Water resources in summer 2003-04 (December 2003 to February 2004)
River flow
River flows for summer were far above normal in many parts of the North Island, near normal in the west of the South Island, and below normal in the east of the South Island. For some rivers in the Manawatu–Wanganui and Taranaki regions, these are the highest summer flows on record, and are a direct result of the February storms. This is a marked contrast to the previous summer, when summer flows were unusually low. Rivers in coastal Canterbury, Otago, and Southland all experienced a dry summer, including some which had the driest summer on record. |
Lake levels
Most of the monitored lakes in the North Island and the west of the South Island were well above their normal levels for summer. Monitored lake levels on the east coast of the South Island were below normal. For many lakes, the levels are a reflection of recent weather. However, for lakes that are managed for uses such as hydro-electric power generation or irrigation, the levels also depend on how the lake is managed. For example, water can be stored during times when inflows are large, and released later, when demand is greatest. Exactly how this is done depends on the timing of the inflows, the patterns of demand for water, and the operating rules for managed lakes. |
|
Water clarity
Water clarity was much lower than normal at many places in the regions hit by the February floods because of the large volumes of sediments that were mobilised. By contrast, water clarity was higher than normal in places where flows were lower than normal (e.g., parts of Otago, Southland), and there were few floods to stir up river sediments. |
Slime index
Maximum values of the slime index over summer were typically no more than 20% over much of the country, which is near normal, or slightly above normal, for the time of year. The slime index records the cover of the bed by 'nuisance' slimes (algae growing as thick mats or filamentous growths) that reduce the recreational and aesthetic value of the river if too prolific (i.e., cover > 30–60%). |
|
Water temperatures
Stream temperatures were typically 2 °C cooler than normal in the central North Island, and a degree warmer than normal in parts of Nelson, South Canterbury, and Otago. |
Groundwater
Groundwater levels were normal in most regions at the start of summer and showed the usual pattern of summer decline. Groundwater levels in some Marlborough wells reached historic lows although overall levels were average in the top of the south. Eastern South Island groundwater levels were lower than normal in the period. Good rainfall from late January onwards, and the associated decline in demand, resulted in a general rise in groundwater levels. |