Stress Coefficients

The use of plant and soil-water-status indicators, thought useful to identify the moment for water applications does not provide information about the amounts needed to replenish the soil reservoir. Usually (see Allen at al, 1998) this is obtained by a daily soil water balance that, under non-restricted water application, that includes the estimation of cumulated ET from the previous irrigation (E ETa), provided ETo and Kc are available.

Under restricted water application, when a moderate water stress is imposed because of water shortage or for the sake of fruit quality, a lower critical threshold value for water applications is considered, being ETa less than ETc (Figure 1). As a consequence, another parameter or relationship has to be included in ETa estimations. The relationship between RT and the sum of ET since last irrigation (E ET) can be useful for this purpose. An example is presented in Figure 15, for a peach orchard: if E ET is calculated since last irrigation, 70% of RT is reached when soil water depletion (SWD) or E ET since last irrigation is about 25 mm. This relationship allows to calculate the ET, during stress periods, if ETo and Kc are available (estimating Es separately, if needed in order to obtain relative ET= Ks).

If the soil surface is dry most of the time, RT (cf. Figure 15) and relative ET (RET) of a stressed plot are similar. However, RET is different from Ks in case of non-daily irrigation, because it is calculated in relation to a plot, whose ET thought well irrigated is also decreasing between two successive irrigations. In the example of Figure 15, ET is simultaneously decreasing in both plots when E ET< 10 mm and Ks = 1. In the diagram of Figure 16a, the seasonal course of daily T, in a well irrigated plot and a stressed plot, as well as RT and Ks for the stressed plot, are shown during two stress cycles to illustrate the comment above.

13 0.4 —'—'—'—'—'—'—'—'—'—'—'—'—'—'—'—'—'—

Figure 15: Relationship between RT (T of a stressed plot over the T of a well irrigated plot) and the total ET since last irrigation, observed in a peach orchard, in central Portugal (Ferreira et al., 1997b).

Figure 16b shows experimental results obtained for Ks - E ETa in two peach orchards, both with similar sandy soil, same region and ET rates, but different irrigation systems and frequency. The sharpest decrease was obtained in an orchard drip irrigated daily, as described in 4.3., while the line with lower slope is for an orchard with sprinkler irrigation, each 3-4 days as presented in Ferreira et al. (1997b). This difference will be later analysed.

Figure 16b shows experimental results obtained for Ks - E ETa in two peach orchards, both with similar sandy soil, same region and ET rates, but different irrigation systems and frequency. The sharpest decrease was obtained in an orchard drip irrigated daily, as described in 4.3., while the line with lower slope is for an orchard with sprinkler irrigation, each 3-4 days as presented in Ferreira et al. (1997b). This difference will be later analysed.

£ ETa (mm)

Figure 16 (a) Diagram of the evolution during a stress cycle of T in a well irrigated plot (dashed line) and stressed plot (black line), with RT and Ks in stressed plot. Black arrows indicate irrigation only in well irrigated plot and grey arrows irrigation in both plots. (b) Experimental relationship between Ks and SET in two experiments with peach orchards, similar soil, climate and ET rates, different irrigation systems and frequency: dashed line - sprinkler irrigation, as described in Ferreira et al. (1997b), full line - daily drip irrigation, as described in 4.3.

In fact, the detailed temporal analysis that micrometeorological methods allow, brings to light a decrease on ET very soon after irrigation, when working with high ET rates and sandy soils. In order to obtain Ks, the value of ET or T for the irrigated plot (used as a reference) is conveniently replaced by another reference: ETo.Kc (with Kc for the day after irrigation). In this case, the platform disappears and a relationship for Ks as the dashed line in Figure 1 is obtained (Figure 16a). This means that the platform effect was, in this case, an artefact due to the simultaneous decrease on ET in both treatments, during the first days, until the irrigated treatment received the second irrigation.

In the general case, if b is the value of E ET for which ET begins to decrease, a relationship can be established as: Ks =1, if E ET< b; Ks = 1- n (E ET - b), if E ET > b. These relationships are able to give an answer to the when and how much and can be auto-sufficient in the sense that, if well adjusted, there is no need for in loco measurements, if a critical threshold value is known, in terms of E ET. An example is given in Table 1.

This relationship changes with the soil characteristics (Puech, 1972), the rate of ET (Denmead and Shaw, 1962) and the root patterns (Lee, 1973). In order to adapt to other soils or rooting depths, the value of E ET (soil water depletion) can be presented in terms of percentage of available water (AW): Ks =1, if E ET/AW < b'; Ks = 1- n' (E ET/AW - b'). Parameters for a and b are shown in Ferreira and Valancogne (1997) for one tomato and one peach orchard experiments.

Table 1:- Simplified calculation of daily Ks for an hypothetic situation

(1)

(2)

(3)

(4)

(5)

(6)

(7)

(8)

Day

ETo

Kc

ETc

S ETa (day i-1)

Ks

ETa

S ETa (day i) =

(i)

mm

0 0

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