UMERICAL GROUND WATER  FLOW  MODELING  OF  ALAMATA  SUB-BASIN  IN  THE  RAYA  VALLEY  (NORTHERN  ETHIOPIA
Creators: name: GETACHEW WELAMO
Publisher: name: National Academic Digital Repository of Ethiopia
Year: 2009

description: Alamata  sub-basin  which  is  found  in  the  Raya  Valley  is  located  in  the  southern 
part  of  Tigray  Regional  State  of  the  Northern  Ethiopia.  The  area  covers  51  %  of 
the  Raya  valley  with  area  coverage  of  1292.57km
2. 
The  sub-basin  water 
resource  is  used  for  irrigation  and  domestic  water  supply  and  is  covered  by 
tertiary  volcanic  in  western  and  eastern  highlands  and  alluvial  deposit  in  the 
valley fill. 
The  main  purpose  of  this  project  work  is  to  make  a  quantitative  estimation  for 
sustainable  use  of  groundwater  resource  for  domestic,  live  stock  and  agricultural 
supply.  To  achieve this  objective  careful  understanding  of  hydrology,  hydrogeology 
and  dynamics  of  groundwater flow  in  and  around the  study  area  is  mandatory. 
As  numerical  groundwater  flow  models  represent  the  simplification  of  complex 
natural  systems,  different  parameters  were  assembled  into  conceptual  model  to 
represent  the  complex  natural  system  in  a  simplified  form.  The  conceptual  model 
was  put  into  the  numeric  model to  examine system  response. 
Numerical  groundwater  flow  was  simulated  in  the  model  by  the  finite-difference 
method  using  MODFLOW,  1996  (McDonald  and  Harabaugh,  1988).  The  finite 
difference  grid  consisted  of  1  layer,  113  rows  and  97  columns.  Two  dimensional 
profile  model  was  developed  considering  the  system  to  be  under  steady  state 
condition  and  assuming flow  system  view  point. 
Model  calibration  was  carried  out  by  trial  and  error  calibration  method  using 
groundwater  contours  constructed  from  heads  collected  in  14  wells.  The 
calibration  showed  that  about  100%  of  simulated  heads  were  within  the  calibration 
target  and  the  overall  root  mean  square  error  for  simulated  hydraulic  heads  is 
about  4.24m.  The  poor  fit  at  some  points  was  due  to  numerous  limitations 
associated  with the  model. 
Model  sensitivity  analysis  was  conducted  by  considering  the  horizontal  hydraulic 
conductivity  and  recharge  because  they  are  sensitive.  A  change  in  hydraulic 
conductivity  by  -55%,-45%,-25%  and  55%,  45%  25%  resulted  in  root  mean  square 
Arba Minch Universty 
xiii 
Alamata sub-basin  ground water modeling 
(RMS)  76 . 31 ,  53.7,  25. 43  24.5,  45. 6  and  51  respectively.  A change  in  recharge  by 
the  same  amount  as  the  horizontal  hydraulic  conductivity  RMS  head  changes  by 
53.3, 40.52,  23,  22.17,  39. 9,  and  47.8. 
As  the  model  is  intended  to  study  the  response  of  the  hydrologic  system  the 
following  four  scenarios  were  assigned:  In  the  first  and  second  scenarios  which 
apply  for  the  sub-basin  and,  withdrawals  were  increased  by  25  and  50%  of  the 
average  existing  withdrawals.  In  the  third  and  forth  scenarios,  25%  and  50% 
reduction  in  recharge  is  applied.  The  effects  of  the  scenarios  are  evaluated  with 
respect  to  change  induced  groundwater  heads  to  the  steady  state  simulated 
heads.  Accordingly,  an  increase  of  withdrawals  by  25%  over  the  whole  area 
resulted  in  an  average  decline  of  steady  state  water  level  with  minimum  decline  of 
3.747m  at  well  084  and  a  maximum  decline  of  7.74  at  well  PZ3.  Again  an 
increase  of  withdrawals  by  50%  results  with  a  minimum  drawdown  of  7.985m  at 
well  084  and  a  maximum of  31 . 472m at  well  PZ3. 
On  the  other way,  reduction  of  recharge  by  25%  results  with  a  minimum decline  of 
20.333m  at  well  8H21  and  a  maximum  decline  of  32.485m  at  well  PZ3.  A 
reduction  of  50%  recharge  resulted  with  a  complete  drying  of  well  PZ3.