Vol 20, No. 4;Apr 2013 54 office@multidisciplinarywulfenia.org Field evaluation of EnviroSCAN performance for monitoring soil water content compared with other soil moisture sensors under arid conditions. Hussein Mohammed Al-Ghobari 1 and Mohamed Said Abdalla El Marazky 1, 2  1 Department of Agricultural Engineering, College of Food and Agriculture Sciences, King Saud University, Riyadh 11451, Kingdom of Saudi Arabia 2 Permanent addresses: Agriculture Engineering Research Institute, Agriculture Research Center (AEnRI), P. O. Box 256, Cairo, Dokki – Giza – Egypt *Corresponding author: elmarazky@yahoo.com. ABSTRACT This study evaluated the accuracy of three commonly-used soil water sensor types (an EnviroSCAN IRR.4-8 probe, a Watermark 200SS sensor and a tensiometer, model R) to monitor volumetric soil water content (θ v ) to develop a means of improving irrigation scheduling. An in situ field test was carried out in a sandy clay loam soil using the gravimetric method (GM) as a reference. The average water contents of the soil profile estimated by the EnviroSCAN capacitance system were used to develop a site specific calibration equation by comparing the sensors’ scaled frequencies for the three depths with soil water content data obtained by gravimetric analysis. The tensiometers and Watermark sensors performed the best with the factory calibrations, with a RRMSE of 6.6, 7.6 and 8.5, and 8.6, 11.1 and 11.0 %, respectively. The results indicated that the tensiometers were more accurate than the EnviroSCAN and Watermark systems without the site specific calibration. While the EnviroSCAN probe overestimated the soil water content, while the Watermark and tensiometer sensors underestimated it. This result indicated that the sensors needed site specific calibration to improve their accuracy in estimating soil water content data. Key words: Soil moisture sensors, sensor performance, irrigation scheduling, calibration, water scarcity. 1. Introduction Soil-based irrigation scheduling techniques involve measuring water content directly or indirectly measuring other soil properties related to water content. Soil water content is considered one of the most important and critical properties of the soil for crop production, irrigation scheduling, and environmental management. Many soil water-monitoring devices are currently available to assist farm managers in the scheduling of irrigation. Generally, these techniques can be classified into the following categories: (i) Tensiometer, measuring the soil water tension or suction in units of kPa (Ling, 2005) (ii) Resistance blocks determining soil water according to electrical resistance measured with an alternating current bridge; (iii) Connector type