DATE FILED: December 29, 2017 3:51 PM FILING ID: 98AB7865D8F01 CASE NUMBER: 2016CW3128       MEMO     28  December  2017     To:     Margaret  Medellin,  PE   Utilities  Portfolio  Manager   City  of  Aspen     From:   Aspen  Global  Change  Institute     Subject:  Potential  Impacts  of  Climate  Change  on  Aspen  Urban  Irrigation  and  Streamflow:   Preliminary  Results     Background     John  Katzenberger  of  AGCI  and  Prof.  Kenneth  Strzepek,  University  of  Colorado  and  MIT’s  Center   for  Global  Change  Research  are  in  the  final  stages  of  an  assessment  of  the  potential  impacts  of   climate  change  on  residential  and  commercial  lawn  irrigation  and  public  greenscape  irrigation   in  years  2061  to  2070  requested  by  the  City  of  Aspen.  Preliminary  results  of  this  work  provide   insights  into  the  nature  of  climate  change  impacts  on  Aspen’s  Urban  Irrigation  demand  and  the   flow  in  Pitkin  County  streams.  A  report  on  this  work  is  forthcoming.     Future  Climate     The  insights  presented  here  reflect  “projections”  of  the  future  climate  for  Pitkin  County   prepared  from  data  provided  by  the  US  Geological  Service  (Thrasher,  et,  2013)  based  on  two   Greenhouse  gas  emission  scenarios  (RCP4.5  –  low  emissions  and  RCP  8.5-­‐  high  emission)  and  30   general  climate  models  (General  Circulation  Model  –  GCM).  Results  for  all  thirty  are  provided  in   Annex  A.  What  this  shows  is  that  there  is  significant  uncertainty  among  the  GCM  projections   about  changes  in  Temperature  and  Precipitation,  but  the  trends  and  central  tendencies  are   similar.  It  should  be  noted  however,  that  in  general  GCMs  are  better  able  to  project  changes  in   temperature  for  a  region  compared  to  changes  in  precipitation.  Further,  projections  for   mountain  regions  are  less  well  represented  in  GCMs  and  their  downscaled  products  such  as   those  used  here.     1     Methods     Irrigation  demand  and  water  supply  is  the  result  of  complex  hydrologic  processes  associated   with  the  timing  of  precipitation  events  and  evapotranspiration  (a  process  that  integrates   temperature,  soil  water  and  climate).  To  assess  future  streamflow  and  irrigation  demand,   mathematical  models  of  these  processes  are  needed.     The  AGCI  analysis  modeled  irrigation  demand  by  using  the  modified  Hargraeves  equation  to   estimate  potential  evapotranspiration  which  was  calibrated  to  data  from  the  Aspen  Golf  Course   and  available  soil  water  using  the  USDA  effective  Precipitation  equation.    Future  runoff   estimates  are  from  a  USGS  water-­‐balance  model  of  the  various  components  of  the  hydrologic   system  coupled  with  the  climatological  projections.  (McCabe  and  Wolock,  2007).   For  this  preliminary  set  of  findings,  a  5-­‐member  representative  sample  of  the  30  GCMs  is   presented.  The  5  are  based  on  what  is  representative  for  the  conterminous  U.S.  providing  a   range  of  temperature  and  precipitation  changes  downscaled  to  Pitkin  County  over  the  decade   2061  to  2070.       Results     Table  1  Percentage  Change  in  Urban  Irrigation  from  Climate  Change  (AGCI  2018)     IWR  RCP  4.5   Apr   May   Jun   Jul   Aug   Sep   Oct   Growing   CCSM4   2%   14%   13%   5%   11%   -­‐1%   6%   8%   CanESM2   27%   31%   9%   5%   -­‐16%   21%   68%   10%   GISS-­‐E2-­‐R   18%   27%   13%   23%   24%   18%   -­‐22%   21%   HadGEM2-­‐ES   97%   39%   27%   17%   30%   -­‐8%   61%   21%   MIROC5   59%   23%   18%   1%   2%   16%   34%   12%   IWR  RCP  8.5     Apr   May   Jun   CCSM4   62%   45%   23%   CanESM2   68%   60%   30%   GISS-­‐E2-­‐R   3%   33%   28%   HadGEM2-­‐ES   124%   68%   39%   MIROC5   59%   47%   21%           Jul   Aug   14%   22%   -­‐7%   -­‐16%   26%   18%   34%   32%   8%   11%   Sep   Oct   Growing   33%   33%   27%   -­‐6%   77%   12%   -­‐9%   -­‐49%   19%   23%   62%   39%   35%   32%   24%   2     Table  2  Percentage  Change  in  Pitkin  County  Runoff  (USGS,  2013)       RUNOFF     RCP  4.5   Jan   Feb   Mar   Apr   May   Jun   Jul   Aug   Sep   Oct   Nov   Dec   ANN   Grow   CCSM4   22%   36%   64%   35%   6%   -­‐14%   -­‐19%   -­‐19%   -­‐23%   -­‐7%   12%   10%   9%   -­‐13%   CanESM   6%   27%   75%   30%   -­‐2%   -­‐14%   -­‐19%   -­‐13%   -­‐23%   -­‐42%   -­‐30%   -­‐10%   -­‐1%   -­‐19%   GISS-­‐E2   34%   21%   31%   18%   -­‐3%   -­‐10%   -­‐18%   -­‐20%   -­‐29%   -­‐10%   23%   27%   -­‐15%   HadGEM   -­‐7%   32%   25%   1%   -­‐9%   -­‐19%   -­‐29%   -­‐31%   -­‐29%   -­‐46%   -­‐16%   -­‐12%   MIROC5   -­‐5%   3%   59%   35%   -­‐6%   -­‐39%   -­‐47%   -­‐44%   -­‐43%   -­‐36%   -­‐10%   -­‐19%   5%   -­‐ 12%   -­‐ 13%   RUNOFF       RCP  8.5   -­‐27%   -­‐36%     Jan   Feb   Mar   Apr   May   Jun   Jul   Aug   Sep   Oct   Nov   Dec       Grow   -­‐1%   6%   23%   18%   -­‐8%   -­‐28%   -­‐38%   -­‐39%   -­‐47%   -­‐55%   -­‐21%   -­‐9%   -­‐ 16%   CanESM   110%   125%   129%   39%   -­‐20%   -­‐45%   -­‐49%   -­‐42%   -­‐38%   -­‐52%   -­‐3%   19%   14%   -­‐41%   GISS-­‐E2   56%   46%   61%   33%   -­‐3%   -­‐22%   -­‐34%   -­‐33%   -­‐31%   27%   53%   54%   -­‐16%   HadGEM2   21%   73%   52%   -­‐2%   -­‐33%   -­‐54%   -­‐63%   -­‐62%   -­‐60%   -­‐69%   -­‐34%   -­‐7%   MIROC5   -­‐1%   14%   115%   46%   -­‐24%   -­‐54%   -­‐61%   -­‐60%   -­‐65%   -­‐60%   -­‐47%   -­‐34%   17%   -­‐ 20%   -­‐ 19%   CCSM4   -­‐36%   -­‐57%   -­‐54%     Table  1  shows  that  all  the  models  project  significant  increases  in  the  irrigation  water  demands  if   current  greenscape  is  maintained.  The  increase  over  the  growing  seasons  for  RCP4.5  shows  a   range  from  8  to  21  percent  with  the  mean  14%  while  the  RCP  8.5  result  suggest  a  range  of  12  to   39  percent  increase  with  a  mean  of  24%  increase.       Table  2  reveals  very  important  results  regarding  runoff  in  Pitkin  County  streams.  The  color   coding  of  monthly  percentage  changes  shows  significant  increases  in  winter  and  early  spring   runoff  and  dramatic  decreases  in  late  spring  and  summer  runoff.     The  mean  annual  change  in  runoff  across  the  5  GCMs  for  the  RCP  4.5  emission  scenario  was  -­‐2   percent  while  over  the  irrigation  season  it  decreased  on  average  22  percent.  For  RCP  8.5,  the   mean  annual  change  in  runoff  across  the  5  GCMs  is  negative  5  percent  while  over  the  irrigation   season  it  decreased  on  average  41  percent.  Annex  B  provides  the  results  for  all  30  CGM  models.     Water  Resource  Management  Implications     The  preliminary  results  above  suggest  that  by  2060  urban  irrigation  demands  will  increase  and   streamflows  will  be  significantly  reduced  over  the  irrigation  season  while  Winter  and  Spring   runoff  will  increase  due  to  changes  in  the  snowmelt  process.  Two  of  the  5  GCMs  are  shown   here  in  Figures  1  and  2.  Each  figure  combines  percentage  change  in  irrigation  demand  and   streamflow.  These  two  results  provide  some  insights  for  water  management  adaptations  to   address  these  simultaneous  changes  from  these  two  extreme  GCM  projections.         3     Figure  1-­‐    GISS  MODEL  RESULTS  FOR  URBAN  IRRIGATION  and  RUNOFF       Figure  2-­‐      MIROC    MODEL  RESULTS  FOR  URBAN  IRRIGATION  and  RUNOFF       4   Both  results  show  significant  increase  in  irrigation  season  water  demand  GISS  21  and  16  and   MIROC  12  and  24  for  the  RCPs  4.5  and  8.5  respectively,  GISS  shows  dramatic  reduction  in   irrigation  season  runoff  while  MIROC  shows  small  increases  in  summer  runoff  but  less  than  the   increase  in  irrigation  demand.    Both  these  wet  and  dry  scenarios  suggest  a  decrease  in  the   water  balance  over  the  irrigation  season.         Recommendation  for  Water  Management     This  limited  preliminary  study  was  focused  on  evapotranspiration  for  the  current  irrigated   landscape  set  in  two  future  climate  scenarios  projected  for  the  2060s.  It  did  not  include  an   analysis  of  the  projected  overall  water  supply  or  demand  for  Aspen.  However,  based  on  the   preliminary  model  results,  the  following  two  points  taken  together  are  indicated  for  Aspen  in   developing  its  long-­‐term  water  management  strategy.     1. Reduce  the  amount  of  greenscape  that  is  being  irrigated  to  reduce  the  net  increase  in   urban  irrigation  demand  or  conversely  increase  the  irrigation  efficiency  with  the  current   greenscape  area.   2. Reservoir  Storage.    The  snow  balance  and  hydrologic  models  projections  for  the  decade   of  the  2060s  indicate  an  increased  winter  and  earlier  and  larger  spring  snowmelt  runoff   for  all  of  the  GCMs  (Annex  B).  With  the  support  of  a  more  extensive  analysis,  storage   could  be  considered  as  an  adaptive  strategy  given  climatological  change  and  potential   growth  in  demand.         Reference       Alder, J. R. and S. W. Hostetler, 2013. USGS National Climate Change Viewer. US Geological Survey http://www.usgs.gov/climate_landuse/clu_rd/nccv.asp doi:10.5066/F7W9575T. Hostetler, S.W. and Alder, J.R., 2016. Implementation and evaluation of a monthly water balance model over the U.S. on an 800 m grid. Water Resources Research, 52, doi:10.1002/2016WR018665. McCabe, Gregory J., and David M. Wolock. "Warming may create substantial water supply shortages in the Colorado River basin." Geophysical Research Letters 34, no. 22 (2007). Thrasher, B., J. Xiong, W. Wang, F. Melton, A. Michaelis, and R. Nemani, 2013. New downscaled climate projections suitable for resource management in the U.S. Eos, Transactions American Geophysical Union 94, 321-323, doi:10.1002/2013EO370002.   5   Annex A Figure A-1. Seasonal Change in Precipitation on Pitkin County Figure A-2. Seasonal Change in Temperature on Pitkin County   6   Annex B Figure B-1. Trends in Runoff on Pitkin County Figure B-2. Change in Seasonal in Runoff on Pitkin County     7