Groundwater availability

Methods - how we monitor

• Where and how we collect the data

  Monitoring sites

  Information used in this indicator has been compiled from groundwater resource consents and estimates of permitted groundwater takes in the Waikato region.

  Waikato Regional Council estimated the number of permitted users based on a database of wells drilled since 1988 (and some before this date) and a model of likely use based on both the number of people and animals in each aquifer zone.  The methodology of the model for calculating the ‘permitted’ water use as described for surface water catchments is detailed in Technical Report 2007/47 - "A model for assessing the magnitude of unconsented surface water use in the Waikato region".

  Monitoring history

  Data for this indicator were collected from groundwater resource consents and estimates of permitted groundwater takes up to 2015.
• How this indicator is compiled

  The natural flow patterns of groundwater systems are complex. It is difficult to accurately estimate the volume of water moving into and out of an aquifer. Waikato Regional Council estimates the amount of groundwater in an aquifer by:

  1. Estimating the amount of available groundwater (annual daily average).
  2. Comparing an aquifer’s volume of ‘available’ groundwater with the amount abstracted by authorised users.

  1. Estimating the amount of available groundwater

  We estimate the annual daily average of available groundwater using the following equation:

  A = (R_R x P_r x G_a x R_a) / 365 days

  A = annual daily average of groundwater availability – cubic metres per day (m³/day).
  R_R = average annual rainfall over the aquifer – metres per year (m/year).
  P_r = estimate of rainfall recharge to groundwater as a fraction of the average annual rainfall over the aquifer. This will usually range from 0.22 (22 per cent) to 0.4 (40 per cent) in the Waikato region.
  G_a = groundwater recharge available for allocation. This is estimated to be 50 percent (0.5 in the equation), allowing the remaining 50 percent of recharge to be lost via springs and submarine discharges.
  R_A = the size (area) of the aquifer – square metres (m²). The size of the aquifers are in line with the aquifers listed in Table 3-6 (Water Allocation chapter) in the Waikato Regional Plan. 

  The location of aquifers can be seen in our Waikato Regional Plan's water allocation maps.

  The above equation assumes:

  • There is no net groundwater recharge from rivers. If rivers are an important source of recharge, estimates of recharge using rainfall will be conservative.
  • The wells monitored intercept groundwater from a vertically uniform, single aquifer.
  • G_a of 50 percent is not conservative enough to adequately protect connected surface water bodies.  In this situation any further allocation from the aquifer may have a minor impact on an aquifer (e.g. subsidence, groundwater levels). However the potential decline in groundwater  discharge to a fully allocated stream may have a greater impact on the allocation status of the stream.  The simple water balance assessment of groundwater availability presented here does not take into account the allocation status of any connected surface water bodies which may restrict any further allocation of groundwater even if the aquifer is in the low stress category. This is assessed on a case by case basis for groundwater consents against Policy 3.3.12 (w) of the Regional Plan.

  2. The percentage of ‘available’ groundwater that is allocated for use

  We compare an aquifer’s volume of ‘available’ groundwater with the amount allocated (the maximum that can be used  by consented and permitted takes). From this we can estimate the level of stress groundwater resources are under. This approach does not take into account the allocation status of any connected streams and rivers which may restrict any further allocation of groundwater even if the aquifer is in the low stress category.

  We divide monitored groundwater resources in our region into three categories of ‘stress’:

  • Low stress areas have less than 10 per cent of available groundwater allocated for use.
  • Medium stress areas have between 10 and 30 per cent of available groundwater allocated for use.
  • High stress areas have more than 30 per cent of available groundwater allocated for use.

  This provides us with a guideline to identify potential problem areas which may need more intensive monitoring to manage impacts within the aquifer. Most of the monitored groundwater areas in our region are under low to medium stress.
• Guidelines and standards

  Proposed National Environmental Standard on Ecological Flows and Water Levels – Discussion Document. Ministry for the Environment March 2008.

  Waikato Regional Plan Policy 3.3.5: Determining Sustainable yields.
• Limitations
  • This indicator is based on maximum potential use (consented and permitted), not actual usage.
  • In catchments with a connection between groundwater and surface water, groundwater allocation may be limited by the availability of surface water.
  • Estimates of aquifer recharge are approximate, as there are few accurate water-balance studies available.
  • The extent of an aquifer is not easily defined. We tend to use a conservative size.
  • We assume that the groundwater wells intercept groundwater from a single, vertically uniform aquifer.
  • The simple water balance assessment of groundwater availability presented here does not take into account the allocation status of any connected surface water bodies. Aquifers listed as having a low allocation stress may not have water available due to the high allocation status of the connected streams and rivers.