Grid-based Parameterization of  Monthly  Water Balance Model for  Simulation of  River Flow at  Ungauged  Sites  in  Abay River  Basin
Creators: name: Birhanu Kebede
Publisher: name: National Academic Digital Repository of Ethiopia
Year: 2009

description: This  paper  describes  the  development  and  validation  of grid-based  regional  prediction  equations 
of  a  conceptual  monthly  water  balance  model  parameters.  The  purpose  of  the  research  is 
generally  to  provide  gridded  values  of the  model  parameters  using  catchment  characteristics  for 
simulating  monthly  runoff  at  ungauged  rivers,  which  at  most  suffer  from  lack  of  hydrological 
information.  The  catchment  indices  (characteristics)  that  are  used  in  developing  the  prediction 
equations  of the  model  parameters  are  mean  annual  rainfall,  average  slope,  NDVI, TWI,  aspect, 
flow  length,  flow  direction,  maximum  soil  moisture,  elevation,  etc.  The  monthly  water  balance 
model  has  three  parameters;  C,  k  and  SC.  Not  all  catchment  characteristics  are  correlated  to  all 
parameters,  but  parameter  C  was  better  correlated  with  mean  annual  rainfall  and  NDVI;  and 
parameter  SC  with  mean  annual  rainfall,  NDVI  and  average  slope.  Parameter  k  was  not 
significantly  correlated  to  any  of  the  indices,  and  hence taken  as  the  average  of  optimized values. 
The  monthly  water  balance  model  was  calibrated  for  a  group  of selected  28  gauged  catchments 
using  FORTRAN  program  that  uses  Shuffled  Complex  Evolution  algorithm  optimization 
technique.  The  model  inputs  used  in  optimizing  the  parameters  in  the  calibration  are  areal 
precipitation,  calculated  using  Thiessen  polygon  method,  and  monthly  potential 
evapotranspiration  calculated  using  Penman-Montieth  and  Thornthwait.  The  monthly  potential 
evapotanspiration  obtained  using  Thorthwait  methed  is  underestimated  and  it  was  adjusted  by 
multiplying each  monthly value  by  mean monthly ratio  of Penman  to  Thornthwaite.  The  R2, IVF 
and  MSE were the  criteria used  to  evaluate the  optimization technique. 
The  optimized  parameter  values  were  related  to  physical  catchment  indices  using  regression 
equations.  The  relationships  were  tested  by  comparing  calibrated  parameters  values  and  the 
predicted  values  using  the  regression  equation  and  evaluated  using  coefficient  of determination, 
R
2, 
for  the  same  catchments  used  in  calibration.  The  result  showed  that  predicted  parameters  are 
not  much  different  from  the  optimized.  After the  parameters  are  optimized  and  evaluated  for  the 
selected  catchments,  the  basin  was  descretized  into  2131  grids  of  size  1  Okm  by  1  Okm  and  the 
three  model  parameter  values  estimated  for  each  grid  using  the  regression  equation.  Then  the 
vii 
model  was  tested  at  kessie  gauging  site  and  the  result  shows  that  the  model  is  promising  and  its 
result will be  improved  by  using grided  inputs  data.  The  correlation  coefficient  between  observed 
and  predicted  discharges  is  0.88. As  the  model  is  developed  mainly for  unguaged  sites,  it  is  also 
tested  for  ungauged  site  taking  Muger sub-basin  as  unguaged  catchment  to  show  how  the  model 
works  so  that  one  can  use the  model to  generate  flow data  any  where  in  the  regio