1. CE 3205 Water and Environmental Engineering Watershed and Introduction to Precipitation Mdm. Norhidayah Rasin

2. Hydrologic Cycle Rain Clouds Precipitation Cloud Formation Evaporation Vegetation Stream Soil Ocean Transpiration Infiltration Percolation Groundwater Flow Runoff Storage Water moves throughout the Earth by different pathways and at different rates

3. Hydrologic Cycle

4. The Watershed or Basin Area of land that drains water, sediments and dissolved materials along a stream channel to a single outlet and is separated from other watersheds by a drainage divide. Rainfall that falls in a watershed will generate runoff to that watershed outlet. Topographic elevation is used to define a watershed boundary Outlet

5. Watershed Watershed – Area of land draining into a stream at a given location Streamflow – Gravity movement of water in channels –Surface and subsurface flow –Affected by climate, land cover, soil type, etc.

6. Watershed – Hydrologic System

7. Watershed/Drainage Basin Terms Catchment Catchment area Catchment basin Drainage area River basin Water basin Watershed

8. Drainage Patterns/Networks Dendritic ParallelTrellis RectangularRadial Centripetal Deranged

9. Watershed Characteristics  Size  Slope  Shape  Soil type  Storage capacity  Land use / cover Reservoir Divide Natural stream Urban Concrete channel

10. Basin size Delineate watershed according to the height of land that separates water draining to the point of interest from water that drains to adjacent basins Delineate watershed according to the height of land that separates water draining to the point of interest from water that drains to adjacent basins Watershed area (km 2, ha) Watershed area (km 2, ha) – smaller watersheds tend to have a more peaked hydrograph, more intermittent water supply – larger watersheds have flatter hydrographs because larger channel network can store more water

11. Watershed Land Slope The slope of the sides of a watershed govern how fast water will drain to the channel The slope of the sides of a watershed govern how fast water will drain to the channel steep slopes - peaked hydrograph steep slopes - peaked hydrograph gentle slopes - flat hydrograph gentle slopes - flat hydrograph slope is vertical over horizontal distance, derived from topographic maps slope is vertical over horizontal distance, derived from topographic maps An objective repeatable formula for land slope: An objective repeatable formula for land slope: where L is the total length of contours, CI is the contour interval and A is the watershed area.

12. Strahler’s Order of Streams A headwater stream with no tributaries is a first order stream A headwater stream with no tributaries is a first order stream When two first order streams join they form a second order stream When two first order streams join they form a second order stream Two second order streams form a third order stream etc. Two second order streams form a third order stream etc. 1 1 2 2 1 11 2 1 1 2 3 3 1

13. Bifurcation Ratio (R B ) The ratio of the number of stream segments of a given order, N n, to the number of segments of the next highest order, N n+1, is called the bifurcation ratio, RB: **Bifurcation-splitting of a main body into two parts

14. Bifurcation Ratio - Example Stream order No.of stream order Bifurcation ratio 11717/6 266/2 322/1 41-

15. Watershed Delineation Upper Bernam Basin Upper Bernam River Basin Catchment Surface SKC Bridge GIS Outlet DEM Data

16. Typical Drainage Basin – Langat River Basin

17. SubWatershed – Bagan and Sat Rivers Sg. Bagan Sg. Sat

18. Watershed Delineation

19. 19 Draw Sub-watersheds

20. Stream Networks

21. 21 Precipitation

22. 22 Introduction All forms of water that reach the earth from the atmosphere is called Precipitation. The usual forms are rainfall, snowfall, frost, hail, dew. Of all these, the first two contribute significant amounts of water. Rainfall being the predominant form of precipitation causing stream flow, especially the flood flow in majority of rivers. Thus, in this context, rainfall is used synonymously with precipitation.

23. 23 Introduction…. In nature water is present in three aggregation states: –solid: snow and ice; –liquid: pure water and solutions; –gaseous: vapors under different grades of pressure and saturation The water exists in the atmosphere in these three aggregation states.

24. 24 Introduction…. Types of precipitation –Rain, snow, hail, drizzle, glaze, sleet Rain: –Is precipitation in the form of water drops of size larger than 0.5 mm to 6mm –The rainfall is classified in to Light rain – if intensity is trace to 2.5 mm/h Moderate – if intensity is 2.5 mm/hr to 7.5 mm/hr Heavy rain – above 7.5 mm/hr

25. 25 Introduction…. Snow: –Snow is formed from ice crystal masses, which usually combine to form flakes Hail (violent thunderstorm) –precipitation in the form of small balls or lumps usually consisting of concentric layers of clear ice and compact snow. –Hail varies from 0.5 to 5 cm in diameter and can be damaging crops and small buildings.

26. 26 Temporal and Spatial Variation of Rainfall Rainfall varies greatly both in time and space –With respect to time – temporal variation –With space – Spatial variation The temporal variation may be defined as hourly, daily, monthly, seasonal variations and annual variation (long-term variation of precipitation)

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29. 29 Measurement of Rainfall Rainfall and other forms of precipitation are measured in terms of depth, the values being expressed in millimeters. One millimeter of precipitation represents the quantity of water needed to cover the land with a 1mm layer of water, taking into account that nothing is lost through drainage, evaporation or absorption. Instrument used to collect and measure the precipitation is called rain gauge.

30. 30 Rainfall measurement… Precipitation gauge 1 - pole 2 - collector 3 - support- galvanized metal sheet 4 – funnel 5 - steel ring 1. Non recording gauge

31. 31 2. Recording gauge / graphic rain gauge The instrument records the graphical variation of the fallen precipitation, the total fallen quantity in a certain time interval and the intensity of the rainfall (mm/hour). It allows continuous measurement of the rainfall. The graphic rain gauge 1-receiver 2-floater 3-siphon 4-recording needle 5-drum with diagram 6-clock mechanism

32. 32 3. Tele-rain gauge with tilting baskets The tele-rain gauge is used to transmit measurements of precipitation through electric or radio signals. The sensor device consists of a system with two tilting baskets, which fill alternatively with water from the collecting funnel, establishing the electric contact. The number of tilting is proportional to the quantity of precipitation, hp The tele-rain-gauge 1 - collecting funnel 2 - tilting baskets 3 - electric signal 4 - evacuation

33. 33 4. Radar measurement of rainfall The meteorological radar is the powerful instrument for measuring the area extent, location and movement of rainstorm. The amount of rainfall overlarge area can be determined through the radar with a good degree of accuracy The radar emits a regular succession of pulse of electromagnetic radiation in a narrow beam so that when the raindrops intercept a radar beam, its intensity can easily be known.

34. 34 Rain gauge Network Since the catching area of the rain gauge is very small as compared to the areal extent of the storm, to get representative picture of a storm over a catchment the number of rain gauges should be as large as possible, i.e. the catchment area per gauge should be small. There are several factors to be considered to restrict the number of gauge: – Like economic considerations to a large extent –Topographic & accessibility to some extent.

35. 35 Raingauge Network….. World Meteorological Organization (WMO) recommendation: –In flat regions of temperate, Mediterranean and tropical zones Ideal  1 station for 600 – 900 km 2 Acceptable  1 station for 900 – 3000 km 2 –In mountainous regions of temperate, Mediterranean and tropical zones Ideal  1 station for 100 – 250 km 2 Acceptable  1 station for 250 – 1000 km 2 –In arid and polar zone 1 station for 1500 – 10,000 km 2 10 % of the rain gauges should be self recording to know the intensity of the rainfall

36. 36 END THANK YOU Next topics to be continued.. -Preparation data -Estimation of missing data -Test for consistency record -Mean Precipitation over an area arithmetic mean the method of the Thiessen polygons the isohyets method -Runoff estimation