Water is central to life in Marlborough—from agricultural landscapes to recreational swimming holes and Mahinga Kai. However, changing landscapes and land uses are the most important factors influencing water quality and quantity, particularly changes away from natural landcover.
Prior to human settlement in New Zealand, most of the country was covered in forests. Since the arrival of humans, there has been a systematic clearance of these forests and as a result many of our waterways are no longer pristine. In Marlborough, much of the North and West remains in native vegetation, particularly at higher altitudes. Native forest, scrub and tussock still cover over 40% of the region. However, most of the river flats have been cleared of native vegetation and are now used agriculturally. Nearly 30% of the region has been converted to pasture with the majority used to graze sheep and beef. The region’s growing viticultural landscape is mainly located on the Wairau Plain and the lower Awatere River, but vineyard development has moved further up through river valleys and into other areas of the region.
The three largest rivers in the Marlborough region are the Te Hoiere/Pelorus in the northwest, the Wairau River in the central part of the region and the Awatere River in the south. The Wairau River has the largest catchment and cumulatively the largest flow of all rivers in Marlborough, spanning the region from the mountains of the St Arnaud Range in the west to the Pacific Ocean in the East. There is also a striking variation in rainfall across Marlborough. The district is located on the eastern side of the South Island, where large parts of the region are in the rain shadow of the Southern Alps. The greatest rainfall (more than 2 metres a year) falls in the Te Hoiere/Pelorus catchment and around the upper reaches of the Waihopai River. The opposite extreme can be found in some areas along the East Coast and in the lower river flats of the Awatere River catchment. The total annual rainfall in these parts of the region is less than 600mm, making the East Coast catchments some of the driest places in NZ. Although the Awatere River catchment is approximately twice the size of the Te Hoiere/Pelorus catchment, the mean flow in the Awatere River is considerably less. During late summer, the eastern parts of the Awatere dry up completely.
Council has a network of rain, climate and river flow monitoring sites across the district.
Marlborough has several groundwater aquifers, with the most significant located beneath the Wairau Plain. In other areas of Marlborough, groundwater is less prominent either because the local geological structure does not naturally store water or there is no significant source of recharge.
Almost all water used by Blenheim and its hinterland is for crop irrigation, industrial processing, municipal and stock supply is sourced from the Wairau Aquifer. Council monitors the state of the region's aquifers based on water levels in a network of 30 wells spread across the district.
Marlborough’s groundwater is generally of very high quality and can normally be used without treatment. However, groundwater in some areas contain substances that can affect human health or aesthetics. Most contaminants found in Marlborough's groundwater, except for nutrients, are from natural sources, such as weathering of rocks. Relative to other parts of the country, pesticides have not been found in high concentrations in Marlborough groundwaters except for isolated cases involving wellhead contamination.
Groundwater levels vary across the district and for several reasons. Some of these reasons are caused by nature, such as earthquakes, or human pressures, such as localised abstraction. In the drier seasons when water is scarce, the council issues consents for water use. In most catchments in Marlborough, the water available for allocation is now approaching unsustainable limits. Some catchments are fully allocated, while others require water storage at high flows to be used in late summer conditions.
In certain catchments, all surface flows dry up over summer and groundwater represents the only viable source of water. In these areas it is essential to irrigate crops over summer to supplement the low rainfall and offset the naturally high evapotranspiration rate. Even in the Marlborough Sounds catchments, with relatively high rainfall, there is an increasing trend towards irrigation of dairy pasture.
Wairau Aquifer levels have been steadily declining for 50 years due to several factors which are currently being investigated as part of a national research programme. These include declining Wairau River channel levels and lower Wairau River summer flows (below 20 m3/second), both of which affect the rate of recharge available from the Wairau River.