1Instituto Superior de Agronomia - ISA, Technical University of Lisbon
Tapada da Ajuda, 1349-017 Lisbon, Portugal 2INRB - Estaçâo Vitivinícola Nacional, Quinta da Almoínha 2565-191
Dois Portos, Portugal
Irrigation management requires information on evapotranspiration (ET). The ET of a well irrigated crop (ETc) is usually approached through an empirical equation using reference ET (ETo) and crop coefficients (Kc): ETc = ETo x Kc. Discrepancies have been observed between Kc measured in several row crops and Kc values recommended in currently used manuals. Therefore, a need arises to address this problem via further field measurements of ET. Some limitations to the application of micrometeorological and hydrological methods to measure ET in small fields or in woody crops are briefly described and sap flow methods (SF) are presented as an alternative to quantify transpiration (T), the major component of ET. A clear underestimation of T with SF data was observed in several conditions, when compared to data from eddy covariance (EC) micrometeorological method. By combining robust, low cost SF methods with reliable EC measurements, an EC-SF relationship can be developed to correct SF measurements. Using this combination, ET and Kc data series can be extrapolated in time (e.g., for a whole season) or space (when micrometeorological methods are not suitable, as in small areas). Some case studies show the combination of complementary EC-SF methods to follow the seasonal variation of ET and Kc (vineyards, kiwi and peach orchards) proving its usefulness in long term studies. A second aspect discussed is the fact that calculating crop ET as ETo x Kc implicitly assumes continuous adequate available soil water for optimum plant growth. However, even for irrigated crops, ET is often below ETc and called actual ET (ETa). Besides, due to growing water scarcity, deficit irrigation strategies are increasingly being explored. Under water stress, the reduction of ET due to stomatal closure is abstract higher for rough canopies and/or woody crops than for low crops and cannot be neglected. Therefore, for practical purposes, ETa can be estimated as ETo x Kc x Ks where Ks is a stress coefficient (Ks = ETa/ETc). The relationship between Ks and soil water depletion (ZET since last irrigation) can be used to estimate how much water to apply and when to irrigate. This relationship varies with soil type, ET rate, and root distribution. Hence, it must be adjusted, which can be done empirically as a function of observations of plant water status. The use of relationships between water stress indicators improves the identification of threshold values for practical purposes.
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