Soil Water Sensing-Focus on Variable Rate Irrigation

Authors: Evett, Steven - Steve; Colaizzi, Paul; Schwartz, Robert; Oshaughnessy, Susan

Submitted to: Proceedings of the Central Plains Irrigation Conference
Publication Type: Proceedings
Publication Acceptance Date: 2/12/2014
Publication Date: 2/25/2014
Citation: Evett, S.R., Colaizzi, P.D., Schwartz, R.C., Oshaughnessy, S.A. 2014. Soil Water Sensing-Focus on Variable Rate Irrigation. Proceedings of the Central Plains Irrigation Conference.

Interpretive Summary: New variable rate irrigation (VRI) systems can apply water where it is most needed and useful in a field. Less water or no water can be applied where it would lost to runoff or deep percolation or applied uselessly on roads, rock outcrops or other non-productive field areas. VRI systems are used with center pivot and lateral move irrigation systems. Scientists with the USDA-ARS Conservation & Production Research Laboratory, Bushland, Texas, studied available soil water sensors for their accuracy and usefulness as guides for VRI scheduling. They determined that only a few available sensors had the accuracy to be useful guides for VRI. These few were, however, able to be used with wireless data transmission, which is a must for practical use in production agriculture. They also found that, depending on the amount of field variation in soil properties and slope, the number of sensors needed could become too expensive. In that case, crop leaf temperature sensors mounted on the laterals of moving irrigation systems provide more complete coverage of the crop water status variation in the field. A few soil water sensors can then be used to provide backup information to avoid over watering.

Technical Abstract: Irrigation scheduling using soil water sensors is an exercise in maintaining the water content of the crop root zone soil 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. The management allowed depletion for a corn crop on a clay loam soil is only about 0.08 inch/inch. To be useful for managing water to prevent over filling the soil or allowing it to dry so much that the crop yield is compromised more than acceptable, soil water sensors must be accurate. The accuracies needed are on the order of 0.02 to 0.04 inch/inch, which is better than many commercial soil water sensors are able to provide. Values of field capacity and permanent wilting point for a particular field, needed for determining available water holding capacity and MAD, may be found from NRCS soil maps, at least to a close approximation. The values are, however, likely to change with depth in the soil and with position in the field, meaning that irrigation management should be site specific to be most effective, and to do that requires sensors be installed in the different soils of the field. Soil water sensing for variable rate irrigation scheduling is clearly possible using presently available accurate soil water sensors and wireless data transmission to the irrigation control center. Present challenges include field installation labor and timing to avoid machine operations that disturb the soil and may damage the sensors and antennas used. Depending on the spatial variation in field soil properties, installing an adequate number of sensors may be overly expensive. Wireless plant water stress sensors mounted on moving irrigation systems may be no more expensive and offer proximal remote sensing that does not get in the way of field operations. Nevertheless, soil water sensing will remain an important tool in irrigation scheduling, including as a check on VRI prescriptions generated using plant water stress sensors