Contrasting January rainfalls and streamflows

River flow records in cubic metres per second from 1 November to early February in the Selwyn and Hurunui Rivers, Canterbury.

The maps of January rainfalls and streamflows reveal interesting contrasts. Exceptional December rainfalls over much of the country, apart from the North Island east coast and the South Island west coast, raised streamflows to high levels and elevated soil moisture conditions to well above normal. January commenced with many streams flowing at high levels and further rain early in the month sustained these flows. The lack of rain from about 12 January saw flows receding steadily to the end of the month. However, the average flows for January 2005 were in many cases well above the average for January, as shown by the map of river flows on Page 2.

The January rainfall map shows that relatively low rainfalls occurred over most of the North Island apart from the southwest, and over Canterbury and the west coast in the South Island. The lack of rainfall over the latter part of January, coupled with the high mid-summer evapotranspiration rates that accompanied the near-normal January temperatures, caused a steady depletion of soil moisture levels, leading to end-of-month soil moisture deficits that were greater than typical end-of-January values (see soil moisture map, Page 2).

In Canterbury, where agriculture is heavily dependent on irrigation, most streams followed the countrywide pattern with steadily reducing flows from about the middle of the month, as illustrated in the flow records for the Selwyn and Hurunui Rivers (below). By the end of the month, some Canterbury rivers were depleted to well below typical January flows, and irrigation restrictions were in force, notably for the Waiau, Hurunui, Ashley, Waimakariri, and Ashburton Rivers. Restrictions were also being imposed on smaller groundwater-fed streams in the region. Overall, however, much less irrigation than usual has been necessary over spring and early summer.

River debris along the beach at Queen Elizabeth II Park on the Kapiti Coast, flushed from southern North Island rivers following the heavy rainfall in early January. (Cover photo: Alan Blacklock)