Gabriola Groundwater Report

Extracted text (beginning)

Department of  
Earth Sciences 
 
 
 
Groundwater Recharge Model for Gabriola Island 
 
R. Burgess and D.M. Allen  
Department of Earth Sciences, Simon Fraser University 
 
Final Report 
Submitted to: 
Regional District of Nanaimo 
 
            
Cover photo: Glenn Jasechko 
December 2016 

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Executive Summary 
The overall goal of this research project was to constrain estimates of groundwater recharge 
on  the  Gulf  Islands.  Gabriola  Island  was   used  as  a  case  study.  Better  constrained  recharge  
estimates will enable better estimates of the water balance components, which are needed 
for water supply and demand studies. This report documents the information collected and 
interpreted  to  formulate  a  conceptual  hydrogeological  model  of  Gabriola  Island,  briefly  
describes the numerical model setup and calibration, and presents the modeling results. 
 
Overall,  there  is  likely  minimal  variability  in  the  climate  of  Gabriola  Island  such  that  
precipitation,  temperature  and  PET  can  all  be  considered  spatially  uniform.  While  there  is  
variability in soil types, vegetation is considered to be relatively uniform (treed over 70% of 
the  island).  There  are  few  surface  water  features,  and  generally  only  ephemeral  streams  
form during the rainy season. There is variability in the hydraulic properties of the fractured 
bedrock on Gabriola Island at a local scale, and with depth. However, on a regional scale the 
fractured   bedrock   is   relatively   homogenous   and   can   be   represented   as   a   single   
hydrogeological unit. A decrease in hydraulic conductivity with depth suggests that below a 
depth of approximately 200 m, groundwater flow is negligible. 
 
A  fully  integrated  land  surface  –  subsurface  numerical  model  was  developed  for  Gabriola  
Island  using  state  of  the  art  software  MIKE  SHE.  The  model  was  first  forced  by  historical  
observed  climate,  and  then  by  projected  climate.  Actual  evapotranspiration,  overland  flow  
(runoff), recharge and groundwater seepage were simulated. Overall there is a good match 
between the averaged simulated and observed WELLS database groundwater levels and the 
model  error  is  randomly  distributed.  However,  the  model  slightly  overestimates  the  
groundwater levels, with a mean error of -4.4 m. The timing of groundwater level variations 
is well  reproduced  by  the  model.  Recharge  increases  from  October  to  January,  where  it  
reaches  a  pseudo-stable  rate,  before  declining  from  April  through  to  September.  The 
simulated  actual  evapotranspiration  (AET)  had  the  widest  range  (35-55%  of  mean  annual  
precipitation), while recharge displayed the least amount of variability (17 to 26% of mean 

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annual  precipitation).  The  simulated  mean  annual  recharge  to  Gabriola  Island  is  20%  of  
precipitation (or 199 mm/year). 
 
The 2050s and 2080s climate change simulatio

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