Precision of soil moisture sensor irrigation controllers under field conditions

New soil moisture sensor systems (SMSs) for irrigation control have been commercialized in recent years. However, limited research has been carried out to evaluate their precision to measure the volumetric soil water content (theta). The objectives of this research were to: (a) determine the relationship between theta and the theta sensed by four commercially available SMSs, (b) quantify the proportion of scheduled irrigation cycles (SICs) that the SMSs bypassed, and (c) determine the theta at which SICs were allowed or bypassed. Sensors from brands Acclima, Rain Bird, Irrometer, and Water Watcher were buried at 7-10 cm depth, on plots with common bermudagrass [Cynodon dactylon (L.) Pers.]. A calibrated ECH2O probe was also installed in every plot, at the same depth, to monitor theta continuously. When comparing the ECH2O readings with theta sensed by the SMSs, significant correlations were found for the three Acclima RS500 (AC) systems tested, and for two of the three systems of Irrometer Watermark 200SS/WEM (IM) and Rain Bird MS-100 (RB). Most of the SMS-based treatments bypassed the majority of the SICs during rainy periods, and allowed irrigation during the dry periods. On average, 71% of the SICs were bypassed by the SMS treatments, without detriment to the turfgrass quality. However, most of the SMSs were not found to be precision instruments, because sometimes they bypassed SICs and sometimes they did not, even when reading the same or a lower theta. Considering the average theta range of over which the different SMS treatments always allowed or always bypassed irrigation, brand AC resulted in the significantly narrowest range (1.4%) followed by RB (3.2%), suggesting that they were more consistent and precise in measuring theta than Water Watcher DPS-100 (WW) and IM (7.4 and 7.8%, respectively). These results are consistent with the reported water savings achieved by these SMSs in related studies. (C) 2009 Elsevier B.V. All rights reserved.