It is possible to get messages from the plant itself or from the plant canopy as a whole indicating that it is time to irrigate. But water stress is not the only stress affecting the plant; diseases, parasite attacks, thermal and light deficits, or even water excess can produce the same type of effect on the crop. Observation of apparent symptoms of water stress cannot be considered a practical guideline for irrigation scheduling because generally it is too late when symptoms appear (see Section 5.5).
Measurements collected on individual plants will require a accurate sampling. Mi-cromorphometric methods allow detection of changes in the dimension of vegetative organs and give interesting information, mainly on trees. Respective sensor devices are not difficult to place on branches or to connected to a logging system. The main problem encountered is that the same response is obtained with excess or lack of water . Methods based on leaf water-potential measurement are difficult, expensive, and cannot be automated. Scholander's chamber and the thermocouple psychrometer are used . The main interest is in linking values of predawn leaf water potential to evapotranspiration . When threshold values are selected, successive measurements allow forecasting of the irrigation date.
Sap-flow measurements are based on the heat capacity of water. Two techniques are available: the sap-flux density technique, which is limited by the need to determine the cross-sectional area of the water-conducting tissue; or the mass flux technique, which is restricted to estimations of small-tree transpiration . However, much progress is to be expected.
Surface-temperature measurements  performed by means of infrared hand-held thermometers have proved feasible and are used in practice. Some indicators, such as the Stress Degree Day (SDD) and the Crop Water Stress Index (CWSI), can be applied but they are crop specific. Efforts to simplify field applications have been carried out [16-20] but they can be used only if weather conditions are somewhat stable because indicators are based on the differences in temperature between the crop and the air. As a result, indicators also are related to specific environmental conditions and require local calibration.
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