CE 3205 Water and Environmental Engineering. Watershed and Introduction to Precipitation. Mdm. Norhidayah Rasin. Cloud Formation. Rain Clouds. Evaporation. Precipitation. Soil. Storage. Runoff. Transpiration. Stream. Vegetation. Infiltration. Ocean. Percolation. Groundwater Flow.
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CE 3205 Water and Environmental Engineering
Watershed and Introduction to Precipitation
Mdm. Norhidayah Rasin
Water moves throughout the Earth by different pathways and at different rates
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
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.
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 (km2, 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
The slope of the sides of a watershed govern how fast water will drain to the channel
steep slopes - peaked hydrograph
gentle slopes - flat hydrograph
slope is vertical over horizontal distance, derived from topographic maps
An objective repeatable formula for land slope:
where L is the total length of contours,
CI is the contour interval and A is the
A headwater stream with no tributaries is a first order stream
When two first order streams join they form a second order stream
Two second order streams form a third order stream etc.
The ratio of the number of stream segments of a given order, Nn, to the number of segments of the next highest order, Nn+1, is called the bifurcation ratio, RB:
**Bifurcation-splitting of a main body into two parts
Upper Bernam Basin
Upper Bernam River Basin
SubWatershed – Bagan and Sat Rivers
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.
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.
Types of precipitation
Rain, snow, hail, drizzle, glaze, sleet
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
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.
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)
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.
1. Non recording gauge
1 - pole
2 - collector
3 - support- galvanized metal
4 – funnel
5 - steel ring
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
4-recording needle5-drum with diagram
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
1 - collecting funnel
2 - tilting baskets
3 - electric signal
4 - evacuation
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.
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.
World Meteorological Organization (WMO) recommendation:
In flat regions of temperate, Mediterranean and tropical zones
Ideal 1 station for 600 – 900 km2
Acceptable 1 station for 900 – 3000 km2
In mountainous regions of temperate , Mediterranean and tropical zones
Ideal 1 station for 100 – 250 km2
Acceptable 1 station for 250 – 1000 km2
In arid and polar zone
1 station for 1500 – 10,000 km2
10 % of the rain gauges should be self recording to know the intensity of the rainfall