Water efficiency and water monitoring technology have advanced significantly in the past decade. There are several systems that allow growers to schedule their irrigation to suit their crop’s water requirements, these can be evapotranspiration based (as discussed above), soil-based (monitoring the progression of water through the soil profile), or plant-based (looking for plant stress).
Soil moisture monitoring systems allow for the management of irrigation timing and volume. If irrigation exceeds the soil water holding capacity in the root zone, then the excess is wasted through leaching, removing nutrients in the process. Soil moisture probes allow irrigation scheduling to target the active root zone.
Soil moisture monitoring systems or irrigation calculators can signal when irrigation is required. Trends in soil moisture determined over time will indicate the rate at which water is taken up by the crop or lost via evaporation or drainage. These trends can then be used to schedule irrigation based on environmental conditions and crop growth stages.
Below the refill point, the soil moisture will impact plant stress and productivity. The refill point can be estimated by measuring soil water potential or by analysing daily water use patterns to determine when the crop is finding it difficult to remove water. As different soils hold varying amounts of water, the refill point will not be the same for different soils. As sandy soils hold less water, they require less water to reach field capacity and therefore the refill point is higher (-8kPa). Clay soils hold more water and therefore the refill point may be lower (-20kPa).
To ensure plant stress doesn’t occur, the refill point should include a buffer. This is important as the decline in soil moisture is usually confirmed followed by a delay in applying irrigation water. To combat this, the refill point should be halfway between field capacity and the permanent wilting point, or when half the readily available water has been lost. For example, in a clay loam soil, field capacity is approximately 375mm of water in 1m of soil with 150mm of readily available water. When the soil moisture content drops to 300mm (half of 150mm), irrigation should commence. This avoids the soil moisture content reaching 225mm or the permanent wilting point. Note that these values will vary for different crops and are a guide only.
Real-time monitoring can use thresholds for soil moisture to trigger irrigation systems. For example, a tensiometer at 30cm depth installed in a sandy soil has a critical soil moisture tension of -8 kPa. When this is achieved, it is suggested that irrigation is applied and hence this is the refill point. This is suggested for mangoes produced on the Swan Coastal Plain in Western Australia under high evaporative conditions (summer) (WA AGRIC). Similarly, soil moisture sensors and capacitance probes can measure soil moisture in mm or percent (%). Refill points or soil moisture thresholds can be established for different sites to trigger irrigation.
For best results when using soil moisture monitors:
Soil moisture monitoring to fine-tune irrigation scheduling on the:
Plant-based irrigation management is driven by plant stress, with sensors that measure an aspect of plant stress. There are several different sensors on the market, some of which require contact with the plant, and others which are contactless.
Plant-based sensors for irrigation management information in the:
Agriculture Victoria fact sheet