Guest post by Paul White, Senior Groundwater Scientist at GNS Science.
Groundwaters are very important water resources in many New Zealand regions — important because they are used for water supplies (urban and rural) and because they supply flow to many springs, streams, rivers and wetlands. The two major inflows to groundwater are from rainfall and from surface water.
We need to know the rates of recharge to groundwater so we can manage groundwater use. For example, groundwater use must be significantly less than groundwater recharge to ensure that groundwater wells and springs do not go dry.
Groundwater recharge from rainfall is the subject of this post which will cover some concepts, how it is estimated, measured, uncertainty and some relevant New Zealand water management polices.
Groundwater recharge from rainfall occurs as rainfall trickles through the soil into aquifers. However, only a portion of all rainfall actually reaches aquifers as recharge. This is because some rainfall evaporates from the ground, and some rainfall is transpired by plants back into the atmosphere – processes termed evapotranspiration. Groundwater recharge first reaches a shallow aquifer (termed the water table or unconfined aquifer). Then the recharge may discharge from the unconfined aquifer to surface waters or to deeper, confined aquifers. Flow paths in aquifers are typically understood using models of groundwater flow systems including geological models and groundwater flow models.
It is common for rainfall recharge to be the largest source of groundwater recharge. In those circumstances sustainable groundwater allocation policies should ensure that allocation is less than recharge and that actual use is less than allocation, i.e.
R > A > U
R = rainfall recharge estimate provided by science, e.g. projects to characterise rainfall recharge and uncertainty undertaken in the Waterscape research programme;
A = allocation of groundwater, which is a policy decision by the groundwater management authority;
U = use of groundwater.
Estimates and measurements of rainfall recharge are very useful for the development of groundwater allocation policies. Regional councils are responsible for policy decisions on groundwater allocation. Central government also has an input to decision-making. For example a National Environmental Standard, proposed by Ministry for the Environment (2008), recommends a default (in lieu of regional policies) maximum groundwater allocation as 35% of groundwater recharge.
Estimates of groundwater recharge from rainfall are often made using computer models that typically consider rainfall, evapotranspiration and soil properties. Measurements of groundwater recharge can be made with lysimeters — this is typically a tube sunk into the ground that encases a soil column. Water flow from the base of the lysimeter column is measured over time. Models of rainfall recharge at the local scale are typically tested against measurements of groundwater recharge at the local scale. Quantifying rainfall recharge at the regional scale involves the use of models and up-scaled measurements from lysimeters.
Rainfall recharge measurements demonstrate significant inflows of rainfall to groundwater. For example, the Canterbury lysimeters measured groundwater recharge in the range 26% (Lincoln) to 37% (Winchmore) of rainfall in the period 1999 — 2000 (White et al. 2003). These results were used to estimate regional rainfall recharge to groundwater in the area between the Waimakariri River and Rakaia River in the range 19.2 to 23.9 m3/s providing a useful indication of groundwater sustainability in comparison with annualised groundwater allocation (approximately 42.6 m3/s) and estimated groundwater use (6.8 m3/s) in the period.
Environment Canterbury established groundwater allocation zones in 2004 and adopted allocation limits based on estimates of land surface recharge (rainfall plus irrigation). Zones in which total allocation exceeded those limits were designated as being fully allocated and referred to as ‘red zones’. The Rakaia-Selwyn groundwater allocation zone has been classified as a ‘red zone’ from the initial adoption of this management policy and Environment Canterbury has recommended decline of further groundwater allocation (e.g. in the Rakaia-Selwyn hearings), reviewed groundwater consents, and placed annual volume limits on groundwater pumping.
This post shows some of the applications rainfall recharge measurements to groundwater resource management and groundwater resource characterisation.
Ministry for the Environment 2008. Proposed National Environmental Standard on Ecological Flows and Water Levels. Discussion Document. 61p.
White, P.A., Hong, Y-S., Murray, D., Scott, D.M. Thorpe, H.R. 2003. Evaluation of regional models of rainfall recharge to groundwater by comparison with lysimeter measurements, Canterbury, New Zealand. Journal of Hydrology (NZ) 42(1), 39-64.
Related Waiology posts:
The low down on groundwater
The importance of groundwater
Where does NZ take its water from?