DSS-Hortimed for On-line Management of Hydroponic Systems

The optimal operation of commercial hydroponic systems depends mainly on the appropriate timely management of the available water sources, especially under saline conditions, as well as on the optimal management of fertilizer injection. In this paper, a decision support system (DSS) for the optimal, on-line, water and fertilizer management of hydroponics is presented. An on-line system, which operates as a supervisor to other greenhouse computers for climate and hydroponics control, implements best management based on real present conditions. An off-line part takes as inputs weather, greenhouse equipment, crop production, salinity tolerance data and water sources available, including their cost, and issues recommendations for optimal water management. The recommendations are used by the grower to make decisions about required investments. The grower can explore various scenarios until he reaches a final decision about the water sources to implement for his own targets. The outputs are also fed into the on-line system as a “guide” to real-time optimal predictive management of water and nutrients supply. INTRODUCTION Several factors have led towards an increasing interest in the commercial application of soilless cultivations in the last decades. The most important ones are the better knowledge on plant nutrition, the technology advances that enable complete control of nutrient injection into the irrigation water and the loss of fertility due to repeated cropping on the same soil. More importantly environmental regulations for some very effective disinfectants, such as methyl bromide, were added recently to the forces of imposing the hydroponic systems, even in underdeveloped countries. The increasing awareness about the environment and the safety of food as well as the depletion of water, has encouraged intensive research activity. In closed-loop soilless cultures, the differences between the composition of the feeding solution and the apparent nutrient uptake concentration (i.e. the ratio between water and mineral uptake by the crop) results in a gradual degradation of the recirculating solution (Li et al. 2001). This appears as salinity build-up or a deficit in nutrients, as the system behaves as a difficult to control pure integrator. The essential and non-essential ions that are in excess to plants’ uptake accumulate in the system. In cases where the incoming water brings along non-essential-ions, the irrigation solution should be replaced (flushed) at specific time intervals, when the salinity or toxicity in the recycling tank exceeds a certain threshold. A closed irrigation system managed in such a way will produce a salt concentration of the form shown in Fig. 1. The average salinity is significantly less than the salinity tolerance (about half the maximum), if the highest point on the curves is the salinity threshold (Fig. 2). With the improvement of crop mineral uptake models a decision support system (DSS) simulates a virtual plant with material balance to allow a better regulation of the recirculating nutrient solution. This system may provide a tool for better management of closed hydroponics with the aim to save water and nutrients and to reduce environmental impact (Sigrimis et al., 2001). The objective of the DSS is to develop a context sensitive strategy for managing irrigation and nutrient supply of greenhouse soilless cultures with constraints on the quantity and quality of water supply. The economical (i.e. quality and Proc. IC on Greensys Eds.: G. van Straten et al. Acta Hort. 691, ISHS 2005 268 quantity of crop yield) and the ecological (i.e. the contamination of water table, nature conservation) factors affecting the strategic decisions are also considered. If the commercial value of crop yield is entered, the DSS seeks the most economical management based on calculated water use efficiency and water cost. Otherwise, the yield value is assumed at very high price and the system runs for maximal yield with minimal water cost. MATERIALS AND METHODS Theoretically, in a closed hydroponic system the total amount of water needed is equal to the water uptake related to plant transpiration and growth and the possible water losses from leakage and evaporation, which usually can be neglected. If a portion of the recirculating water is frequently leached and replaced by fresh nutrient solution (semiclosed hydroponic systems), then it is possible to maintain equilibrium at the salinity level of the irrigation system, close to the maximum salinity tolerance EC (EC ≤ threshold EC; Fig. 1 and 2). If the salinity equilibrium is kept close to the threshold point of the yield response curve to salinity (Fig. 2), then it is possible to achieve maximum production with minimum water requirements. The leaching requirements in semi-closed hydroponics depends on the salinity (namely electrical conductivity or EC) of the water source, the composition of the nutrient solution and the water and nutrient uptake by the crop that may be simply represented by the apparent nutrient uptake concentration. The negative effect of salinity on crop yield (Li et al., 2001) may be circumvented or lessened by reducing transpiration (Li et al., 2001; Lorenzo et al., 2003). The optimization function of the DSS accepts inputs such as the cost of the increase in humidity versus the effect of reduced transpiration, and runs for estimating optimal humidification. Experimental data from the HORTIMED research project were used to determine nutrients uptake and plants tolerance to salinity (D’Amico et al., 2003; Heuvelink et al., 2003; Li et al., 2001; Lorenzo et al., 2003; Sigrimis et al., 2001), as well as results from Yu et al. (2001). The DSS process consists of two cooperative systems: off-line and on-line. The off-line DSS aims to calculate the best price-performance water management for a given crop on a yearly basis (crop data) and in a given place (climate conditions), making all the possible combinations of the available water sources according to the calculated water needs. The water needs depend on the climate conditions, the crop and its developmental stage. The system to be used (open or closed soilless culture) does not affect the off-line DSS, because the total water demand by the crop is theoretically the same under the minimum-drain policy (Sigrimis, 2000). When a closed system is adopted, it provides some technical advantages, since over-irrigation may be used in order to compensate for the unequal transpiration of individual plants, the differences in water flow rate of individual trickle nozzles and the related uneven water distribution per plant, as well as to prevent salt accumulation and the imbalance in the nutrient solution in the root environment, which both may depress crop growth and production. Nevertheless, the DSS does not advice the grower about the proper system to be used due to the influence of parameters that are quite complicated (type of substrate, monitoring aids, etc) and subjective as well (grower’s experience). Unlike the off-line system, the on-line DSS takes into account the current weather conditions, after been fed with the estimated projections of the off-line DSS. The on-line DSS uses the same plant model (including nutrient uptake) that was presented in off-line DSS, but for a different reason. The model’s outputs are fed to a Risk Management scheme that uses specific probabilities for several possible weather conditions and forms the appropriate risks of feasible actions. These risks, combined with the plant model’s outputs of water requirements and salinity tolerance, the characteristics of growing systems (open or closed aggregate culture, NFT, etc.) as well as the data for nutrient compensation in salinity water or recycled water, develop an advanced Water Source and Nutrients Management System.