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- O que fazem as oliveiras de noite ou redistribuição hidráulica num olival de sequeiro no AlentejoPublication . Ferreira, Maria Isabel; Conceição, N.; Pacheco, C.A.; Green, S.Hydraulic redistribution is a mechanism that ensures the survival of superficial roots in dry soil during late summer, as they are submitted during part of the day to a negative flux, coming from roots with access to deep water. This mechanism also enables the plant to continue exploring these upper soil layers, richer in minerals and organic matter and has been observed in many ecosystems, including Montado, in Portugal. A critical question arises: to which extent is the survival of traditional rainfed ecosystems dependent on the water table condition? In order to identify the importance and dynamic of this process in olive groves, a study was conducted in South Portugal using two different, independent methods to follow sap flow in roots. We report the first results obtained with a modified heat pulse method, in a rainfed olive grove. By following roots sap flow from wet spring, 6 months without significant rain and early autumn, we found experimental evidence of hydraulic redistribution from some roots to others
- Transpiration reduction as an answer to water stress: models versus measurements for irrigated olive trees in South PortugalPublication . Lourenço, S.; Conceição, N.; Losada, H.P.; Silvestre, J.; Siqueira, J.; Santos, F.; Rolim, J.; Ferreira, Maria IsabelThe so-called stress functions, allowing quantification of transpiration or evapotranspiration reduction, as an answer to water deficit stress (stress coefficient, Ks), are useful tools for irrigation scheduling. It is known that this coefficient depends on several variables and parameters, such as evapotranspiration rate, root patterns/density and soil properties. However, the most common Ks models with practical applicability do not include some of these factors and the experimental work on this matter is not abundant. A well-known model proposed in the FAO 56 manual uses two parameters: the allowable depletion (p) and the total available soil water (TAW) in the root zone, the input variable being the soil water depletion (SWD). We discuss the application of this model in olive trees under specific natural conditions. Two studies were conducted in Alentejo, South Portugal (2011 and 2017), in one intensive and one super intensive drip irrigated olive orchards (Olea europaea 'Arbequina'). Transpiration reduction was obtained from sap flow measurements. For both experiments, Ks was related to the estimated SWD. The experimental relationship was compared with modelling outputs. Experimental results show that, in both experiments, the Ks FAO 56 model only fits using parameters such as p= 0.05 and TAW = 350 mm. The suggested p in that manual is 0.7. Furthermore, for such soils, anisotropic canopy and small wetted area (drippers), 350 mm is much more than first approach estimates would suggest. The results can be related with the root system functioning and resilience of olive trees
- Análise de indicadores de desconforto hídrico durante ciclos de stress num olival intensivo no AlentejoPublication . Ferreira, Maria Isabel; Conceição, N.; Pacheco, C.A.; Hausler, M.The identification of a reliable and practical water stress indicator and the corresponding threshold value, for irrigation scheduling, is a classical concern in water management in agriculture. This is not well solved for woody crops, especially those not traditionally irrigated, usually with large roots systems. Aiming to contribute to these studies, we measured water fluxes and some water stress indicators in an olive grove in south Portugal, during successive stress cycles. We report the first preliminary results during a stress cycle in August-September 2011, using sap flow, leaf water potential at predawn, stem diameter variations and soil water content, from which some indicators were derived. From the preliminary information obtained we could not identify a threshold value but very good relationships were found, between the selected automated and not automated variables, useful for farmers and engineering applications. These first encouraging results need confirmation and further analysis
- Peach orchard evapotranspiration in a sandy soil: comparison between eddy covariance measurements and estimates by the FAO 56 approachPublication . Paço, Teresa; Ferreira, Maria Isabel; Conceição, N.The evapotranspiration from a 3 to 4 years old drip irrigated peach orchard, located in central Portugal, was measured using the eddy covariance technique during two irrigation seasons, allowing the determination of crop coefficients. These crop coefficient values differed from those tabled in FAO Irrigation and Drainage Paper 56. In order to improve evapotranspiration estimates obtained from FAO tabled crop coefficients, a dual crop coefficient methodology was adopted, following the same guidelines. This approach includes a separation between the plant and soil components of the crop coefficient as well as an adjustment for the sparse nature of the vegetation. Soil evaporation was measured with microlysimeters and compared with soil evaporation estimates obtained by the FAO 56 approach. The FAO 56 method, using the dual crop coefficient methodology, was also found to overestimate crop evapotranspiration. During 2 consecutive years, measured and estimated crop coefficients were around 0.5 and 0.7, respectively. The estimated and measured soil evaporation components of the crop coefficient were similar. Therefore, the overestimation in evapotranspiration seems to result from an incorrect estimate of the plant transpiration component of the crop coefficient. A modified parameter to estimate plant transpiration for young, yet attaining full production, drip irrigated orchards is proposed based on field measurements. The method decreases the value of basal crop coefficient for fully developed vegetation. As a result, estimates of evapotranspiration were greatly improved. Therefore, the new approach seems adequate to estimate basal crop coefficients for orchards attaining maturity established on sandy soils and possibly for other sparse crops under drip irrigation conditions