The TopNet hydrological model was calibrated based on a combination of Virtual Climate Network stations and existing sub-daily precipitation information located within Lake Waahi surface water catchment. The hydrological models were calibrated at one continuous monitoring streamflow station in each of the surface water catchments discharging to the lakes. Due to the large potential impact of water consented activities above Awaroa at Sansons Br (within Lake Waahi surface water catchment), calibration for the Lake Waahi model was carried out taking into account only winter flows. Analysis of the calibration indicates that the models are able to better represent low flow conditions than high flow conditions when compared with streamflow observations. This result is expected due to the low density of observed precipitation gauges across the catchments and the area of the surface water catchments. Validation of the models over the whole period of record indicates that the hydrological models reproduce the range of hydrological characteristics observed at the gauging stations. Over the period 1973-2013 Lake Whangape inflows are estimated to average 5.81 cumecs (ranging from 3.80-8.8 cumecs) while Lake Waahi inflows are estimated to average 1.88 cumecs (ranging from 1.00-2.57 cumecs). Inter-comparison of the calibrated Topnet parameters used for both catchments indicates that most of the parameter multipliers are similar across both models. This indicates a consistency of the hydrological process representation across the large watershed. The TopNet hydrological model is routinely used for hydrological modelling applications in New Zealand. It is a spatially distributed, time-stepping model of water balance. It is driven by time series of precipitation and temperature data, and of additional weather elements where available. TopNet simulates water storage in the snowpack, plant canopy, rooting zone, shallow subsurface, lakes and rivers. It produces time series of modelled river flow (under natural conditions) throughout the modelled river network, as well as evaporation. TopNet has two major components, namely a basin module and a flow routing module. Climate information was obtained from Virtual Climate Station Network. Annual average precipitation and evaporation were estimated by NIWA. Spatial information in TopNet is provided by national datasets on catchment topography (i.e. 30m digital elevation model), physical (Land Cover Database, Land Resource Inventory, and hydrological properties (River Environment Classification). The method for deriving TopNet initial parameter estimates from GIS data sources in New Zealand is given in Table 1 of Calrk et al. (2008). [Clark, M.P.;Rupp,D.E.;Woods,R.A.;Zheng,X.;Ibbitt,R.P.;Slater,A.G.;Schmidt,J.;Uddstrom,M.J.(2008). Hydrological data assimilation with the ensemble Kalman filter: Use of streamflow observations to update states in a distributed hydrological model. Advances in Water Resources 31(10):1309-1324.]

This layer is based on the New Zealand River Environment Classification REC2 V5 and describes the larger catchments of New Zealand that are a Strahler order five and greater. It also has the associated names that belong to the parent sea draining catchment. It was originally derived by dissolving local Strahler Order 1 watersheds. REC2 (River Environment Classification, v2.5) - June 2019 [Hosted Feature Layer]This service depicts catchments as polygons The River Environment Classification (REC) is a database of catchment spatial attributes, summarised for every segment in New Zealand's network of rivers. The attributes were compiled for the purposes of river classification, while the river network description has been used to underpin models. Typically, models (e.g. CLUES and TopNet) would use the dendritic (branched) linkages of REC river segments to perform their calculations. Since its release and use over the last decade, some errors in the location and connectivity of these linkages have been identified. The current revision corrects those errors, and updates a number of spatial attributes with the latest data. REC2 provides a re-cut framework of rivers for modelling and classification. It is built on a newer version of the 30m digital elevation model, in which the original 20m contours were supplemented with, for example, more spot elevation data and a better coastline contour. Boundary errors were minimised by processing contiguous areas (such as the whole of the North Island) together, which wasn't possible when it was originally created.Major updates include the revision of catchment land use information, by overlaying with the land cover database (LCDB3, current as at 2008), and the update of river and rainfall statistics with data from 1960-2006. The river network and associated attributes have been assembled within an ArcGIS geodatabase. Topological connectivity has been established to allow upstream and downstream tracing within the network. This is based on REC2 (Version 5) , June 2019 - a publicly available dataset from NIWA Taihoro Nukurangi.NIWA acknowledges funding from the MBIE SSIF towards the preparation of REC v2.5Coordinate Reference System: NZTM (New Zealand Transverse Mercator, EPSG: 2193) Geometric Representation of Rivers: Lines Geometric Representation of Catchments: Polygons Extent (Bounding Box): Top(Latitude) -33.9534Bottom(Latitude) -47.4867 Left (Longitude) 166.2634 Right (Longitude) 178.9733 Available Fields: Shape,FID_1,diss,FID_2,HydroID,CATAREA,CUM_AREA,nzsegment,StreamOrde,upElev,downElev,upcoordX_1,downcoor_1,downcoor_2,upcoordY_1,RivName,Distance,ord_diff,Distance_1,Shape_Length,Shape_Area

A map service and web application where users can extract flood statistics from a regional flood model, at-site flood statistics and annual maximum flows from flow recorders used in the model development, flood estimates based on rain intensity for use in the Rational Method.  1. Modelled flood frequency estimates on river lines. NZREACH NZ digital network 1 ID no. River name where available Area km2 catchment area draining to the downstream end of this reach (square kilometres)q100_reach ratio of Q100 (1% aep flood) to mean annual flood C18_MAF mean annual flood (cumecs) C18_5-yr 5-yr (20% aep) flood (cumecs) C18_10y 10-yr (10% aep) flood (cumecs) C18_20y 20-yr (5% aep) flood (cumecs) C18_50y 50-yr (2% aep) flood (cumecs) C18_100y 100-yr (1% aep) flood (cumecs) C18_1000y 1000-yr (0.1% aep) flood (cumecs) HCse_MAF standard error of mean annual flood (cumecs) HCse_5y standard error of 5-yr (20% aep) flood (cumecs) HCse_10y standard error of 10-yr (10% aep) flood (cumecs) HCse_20y standard error of 20-yr (5% aep) flood (cumecs) HCse_50y standard error of 50-yr (2% aep) flood (cumecs) HCse_100y standard error of 100-yr (1% aep) flood (cumecs) HCse_1000y standard error of 1000-yr (0.1% aep) flood (cumecs) Strm_Order Strahler stream order (1=headwater, two ones join to make an order 2, etc.) 2. At-site flood statistics at flow recorders used in the model development Siteno NIWA reference number for flow recorder Name Flow recorder name generally 'river' at 'location' NZTM_E NZTM easting NZTM_N NZTM northing Region Regional council or unitary authority territory Operator Organisation that operates the flow recorder Funder Organisation that funds the flow recorder (can be more than one)Area_km2 catchment area draining to the flow recorder (square kilometers) No_years Number of years in the annual flood series (allowing for years with missing data) L1_mean Mean of the annual flood series (linear moment 1)L2 Linear moment 2 of the annual flood series (analogous to standard deviation)Lcv Linear CV of the annual flood series (L2/L1) T3_Lskew Linear skew ratio of the annual flood series T4_Lkurt Linear kurtosis ratio of the annual flood series Gumb_u Gumbel distribution u of the annual flood series Gumb_alpha Gumbel distribution alpha of the annual flood series GEV_u Generalised Extreme Value u of the annual flood seriesGEV_alpha Generalised Extreme Value alpha of the annual flood series GEV_k Generalised Extreme Value k of the annual flood series GEV_z Hosking-Wallis normal standard variate to test significance of GEV-k (is the at-site distribution Gumbel or not?)Data 2.33y mean annual flood (2.33-yr assuming Gumbel, or 43% aep) Data 5y 5-yr (20% aep) flood (cumecs)Data 10y 10-yr (10% aep) flood (cumecs) Data 20y 20-yr (5% aep) flood (cumecs)Data 50y 50-yr (2% aep) flood (cumecs) Data 100y 100-yr (1% aep) flood (cumecs)Data 250y 250-yr (0.4% aep) flood (cumecs) Data 500y 500-yr (0.2% aep) flood (cumecs) Data 1000y 1000-yr (0.1% aep) flood (cumecs) se_2.33y standard error of mean annual flood (%) se_5y standard error of 5-yr (20% aep) flood (%) se_10y standard error of 10-yr (10% aep) flood (%)se_20y standard error of 20-yr (5% aep) flood (%) se_50y standard error of 50-yr (2% aep) flood (%) se_100y standard error of 100-yr (1% aep) flood (%) se_250y standard error of 250-yr (0.4% aep) flood (%) se_500y standard error of 500-yr (0.2% aep) flood (%) se_1000y standard error of 1000-yr (0.1% aep) flood (%) 3. Return period estimates of the area-rain intensity values for small catchments. Where results display '-1' the catchment is greater than 30 square kilometres and no answer is available. Numbers need to be multiplied by the Rational Method 'C' factor to give the required flood magnitude. NZREACH NZ digital network 1 ID no. AREA_km2 catchment area draining to the flow recorder (square kilometers) QIA_5y 5-yr (20% aep) IA estimateQIA_10y 10-yr (10% aep) IA estimate QIA_20y 20-yr (5% aep) IA estimateQIA_50y 50-yr (2% aep) IA estimateQIA_100y 100-yr (1% aep) IA estimate QIA_1000y 1000-yr (0.1% aep) IA estimate 4. Annual data values at flow recorders used in the model development. Owner: NIWA_OpenData