Irrigation scheduling is as important to optimal onion yield and quality as irrigation methods are, and there are several simple soil moisture monitoring systems that can be used. The measurement of soil water tension (SWT) is one of the most effective methods for scheduling onion irrigation. Drip irrigation and soil water tension work together well in areas that need to maximize water use efficiency. Two tools often utilized with SWT are granular matrix sensors and tensiometers, both of which indicate soil moisture on a scale of 0-80 centibars (0 being the wettest and 80 being the driest). Granular matrix sensors measure soil water potential and tension—the “force necessary to remove water from the soil” (Shock, n.d.)—and the amount of water in the soil. Tensiometers measure the strength that water is held in the soil. Onions in excessively wet soil are more likely to contract diseases; onions in excessively dry soil are less productive. There is only a small margin of error between too wet and too dry. For example, in the Treasure Valley, it is best to irrigate a drip-irrigated onion crop on silt loam soils when the SWT reaches 20 centibars to maximize onion yield, size, and profit (Shock, C. C., Flock, R., Feibert, E., Shock, C. A., Jensen, L., & Klauzer, J., 2013; Shock, C.C., 2013).
The checkbook irrigation scheduling method for crop evapotranspiration takes into consideration how much water will evaporate from the soil, how much will transpire from plants each day, how much water has been applied, how much soil water depletion is allowed, and how much rain is received (Shock, Feibert, Jensen, & Klauzer, 2005). After carefully recording this information, growers decide how much water to use and when. This information is provided by local weather stations (like Agrimet for the Pacific Northwest) and other weather data resources (like NOAA's National Climatic Data Center). SWT is the preferred scheduling method over crop evapotranspiration due to its precision, while crop evapotranspiration depends on local weather stations whose information may not apply over large geographic areas. The checkbook method also requires a rain gauge in each field, without which the method is not as precise (Shock et al., 2010). However, that is not to say the check book method is not an effective scheduling method.