The increasing worldwide shortage of water and the high cost of irrigation have already led to the use of precise irrigation methods. An example is the introduction of trickle irrigation in agriculture and horticulture. This recent evolution, however, has also highlighted the urgent need for precise irrigation control and scheduling (Jones, 2004). This need is certainly true in the case of substrate-grown greenhouse crops, because the water storage in a substrate slab is quite limited. The most accurate irrigation control systems have relied on algorithms, which are frequently based on the Penman-Monteith model (e.g., Boulard and Jemaa, 1993; Hamer, 1996; Medrano et al., 2005; Pollet et al., 2000). Although these algorithms use conventional environmental data to estimate the crop water requirements, it is often hard to implement them in a practical irrigation control system, because these algorithms must be calibrated for each specific crop at different stages of crop development (Medrano et al., 2005). Therefore, new approaches have recently been introduced in irrigation management, which can directly reveal the actual crop water requirements without knowing all crop–environment interactions. As a result, direct crop monitoring is an important objective in current horticultural research programs (Ehret et al., 2001).
One of the techniques to determine water uptake directly is by using a lysimeter or an electronic balance. This device is commonly used in the scientific community to calibrate and validate alternative methods or climate-based transpiration models. Recently, de Graaf et al. (2004) and Helmer et al. (2005) successfully tested this so-called “mass-balance” technique in a commercial greenhouse setting on vine crops such as tomato. Nevertheless, Helmer et al. (2005) noticed that information extraction from weight measurements must be done very carefully, because incremental changes resulting from water uptake over a short time interval are very small in comparison with the total substrate mass. Hence, attention must be paid to the precision of the used weighing equipment.
A totally different technique originally developed by Sakuratani (1981) is based on the thermal energy balance of a stem segment for which the sap-flow rate is determined. This so-called “heat-balance” technique has already been examined for potential use in a wide range of crops, including cotton (Gossypium hirsutum) (Dugas, 1990), cucumber (Cucumis sativus) (Kitano and Eguchi, 1989), potato (Solanum tuberosum) (Gordon et al., 1997), and roses (Rosa hybrida) (Rose and Rose, 1998). Unfortunately, little extended information is available on the application of this technique on a tomato crop, so an evaluation of the heat-balance technique is difficult for this crop.
As a result, the objective of this study is to evaluate both techniques by using them in a semipractical greenhouse and to overcome the possible problems to measure tomato water uptake in a direct way.
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