1. 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), …

2. Formation Of Precipitation
Conditions:
Humid air cooled to dew-point T
Nuclei
Droplets to raindrops
Size of raindrops

3. Standard Rain Gages (SRG)
8 in (20.32 cm)
Standard Rain Gages (SRG)
30 in (76.2 cm)

4. Effective Depth Of Precipitation (UED)
Arithmetic average:
for evenly distribute stations (uniform density)
Thiessan method
area-weighted averaging
Isohyetal lines
contouring

5. Areal Estimation of P from a network of gages
13.97 mm
22.1 mm
137.2 mm
59.2 mm
48 mm

6. (1) Arithmetic average Pa = 1/N ∑ Pi
( )/5
= 56.1

7. (2) Thiessan Polygon Method
Area-weighted average
(every gage represents best the area immediately around the gage)
Constructing Thiessan Network:
Plot stations on a map
Connect adjacent stations by straight lines
Bisect each connecting line perpendicularly
Perpendicular lines define a polygon around each station
P at a station is applied to the polygon closest to it

8. Weight-ed P Weighted area Polygon area P St.No 1.788 0.128 15 13.97 1
9.273
0.281 33
22.1 2
14.5
0.245
28.8
59.2 3
6.672
0.139
16.4 48 4
28.4
0.207
24.3
137.2 5
60.633
1.00
117.5
280.47
Totals

9. )3) isohyetal method Based on areas calculated from contoured P map
(check first for effect of elevation by plotting P vs elevation)
STEPS:
Plot a contour map of P based on gage readings at station
Compute area between each successive contour lines
Pa = PaiAi/  Ai

10. Isohyetal method

11. 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 30 25 20 28 26 15
(4) 22
(3) 17
(2)
(1) 11

12. 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 in clay 
Low infiltration capacity causes more runoff and more erosion.

13. Depression storage
Overland flow (P rate > F rate)
Interflow (horizontal flow in unsaturated zone)
Baseflow

14. 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 saturate 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

15. Standard: US weather service class A Pan
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
Eo = I - O (+/-) S
Evaporation pan
Standard: US weather service class A Pan

16. Evaporation estimation
4 ft (1.22 m)
10” (25.4 cm)

17. Evaporation Pan operation:
Pans placed on supports to allow air circulation
A water depth of 7-8 in ( 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)

18. 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

19. 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

20. Potential ET

21. Theoretical estimation (empirical formulas)
Thornthwaite's method
Blaney and Criddle method
Penman Method
field measurement
Neutron-Probe method (most accurate method)
Lysimeter
Large watertight caisson buried in ground, filled w/ soil and planted with vegetation.

22. Thornthwaite Eqn.

23. Infiltration, Overland Flow, and Interflow

24. Infiltration, Overland Flow, and Interflow

25. Infiltration, Overland Flow, and Interflow
Infiltration: process of downward entry of water into soil
Infiltration rate depends on:
Sensitive to soil condition, Antecedent water content of soil
Influenced by soil use and management
Decreases with increasing time
Overland flow = surface runoff: flow across land surface to channels
Inflow: lateral flow of water above water table

26. Infiltration, Overland Flow, and Interflow

27. Infiltration, Overland Flow, and Interflow
Infiltration: process of downward entry of water into soil
Infiltration rate depends on:
Sensitive to soil condition, Antecedent water content of soil
Influenced by soil use and management
Decreases with increasing time
Overland flow = surface runoff: flow across land surface to channels
Inflow: lateral flow of water above water table