Net change in lake water storage for the North Island, New Zealand

Evapotranspiration: Transfer of water from the Earth’s surface to the atmosphere by evaporation of liquid or solid water plus transpiration from plants.
Precipitation: Water in any form (including rain, snow, hail, sleet, and mist) that leaves the atmosphere and reaches the Earth’s surface.
Outflow to sea: The total volume of water that flows to the sea (does not consider prior water abstraction).
Outflow to other regions: The total quantity of surface water that leaves a region and flows to another region
Inflows from other regions: The volume of water that enters a region from outside that region (includes non-riverine water transfers).
Hydro-generation abstraction The total volume of water abstracted from surface water for electricity production by hydrogeneration companies.
Discharge by Hydrogeneration: The total of water discharged by hydro-generation companies. It is equivalent amount to water abstracted for hydro-generation.
Change in lakes: The change in volumes of lakes and reservoirs.
Change in soil moisture: The change in volume of water stored in soil.
Change in snow: The change in quantity of water stored as frozen water (permanent and seasonal snow/ice).
Change in ice: The change in quantity of water stored in ice.

Change in ice, lakes, snow and soil moisture represent a change from the end of the previous June year to the end of the current June year.
Precipitation and Inflow from other regions represent water gained in the region. All other measures are outflows ie a loss of water.
Water used in hydroelectricity generation is returned to the hydrological system. Discharges match abstraction, meaning that 'net' abstraction is zero.

Environmental-economic accounts: Water physical stocks, year ended June 1995–2020 https://www.stats.govt.nz/information-releases/environmental-economic-accounts-water-physical-stocks-year-ended-june-1995-2020
New Zealand water accounts: Update 2020 https://www.stats.govt.nz/methods/new-zealand-water-accounts-update-2020
Update of the national groundwater volume stock account: 1994 to 2020 https://www.stats.govt.nz/methods/update-of-the-national-groundwater-volume-stock-account-1994-to-2020

All of the values used in Groundwater data are based on a mixture of often scarce measurements and expert knowledge applied over large scales. Therefore, all results must be considered in the qualitative sense. And similarly, the uncertainty of these results must also be considered in qualitative terms such as low, medium and high reliability rather than attributing standard error terms to these results. It is also important to keep in mind that the results represent analyses at a regional scale and cannot be considered indicative at sub-regional scales, except in an average sense. Finally, it is important to realise when combining uncertainties, that they can propagate through to the uncertainty of the final product. For example, combining a high reliability result and a low reliability result could give a low reliability result.

There are several differences between the 2015 and 2020 water accounts. These differences are due to a number of changes in accounting methodology since 2015 including:
i) changes to hydrological process conceptualisation within the surface water model;
ii) changes to the reporting frequency of some variables;
iii) changes in the methodology used to calculate specific components of the water account; and
iv) changes in datasets used to calculate the water account.

In the 2020 account, the ice modelling team used a new methodology to calculate ice storage and ice storage change. This methodology overcomes methodological issues associated with the method used in previous accounts and it takes into account the latest technological developments that have occurred between 2015 and 2020.

Due to updates to the digital river network, a new method was used to identify surface water interregional flow transfers. To do this, for all sub-catchments in New Zealand, two points were defined, a centroid and an outlet. The centroid is defined from the sub-catchment boundary and the outlet is defined as the most downstream point of the sub-catchment. Inter-regional flow is then identified where the centroid and outlet of any sub-catchment lie in different regions. For example, interregional flow occurs where the bulk of a sub-catchment lies in one region and the outlet of the catchment lies in another. It should be noted that the existence of inter-regional flow may lead to higher total regional outflow (to oceans) than inflows (precipitation), (e.g. Otago). A visual inspection of identified inter-regional flow transfers was performed to verify the above algorithm. As a result, the predicted inter-regional discharge estimation is larger than that of previous water accounts.

At URL provided, download 'Water physical stock account, year ended June 1995–2020 – CSV'.

The water physical stock accounts present information on the hydrological cycle based on the System of Environmental-Economic Accounts (SEEA).
The water physical stock accounts focus on water quantity and provide estimates of national and regional water flows (precipitation, evapotranspiration, and rivers) and change in storage (ice, snow, soil moisture, groundwater, lakes and reservoirs, and hydro-electric generation water use)analyse seasonal patterns, and any changes in seasonality analyse extreme events which may be obscured in annual data compare water statistics with other quarterly data, such as agricultural output or electricity generation

Analyse seasonal patterns, and any changes in seasonality
Analyse extreme events which may be obscured in annual data
Compare water statistics with other quarterly data, such as agricultural output or electricity generation

Information on how much water is gained through precipitation and lost through evapotranspiration summarises the surface component of the water stocks of New Zealand. The data provider derived this from a combination of direct measurement and modelled data.
To calculate the volume of groundwater in an aquifer, the volume of saturated rock (areal extent multiplied by saturated thickness) is multiplied by the water storage parameter.
The change in groundwater volume in an aquifer over time is estimated using water level measurements from an indicator well to calculate variations in saturated thickness compared to the reference year (1994) using the following equation: Change in groundwater volume (m3) = Groundwater volume (m3) x Change in indicator well water level (m).

Precipitation: New Zealand Water Model (NZWaM) rainfall input, derived from the Virtual Climate Station Network (VCSN).
Evapotranspiration: Calculated by NZWaM, based on temperature dependent potential evapotranspiration, water availability (calculated soil moisture) and vegetation characteristics.
Outflow to sea: Output of surface water component of NZWaM.
Outflow to other regions: NZWaM flow data output with GIS analysis of river networks and administrative regional boundaries.
Inflows from other regions: NZWaM flow data output with GIS analysis of river networks and administrative regional boundaries.
Hydro-generation abstraction: Derived from measured power station machine flows (does not include spill flows).
Discharge by Hydrogeneration: Is equivalent amount to water abstracted for hydro-generation.
Change in Lakes: Derived from recorded hydro-lake level data
Change in Soil Moisture: Derived from NZWaM output.
Change in Snow: Derived from NZWaM output.
Change in ice: Derived from end-of-summer snowline survey (EOSS), and observation of pro-glacial lake development and down-wasting.