Source
https://niwa.co.nz/climate-and-weather/mean-monthly-rainfall-mm — original source (opens in a new tab; the file is not redistributed)
NIWA — Mean Monthly Rainfall (mm), 1991–2020
The monthly seasonal distribution of rainfall — the missing input for the NI seasonal water balance
NIWA’s official 1991–2020 monthly-rainfall normals for 30 NZ stations (Jan–Dec + annual + station metadata). Complements RD_022 (which carries the annual
rainfall_mm); RD_023 adds the within-year shape needed to maketank_m3drivewater_ssi(the seasonal storage balance, model_design.md §5 ⑤). Authoritative primary download (data_quality verified).
Summary
NIWA mean monthly rainfall (mm) for the 1991–2020 normal period, 30 stations with ≥5 complete years. Downloaded direct from niwa.co.nz (mean_monthly_rainfall_1991-2020_0.xlsx). Two sheets: Data (LOCATION × JAN…DEC + YEAR) and Station_metadata (agent/network number, lat/long, height). This is the seasonal distribution the Neobiome Intelligence water domain needs: the annual-ratio MVP scored water_ssi = 1.0 even where a community runs dry every summer (the Earthsong failure mode, Interview VI [INT_006] “tanks run dry several times each summer”). With monthly fractions, a tank-storage simulation makes water_ssi honest and tank_m3-dependent.
Key claims
- claim: "NIWA mean monthly rainfall (mm) 1991–2020 normals, 30 stations. NZ rainfall seasonality is strongly regional: NORTHERN/EASTERN North Island is winter-wet / summer-dry — Auckland winter (JJA) 33.4% vs summer (DJF) 18.1% of annual (seasonality max/min month 2.36×), Kaitaia 2.24×, Wellington 2.65×; the rain-bearing westerlies move north in winter. The PILOT (Nelson/Tasman) is mild — DJF 23.5% / JJA 26.8%, 1.58× — so storage matters less there than in Auckland/Northland. Central Otago (Alexandra) is summer-MAX (DJF 33.6%, continental). West Coast (Hokitika 2,921 mm) is very wet and near-even."
source_location: "Data sheet — LOCATION × JAN–DEC; 1991–2020 normals"
- claim: "Annual totals (mm, 1991–2020) span an order of magnitude: Alexandra 365, Lake Tekapo 535, Timaru 543, Christchurch 618, Blenheim 639 (driest east); Nelson 976, Auckland 1,119; up to Westport 2,074, Hokitika 2,921, Mt Cook 4,132, Milford Sound 6,545 (wettest west). Confirms the orographic west-wet / east-dry contrast across the Southern Alps."
source_location: "Data sheet — YEAR column"Neobiome Intelligence relevance
Fills the seasonal water-balance data dependency (model_design.md §5 ⑤ water). Each NI region reads a 12-value month_frac (fractions of annual rainfall, summing to 1), applied to the annual rainfall_mm (RD_022) to give monthly capture; the tank simulation (capture → store ≤ tank_m3 → draw → spill/deficit, 24-month/year-2 steady state) then yields a storage-dependent water_ssi.
16-region month_frac (derived — station→region mapping; Jan…Dec):
| region | J | F | M | A | M | J | J | A | S | O | N | D | station(s) |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Northland | .059 | .065 | .078 | .074 | .093 | .112 | .131 | .102 | .090 | .064 | .058 | .074 | Kaitaia, Whangarei |
| Auckland | .052 | .056 | .067 | .078 | .107 | .107 | .122 | .105 | .089 | .082 | .062 | .073 | Auckland |
| Waikato | .067 | .058 | .067 | .082 | .092 | .105 | .111 | .095 | .089 | .076 | .070 | .088 | Hamilton |
| Bay of Plenty | .067 | .072 | .076 | .104 | .096 | .099 | .106 | .093 | .075 | .068 | .059 | .086 | Tauranga, Rotorua |
| Gisborne | .071 | .066 | .094 | .107 | .084 | .107 | .119 | .078 | .073 | .076 | .065 | .060 | Gisborne |
| Hawke’s Bay | .081 | .069 | .078 | .103 | .079 | .099 | .123 | .072 | .074 | .077 | .073 | .073 | Napier |
| Taranaki | .054 | .064 | .065 | .083 | .106 | .102 | .100 | .091 | .087 | .090 | .073 | .085 | New Plymouth |
| Manawatū-Whanganui | .061 | .072 | .061 | .087 | .087 | .096 | .090 | .087 | .086 | .096 | .084 | .093 | Whanganui, Palmerston Nth |
| Wellington | .060 | .047 | .077 | .090 | .099 | .097 | .118 | .100 | .082 | .091 | .071 | .069 | Wellington, Masterton |
| Tasman | .075 | .064 | .073 | .087 | .090 | .102 | .081 | .086 | .087 | .091 | .070 | .095 | Nelson ⚠ |
| Nelson | .075 | .064 | .073 | .087 | .090 | .102 | .081 | .086 | .087 | .091 | .070 | .095 | Nelson |
| Marlborough | .067 | .070 | .062 | .084 | .089 | .107 | .101 | .091 | .085 | .089 | .077 | .078 | Blenheim |
| West Coast | .084 | .063 | .071 | .082 | .088 | .092 | .080 | .087 | .086 | .098 | .076 | .094 | Hokitika, Westport |
| Canterbury | .080 | .079 | .072 | .092 | .084 | .093 | .091 | .088 | .065 | .084 | .083 | .088 | Christchurch, Timaru |
| Otago | .104 | .089 | .074 | .077 | .096 | .084 | .073 | .068 | .072 | .084 | .086 | .091 | Dunedin, Queenstown, Alexandra |
| Southland | .080 | .072 | .079 | .079 | .096 | .084 | .082 | .069 | .088 | .099 | .089 | .086 | Invercargill, Manapouri |
⚠ Mapping caveats (derived, not NIWA): ① Tasman has no separate station → uses Nelson Aero (geographically adjacent; reasonable for the pilot, but Tasman’s wetter hill country is under-represented). ② Multi-station regions are a simple mean of the listed stations’ monthly mm (e.g. Otago averages coastal Dunedin + inland Queenstown/Alexandra — which have opposite seasonality, so the blend is a compromise). ③ Station = the main aero/EWS site, not a regional average. For a parcel-GIS upgrade (§11) the nearest-station or a gridded NIWA monthly surface would replace this.
Research targets
Research gaps
- A gridded NIWA monthly-rainfall surface (vs the 30-station points) would let the parcel-GIS layer (§11) read monthly fractions at any coordinate, and resolve the Tasman/multi-station blending compromises — noted, no new RT (the station normals are sufficient for the 16-region MVP).
Connections
Links to
Sources (1): RD_022