ARS | Publication request: Irrigation scheduling by ET and soil water sensing

Submitted to: Irrigation Associations Exposition and Technical Conference Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: November 5, 2012
Publication Date: November 5, 2012
Citation: Evett, S.R., Schwartz, R.C., Howell, T.A. 2012. Irrigation scheduling by ET and soil water sensing[abstract]. Irrigation Associations Exposition and Technical Conference, November 3-7, 2012, Orlando, Florida, 2012 CDROM.

Technical Abstract: Irrigation scheduling is the process of deciding when, where and how much to irrigate, usually with the goal of optimizing economic return on investment in land, equipment, inputs and personnel. This hour-long seminar presents methods of irrigation scheduling based, on the one hand on estimates of the crop water use (ET, evapotranspiration), and on the other hand based on soil water sensing. Soil water based irrigation scheduling typically relies on a management allowed depletion (MAD) concept that involved maintaining the soil water content between the field capacity and a drier point that is determined by the management allowed depletion. Today, many sensors are availble to irrigators, but most are not accurate enough to use in a MAD-based schedulign scenario without risk of approaching the permanent wilting point, which would cause serious drops in crop yield and quality or plant appearance. Differences between major soil water sensor types are explained and reasons for avoiding some sensor types are given. Conversely, some sensors that have proven accurate and stable in a range of environments are discussed. Alternative sensors that report the soil water potential are also discussed since they are appropriate for some crops and soils. In contrast to the soil water sensor-based scheduling approaches, scheduling based on ET estimates is also widely used. Modern ET-based methods typically use the Penman-Monteith reference ET method, which calculates the daily reference ET from wind speed, solar radiation, humidity of the air and air temperature. The reference ET is then multiplied by a crop coefficient, which is itself dependent on the crop growth stage, resulting in a daily crop ET estimate. Using a simple checkbook method of accounting for water additions through irrigatoin and water depletions through crop ET, an irrigator may determine when and how much to irrigate. An ubiquitous problem with the ET approach is that, due to inaccuracies in the daily ET estimates, the soil water content may deviate over time from that predicted by the checkbook calculation. Thus, an occasional check on soil water status is still needed. Several examples of ET-based weather networks for irrigation scheduling in different states or regions are presented. The seminar concludes with a discussion of impediments to use of these irrigation scheduling methods and possible solutions.