**3.2. Olive**

(RDTS), where the relative value of daily amplitude in diameter is divided by the correspondent in well-watered plants, obtaining an indicator practically independent from atmospheric variations, as required. Sometimes, maximum and minimum trunk diameters are used indi-

The success of SDV-derived variables depends on plants' behavior. Its application seems to be more successful when applied to conditions of anisohydric behavior [57]. Unfortunately, the outputs often are of difficult interpretation [56, 58], sometimes being the use based on visual

Also, as diameter changes, sap flow rate can be continuously and automatically recorded with high resolution across large temporal scale. Sap flow sensors became popular in last decades, and by measuring fluxes, for the same reasons of independence from atmospheric demand, they only can be directly linked to water status indicators, provided relative transpiration (RT) [48] and the absolute values are not used. The inconvenience of requiring well-watered

As the stomatal conductance is reduced to prevent excessive transpiration, the temperature of leaves and canopy rises. Therefore, the temperature of the canopy in relation to the air is linked to the level of water stress, due to the effect of transpiration evaporative cooling. Several indexes have been proposed and applied in different conditions, space and temporal scales, mainly following the work of Jackson et al. in early 1980s [59], to derive the crop water stress index (CWSI). Measuring canopy temperature is a simple procedure using inexpensive infrared thermometers or any other optical devices that can take many observations rapidly without disturbing the plant. However, canopy temperature is affected by multiple factors,

Overviews and results on remote sensing approaches have been presented [60, 61]. The "advantages and pitfalls" of plant-based methods in the perspective of irrigation scheduling have been discussed by Jones [36]. Fernández [57] recently presented a review of soil or plant water status and other variables used as other water stress indicators for irrigation scheduling. In general, technologies have greatly improved over the years, sensors are more affordable but sampling is still a limitation. In all cases where the relative independence from daily variations in atmospheric demand requires well-watered plants as a reference, this represents a practical disadvantage, limiting its use to the field of research. Unfortunately, these affects many possible indicators and the number of those remaining that are not excessively time

Therefore, the combination of these indicators with models for water balance is advisable [48]. In fact, the most popular variables in irrigation scheduling practices, used at present, either by farmers or enterprises, providing irrigation scheduling services, often include soil moisture quantification, sometimes as a complement to water balance models based on estimated ETc, for example, Ondrasek [1]. This is related to easiness, cost, rapidity to obtain the outputs, simplicity of data treatment/interpretation, and significance. Furthermore, the advantage of

heterogeneity and the quality of the measurements are often disregarded, meaning that a

qualitative use of these outputs is often accepted and considered useful.

with the outputs from water balance is crucial. The problems of spatial

namely VPD, turning it complex to relate with soil water availability.

vidually (MXTD and MNTD).

plants as reference limits its use to research.

consuming, is reduced to a few.

directly linking θ<sup>s</sup>

and qualitative analysis.

56 Irrigation in Agroecosystems

In general, plant water potential seems to be a better indicator than the SDV-derived variables, when full irrigation scheduling is applied. Moriana et al. [62] suggested that values of Ψstem > −1.65 MPa in field conditions provide the maximum g<sup>s</sup> and when Ψstem > −1.8 MPa, maximum yield was obtained [63]. Pérez-López et al. [64] suggested that a threshold value Ψstem of −2.0 MPa (moderate water deficit) may be used to DI. Nevertheless, Ψstem in DI trees was affected by crop load and environmental conditions. Indeed, Moriana and Fereres [65] reported that VPD produced a variation on Ψstem from −0.8 to around −1.4 MPa in fully irrigated olive trees of different ages and fruit load. A threshold value of Ψpd > −0.9 MPa was often proposed to FI [66–68].

It has been observed that SDVs are affected by seasonal growth patterns, crop load, plant age and size, and other factors, apart from water stress [58]. So, the use of SDV needs expert interpretation, which limits their potential for automating the calculation of irrigation depth (ID). Despite this, they refer that, when combined with aerial or satellite imaging, SDV measurements are useful for scheduling irrigation in large orchards with high crop-water-stress spatial variability.

Alcaras et al. [69] reported that the increase in MXTD showed strong relationships with REW, Ψstem and gs . Trunk growth rate (TGR) showed a very early response to water-withholding and it decreased along with Ψstem until it reached a constant negative growth rate, at Ψstem of −2.7 MPa. In their study, DTS was much less responsive to irrigation than either MXTD or TGR. They suggest the use of automated soil moisture sensors if reliable soil moisture values can be obtained, and indicate that a continuous recording of trunk diameter has some potential, but further investigation of MXTD and TGR is warranted.
