Soil moisture is a fundamental part of the hydrological cycle, linking hydrological, biological and biochemical processes. Information on soil moisture status is of interest to farmers because soil water balances are important for irrigation, effluent application, drainage, livestock grazing management, seed sowing and agronomy. The information is also important to climate scientists and hydrologists for climate projections as well as flood warning and drought modelling and monitoring.

A soil moisture monitoring network was established by Auckland Council in 2014. The purpose of this network was to assist with agricultural farm management and for drought information purposes. Ten soil moisture AQUAFLEX sensors were installed across the Auckland region between 2014 and 2016 and in accordance with the National Environmental Monitoring Standard. Soil water. Measuring, Processing and Archiving of Soil Water Content Data (NEMS, 2013, 2016a).

The 10 soil moisture sites were spread across north, south and central Auckland and were selected to predominantly represent local soil types and climates. All sites have grass cover, either mown or grazed. Seven of the 10 sensors at the soil moisture sites have been operational for three or more years and have been field calibrated using a neutron probe sensor. Soil moistures across these seven sites were variable, which is likely to be a product of differences in local climate and soil physical characteristics, as well as site management and landscape position.

This report provides an overview of the soil moisture monitoring network, including site selection, installation methodology, calibration and initial results.

Recommendations for future monitoring and research include undertaking additional laboratory analysis to determine field capacity (at -10kPa) and permanent wilting point (at - 1500kPa) measurements that better reflect the soil depths monitored by the soil moisture sensors at each of the sites. This additional sampling will also contribute towards gravimetric measurements of soil water contents across the sensor soil depths for additional calibration purposes that will provide validation or necessary adjustment to the current neutron probe calibration.

The ongoing monitoring and calibration of the soil moisture sites are recommended in order to contribute towards establishing longer-term datasets that will aid scientific understanding of how Auckland's diverse soils store and release water.

Auckland Council technical report TR2018/012