1. Precipitation Single strongest variable driving hydrologic processes
Formed water vapor in the atmosphere
As air cools its ability to ‘hold’ water decreases and some turns to liquid or ice

2. Hydrologic Cycle

3. Causes of Precipitation

4. U.S. Annual Precipitation
Figure 2.6
Have students explain differences in precip due to air masses
Rainfall type

5. U.S. Annual Precipitation
Figure 2.6
Have students explain differences in precip due to air masses
Rainfall type

6. Illinois. Annual Precipitation
Average annual precipitation pattern for Illinois with average monthly amounts (inches) for selected weather stations,

7. Forms of Precipitation
Rain - Rain is liquid precipitation that reaches the surface in the form of drops that are greater than 0.5 millimeters in diameter
Drizzle - Drizzle is liquid precipitation that reaches the surface in the form of drops that are less than 0.5 millimeters in diameter.
Snow - Snow is an aggregate of ice crystals that form into flakes.

8. Forms of Precipitation
Hail - Hail is dense precipitation ice that is that least 5 millimeters in diameter.
Graupel - Graupel forms in the same way as hail except the diameter is less than 5 millimeters.
Freezing Fog - Freezing fog is a fog composed of supercooled water drops that freeze just after they wet the earth's surface

9. Illinois Annual Snowfall
Average annual snowfall pattern for Illinois with average monthly amounts (inches) for selected weather stations.

10. Snow Measurement Determine the water equivalent
5%-60% of snow depth may be water equivalent-- “density”
Snow pillows use antifreeze solution and pressure measurement to measure water equivalent

11. Location of Gages Gauges measure point rainfall
True precipitation unaffected by surroundings-winds, trees, buildings
Clearance distance 2 times height of object
For large areas multiple gauges are needed for more accurate estimates

12. Placement of Rain Gauges
Gauges are affected by wind pattern, eddies, trees and the gauge itself, therefore it is important to have the gauge located and positioned properly.
1m above ground level is standard -
all gauges in a catchment should be the same height
2 to 4 times the distance away from an isolated object (such as a tree or building) or in a forest a clearing with the radius at least the tree height or place the gauge at canopy level

13. Placement of Rain Gauges
shielded to protect gauge in windy sites
or if obstructions are numerous they will reduce the wind-speed, turbulence and eddies.

14. Placement of Rain Gauges
For sloping ground the gauge should be placed with the opening parallel to the ground
The rainfall catch volume is then divided by the opening area that the rain can enter

15. Number of Gauges Depends on Storm type
Frontal storms (large areas, low intensities) -small number of gauges may be O.K.
Convective storms (local, intense, uneven distribution) -denser network needed. Convective storms may have seasonal dominance -need to consider this as well.
Orographic rainfall due to mountains (not fronts) -may need denser network than flatter area.

17. Methods of Measuring Rainfall: Manual
Often have a funnel opening into a cylinder gauge.
Come in a variety of shapes and sizes
Calculate the rainfall ( inches) by dividing the volume of water collected by the area of the opening of the cup. (The gauge marking often accounts for this).

18. Methods of Measuring Rainfall: Remote
Tipping bucket rain gauge -The bucket tips when precipitation of 0.2 mm, 0.5 mm, 1.0 mm has been collected. Each tip is recorded by a data logger.
Weather Station - Records rainfall, but also evaporation, air pressure, air temperature, wind speed and wind direction (so can be used to estimate evapo-transpiration)
Radar - Ground-based radar equipment can be used to determine how much rain is falling and where it is the heaviest.

19. Weather Station

20. Methods of averaging rainfall data
Arithmetic average
Theissen polygons
Isohyetal method
Although, most of these calculations are done with computer mapping programs, it is still useful to understand these methods.

21. Isoheyetal method for Mapping Rainfall
The most basic method of representing the spatial distribution. This is generally the most accurate method but is also the most laborious.
Locate all rainfall stations on a base map and record the rainfall amount.
Draw isohyets (lines of equal rainfall) by proportioning the distances between adjacent gauges according to differences in catch.

22. Thiessen Method for Average Rain
Step 1
Figure 2.15

23. Thiessen Method for Average Rain
Step 2
Figure 2.15

24. Thiessen Method for Average Rain
Figure 2.15
Step 3

25. Thiessen Method for Average Rain
Step 4
Figure 2.15
After summing areas and rainfall Thiessen predicts 2.08 inches, and the mean is 1.97 inches. This is because the heavier rainfall amounts in center of study area get more carry more area and weight in the calculation.

26. Interpolating Rainfall Data
Nearest neighbor
Inverse distance weighted method
Kriging

27. INTERPOLATION
Procedure to predict values of attributes at unsampled points within the region sampled
Why? Examples:
Can not measure all locations: - temperature
Time acid rain deposition
Money soil characteristics
Impossible (physically, legally) - mining: gold deposits
- Changing cell size
- Missing/unsuitable data
- Past date (e.g. temperature)

28. INTERPOLATION Many different methods
All methods use location and value at sampling locations to estimate the variable of interest at unmeasured locations
Methods differ in weighting and number of observations used
Each method produces different results (even with same data)
No method best for every application
Accuracy is often judged by withheld sample points (difference between the measured and interpolated values)

29. INTERPOLATION Thiessen Polygon
Assigns interpolated value equal to the value found at the nearest sample location
Conceptually simplest method
Only one point used (nearest)
Often called nearest sample or nearest neighbor

30. Thiessen Polygon
Start: 1) 2) 3)
Draw lines connecting the points to their nearest neighbors.
Find the bisectors of each line.
Connect the bisectors of the lines and assign the resulting polygon the value of the center point 1 2 3 5 4

31. Sampled locations and values
Thiessen polygons

32. INTERPOLATION Thiessen Polygon Advantage: - Ease of application
- Appropriate for discrete (i.e., categorical) variables
Disadvantages:
- Accuracy depends largely on sampling density
- Boundaries often odd shaped at transitions
- Continuous variables often not well represented

33. INTERPOLATION Inverse Distance Weighted (IDW)
Estimates the values at unknown points using the distance and values to nearby know points (IDW reduces the contribution of a known point to the interpolated value)
Weight of each sample point is an inverse proportion to the distance.
The further away the point, the less the weight in helping define the unsampled location

34. INTERPOLATION Inverse Distance Weighted (IDW)
Zi is value of known point
Dij is distance to known point
Zj is the unknown point
n is a user selected exponent
(often 1,2 or 3)
Any number of points may be used up to all points in the sample; typically 3 or more

35. INTERPOLATION Inverse Distance Weighted (IDW)

36. INTERPOLATION Inverse Distance Weighted (IDW)
Factors affecting interpolated surface:
Size of exponent, n affects the shape of the surface
(larger n means the closer points are more influential)
A larger number of sample points results in a smoother surface

37. INTERPOLATION Inverse Distance Weighted (IDW)

38. INTERPOLATION Inverse Distance Weighted (IDW)