Precipitation

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 • Adiabatic expansion due to pressure change

4. 8 in (20.32 cm) Standard Rain Gages (SRG) • Standard Rain Gages (SRG) Accuracy? 30 in (76.2 cm)

5. ESTIMATION OF PRECIPITATION OVER AN AREAEffective Depth of Drecipitation (EUD) • Arithmetic average: for evenly distribute stations (uniform density) • Thiessen method area-weighted averaging • Isohyetal lines contouring

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

7. (1) Arithmetic average Pa = 1/N ∑ Pi (13.97 + 22.1 + 59.2 + 48.0+ 137.2)/5 = 56.1

8. (2) Thiessen Polygon Method • Area-weighted average (every gage represents best the area immediately around the gage) Constructing Thiessen 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

10. )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

11. Isohyetal method

12. Isohyetal method procedure 30 • 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 25 20 28 26 15 (4) 22 (3) 17 (2) (1) 11

13. Example • P was measured at several stations. Characterize the precipitation over the area as the arithmetic average, Thiessen-weighted average, and isohyetal average.

14. Example, solution • Arithmetic average: Pa = (4.5+4.4+4.4+4.1+4.0+3.7+3.5+2.5+2.7)/9 = 3.76in

15. Example, solution 2. Thiessen-Weighted average:

16. Example, solution 3. Isohyetal Method:

17. 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.

18. Depression storage • Overland flow (P rate > If rate) • Interflow (horizontal flow in unsaturated zone) • Baseflow

19. Evaporation

20. 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

21. 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

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

23. 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)

24. 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

25. 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

26. Potential ET

27. 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

28. ET measurement • field measurement Large watertight caisson buried in ground, filled w/ soil and planted with vegetation. Example 2.2

29. Infiltration Overland Flow Interflow

30. 2.3 Infiltration, Overland Flow, and Interflow

34. 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)

36. Example 2.3 For a drainage area of 80 acres 30% 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.

37. Example 2.3 solution We need C, Tc, I C = 0.43 Tc from figure 2.8 = 14 min i 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

38. Figure 2.8Nomograph for estimating Tc for a small basin

40. Chapter Highlights • Precipitation can occur in different forms-rain, drizzle, snow, and sleet. • The standard U.S. precipitation gage and various types of recording gages (for example, tipping bucket) form the basis for regional, station-based measurements. • New NEXRAD radars provide a capability of continuous precipitation measurements over thousands of square miles. • 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.

41. Chapter Highlights • 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. • 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. • 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).