The National Institute of Water and Atmospheric Research (NIWA) was commissioned by Statistics New Zealand to estimate eleven components of the national and regional water balance of New Zealand for each of the sixteen years from 1 July 1994 to 30 June 2010.
Subsequent requirements extended the modelling window to mid-2014 and the TopNet model was re-run to include this additional time period. The modelling information was used by Statistics New Zealand in a set of annual national water accounts they are developing, as part of a set of environmental accounts for New Zealand. Specifically, this study contributed to the Water Physical Stock Accounts.
The study focused on eleven components of the accounts which were; Precipitation, Inflows from rivers, Evapotranspiration, Abstraction by hydro-generation companies, Discharges by hydro-generation companies, Outflow to sea from surface water, Outflow to other regions (regional scale only), Net change in lakes and reservoirs, Net change in soil moisture, Net change in snow storage, and Net change in ice storage.
The accounts were presented at two levels of spatial detail: national and regional. The 16 areas administered by regional councils and unitary authorities define the regional boundaries.
Combinations of three methods were used to estimate the eleven components of the accounts listed above: direct calculation from measurements, spatial mapping, and simulation modelling. A significant task within the project was the operation of a national hydrological model, using NIWA’s TopNet modelling system. This was necessary to estimate some of the account components for which very few measures are available including;
The study produced eleven component values (measured in m3) for each of the 16 regions and the whole New Zealand over the new time period of 20 years (1995 -2014).
TOPNET - Annual National Water Fluxes
The studies resulting accounts capture the broad temporal and regional variability of water movement and storage in New Zealand. They show that precipitation is the dominant component, and that, when taken together, river flow to the sea and evapotranspiration accounts for most of this precipitation. The use of water for hydrogenation is a significant component of the national accounts, but it should be noted that this is a non-consumptive use, and that the water has been counted each time it is used for hydrogenation. By comparison, inter-regional flows and the various net changes in storage are relatively small at the annual-national scale.
TOPNET - Precipitation by Region
The estimation of precipitation across New Zealand remains a key step in the development of the accounts, and the latest accounts use new methods for this. Further development is needed to improve on both the precipitation, and estimates of actual evaporation in many areas of New Zealand. However, in their present form, they already provide detailed information on year-to-year trends around the nation.
Although outside the scope of this study, two outstanding issues remain: to integrate changes in groundwater storage with these surface water accounts, and to obtain data on water use and link it to these accounts. Research in the Waterscape research programme funded by the Ministry of Science and Innovation, will make contributions to both issues, but coordinated input from many other parties is needed to address these two outstanding issues adequately.
TopNet is a semi-distributed hydrological model for simulating catchment water balance and river flow.
Daniel Collins, et al (2015): Surface water components of New Zealand’s National Water Accounts, 1995-2014. Prepared for Ministry for the Environment May 2015. NIWA Client Report No: CHC2015-013-v2