Crop water use efficiency

Some crop plants and varieties are more water efficient than others, for example C4 plants such as sorghum generally have a greater crop WUE than C3 plants such as wheat. C3 and C4 plants are classified by how they turn carbon dioxide into energy (photosynthesis). C3 plants are those which are typically adapted for temperate climates, while C4 plants are more adapted for conserving water in hot and dry environments.

Soil type plays an important role in the amount of plant available water and water use efficiency. Water drains easily through the large pores between aggregates and particles in sandy soils and therefore have a low water holding capacity, while the smaller pore size of clay soils allows them to retain more water (high water holding capacity). However, due to the tension at which this water is held in clay soils, plants cannot easily take up this water for use, compared to water retained in sandy soils. The soil water content at which plant can no longer uptake water from the soil due to the tension at which it is held is called the ‘wilting point’. Clay soils therefore have a higher wilting point than sandier soils.

Other factors such as a shallow or stony soil, soil compaction, or inherent chemical constraints to plant growth (e.g. high levels of salt, sodium, boron, acidity) can limit WUE.

The main goal is to implement land management practices which reduce water loss via evaporation and facilitate plant uptake of water.

Irrigation management, including irrigation method and timing of irrigation influences crop water use efficiency. Drip irrigation increases WUE compared to flood irrigation as water as it minimises soil evaporation and runoff, by applying water slowly and directly to the plant root. Flood irrigation applied at the middle of the day maximises the potential for water evaporation and runoff.

Maximising soil cover and using mulching to reduce water evaporation can improve crop water use efficiency. 

Crop water use efficiency (WUE) in wheat has been estimated for the 1982 – 2020 period as a broad indicator of soil productive health on cropping land in South Australia. This was estimated using annual wheat yield estimates produced for the cropping districts by the Department of Primary Industries and Regions (PIRSA) and monthly gridded rainfall data from the Bureau of Meteorology (spatially trimmed to cropping land), using a modified French – Schultz yield potential model. This analysis showed the trends in WUE over time in each district and weighted average WUE by region and for the state, as well as differences in WUE between districts and regions. It has helped to identify where soil or agronomic constraints to production may exist as well as opportunities to improve productive performance.