This project was initiated as part of the Smart Aquifer Characterisation (SAC) Programme, which aims to develop innovative methods to characterise groundwater systems in a cost-effective and rapid way. The project aims to model how much rainwater enters the groundwater across New Zealand. Rainwater which drains through the soil and recharges the groundwater is known as rainfall recharge. Rainfall recharge is important for water management and policy purposes to understand how much water is replenishing groundwater systems. A consistent overview of rainfall recharge estimates across New Zealand does not exist, as these estimates vary between regions where different methods have been used.

Background

Rainfall can either:

  • evaporate back into the atmosphere from the ground surface or vegetation
  • recharge the groundwater by draining through the soil and entering the water table
  • runoff across the ground surface or through shallow soils and enter streams

To calculate rainfall recharge we need to know how rainfall and evapotranspiration change over space and time. Secondly, in order to understand runoff processes, information on how different types of land cover respond to rainfall are needed. Lastly, soil properties and underlying geology, determining where water can be stored within the earth, must be understood.

Rainfall

New Zealand has an excellent nationwide record of rainfall information based on the interpolation of measured climate station data provided by NIWA. This dataset is known as the virtual climate station network and runs from the year 1972 to current.

Evapotranspiration

Actual evapotranspiration (AET) describes the amount of water evaporated from plants or the land surface which is typically around 50% of rainfall (long-term average for New Zealand). This project calculates AET with the use of data from sensors aboard satellites which measure the properties of reflected light from the earth’s surface to determine land cover, albedo and vegetation properties. This is integrated with ground-estimated temperature, radiation, air humidity to calculate the spatial and temporal variability of evapotranspiration. This broad approach allows for large scale estimates of AET with a better known uncertainty.

Recharge or Runoff

The remaining water, after evapotranspiration has taken place, can either recharge the groundwater or runoff the surface. The amount of rainwater that becomes recharge or runoff depends on many factors including the condition of the ground surface, soil properties, geology and rainfall intensity. The nation-wide model calculates recharge and runoff at a monthly time step using existing datasets of land elevation which indicates the slope angle of the ground surface, water holding capacity and permeability of the soil and underlying geology. The relationship between these factors describes the amount of rainfall recharge and runoff. The nation-wide map of estimated rainfall recharge (mean annual values) are shown in figure 1.

Nation-wide rainfall recharge compiled to mean annual values

Figure 1: Nation-wide rainfall recharge compiled to mean annual values.

Summary

Rainfall recharge is important for water management and policy purposes to understand how much water is available within groundwater systems. Calculating rainfall recharge is complex as it involves the interaction of many climatic and land based parameters, which vary spatially. This modelling approach provides rainfall recharge estimates across New Zealand by using a combination of ground-observed and satellite-derived parameters. A more consistent overview across New Zealand is found, which can help make initial estimates in areas which are data-sparse, or can aid in indicating formerly unknown areas of rainfall recharge.

Data

The data are available in NetCDF (see Table 1)

(here could go little map from the SMART Mapviewer with ncWMS)

Table 1: Specifications of NZ_recharge_monthly data file for New Zealand

Info Spec
Format NetCDF UCAR (2014), CF-1.6
Spatial resolution 30 arc sec
Coordinate system WGS84 (EPSG:4326)
Start Jan-2000
End Dec-2013
Time interval Monthly
Unit mm/month
Data layers Monthly ET MOD160
download NZ_recharge_monthly
size 3 GB