The total water flux from vegetated surfaces to the atmosphere, or evapotranspiration (ET), includes transpiration (T), soil evaporation (Es) and interception (evaporation of water from the leaves outer surface, due to rain, irrigation or dew). Through quantification of ET and water balances it is possible to better understand the role of transpiration and evaporation in the water cycle and to enable more efficient use of water in irrigated crops. The validation of water use models and the understanding of the related physical and physiological processes require good methods for long-term ET measurements. Water management in irrigation has been the motivation for most of the studies of ET and its relationship with water stress.

Irrigation is essential to achieve good crop growth and economic yields whenever precipitation is low or negligible during the period of active vegetation growing, as in Mediterranean climates. Under such climate, most precipitation occurs during winter, when temperatures and ET are low. Therefore, water loss through interception represents only a small part of annual ET. Transpiration, T, becomes the most important term in ET, because high Es losses are often avoided by good irrigation or cultural practices. Another consequence of summer water scarcity is that it results in dominant stands of woody plants that can withstand water deficits while extracting water from deep below ground sources that are recharged during the winter. In such cases, sustainable water management means using irrigation as a supplement to rainfall.

Irrigation planning and management requires information on maximum ET for the specific climatic and crop conditions (ETc), often approached through an empirical equation using reference ET (ETo) and crop coefficients (Kc): ETc = ETo x Kc, (e.g. Doorenbos and Pruitt, 1977; Allen et al., 1998) with Kc accounting for both T and Es (Ritchie, 1972; Tanner and Jury, 1976; Kanemasu et al., 1979, Allen at al., 1998). ETo can be estimated using meteorological data (vd Doorenbos and Pruitt, 1977; Burman, 1983; Rosenberg et al., 1983; Jensen et al., 1990). The use of Penman-Monteith (PM) equation adapted to practical uses and with grass parameters, according to Allen et al. (1998), has been increasingly used. Kc values vary with crop phenological stages to accommodate crop changes. However, limitations in determining Kc, and more importantly, implementing ET-based irrigation scheduling, arise when agro-technical differences cause differences in crop growth and water use. Discrepancies between Kc measured in several row crops and Kc from currently used manuals (as in Allen et al., 1998) show the need for additional field measurements of actual ET in order to improve estimation methods.

Limitations to the application of ET measurement methods in small fields and/or woody crops have been often described, as discussed in Section 2. Due to such limitations, measurements of water use of individual plants with sap flow methods (SF) are an alternative to quantify T, the major component of ET. An underestimation of T from SF data has been observed under several conditions, when compared to data from eddy covariance (EC) micrometeorological method. Even where the size of the plot is large enough to allow EC measurements, it is often much easier to make long term measurements of sap flow. When a complete soil water balance is not feasible (deep, sparse roots), a combination of those methods (SF and EC) can provide reliable, long term ET estimates. This chapter shows how to combine these methods to improve ET estimates.

Two basic questions in irrigation water management in agriculture are when to irrigate and how much water to apply when irrigation occurs. When to irrigate can be indicated by determining a critical value for soil water depletion or other direct or indirect indicator of plant water status. The soil water depletion is the result of the cumulated actual ET (ETa) since last irrigation event. Therefore, it can be approached via the soil water balance, using ETa estimations. Calculating ETa as ETo x Kc implies that irrigation occurs before a critical value for plant water status is attained, meaning that irrigation takes place before ETa starts to decrease (black arrow in Figure 1) from its maximum value, ETc.

The relationship between the relative value of T or ET and the reduction in available soil water has been studied for decades (Penman, 1940; Hallaire, 1960; Denmead and Shaw, 1962), where the available soil water is the difference between the field capacity, FC, and the permanent wilting point, PWP (Figure 1). This relationships, valid in irrigation conditions with moderate stress, can exhibit a continuous decrease since irrigation occurs (dashed line) or a platform (ETc=ETa) followed by a sharper slope (full line).

Figure 1: Possible relationships between Ks = ETa/ETc and available water in soil. Arrows indicate possible choices for irrigation applications.

Available water

Figure 1: Possible relationships between Ks = ETa/ETc and available water in soil. Arrows indicate possible choices for irrigation applications.

Experimental evidence based on ET measurement methods that provide information over detailed time scales, suggest that ETa is often less than ETc, in irrigated rough canopies and/or woody crops. In fact, as pointed out by Jarvis (1985), water stress inducing stomatal closure has, in general, more impact on ET reduction in tall crops than for low crops. Given the extreme complexities of deterministic plant ET models, an operational alternative is to establish parameters of empirical models for ETa estimation accounting for water stress by using a stress coefficient (Ks) given by ETa/ETc, which needs to be adjusted to specific conditions. Relationships between Ks and other water stress related variables allow to perform this adjustment as suggested in Section 5.

This chapter describes basic concepts and assumptions concerning the application of ET estimations and the water-stress indicators related to irrigation scheduling, with emphasis on the specificities of woody species.

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