Applying Methods for Assessing the Costs and Benefits of CCA
2 nd Regional Training Agenda, 30 September – 4 October 2013
Priyanka Dissanayake- Regional Coordinator
Global Water Partnership South Asia
Prof Robert Mendelsohn - Yale University
October 3, 2013 Bangkok , Thailand
What is Hydrology?
– It is a science of water.
– It is the science that deals with the occurrence,
circulation and distribution of water of the earth
and earth’s atmosphere
– Estimation of water resources
– The study of processes such as
precipitation, evapotranspiration, runoff
and their interaction
– The study of problems such as floods and
droughts and strategies to combat them
Hydrologic Cycle
Water exists on the earth in all its three
states, viz. liquid, solid, gaseous and in
various degrees of motion
Hydrologic Cycle
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Water, irrespective of different states,
involves dynamic aspect in nature.
The dynamic nature of water, the
existence of water in various state
with different hydrological process
result in a very important natural
phenomenon called
cycle
Hydrologic
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Hydrologic Cycle
Evaporation of water from water bodies, such as oceans
and lakes, formation and movement of clouds, rain and
snowfall, stream flow and ground water movement are
some examples of the dynamic aspects of water
Hydrologic Cycle
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Evaporation from water
bodies
Water vapor moves
upwards
Cloud formation
Condensation
Precipitate
Interception
Transpiration
Infiltration
Runoff–stream flow
Deep percolation
Ground water flow
Hydrologic Cycle
The hydrologic cycle has importance influence
in a variety of fields agriculture, forestry,
geography, economics, sociology, and political
scene
Engineering application of the knowledge are
found in the design and operation of the
projects dealing with water supply,
hydropower, irrigation & drainage, flood
control, navigation, coastal work, various
hydraulic structure works, salinity control and
recreational use of water
Catchment area/Watershed
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The area of land draining in to a stream
or a water course at a given location is
called catchment area / drainage area /
drainage basin / watershed.
A catchment area is separated from its
neighboring areas by a ridge called
divide / watershed
Watershed and watershed divide
Watershed/
catchment
Watershed/
catchment
Watershed &
Water Budget Equation
A watershed is a geographical unit in which the
hydrological cycle and its components can be
analyzed.
Usually a watershed is defined as the area that
appears, on the basis of topography, to contribute
all the water that passes through a given cross
section of a stream
The equation is applied in the form of waterbalance equation to a geographical region, in order
to establish the basic hydrologic characteristics of
the region.
Water Budget Equation
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For a given catchment, in an interval of time
∆t, the continuity equation for water in its
various phases can be given as:
Mass inflow – Mass outflow = change in mass storage
(in soil or bedrock)
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If the density of the inflow, outflow and
storage volumes are the same:
Vi -
Inflow volume in to the catchment, Vo - Outflow volume
from the catchment and ∆S - change in the water volume
Water Budget Equation
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Therefore, the water budget of a catchment
for a time interval ∆t is written as:
P – R – G – E – T = ∆S
P = Precipitation, R = Surface runoff, G = net ground water flow out of
the catchment, E = Evaporation, T = Transpiration, and ∆S = change
in storage
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The above equation is called the water
budget equation for a catchment
The purpose of the water balance is to
describe the various ways in which the
water supply is expended
Components of hydrologic cycle
Evapo transpiration
Precipitation
Stream flow
(Runoff)
Inter flow
Infiltration
Base flow
Groundwater flow
Data for collection
Climate Model & Input Data
• Predicts precipitation by month in watershed
– Hourly precipitation if tracking floods
• Predicts temperature by month
• Land cover over watershed is an input
• Land cover and temperature used to predict
monthly evapotranspiration
• Snow and ice can hold (store) water
depending on temperature
• Precipitation, evapotranspiration, and change
in snow and ice determines flow
Water supply per person for individual
river basins as of 1995 WRI
water supply per person for individual
river basins projections for 2025 WRI
Observed Changes
Increases in
some places
and
decreases in
others
No clear
trend
Trend in Annual Runoff, 1971-1998 as a % of 1900-1970 Average
reconstructed by climate models
Source: Milly et al. 2005
Water for a food-secure world
www.iwmi.org
Projected Changes
Positive: Sri
Lanka,
Bangladesh,
India, parts
of Nepal and
Bhutan
Negative:
Pakistan and
Afghanistan
Change in Annual Runoff in 2090-2099 relative to 1980-1999
Source: Milly et al. 2005
Water for a food-secure world
www.iwmi.org