Sensors and wireless sensor networks for irrigation management under deficit conditions (FLOW-AID)

FLOW-AID has the objective to develop and test an irrigation management system that can be used under deficit, when water availability and quality are limited. It intends to use innovative, but simple and affordable concepts usable for a variety of irrigation set-ups and constraints. Amongst others, it focuses on development of low-power wireless sensor networks for soil water monitoring. Six sensor nodes equipped with SM200 soil moisture probes and 3 repeaters were built and evaluated during 5 months under practical Mediterranean conditions for a container grown crop. The work is still on-going and only intermediate results are reported. The nodes were capable of relaying data to a repeater over a distance up to 20 m, but the requirement of a maximum of 5% data loss could not be fulfilled. The battery lifetime of the sensor nodes was adequate providing that the nodes were configured correctly. Remote access and data transport over the internet worked very stable and fluently. Weak points of the system were identified for packaging, signal losses, high cost and sensor performance. Enhancement recommendations are reported and form the basis for the next generation WSN, currently being developed and to be used during the two upcoming growing seasons. INTRODUCTION In the developing world, water allocated to irrigation is about (or exceeds) 69% of water resources (Fry, 2005). In view of increased domestic competition for resources and the need for larger agricultural production to ensure food security, such a fraction is unsustainable. Therefore, future water security can only be warranted by a considerable increase of the water use efficiency. As a consequence, the demand for new water saving irrigation techniques is growing rapidly. To produce “more crop per drop”, growers in (semi) arid regions currently explore irrigation techniques in the range from using less fresh water to using marginal water resources like recycled household water. Doing so, they enter the domain of producing crop under sub-optimal or even stressed conditions for the crop, commonly referred to as deficit irrigation (Lamaddalena, 2007). As such, the margin the grower has to control his crop becomes smaller, and information about actual crop status plays a crucial role. Especially soil water content or soil water tension as well as electrical conductivity (EC) are important parameters to monitor, since highly saline soils invoke crop stress and yield reduction. Current research focuses on monitoring systems that regularly supply information to growers about growing conditions (soil, climate and crop status) and decision support systems (DSS) with automated control that take adequate and timely irrigation or fertigation actions based upon water availability and crop needs. The concept of using soil water sensors to activate irrigation scheduling is a well-known concept and has become a common practice since the introduction of dielectric (TDR, FDR) soil water sensors (Balendonck et al., 1998). A large number of soil moisture sensors have become available over the past decade, however only the WET-sensor (Hilhorst, 2000) and the new ECHO-EC5 probe (Decagon, US) are capable op monitoring both EC and soil water content. For precision real-time irrigation control, controllers and sensors are installed at each plot or at least at every group of sprinklers or drippers in the field. Each controller performs an individual irrigation schedule which is set and reprogrammed on a regular basis. Due to soil spatial variability, a large number of sensors are needed in each irrigation zone to obtain a reliable mean soil moisture reading. Since many controllers and sensors are involved, the high cost for investment, installing wiring, maintenance, data-handling and use is becoming a large bottleneck, which forces growers to look for new improved and costeffective monitoring and control systems. The use of wireless sensor networks (WSN) saves a lot of installation and management cost (Panchard, 2006; Ning Wang et al., 2006; Kim et al., 2006; Baggio, 2005), and companies like Delta-T Devices (UK), Netafim (IS), Decagon (US) and Crossbow (US) start offering systems or components of wireless sensor systems for irrigation management. However, equipment is still expensive and uses a lot of energy to overcome the variable damping of electro-magnetic waves in crops under fluctuating weather conditions (Thelen et al., 2005). Numerous publications can be found on all the different aspects of WSN and the protocols for ubiquitous communication. Especially in the last 3 years the number of publications has increased rapidly. For a comprehensive survey and state of the art on wireless sensor networks with special attention to the ZigBee standard we refer to Baronti et al. (2007). Most research about the use of WSN in the field of agriculture and horticulture are so far carried out in Australia and North America. Quite often the applications are related to irrigation and water management issues. A number of publications confirm that at the current development stage WSNs are not reliable enough, can’t withstand outdoor climate conditions, lose communication, are not fault tolerant, use too much power, are damaged too quickly and are riddled with new problems not foreseen by manufacturers or end users. Tuijl et al. (2007) published a review on WSNs with focus on agricultural and horticultural applications and confirm that short communication distances (10 – 30 m), maintenance cost for frequent replacement of batteries, and above all price, are still the biggest hurdles for implementing dense WSNs. Currently we see that the end-user price of wireless nodes lies between €100 and €350 and for soil moisture sensors between €50 and €800. The use of solar power instead of batteries is sometimes not possible due to sun-blocking by crop foliage. Battery operated equipment is more reliable and still favorable, which makes it needed to improve both equipment and communication protocols in such a way that they become low-power and work reliably under outdoor agricultural conditions. This work, performed within the framework of an EC project called FLOW-AID, focuses on the development of a prototype low-power sensor/communication device (receiver/transmitter pair called a MOTE), incorporating a robust multi-node communication protocol (MESH-network) for transmission of data-signals from monitoring and control devices for irrigation management. With multiple prototypes of these MOTES, during 3 years, tests will be performed under practical conditions for a container grown crop in Pistoia, in the Tuscany region in Italy. The objectives of the 1 year experiment were to assess the long term robustness and reliability of a WSN with focus on: 1. Communication robustness (failure of nodes/repeaters), 2. Maximum range of wireless communication between nodes, 3. Battery life time / power consumption, 4. Suitability for outdoor usage (packaging as well as radio communication), 5. Connectivity to other interfaces, 6. Sensor performance, and 7. Cost price. This paper describes the FLOW-AID concept and the progress of the work on the wireless sensor network during the growing season of 2007. FLOW-AID system FLOW-AID (Farm Level Optimal Water management, Assistant for Irrigation under Deficit) is an on-going European project that aims to make irrigation sustainable by improving deficit irrigation practices, and by helping growers to safely, more efficiently and cost-effective manage irrigation. It aims at integrating innovative, but simple and affordable, monitoring and control technologies within an appropriate DSS (Balendonck et al., 2007; Balendonck, 2008). It focuses on the various and typical (protected as well as non-protected) growing systems found in the semi-arid regions of the Mediterranean. Testing and calibrating the system under the various local constraints of farm and basin management, helps to ensure that the technical, environmental and economical performance of irrigation systems is improved.