SWAT Hydrological Model for Assessing Climate Change Impact on Water Resources

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SWAT Hydrological Model for Assessing Climate
Change Impact on Water Resources

• Sandhya Rao

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SWAT (Soil and Water Assessment Tool) - Model

• Features
• Physically based
• Distributed model
• Continuous time model (long term yield model)
• Uses readily available data
• Used for long term impact studies

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SWAT System and Processes Modeled

• Weather
• Hydrology
• Sedimentation
• Plant Growth
• Nutrient Cycling
• Pesticide Dynamics
• Management
• Bacteria

• Land phase of the hydrological cycle
• Water, sediment, pesticide loading to the main
  channel in each subbasin
• Routing (water) phase of the hydrological cycle
• Movement of water, sediment, pesticide loading
  through the channel network of the watershed to
  the outlet

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SWAT Strengths

• Upland Processes Comprehensive Hydrologic
  Balance
• Physically-Based Inputs
• Plant Growth Rotations, Crop Yields
• Nutrient Cycling in Soil
• Land Management BMP Tillage, Irrigation,
  Fertilizer, Pesticides, Grazing, Rotations,
  Subsurface Drainage, Urban-Lawn Chemicals, Street
  Sweeping

• Channel Processes
• Flexible Watershed Configuration
• Water TransferIrrigation Diversions
• Sediment Deposition/Scour
• Nutrient/Pesticide Transport
• Pond, Wetland and Reservoir Impacts

SWAT variable comparable to stream flow is
calculated as sum of Direct surface runoff,
Lateral flow (subsurface runoff) from soil
profile, GW flow from shallow aquifer
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Crop Growth Simulation

• Crop cover / vegetation cover determines
• Transpiration (LAI)
• Interception ? Surface runoff (soil cover)
• Soil erosion (soil cover)
• Additional water consumption (aET) due to
  irrigation
• Nutrient uptake (biomass production)
• Indirect impact on water quality due to farming
  practice (i.e. fertilization, pesticide
  application

• a generic crop growth model, one model simulates
  many crops
• The crops are differentiated by different
  parameter values
• Phenological development of the crop is based on
• daily heat unit accumulation
• Potential biomass increase, leaf-area index
  (LAI), plant size, water use and plant-growth
  constraints due to water, temperature, aeration
  and radiation

useful for evaluating some agronomic adaptations
to climate change, such as changes in planting
dates, modifying rotations (i.e., switching
cultivars and crop species), changing irrigation
practices, and changing tillage operations
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Application Impact of Climate Change on water
resources - India

• Initial National Communication of India to UNFCCC
• For Ministry of Environment and Forests
• To quantify the impact of the climate change on
  the water resources of India, Identify Hotspots,
  Identify Adaptation Coping strategies
• 12 river basins modelled
• Flood and drought analyses have been performed

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Tools Used

• Modelling SWAT (Soil and Water Assessment Tool)
  model used - provides opportunity for scenario
  generation
• GIS framework acts as a pre-processor for the
  distributed modelling and is also a powerful tool
  for visualization of the outputs/results in terms
  of V A

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Data Used for Modeling

• Digital Elevation Model 1km grid, generated
  using 1250,000 topographic map
• Land use Global data, 12M USGS
• Soil Global data, 15M FAO
• Drainage 1250,000
• Weather Data generated in transient experiments
  by the Hadley Centre for Climate Prediction
  U.K. at a resolution of 0.44 X 0.44 latitude by
  longitude grid points obtained from IITM, Pune
• HadRM2 IS92a IPCC Scenario
• 20 years current and 20 years GHG

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River Basins Modeled
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Layers

• DEM
• Delineated Basins
• Landuse
• Soil
• Weather

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Impact studied

• Impact on annual water availability
• Impact on seasonal water availability
• Impact on inter annual water availability
• Regional Variability of Water availability

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Annual mean water balance for Control and GHG
climate scenarios in different river basins
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Percent change in mean annual water balance for
Control and GHG climate scenarios
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Trend in Precipitation, Runoff and
Evapotranspiration for Control and GHG Climate
Scenarios

• Increase in precipitation in Mahanadi, Brahmani,
  Ganga, Godavari, and Cauvery, for the GHG
  scenario
• the corresponding total runoff for all these
  basins has not necessarily increased
• Cauvery and Ganga show decrease in total runoff.
  This may be due to increase in evapotranspiration
  on account of increased temperatures or variation
  in the distribution of the rainfall
• In the remaining basins decrease in precipitation
  has been expected
• The resultant total runoff has decreased in
  majority of the cases but for Narmada and Tapi

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Vulnerability Assessment Drought Flood

• Soil Moisture Index to monitor drought severity
• focuses on the agricultural drought where
  severity implies cumulative water deficiency
• weekly information has been derived using daily
  SWAT outputs to incorporate the spatial
  variability
• Daily outflow discharge taken from the SWAT
  output
• Maximum daily peak discharge has been identified
  for each year and for each sub-basin
• analysis performed to identify those basins where
  flooding conditions may deteriorate in the GHG
  scenario
• two vulnerable river systems Mahanadi and
  Brahmani

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Vulnerability Assessment Procedure

• Palmer Drought Severity Index (PDSI) widely used
  index
• incorporates information on rainfall, land-use,
  and soil properties in a lumped manner
• PDSI value
• below 0.0 indicates the beginning of drought
  situation
• A value below -3.0 as sever drought condition
• Soil Moisture Index to monitor drought severity
  using SWAT
• output to incorporate the spatial variability

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Drought Analysis

• Krishna Subbasins with maximum Monsoon Non
  monsoon events in Control GHG Scenario

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Spatial and temporal distribution of drought
conditions

• graduated colour depicts spatial variability of
  concurrent severity of drought, number of
  sub-basins where severe concurrent conditions
  prevailed in that year
• size of the green dot reveals the number of
  drought weeks experienced in each sub-basin over
  the 20 years

• Sabarmati and Mahi, sever drought conditions in
  comparison to control scenario
• Mahanadi and Brahmani , the drought conditions
  seem to improve in the GHG scenario

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Flood Analysis
Mahanadi River Basin

• Annual maximum daily peak discharges for two
  sub-basins of Mahanadi and Brahmani river basins
  for Control and GHG scenarios

Brahmani River Basin
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Spatial Distribution - Mahanadi River Basin

• spatial distribution of annual maximum daily peak
  for 19th year as a sample year for control
  scenario
• and 20 year bar charts for control and GHG
  scenarios for each of the sub-basins of the
  Mahanadi River

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Key Findings

• Under the GHG scenario the conditions may
  deteriorate in terms of severity of droughts in
  some parts of the country and enhanced intensity
  of floods in other parts
• there is a general overall reduction in the
  quantity of the available runoff under the GHG
  scenario
• Luni with the west flowing rivers Kutch
  Saurastra which occupies about one fourths of the
  area of Gujarat and 60 percent of the area of
  Rajasthan shall have acute physical water scarce
  conditions
• River basins of Mahi, Pennar, Sabarmati, Krishna
  and Tapi shall face constant water scarcities and
  the water shortage conditions
• River basins belonging to Cauvery, Ganga, and
  Narmada shall experience seasonal or regular
  water stressed conditions
• River basins belonging to Godavari, Brahmani and
  Mahanadi shall have rare water shortages and if
  exist are only confined to few locations

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Uncertainties

• Uncertainties in Climate Simulation
• Assumptions and Coarseness of the Data
• One GCM and one IPCC scenario used
• Landuse has been coarse
• detailed data on the agricultural land use and
  the cropping pattern has not been used
• Soil type and profile has also been scanty
• Water bodies including reservoirs were not
  incorporated due to lack of data on their
  capacities and the operation rules

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Future Improvements

• Use HadRM3 simulation daily
• Use more SRES scenarios
• Incorporate man made interventions like
  reservoirs, dams etc
• Identify hotspots with respect to drought,
  floods, incorporating socioeconomic and other
  desired parameters
• Pilot level flood zone mapping for river basin
• Integration of the results from water sector with
  other sectors to formulate coping strategies

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Thank You