1. Hydrology CIVL341

2. Precipitation

3. Forms of Precipitation
Precipitation is all forms of water that reach the earth from atmosphere Common forms of precipitation are: Rain : the term rainfall is used to describe precipitations in the forms of water drops of sizes larger than 0.5 mm. Snow : another important form of precipitation . It consists of ice crystals which usually combine to form flakes.

4. Forms of Precipitation
Drizzle : fine water droplets of size less than 0.5 mm. the drops are so small that they appear to float in the air. Glaze : when rain or drizzle comes in contact with cold ground at around 00 C, the water drops freeze to form an ice coating called glaze or freezing rain. Hail : it is showery precipitation in the form of irrigular lumps of ice of size more than 8 mm. occurred in violent thunderstorms.

6. Why do we need to measure rainfall?
Rainfall Measurement
Why do we need to measure rainfall?
Engineers - to design structures for runoff control i.e. storm-water drains, bridges etc.

7. What if you need to know the rainfall in a catchment?
Measure it yourself….
Type of rain gauges?
Where to put gauges?
How many gauges?
How do you map it?

8. 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 (in mm) by dividing the volume of water collected by the area of the opening of the cup. (The gauge marking often accounts for this).

9. 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.

10. Number and Distribution of Gauges
Need to consider:
size of area
prevailing storm type
form of precipitation
topography
aspect
season

11. Distribution of Gauges
The distribution of gauges should not be random.
only fixed characteristics of areas can be sampled randomly. Random events must be sampled by a systematic arrangement of sampling points
Practical considerations of access and exposure mean that some pragmatism is required in designing a network.
It is useful to locate gauges so that isohyetal maps can be drawn. Some gauges need to be near, or outside the catchment boundary in order to cover the catchment completely.

12. Number of Gauges Depends on Storm type
Cyclonic 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.

13. Methods of Computing a Rainfall
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.

16. Example 7: Estimate the mean precipitation for the catchment area were tabulated as below:
Solution :

18. Example 8: for the catchment shown below, If the rainfall depths recorded by Gauge A, B and C are 10mm, 8mm, 7mm and the corresponding areas are 2.1km2, 9.1km2 and 2.4km2, estimate the catchment average rainfall depth and the total volume of water from this rainfall event. Apply Thiessen Method of Analysis.

19. Solution :

27. Ex 9: point rainfalls due to a storm at several rain-gauge stations in a basin are shown in Figure EX.9. Determine the mean areal depth of rainfall over the basin by the three methods.

28. Figure EX.9

31. Optimum Rain Gauge Network Design
Where :
N: optimum number of rain gauge station to be establish in the basin
Cp: coefficient of variation of the rainfall of the existing rain gauge stations
P: desired degree of percentage error in the estimate of the average depth of rainfall over the basin

32. Ex 10: for the basin shown below, the normal annual rainfall depths recorded and the isohyetals are given. Determine the optimum number of rain gauge stations to be established in the basin if it is desired to limit the error in the mean value of rainfall to 10%. Indicate how you are going to distribute the additional rain gauge stations required, if any. What is the percentage accuracy of the existing network in the estimation of the average depth of rainfall over the basin?

35. Thus additional rain gauge stations to be established = N-n=11-5=6
The additional six rain gauge stations have to be distributed in proportion to the areas between the isohytals as shown below:

36. The percentage error P in the estimation of average depth of rainfall in the existing network: :
The percentage accuracy = 85.2%

37. Ex 11: For the basin shown in Figure below, the normal annual rainfall depths recorded and the isohyetals are given. Determine the optimum number of rain-gauge stations to be established in the basin if it is desired to limit the error in the mean value of rainfall to 10%. Indicate how you are going to distribute the additional rain-gauge stations required if any. What is the percentage accuracy

40. EX12: The figures below show the spatial distribution of rainfall stations in Faria Catchment along with the long-term annual rainfall depth. You need to find out the average rainfall in the catchment and the total rainfall volume using a. The arithmetic-mean method b. Thiessen method c. Isohyetal method

41. Name
Rain (mm)
Area (km2)
Tammun
322.3 49
Nablus
642.6 36
Beit Dajan
379.1 75
Al Faria
198.6
100
Tubas
415.2 19
Talluza
630.5 52

42. EX. 13 A rain gauge network is shown in Fig
EX. 13 A rain gauge network is shown in Fig. below Determine the mean areal rainfall depth for the area by Thiessen polygon method. The area is composed of a square plus an equilateral triangular plot of side 2 km. Rainfall readings are in cm at the various stations indicated.