Wireless Soil Sensing Technologies Supporting Variable Rate Water Management.

Irrigation is the largest global user of allocated freshwater, which enhances crop growth to meet global food demand. However, there is little scope for greater allocation of freshwaters due to unprecedented expansion since the 1950s, plus other multiple demands on freshwater resources to meet higher living standards. It is timely to focus on improving the efficiency of existing irrigation systems to improve the conversion of each drop of irrigation water to crop production. Improving the precision of irrigation to crop requirements is one way to do this.

Spatio-temporal modelling of proximally sensed soil information at very fine scales (5–10 m) has been used to quantify the soil and topographic variations impacting on crop water utilisation. This real-time modelling aims to provide an irrigation scheduling decision support tool for precision irrigation hardware; and has been tested in a series of agronomic trials in New Zealand. The trials, conducted over 2 years, have compared a uniform rate of irrigation (URI) with variable rate irrigation (VRI), i.e. irrigation tailored to the site specific conditions of plant-available water supply in (i) a 110-ha irrigated mixed arable field, (ii) a 170-ha irrigated dairy pasture, and (iii) a 75-ha irrigated arable field. At each farm, wireless soil moisture sensor networks were used to monitor soil moisture in soil management zones, defined from EM (electromagnetic) mapping. These data, accessible remotely over the web, have been used for spatial modelling of plant water needs. Finally, the sprinkler irrigation systems have been modified with individual nozzle control for precision water application; regulated by prescription maps loaded into customised software.

Results from these trials underline the importance of being able to manage within-paddock variability, with a two to three-fold difference in available water-holding capacity at all case study sites. VRI has enabled water savings of 15%, 29%, and 22% at the three sites over the 2 years of trials, with water use efficiencies (WUE) of 6.2, 47.8 and 17.2 kg/mm compared with 5.8 kg/mm, 41.4 kg/mm, and 14.1 kg/mm under URI, respectively.

Results show that yields were maintained or increased using VRI compared with URI. VRI scheduling withheld irrigation to zones that were too wet (<10 kPa), or adequately moist (10–100 kPa), while simultaneously irrigating adjacent areas where plant-available soil moisture had decreased to ≥100 kPa and irrigation was required to maintain potential plant growth.

Future research aims to improve the scheduling decisions by including crop type, growth stage and time-dependent variable infiltration rates in the decision support tool, because crops, at certain growth stages can withstand different levels of soil moisture deficit without yield depletion. In addition our research aims to automate the further acquisition and processing of soil moisture, plant, and climate forecasting data to improve direct updating of VRI controlling software.