Using satellite data for a nationwide estimate of rainfall recharge in New Zealand

In many parts of New Zealand, rainfall directly recharges shallow and unconfined aquifers. Measurements and calculations of rainfall recharge (‘recharge’) are considered important since they help policymakers to decide on volumes of groundwater allocation. However, ground-level observations of recharge (using lysimeters) are sparse.

Photo: Mt. Ruapehu, Tongariro National Park.

Volcanic rocks are heterogeneous and can be very hydraulically conductive.

Local models inadequate

As a result, local models (of which there are many in New Zealand) are used to estimate recharge in many cases. This situation leads to a range of recharge estimates based on different input data. In addition, the input data (rainfall, evapotranspiration, soil moisture, soil type) include uncertainties, with potentially large model errors as a result. Furthermore, there are large areas that are not covered by any model. Last but not least, water data and policy are mostly managed by regional councils, resulting in differences in input data and the resulting model outputs. A nationwide estimate of rainfall recharge may help to resolve these issues.

Using satellite data

Deltares is therefore studying the possible contribution that satellite data can make to a nationwide estimate of recharge. Satellite data covering evapotranspiration and soil moisture are being used in combination with ground-level observations in a soil-water-balance model. The model provides estimates of 1×1 km monthly nationwide rainfall recharge to groundwater (from January 2000 to December 2014) with help from the satellite data.

By linking satellite data to ground-level observations and by looking at error propagation in evapotranspiration and soil moisture models, uncertainties can be defined better. An additional spin-off is the establishment of a long-term average for a nationwide water table.