Figures showing daily water balance at over 100 climate stations around New Zealand are available on ClimateExplorer. The model that generates the figures makes some broad assumptions. The available water capacity, for example, is taken to be 150 mm, which is about right for the pasture root zone of an average silt loam soil. The model works best for level paddocks under pasture. Evapotranspiration is assumed to continue at a potential or atmospheric demand rate until about half of the available water is used up, at which point it decays linearly to near zero as the water content approaches permanent wilting, at 150 mm deficit.
The figure left shows the daily July to June water balance at Winchmore, Canterbury. The black curve gives the average daily plant-available water, in mm; the green, blue, and red curves track the 2003–04, 2004–05, and 2005–06 seasons respectively. Field capacity is shown at 0 deficit. Rain falling after the soil reaches field capacity is assumed to be lost by surface runoff or by rapid gravitational drainage through the soil profile, and is therefore not available to pasture for subsequent growth.
The assumptions and parameterisation of the model are based on field measurements on a number of farms, and can therefore be used with confidence in comparing relative soil moisture availability between seasons, and with long-term average conditions.
The model can be used at any site with a record of daily rainfall. The water balance maps shown on page 2 of this publication are generated from a base map of over 1900 rainfall stations.
A range of soil moisture maps created from the model is available from ClimateExplorer. An example, shown figure right, gives the median number of soil moisture deficit days for December. These are days when the moisture available in the soil is insufficient to meet the atmospheric demand, leading to plants starting to wilt and produce less than their potential growth.
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