Submitted to: Irrigation Associations Exposition and Technical Conference Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 11/1/2014
Publication Date: 12/19/2014
Citation: Evett, S.R., Oshaughnessy, S.A., Colaizzi, P.D., Schwartz, R.C. 2014. Soil water sensing: Implications of sensor capabilities for variable rate irrigation management. Irrigation Associations Exposition and Technical Conference Proceedings.
Interpretive Summary: Fresh water supplies are decreasing world-wide, while demand is increasing. To feed an increasing global population, envirornmental agriculture use needs to be water smart, agricultural fields are not uniform in soil type and slope. Thus, crop water needs may vary across a field. Producers of irrigated crops are now able to purchase variable rate irrigation (VRI) systems from dealers. These VRI systems are capable of applying water to the crop just where it is needed and in the amount needed. But knowing where, how much and when to apply irrigations is still an unanswered question for producers. A team of scientists at the USDA-ARS Conservation and Production Research Laboratory at Bushland, Texas, studied several soil water sensors and evaluated their usefulness for providing the kind of information about crop water needs that is needed for VRI management. They concluded that current commercially available soil water sensors are too costly and inaccurate to be useful for VRI management. The team also looked at other methods of VRI management and concluded that plant canopy temperature sensing, using sensors spaced along irrigation system laterals, could in many cases provide more useful information for VRI management at less cost. They also concluded that the canopy temperature sensing system could be combined with a few, accurate soil water sensors to provide better information than either canopy or soil sensors could achieve on their own. Sensors and sensing and irrigation control systems developed by the team are now being transferred to private firms for commercial production and sale to producers.
Technical Abstract: Irrigation scheduling using soil water sensors aims at maintaining the soil water content in the crop root zone above a lower limit defined by the management allowed depletion (MAD) for that soil and crop, but not so wet that too much water is lost to deep percolation, evaporation and runoff or that the crop quality is impaired. To be useful for managing water to prevent over filling the soil or allowing it to dry so much that crop yield is compromised, soil water sensors must be accurate to the order of 0.02 to 0.04 inch/inch. Issues of sensor performance, numbers of sensors required for effective variable rate irrigation (VRI) management, and other factors complicating sensor application to VRI management all hamper adoption of soil water sensing systems for VRI. An alternative to soil water sensing that may be helpful is soil bulk electrical conductivity mapping, which can delineate field zones of different soil textures if salinity is not a factor. Another alternative, soil canopy temperature sensing, has been shown to accurately reflect plant water status and thus soil water status in semi-arid and arid irrigation regions; and this method is approaching commercial availability. A combination of crop canopy temperature sensing, which effectively uses the crop as many thousands of biological soil water sensors, and a few accurate soil profile water content sensors may prove to be the most practical approach to variable rate irrigation management in many regions.