2. 1. Introduction
1.1 Hydrologic cycle
1.2 Global water distribution
1.3 Atmospheric subsystem
1.4 Drainage basin
Course Outline
3. 2. Precipitation
2.1 Precipitation mechanism
2.2 Precipitation measurement
2.3 Mean basin rainfall
2.4 Depth – Area – Duration relationship
2.5 Intensity – Duration – Frequency analysis (IDF)
4. 3. Evaporation and Evapotranspiration
3.1 Factors controlling evaporation
3.2 Evaporation from free water surface
3.3 Evapotranspiration
3.4 Estimation of evaporation and evapotranspiration
5. 4. Runoff and streamflow
4.1 Sources and components of runoff
4.2 Hydrograph separation
4.3 Streamflow estimation
4.4 Flood frequency analysis
4.5 Analysis of drought
4.6 Rainfall and runoff relationship: Introduction to hydrologic models
6. 1. Introduction
✓ Why do we need to study hydrology?
➢ Hydrology is the study of the cycling of water through different reservoirs
on earth. Hydrology focuses on the distribution of water in the subsurface,
surface and atmosphere, the chemistry of that water, and the effects of
climate on water cycle.
➢ The science dealing with the occurrence, circulation, distribution, and
properties of the waters of the earth and its atmosphere.
7. 1.1 Hydrologic cycle
➢ Hydrologic cycle involves the continuous circulation of water in the earth-atmosphere
system.
➢ Engineering hydrology takes a quantitative view of the hydrologic cycle.
➢ The quantification of the hydrologic cycle can be represented by a mass balance
equation and it can be applied either on global or regional scale.
Inputs (I) minus outputs (O) are equal to the change in storage (ΔS)
I - O = ΔS
8.
9.
10. ✓ Precipitation
✓ Evaporation
✓ Transpiration
✓ Infiltration
✓ Overland flow
✓ Surface runoff (streamflow)
✓ Groundwater flow
Hydrological Processes
Water Balance Components
This occurs as change in:
✓ Precipitation
✓ Surface runoff inflow into a given area
✓ Groundwater inflow from adjoining areas
Inflow:
Outflow:
✓ Surface runoff outflow
✓ Evaporation
✓ Transpiration
✓ Groundwater outflow
Change in Storage:
✓ Groundwater storage
✓ Soil moisture storage
✓ Surface reservoir water storage
11. ✓ Stainable water resources planning and management
✓ Environmental protection and management
✓ Prevention and control of natural disasters
✓ Mitigation of the negative impacts of climatic change
❑ Understanding the hydrologic cycle is essential for:
❑ Water balance of a drainage basin
SM + GWS = P - I - AET - OF - GWR
SM: soil moisture storage change
GWS: groundwater storage change
P: precipitation
I: interception
AET: evapotranspiration
OF: Overland flow
GWR: groundwater runoff → baseflow
12. ❑ Case study
➢ The water table fluctuates with
climate. The high water table brought
high-salinity water to the root zone,
which resulted in “physiological
drought” caused by osmosis.
➢ The osmotic force drives the flow of
water across cell walls from low salinity
regions to high-salinity regions. When
soil salinity is too high, plant roots
cannot extract water from the soil
against the strong osmotic force.
➢ Water table: the boundary between moist and saturated soil
13. ➢ Global water is unevenly distributed around the world.
➢ Only 3% of water on the surface is fresh; the remaining 97% resides in the ocean.
➢ Of freshwater, 69% resides in glaciers, 30% underground, and less than 1% is located
in lakes, rivers, and swamps.
1.2 Global Water Distribution
❑ Why is there uneven distribution of water in the world?
➢ Global warming will change patterns of rainfall and water distribution
➢ Regions that currently receive an adequate water supply of rainfall may shift in
the future under the changing climate.
Oceans 97.2%
Rivers and lakes 0.01%
Ice caps and glaciers 2.1%
Soil water 0.005%
Groundwater 0.61%
Distribution of water in the global hydrologic cycle
14. Change in average annual runoff
by 2050 under:
a) HadCM2 ensemble mean and
b) HadCM3 (Arnell, 1999b).
15. 1.3 Atmospheric Subsystem
Temperature
Evaporation
Precipitation
➢ Precipitation and evaporation are the hydro-meteorological processes in the
atmosphere that interact with surface water.
➢ The water holding elements of the hydrological cycle are:
a) Atmosphere
b) Vegetation
c) Snow packs
d) Land surface
e) Soil
f) Streams, lakes and rivers
g) Aquifers
h) Oceans
17. 1.4 Drainage Basin Catchment?
Watershed?
➢ A drainage basin is any area of land where precipitation collects and drains off into a
common outlet, such as into a river, bay, or other body of water.
➢ A catchment is a portion of the earth’s surface that collects runoff and concentrates
it at its furthest downstream point, referred to as the catchment outlet.
➢ The terms watershed and basin are commonly used to refer to catchments.
Generally, watershed is used to describe a small catchment (stream watershed),
whereas basin is often used for large catchment (river basins).
18.
19. ➢ Drains surface water to a common outlet
➢ Drainage divide - how is it defined?
➢ Scale effects?
➢ Stream orders?
➢ Stream gauging station at the outlet
20. ➢ Palmate channel network (Fig. a and b)
Stream discharge gradually increases downstream
➢ Pinnate channel network (Fig. c)
Stream discharge suddenly increases
✓ Drainage density = total channel length (km) / Basin area (km)
How does this affect the hydrologic response?
✓ Relief ratio = elevation difference, highest – lowest /length of the basin, parallel to
the main stream
Relief ratio indicates the average slope of the basin.
21. This figure shows that the geometry of the
drainage network will have a strong influence
on the shape of the catchment hydrograph.
➢ The basin shape and the pattern of the drainage network could influence the size
and shape of flood peaks at the basin outlet.
➢ This is important to understand how the changes within a basin, such as land cover,
may (or may not) affect the pattern of flows at the drainage basin outlet.
22. ❑ Stream orders: How do we classify the rivers from a hydrological view point?
➢ Horton suggested a classification of stream order as a
measure of the amount of branching within a basin.
❑ Scale effects
✓ A first order stream is a small, un-branched tributary,
✓ A second order stream has only first order tributaries,
✓ A third order stream has only first and second order
tributaries and so on.
✓ “Zero-th” order channels: They do not show on
map sheets as water courses, but becomes
channels during storms.
23.
24. Weekly Quiz
1. Figures show the channel patterns of two watersheds. Briefly comment on differences
between the hydrologic response of the two watersheds to a heavy rain event.
A. Head water areas of stream A are located at various distances from the main channel. Therefore, storm
flow enters the main channel at different timings, resulting in gradual increase of flow downstream. In
contrast, head water areas of stream
B. is located at similar distances from the main channel, resulting in a sudden increase of flow, particularly
at the confluence of multiple streams.
2. Write a water balance equation of a watershed including storage, precipitation,
interception, evapotranspiration, surface runoff, and groundwater runoff.
o Change in storage = Precip - Interception -ET - Surface runoff - Groundwater runoff
3. List and describe five major factors that determine a catchment’s response to a given
rainfall
A B
