Assessment of Precision Technologies for Accurate Delivery of Crop Inputs

Abstract

At the Tennessee Valley Research and Extension Center (TVREC), Belle Mina, Alabama, Global Positioning System (GPS)-assisted machine guidance and subsurface drip irrigation (SDI) have been implemented to study precision agriculture practices. Crop yield can be adversely affected if 1) not planted optimally relative to SDI product placement, or 2) the SDI system performance has degraded. Recent evidence of guidance-system drift and soil moisture variability after irrigation at TVREC prompted performance evaluations of both systems. Use of an optical total station was investigated to quantify positioning error in a GPS-based autoguidance system when using different correction-signal services. Also developed was an assessment technique for in-situ pressure-compensating SDI application uniformity at variable operating pressures or slopes. The total station provided an absolute frame of reference for comparing accuracy and repeatability of Wide-Area Augmentation System (WAAS), John Deere® SF1 and SF2, and Real-Time Kinematic (RTK) correction signals. Recommended techniques for proper correction-signal testing were to perform site calibration, establish semipermanent reference monuments, conduct guidance tests in northward/southward or eastward/westward path orientations, and account for tracking-prism tilt. Measured cross-track error (XTE) was influenced by drive-path orientation for RTK, SF2, and WAAS; SF1 performance was consistent regardless of orientation. XTE for SF1 and WAAS were more representative of manufacturer’s claims. Unsigned 95% short-term and long-term error probability was preferred to describe guidance performance over signed pass-to-pass (P2P) and year-to-year (Y2Y) errors, respectively. A field-testing apparatus was fabricated to interface with the in-situ SDI product to deliver water over a range of operating pressures (48, 83, 117, and 138 kPa) while measuring sample discharge. Apparatus discharge rates agreed to within a half-percent of per-emitter laboratory discharge rates. Lowest discharge was observed for the irrigated SDI product operating at 48-kPa. Discharges from the non-irrigated and irrigated SDI treatments at 83 and 117 kPa were not different (a=0.05). In summary, water delivery is dictated by SDI placement and functionality; as a consequence, other farming practices have to be conducted with a high level of spatial accuracy in relation to the SDI system in order to maximize crop yield, like the RTK correction signal. Techniques and equipment developed in this study provide autoguidance and SDI researchers with innovative approaches to evaluate long-term performance of these precision-agriculture systems.