Precipitation

P: water (solid, liquid) falling from atmosphere to ground P includes Rain, Drizzle, Snow, Hail, Sleet, Ice crystals

Measurement: Container to collect P in a storm event Radar, digital recorders Accuracy depends on physical setting, disturbances…etc Weather data available from government agencies:

PME (MEPA), MWE (MAW), …

Formation Of Precipitation

Conditions:

1. Humid air cooled to dew-point T

2. Nuclei

3. Droplets to raindrops

4. Size of raindrops

Adiabatic expansion

due to pressure change

Standard Rain Gages (SRG)

Standard Rain Gages (SRG)

8 in (20.32 cm)

30

in (

76

.2 c

m)

Accuracy?

ESTIMATION OF PRECIPITATION OVER AN AREA

Effective Depth of Drecipitation (EUD)

1. Arithmetic average:

for evenly distribute stations (uniform density)

3. Thiessen method

area-weighted averaging

2. Isohyetal lines

contouring

Areal Estimation of P from a network of gages

13.97 mm

48 mm

59.2 mm

22.1 mm

137.2 mm

(1) Arithmetic average

Pa = 1/N ∑ Pi

(13.97 + 22.1 + 59.2 + 48.0+ 137.2)/5

= 56.1

(2) Thiessen Polygon Method

Area-weighted average

(every gage represents best the area immediately around the gage)

Constructing Thiessen Network:

1. Plot stations on a map

2. Connect adjacent stations by straight lines

3. Bisect each connecting line perpendicularly

4. Perpendicular lines define a polygon around each station

5. P at a station is applied to the polygon closest to it

St.No P Polygon area

Weighted area

Weight-ed P

1 13.97 15 0.128 1.788

2 22.1 33 0.281 9.273

3 59.2 28.8 0.245 14.5

4 48 16.4 0.139 6.672

5 137.2 24.3 0.207 28.4

Totals 280.47

117.5 1.00 60.633

)3) isohyetal method

Based on areas calculated from contoured P map

(check first for effect of elevation by plotting P vs elevation)

STEPS:

1. Plot a contour map of P based on gage readings at station

2. Compute area between each successive contour lines

Pa = PaiAi/ Ai

Isohyetal method

Isohyetal method procedure

Determine contours of equal P:

(Isohyetal lines) Estimate representative P

for each region Calculate Pav

P = Pi*Ai/AT

= P(1)*A(1)/AT + P(2)*A(2)/AT + P(3)*A(3)/AT +P(4)*A(4)/AT

28

22

17

11

30

25

20

1526

)1()2(

)3(

)4(

Example

P was measured at several stations.

Characterize the precipitation over the area as the arithmetic average, Thiessen-weighted average, and isohyetal average.

Example, solution

1. Arithmetic average:

Pa = (4.5+4.4+4.4+4.1+4.0+3.7+3.5+2.5+2.7)/9 = 3.76 in

Example, solution2. Thiessen-Weighted average:

Example, solution3. Isohyetal Method:

Events During Precipitation

Interception (8- 35% for densely vegetated) Stem flow Infiltration

infiltration capacity how fast water is absorbed into soil.  effected greatly by soil type.  Water infiltrates faster into sand than it does

clay.  Low infiltration capacity causes more runoff and

more erosion.

Depression storage

Overland flow (P rate > If rate)

Interflow (horizontal flow in unsaturated zone)

Baseflow

Evaporation

Evaporation

It’s the physical process by which liquid is transformed to gas (vapor) due to the release of the bonds holding the molecules together

In Hydrology: it’s the amount of water lost from soils and open water bodies to the atmosphere

Evaporation stops when air is saturated with moisture Absolute humidity:

Grams of water/cubic meter of air Saturation humidity

max amount of moisture air can hold at any T

Relative humidity:Absolute humidity/saturation humidity

H = H

HX100%R

A

S

Evaporation contd.

Dew point:T at which condensation will begin

Rate of E depends on: T (air), T(water), absolute humidity, wind.

Estimation and Measurement: No direct measurement Water budget method

E = I - O (+/-) S Evaporation pan

Standard: US weather service class A Pan

Evaporation estimation

4 ft (1.22 m)

10” (25.4 cm)

Evaporation Pan operation:

Pans placed on supports to allow air circulation A water depth of 7-8 in (18 - 20 cm) is maintained. Water depth in pan is measured with time Max, min, T recorded Water is added or removed from pan to adjust for

rain and E, its volume recorded E from a pan is higher than actual E from a lake Pan coefficient:

Empirical correction factor 0.58 – 0.78 (depending on month)

Transpiration

Mass transfer of water from ground to the air through plants

Transpiration can exceed evaporation in heavily wooded areas

Transpiration is only important during growing season (in cultivated areas)

wilting point: soil moisture is low causing surface tension of soil-water interface to exceed osmotic pressure water will not enter roots

Transpiration is measured in carefully controlled lab conditions Phytometer: is a sealed container partially filled w/ soil.

Transpiration is measured as the increase in humidity in the air space around the plant

Evapotranspiration (ET)

any transfer of moisture to the air 90% of P in arid regions! In field conditions, not possible to separate

evaporation from transpiration Potential ET: maximum evapotranspiration if

there is infinite supply of water available in the soil for the vegetation.

Actual ET: amount of ET under field conditions

Potential ET

Theoretical estimation (empirical formulas)

Thornthwaite's method

a =0.492+0.0179I – 0.0000771I2 + 0.000000675I3

ET is potential evapotranspiration in cm/month

Tai is mean monthly air temperature in C for month I

I is annual heat index

a is s constant

a

aiT I

TE

10

62.1

12

1

5.1

5i

aiTI

ET measurement

field measurementLarge watertight caisson buried in ground, filled w/ soil

and planted with vegetation.

Example 2.2

Infiltration

Overland Flow

Interflow

2.3 Infiltration, Overland Flow, and Interflow

2.5 Runoff Estimation- Rational Method

Q = C i A

Q: Peak discharge ft3/s

C: runoff coefficient

i : average intensity of rainfall (in/hr) for a selected frequency of occurrence or return period for the time of concentration (Tc) (min)

Tc: estimated time required for runoff to flow from most remote part of the area under consideration to the point under consideration

A: drainage area (acres)

Example 2.3

For a drainage area of 80 acres30% rooftops

10% streets and driveways

20% lawn on sandy soil

40% woodlands

Height of most remote point is 100 ft

Maximum travel length of 3000 ft

Calculate peak runoff from a 10-year frequency storm.

Example 2.3 solution

We need C, Tc, I

C = 0.43Tc from figure 2.8 = 14 mini from fig. 2.9 (for 10 year return period, 14 min duration) =

4.9 in/hr

Q = C i A = 0.43 x 4.9 x 80 = 169 cfs

Figure 2.8 Nomograph for estimating Tc for a small basin

Chapter Highlights

1. Precipitation can occur in different forms-rain, drizzle, snow, and sleet.

2. The standard U.S. precipitation gage and various types of recording gages (for example, tipping bucket) form the basis for regional, station-based measurements.

3. New NEXRAD radars provide a capability of continuous precipitation measurements over thousands of square miles.

4. Various statistical techniques are available to estimate average precipitation from several stations scattered across an area. Simplest technique is arithmetic average. More sophisticated Thiessen polygon and isohyetal methods provide different ways of weighting the individual data points according to their area of influence.

Chapter Highlights6. Evaporation and evapotranspiration lead to water losses from the

surface and subsurface. Evaporation from surface-water bodies is estimated from pan measurements. Evapotranspiration can be measured using a lysimeter. Empirical methods (for example, Thornthwaite, 1948) provide a useful alternative for estimating potential evapotranspiration.

7. Infiltration, overland flow, and interflow processes occur at or close to the ground surface. Of the precipitation falling to the ground, some fraction moves downward and enters the soil as infiltration. Sometimes a small proportion of infiltrated water flows in the unsaturated zone to a nearby stream as interflow. As a rainstorm continues, overland flow moves some of the water on the ground surface to nearby streams. All of these processes vary with time and space and depend on rainfall rate, soil characteristics, vegetation, and topography.

8. A key parameter of interest to hydrologists is the stream discharge, which is defined as the volume of water flowing past some location per unit time (units like ft3/s).