Using NIWA WRENZ To get Waiheke Climate Plots

NIWA’s Water Resources Explorer (WRENZ) is a GIS based web application that allows the user to display selected layers of
water resource related spatial information on a map of New Zealand. Its main use is to display the locations of water resources stations such as rivers, catchments, water level, ground water and soil moisture. In most respects it just shows some metadata associated with a station and identifies when historically that station was in use.

WRENZ has just been enhanced to show all of NIWA’s climate stations, both real ones and the Virtual Climate station network – a grid of virtual stations generating data by interpolation. You can access real data from these stations using CliFlo but now you can rapidly get some useful plots from locations directly from WRENZ. http://wrenz.niwa.co.nz

Obtaining station plots

Select the VCS Network

Select “Map layers” (function tools to the right of the map);
Click on Stations & Virtual Climate Stations then Refresh map (should put some red dots on the map);

Select the Station

Zoom into your favourite area of NZ

Click on a red dot (VCS)

The balloon that pops up has 3 tabs – the first tab is “station” info, the second tab is “plots”, the third tab is data “download”
Click on the plots tab, then on any of the pre-generated plots specific to this VCS site (the plots are the same ones we do for ClimateExplorer.

Understanding the Plots

Click on each image to see larger version. The plots are automatically updated every weekend

Daily water balance plot

The daily water balance keeps track of water (rainfall) entering the pasture root zone in the soil, and being lost from this zone by evapotranspiration or use of water by the plants. The balance is done for an average soil type where the available water capacity (the amount of water in the soil ‘reservoir’ that plants can use) is taken to be 150 mm.

When there is a surplus of rain and this reservoir of water overflows, the excess water is assumed to be lost from the reservoir, and is therefore deleted from the water balance. After the overflow, the reservoir is assumed to return to field capacity in a day or two, depending on how large the surplus was.

Evapotranspiration is assumed to continue at its potential rate until about half of the water available to plants is used up, whereupon it decreases, in the absence of rain, as further water extraction takes place. Evapotranspiration is assumed to cease if all the available water is used up.

The lines on the plot show the daily soil water balance from 1 July to 30 June, in the current season (red curve), and last season (blue curve. The solid red and blue curves represent the water balance in the top soil—while the curves track above the 75mm deficit line there is unlikely to be a shortage of water for pasture growth. The dashed lines represent the water balance for the total profile, including the black curve which represents the historical average soil water balance. The red and blue bars at the bottom of the figure show the days of deficit—taken to be days when the profile water balance was at or below the 75 mm deficit threshold.

Accumulated rainfall plot

The lines show accumulated rainfall for the growing season from 1 July to 30 June, in the current season (red curve), and last season (blue curve). The black curve indicates the historical 50 percentile accumulation, and the historical 10 to 90 percentile accumulation range is indicated by the grey shaded band. The scale for the daily rainfall total is shown on the right hand vertical axis.

The bars represent the monthly rainfall total for the current season (red), last season (blue) and the historical average (grey). The scale for the monthly totals is on the left vertical axis.

Mean daily temperature and frost occurrence plot

The lines show mean daily temperatures for the growing season from 1 July to 30 June, in the current season (red curve), and last season (blue curve). The black curve represents the 50 percentile mean temperature. The mean temperatures are calculated from (maximum + minimum)/2 on each day, and are statistically smoothed. The scale for the daily mean temperature is shown on the right hand vertical axis.

If there have been air frosts (minimum air temperature less than 0.0°C) during the current and immediate past season they are shown as red and blue triangles.

The bars represent the monthly mean temperatures for the current season (red), last season (blue) and the historical average (grey). The scale for the monthly means is on the left hand vertical axis.

To get the underlying data

Click on the download tab – this provides instructions on how the user can get the VCS data from cliflo.

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