Evaluating Methods of Crop Water Use Estimation for Refining Irrigation Scheduling

Irrigating cotton for high yields is a balancing act between providing abundant water leading to excessive vegetative growth and minimising plant water stress that impacts on yield. In Australian furrow irrigated systems the target soil moisture deficit (refill point) is approximately 50% of plant available soil water (PAWC). In high yielding systems the impact of exceeding the optimum deficit can be significant with up to 3% yield loss per day of stress during flowering so it is important for producers to have an accurate method to predict when the soil water will reach this target deficit to determine when to irrigate. The availability of ETo (reference evapotranspiration) forecasts may enable for better predictions of crop water use to be made than traditional estimates of crop evapotranspiration (ETc). We compare three methods of predicting ETc against measurements of soil water the day of each irrigation even across three years in a fully irrigated furrow irrigated crop. 1. ETc weather based with crop coefficient using published values for crop stages.

2. ETc weather based with crop coefficient based on LAI measurements

3. ETc weather based with crop coefficient based on LAI measurements and corrections based using soil moisture probe readings.

The comparison showed that the 3^rd method was the most accurate with predictions of ETc and hence irrigation date varying from the actual soil moisture measurements by up to 3 days. In particular the measurement of soil water soon after an irrigation was particularly important to keep the ETc predictions from varying significantly from the actual soil water. With new technologies using NDVI satellite imagery making the use of calibrated crop coefficients more accessible to producers this study showed the importance of using soil moisture or some other measure of real-time crop stress/water use to accurately predict irrigation timing.