount or intensity and duration. For example, rainfall return periods are referred to as 100 year-1 hour rainfall or 100 year-24 hour rainfall to define the probability that a given amount will fall within a given time period.

- The frequency of occurrence of a storm of given magnitude and duration is important to establish a measure of risk. - For a given storm duration, the probability that an event of certain magnitude has of being equaled or exceeded in any one year is termed the probability of exceedance - Frequency can be represented by the return period, which is the average number of years between events of a given magnitude or greater. The return period is related to the probability of exceedance by

1 Tr  Prob Exceedance  Where Tr is the return period and P is the probability of exceedance.

Frequency characteristics of storms are generally summarized in Intensity-DurationFrequency (IDF) Curves. In general, for the same return period, short storms are more intense than long storms. Similarly, for a given intensity, longer storms are associated with greater return periods In hydrologic design, frequencies are needed to select appropriate rainfall values that will result in design streamflows. A storm of a given frequency does not generally produce a peak discharge of the same frequency. However, these frequencies are commonly assumed to be the same, especially if models are used to estimate runoff from precipitation.

Intensity Duration Frequency (IDF) Curves

IDF is a plot of precipitation intensity on the y-axis and duration on the x-axis with return period indicated on each intensity-duration curve. Rainfall Intensity - is the amount of rainfall for a given rainfall event recorded at a station divided by the time of record, counted from the beginning of the event. Return Period - is the time interval after which a storm of given magnitude is likely to recur. This is determined by analysing past rainfalls from several events recorded at a station.

Intensity in inches/hour

Intensity-Duration-Frequency IDF-Curves

Time in minutes

The seasonal distribution of rainfall refers to the time of year when various rainfall amounts occur. The seasonal distribution determines when surface runoff or deep percolation are most likely to occur or if irrigation is needed. Since the seasonal distribution of rainfall varies in different parts of the country, practices used or recommended in one part of the country may not necessarily be appropriate in another.

•

A hyetograph is also used to describe the variation of the storm with time.

• The time distribution of the storm affects the shape of the direct runoff hydrograph. •

Early, Center, Late peaking precipitation

Rainfall Intensity in in/hour

Rainfall Hyetograph

Time in hours

Spatial Distribution • A localized storm would likely produce smaller peaks and a shorter hydrograph than if the same storm covered the whole watershed. • A storm moving away from the outlet will produce an earlier and smaller peak than if the storm moves towards the outlet. • Storm location, aerial extent, and storm movement are usually determined by the origin of the storm. • For instance, cold fronts produce localized fast-moving storms. Warm fronts give origin to slow-moving widespread precipitation.

Spatial Distribution • A storm taking place far from the outlet would produce longer hydrographs and lower peaks than if the same storm occurred near the outlet. • In most circumstances, it is assumed that rainfall is uniform over the entire watershed for the duration of the time increment.

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 tippingbucket type. All of the above gages measure precipitation at a point. Another method of measuring precipitation is through the use of radar. World Meteorological Organization (WMO) suggests the number of rain gauges: Ideal

Tipping Bucket Raingauge

Acceptable

For Flat Regions of temperate, Mediterranean and tropical zones

1 Station for 600 1 Station for 900 – 900 km2 – 3000 km2

For Mountainous Regions of temperate, Mediterranean and tropical zones

1 Station for 100 1 Station for 250 – 250 km2 – 1000 km2

Manual Raingauge

Selection of RAINFALL Gauging Station Location 1. Site is safe and accessible for installation and operation. 2. An area upstream of proposed intake and within river catchment. 3. Objects should not be closer to the gauge than a distance twice their height above the raingauge orifice. 4. Sites on slope or on the roof of a building should be avoided. 5. Best sites are often found in cleanings within forest or orchards, among trees, in scrub or shrub forests, or where other objects act as an effective wind-break for winds from all directions.

6. The surface surrounding the precipitation gauge can be covered with short grass, gravel or shingle, but hard, flat surfaces, such as concrete, should be avoided to prevent excessive in splashing.

Precipitation Patterns in Space and Time: 1. Geographic Range of Precipitation Rainfall tends to be heaviest near the equator and to diminish as the air flows toward higher latitudes. The geographical distribution of precipitation depends on orographic factors as well as how far an air mass has moved away from its source. 2. Seasonal Variations in Precipitation Precipitation varies with the sun’s position. Many parts of the world have one definite rainy season, which may occur in summer or in winter depending on the general circulation.

Source:http://kidlat.pagasa.dost.gov.ph/c ab/climate.htm

Precipitation Patterns in Space and Time: 3. Long Term Variations in Precipitation Precipitation have a predictable pattern of fluctuation around its long term average value or mean.

Variation of Rainfall Rainfall may vary from a given point to the other. Average Rainfall Depth (ARD) – is the mean depth of rainfall for a particular basin. For any given time duration, the average depth of rainfall falling over the basin can be computed using three methods: 1. Arithmetic Mean – taken as the average of all rainfall depths.

Time (hrs)

15  12  8  5  10.0 4

mm

Rainfall (mm)

Total

1st

2nd

3rd

4th

A

15

10

3

2

30

B

12

15

8

5

40

C

8

10

6

4

28

D

5

8

2

2

17

2. The Thiessen Polygon Method

This method is proposed by Theissen in 1911, considers the representative area of each rain gauge. These area are found out using a method consisting of the following three steps:

1. Joining the rain gauge station locations by drawing straight lines to form triangle. 2. Bisecting the edges of the triangles to form the so called “Thiessen Polygons”. 3. Calculate the area enclosed around each rain gauge station to find the area of influence corresponding to the rain gauge.

For example, the weighted average rainfall over the catchment is determined as,

(55x15)  (70x12)  (80x8)  (35x5)  10.33mm (55  70  80  35)

2. The Isohyetal Method

This is considered as one of the most accurate method. The method requires the plotting of isohyets as shown in the figure and calculating the areas enclosed either between the isohyets or between an isohyet and the catchment boundary. The areas II and III fall between two isohyets each. Hence, these areas may be thought of as corresponding to the following rainfall depths: Area

Area II : Corresponds to (10 + 15)/2 = 12.5 mm rainfall depth Area III : Corresponds to (5 + 10)/2 = 7.5 mm rainfall depth

(40x15)  (80x12.5)  (70x7.5)  (50x5)  9.896 mm 240

Area I

40 km2

Area II

80 km2

Area III

70 km2

Area IV

50 km2

Total

240 km2

Isohyetal method • Steps • Construct isohyets (rainfall contours) • Compute area between each pair of adjacent isohyets (Ai) • Compute average precipitation for each pair of adjacent isohyets (pi) • Compute areal average using the following formula

1 N PP  A p  Ai Pi  A i 1 M

i 1

P

i

i

5  5  18 15  12  25  12  35  21.6 mm 47

P1

P2

P3

Probability and Statistics

The concept of probability is paramount in the field of hydrology. The following are techniques of probability and statistics used to analyze random events. 1.

The probability of obtaining either outcome A or B, with A and independent and mutually exclusive, is the sum of the probability of obtaining each, thus

P( AorB)  P( A)  P ( B) Where: P(A or B) = P(A) = P(B) =

probability of obtaining either A or B probability of obtaining A probability of obtaining B

Return Period

Typical Rainfall Intensity Duration Frequency (IDF) Curve

1

2

3

6

12

24

62.67

38.83

28.01

15.11

7.77

3.88

79.50

49.26

35.53

19.17

9.85

4.93

20

101.34

62.79

45.29

24.43

12.56

6.28

50

115.17

71.37

51.47

27.77

14.27

7.14

100

125.54

77.79

56.11

30.27

15.56

7.78

135.876

84.195

60.724

32.763

16.840

8.422

159.809

99.025

71.419

38.534

19.806

9.905

2

Davao City – Sasa Rainfall Station

Rainfall Intensity (mm/hr)

5

Years of Record: 1952 - 2011

200 1000

Rainfall Intensity Duration Frequency (RDIF) Curves

Rainfall Intensity (mm)

200

150

2 year 5 year

100

20 year 50 year

50

100 year 200 year

0

1000 year

0

2

4

6

8

10

12

14

Time (Hours)

16

18

20

22

24

Analysis for Anomalous Rainfall Record

Rainfall recorded at various rain gauges within a catchment should be monitored regularly for any anomalies. These two major types of anomalies in rainfall are categorized as: 1. Missing rainfall record To approximate the missing record,

1  N4 N4 N4  P4   P1  P2  P3  3  N1 N2 N3  P4

= precipitation at the missing location N1, N2, N3 and N4 = normal annual precipitation of the four stations P1, P2 and P3 = rainfalls recorded at the three stations 1, 2 and 3 respectively.

2. Inconsistency in rainfall record To determine the possible inconsistencies, use double mass curve.

Example: Find out the missing storm precipitation of station 'C' given in the following table:

Double Mass Curve - tests the consistency of the record at a station by comparing its accumulated annual precipitation with the concurrent accumulated values of mean precipitation for a group of surrounding stations. -used to check the consistency of a rain gauge record: - compute cumulative rainfall amounts for suspect gauge and check gauges Cumulative rainfall at other gauges - plot cumulative rainfall amounts against each other (divergence from a straight line indicates error) - multiplying erroneous data after change by a correction factor k where

end

end

Example: X and Y are two neighboring rainfall stations. Station X has complete records and station Y has some missing values. Find the linear correlation equation between the two series as mentioned in columns 2 and 3 of the following table (8) and then check the correlation by computing both the correlation and regression coefficients, showing the correlation line on an X-Y diagram? Using the derived equation, find the missing data of Y if the observed data at X for the same years are (110, 170 and 166 mm).

Annual precipitation amounts as recorded by stations X and Y.

Computations of the Components to be Used in the Least Squares Method

Computation of the slope of the line:

Computation of the intercept of the line:

Computation of the correlation coefficient of the line:

Runoff and Streamflow

Selection of STREAMFLOW Gauging Station Location 1. A General course of stream is straight for about 10 times the stream width. 2. Total flow is confined to one channel at all stages and no flow bypasses the site as subsurface flow. 3. Stream bed is not subject to scour and fill and is relatively free of aquatic vegetation.

4. Banks are permanent, high enough to contain floods, and are free of brush. 5. Unchanging natural controls are present in the form of a bedrock outcrop or other stable riffle. 6. A pool is present upstream from the control at extremely low stages to ensure a recording of stage at extremely low flow, and to avoid high velocities at the stream ward end during periods of high flow. 7. Sensitivity of control should be such that any significant change in discharge shall result in a measurable change in stage.

Selection of STREAMFLOW Gauging Station Location 8. Gauge site is far enough upstream from the confluence with another stream or from tidal effect 10.A satisfactory reach for measuring discharge at all stages is available within reasonable proximity of the gauge site. It is not necessary for flow and high flows to be measured at the same stream cross section.

11.Site is safe and readily accessible for ease in installation and operation of the gauging station. 12.Good conditions for discharge measurements at all stages. 13.Instruments, shelter and housing above all flood levels.

14.Two reference point must be set located sections Upstream and Downstream of the gauged cross section for large flow events. A number of these flood peak water surface profiles should be recorded to ensure that the variation in water surface profile is understood across the full range of observed flows.

Streamflow Gauging Instruments

1.

Staff gauge is a graduated scale anchored in the water and read by observing the level of the water surface in contact with it.

2.

Current meter is suspended in the water using a cable with sounding weight or wading rod and will accurately measure streamflow velocities from 0.1 to 25 feet per second (0.025 to 7.6 meters per second).

Streamflow Gauging Instruments

3.

Smart PT2X water level sensor is a submersible pressure/temperature sensor and data logger combined in one small diameter unit.

4.

Acoustic Doppler Current Profiler (ADCP) is a more sophisticated streamflow gauging instrument and more suited for big-deep rivers. This instrument is used to measure how fast water is moving across an entire water column.

Streamflow Gauging Procedures A. Using Current Meter 1. Fill out the appropriate Streamflow Gauging Form. 2. The line of flow measurement should be perpendicular to the river flow. Put a marker (ex. rope or string) across the river marked at every desired interval. 3. Do not let the marker get weighed down by the current. 4. Measure the depth of water at every point of flow measurement, from the river bed to water surface using the wading rod. In measuring the depth of water, put on a base that the rod will not penetrate sandy bottoms.

Streamflow Gauging Procedures

A. Using Current Meter (continuation) 5. When the depth is below 0.50 meters (50cm), place the bucket wheel at the proper operating depth which is 40% of the total depth from the river bed. If the depth is at 0.50 meters or more, place the wheel at 20% and 80 % of the total depth. Let the bucket wheel get stable or used to the current and then begin the measurement. 6. Using the current meter, the velocity of the river at a minimum of 40 seconds will be measured.

Streamflow Gauging Procedures B. Using Floatation 1. Estimate the cross sectional area (A) of the stream by using a steel tape to measure both the stream width and a few depth measurements. 2. Measure a stream reach of adequate length (L) to allow a travel time of over 20 seconds for the float. Mark the starting and finishing points of this reach with a stake or a string across the stream. 3. Choose a float that is only slightly buoyant to reduce wind effects. An orange, chunk of ice, half-filled fishing float or a waterlogged stick are good options.

4. Place the float upstream of the defined starting point of the reach, so that the float is travelling at the velocity of the stream by the time it reaches the starting marker. 5. Measure the time that the float takes to travel between the upstream and downstream markers using a stopwatch. 6. An average time (t) is obtained by taking multiple readings. 7. Use a correction factor (k) to account for surface velocity being faster than the average stream velocity.

Streamflow Gauging Procedures

B. Using Floatation

Rating Curve

After the entire gauging process has completed, a rating curve will be developed. The Rating Curve is a graphical representation of the relationship between the stage height and the discharge (flow) for a given point on a stream or gauging stations. This curve is thus utilized to convert stage height readings to flow discharges.

Flow Duration Curve

The flow-duration curve is a cumulative frequency curve that shows the percent of time during which specified discharges were equaled or exceeded in a given period. (Searcy, J.) Flow Duration Curve 350.00

300.00

Discharge m3/sec

250.00

200.00

150.00

100.00

50.00

0%

10%

20%

30% 40% 50% 60% 70% % of Time Indicated that Discharge is Exceeded or Equalled

Ilog River, Negros Oriental Flow Duration Curve

80%

90%

100%

Example

hydrograph

Overflow and Interflow Interception – a phenomenon when rainfall is intercepted by vegetation before it reaches the ground. Depression or storage - flow as a thin sheet of water across the land surface Overland flow - the amount of rainfall in excess of the infiltrated quantity flows over the ground surface following the land slope

Water Table - free surface of a fully saturated region with water (the ground water reserve) Interflow - Part of the water in the unsaturated zone of the soil (also called the vadose zone) moves in a lateral direction.

Runoff is water that flows across the land surface after a storm event. The geographical area that contributes to the flow of a river is called a watershed (basin). The river basins of Davao City are the following: Tuganay River Lasang River

River Basin

Drainage Area, km2

1. Davao River

1757.76

2. Talomo River

215.78

3. Lasang River

453.9

4. Lipadas River

167.96

5. Sibulan River

282.13

6. Matina River

78.79

7. Tuganay River

757.47

8. Bunawan River

252.13

TOTAL

3965.92

Davao River Watershed

Bunawan River

Talomo Watershed Matina River

Lipadas Watershed

Sibulan River

Source: pceemdavao.weebly.com

Hydrograph and Hyetograph Hydrograph is the plot of the stream flow at a particular location as a function of time. Hyetograph is the graphical plot of the rainfall plotted against time.

Factors affecting runoff: 1. Climate • Climate determines not only the ultimate water supply through precipitation, but also the extent to which that precipitation is return to the atmosphere before it can participate in stream flow. • Where a lot of precipitation occurs, obviously a lot of runoff is also apt to occur.

2. Physical Characteristics of the Drainage Basin a. Shape of the Catchment

A catchment with the narrow end towards the upstream and the broader end nearer the catchment outlet shall have a hydrograph that is fast rising and has a rather concentrated high peak.

A catchment shaped with its narrow end towards the outlet has a hydrograph that is slow rising and with a somewhat lower peak.

b. Elevation and orientation of the basin • The main effect of elevation are related to temperature. At higher elevations, cooler temperature result in less water loss by evapotranspiration. • Basin orientation in relation to the prevailing storm tracks may also have a pronounced effect on runoff. c. Slope and Topography • Larger slopes generate more velocity than smaller slopes and hence can dispose off runoff faster. d.

Vegetation and Soil Type • The existence of a vegetation cover retards overland flow, giving the water more time to infiltrate the soil. This can make substantial difference in the rate at which stream levels rise during the storm. • Soil type influences vegetative cover just as vegetation tends to modify soil to increase its infiltration capacity and permeability.

e.

Geology • Geologic factors also largely determine the storage time during which water is held between precipitation and eventual runoff as stream flow.

f.

Land Use: Human Alterations of the Environment • Invention of the city caused local problems in drainage and flooding where people occupied areas on river floodplains. • Modern farming methods have sometimes accelerated erosion of bare soil and have resulted in minor changes in natural drainage nets.

End of topic

B F D A

I

II

III

IV

VI

V

C

VII

H

E G

VIII

500

A

C

I

II III E

IV

V VI

D

9.8 mm Isohyet

B

E VII

VI

C V

D

III

IV

II

I

A B

F