Dr Vince Bidwell
24th July 2006
Many of us have contemplated the Canterbury Plains on a hot norwest day during a summer drought and wondered about schemes for importing some of that excessive rainfall falling on the West Coast. In fact, such a scheme already exists naturally. The Waimakariri and Rakaia Rivers that border the Central Plains receive most of their water from near the Main Divide where precipitation is measured in metres. As the water in these rivers crosses the plains towards the coast, about 5-10 % of the flow seeps through the river bed into the groundwater aquifers below. This river recharge of the aquifers is supplemented by drainage of water vertically downwards from soil all over the Central Plains, called land surface recharge. The amount of land surface recharge occurring in each year is highly variable because it depends on the difference between rainfall and the evaporation from pasture, crops, forests, soil and other land uses.
The aquifers of the Central Plains supply almost all the water for irrigation and human use in the region, including Christchurch city. The world-class quality of Christchurch water depends on its origin as recharge from the alpine-fed Waimakariri River and, for protection against contamination, on the upward flow of groundwater through the aquifers beneath the city. This upward flow is caused by a combination of the groundwater levels in the Central Plains aquifers and the particular geology of the Plains near the coast. The simple message here is that the use of groundwater in the Central Plains has the potential to affect the water resources of Christchurch City, including flow in the groundwater-fed streams such as the Avon and Heathcote Rivers.
The groundwater of the Canterbury Plains is a large, continuously-flowing body of water within layers of silts, sands and gravel down to depths of up to 500 metres. This is not a static pond of water that we can “save for our grandchildren”. What we do need to pass on to succeeding generations is wise management of this dynamic resource. This involves recognising the natural variability of groundwater levels, and the consequent condition of springs, streams, wetlands and lakes. Our own Canterbury mudfish long ago adapted to living in lowland waterways that tend to dry out during the summer in this highly variable regime. The high natural variability of groundwater levels is caused primarily by our variable rainfall climate and its influence on land surface recharge. This is the lesser of the two recharge sources but accounts for most of the obvious signs of drought such as lower flows in streams and low water levels in wells. The Central Plains aquifers provide useful water storage which smooths out much of the variation in recharge. The resulting effect is that groundwater storage replenished by land surface recharge loses about five percent of its volume each month in discharge to surface waters.
The other side of this story is that increasing use of groundwater in the region lowers this envelope of variability so that low flows in streams, for example, occur more frequently. Another effect of decreasing groundwater flow is that the interface between fresh groundwater and the ocean moves in a landward direction, with the consequent risk of brackish water entering wells near the coast. A third potential effect of decreasing groundwater flows is associated with Central Canterbury being a productive agricultural region. Agricultural land use results in nitrate contamination of the land surface recharge that contributes to groundwater – an effect that usually increases with farming intensity. We are fortunate in Central Canterbury that the greater proportion of groundwater is from uncontaminated river recharge, and there is only limited mixing with the contaminated land surface recharge. However, increasing groundwater abstraction for more intensive farming could alter the balance between contaminated and uncontaminated zones in the aquifers.
We are not beginning from a state of pristine groundwater and surface waters. Nitrate would have begun draining towards groundwater from the first turning of the sod in 19th century Canterbury Plains agriculture. Today, the nitrate status of our “natural waters” is the result of many years of the agricultural production that has contributed so much to our current economic wealth. It is doubtful that the recent rapid increase in dairy farming in Canterbury has yet contributed significantly to the nitrate content in groundwater, because of the transit time from soil down to the watertable. Dairy farming has, however, had an immediate impact on the consumptive use of the groundwater resource and quickly brought to a head the need to implement effective decision making about how to allocate the “sustainable” level of use.
So the simple answer to the question, “Do we have enough water in Central Canterbury?” is that there is not enough for all potential users. There is clearly a balance to be sought between abstraction of groundwater for economic and social benefit and the consequent effects on access to high quality drinking water and the ecology of surface waters. The key elements of this decision are a clear understanding of the dynamics of the groundwater resource and robust public consultation within the framework of the Resource Management Act.
Dr Vince Bidwell is a senior research engineer at Lincoln Ventures Ltd, a subsidiary of Lincoln University.