Water Supply Modeling. 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.
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Water Supply Modeling 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 • 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 Hydrologic cycle
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 • 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 • 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 divide
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 water-balance equation to a geographical region, in order to establish the basic hydrologic characteristics of the region.
Water Budget Equation • 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) • 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 • 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 • The above equation is called the water budget equation for a catchment • The purpose of the water balanceis 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
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 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
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