Luca Testi

Measurement and modeling of evapotranspiration of olive (Olea europaea L.) orchards

Abstract Efficient irrigation management requires a good quantification of evapotranspiration. In the case of olive orchards, which are the dominant crop in vast areas of southern Europe, very little information exists on evaporation. Measurements of aerodynamic conductance and evaporation above and below an olive orchard allowed the calibration of a transpiration model of olive trees based on the Penman–Monteith equation. The model was combined with Ritchie’s soil evaporation model and tested against an independent data set, indicating its validity unless a substantial fraction of the soil surface is wetted by irrigation emitters, which is not taken into account by the model and deserves further research. Simulated crop coefficients of olive orchards in southern Spain changed during the year in response to changes in vapor pressure deficit (VPD) and evaporation from the soil surface. The average annual crop coefficient (0.62) was rather low due to the low ground cover and to the enhanced control of canopy conductance by stomatal responses to VPD. According to our results the crop coefficient will vary among locations and even among years, depending on rainfall and temperature.

Water requirements of olive orchards–II: determination of crop coefficients for irrigation scheduling

Intensification of olive cultivation by shifting a tree crop that was traditionally rain fed to irrigated conditions, calls for improved knowledge of tree water requirements as an input for precise irrigation scheduling. Because olive is an evergreen tree crop grown in areas of substantial rainfall, the estimation of crop evapotranspiration (ET) of orchards that vary widely in canopy cover, should be preferably partitioned into its evaporation and transpiration components. A simple, functional method to estimate olive ET using crop coefficients (Kc=ET/ET0) based on a minimum of parameters is preferred for practical purposes. We developed functional relationships for calculating the crop coefficient, Kc, for a given month of the year in any type of olive orchard, and thus its water requirements once the reference ET (ET0) is known. The method calculates the monthly Kc as the sum of four components: tree transpiration (Kp), direct evaporation of the water intercepted by the canopy (Kpd), evaporation from the soil (Ks1) and evaporation from the areas wetted by the emitters (Ks2). The expression used to calculate Kp requires knowledge of tree density and canopy volume. Other parameters needed for the calculation of the Kc’s include the ET0, the fraction of the soil surface wetted by the emitters and irrigation interval. The functional equations for Kp, Kpd, Ks1 and Ks2 were fitted to mean monthly values obtained by averaging 20-year outputs of the daily time step model of Testi et al. (this issue), that was used to simulate 124 different orchard scenarios.

Balancing crop yield and water productivity tradeoffs in herbaceous and woody crops

The links between water and crop yield are well known. In agricultural systems, maximum yield and maximum water productivity (WP; yield divided by water use) are not always compatible goals. In water-limited situations, optimal solutions must be reached by finding a compromise between the levels of crop production and WP. The tradeoffs between production and WP are reviewed here and the dominant effects of the environment on WP are examined. Genetic improvement for WP generally has yield tradeoffs, whereas management measures devised to improve WP also enhance yield. It is shown that partial closure of the stomata in response to environmental stimuli has a variable impact on canopy transpiration, depending on the degree of coupling between the canopy and the atmosphere. In contrast to the behaviour of the major herbaceous crops, WP increases in some woody crops in response to water stress, suggesting that biomass and transpiration are not linearly related, and that deficit irrigation should be successful in these species. Avoiding high evaporative demand periods (e.g. through tolerance to low temperatures) is an important option that aims to increase production and WP. A case study is presented for improving sunflower (Helianthus annuus L.) yield and WP in temperate environments.

Transpiration of young almond trees in relation to intercepted radiation

Increased water scarcity demands more efficient use of water in the agricultural sector which is the primary consumer of water. Precise determination of irrigation requirements based on specific crop parameters is needed for accurate water applications. We conducted a 4-year study on almond evapotranspiration using a large weighing lysimeter. Tree canopies changed from 3 to 48 % ground cover during the course of the study. Sap flow measurements made on the lysimeter tree provided a continuous record of tree transpiration. We propose to use the daily fraction of photosynthetically active radiation intercepted by the canopy (fIRd) as a predictor of almond orchard maximum transpiration. The transpiration coefficient (T/ETo or KT) was related to the fIRd of the last two years, and the ratio between fIRd and KT stayed more or less constant around a value of 1.2. Such value extrapolated to the size of a mature orchard with 85 % intercepted radiation gives a KT of around 1.0, a number above the standard recommendations, but fully compatible with the maximum Kc values of 1.1–1.15 recently reported.

A soil-plant-atmosphere continuum (SPAC) model for simulating tree transpiration with a soil multi-compartment solution

AimsA soil-plant-atmosphere continuum (SPAC) model for simulating tree transpiration (Ep) with variable water stress and water distribution in the soil is presented. The model couples a sun/shade approach for the canopy with a discrete representation of the soil in different layers and compartments.MethodsTo test its performance, the outputs from the simulations are compared to those from an experiment using trees of olive ‘Picual’ and almond ‘Marinada’ with the root system split into two. Trees are subjected to different irrigation phases in which one side of the root system is dried out while the other is kept wet.ResultsThe model is able to accurately predict Ep (R2 and the efficiency factor (EF) around 0.9) in the two species studied. The use of a function that modulates the uptake capacity of a root according to the soil water content was necessary to track the fluxes observed from each split part. It was also appropriate to account for root clumping to match the measured and modelled leaf water potential.ConclusionsCoupling the sun/shade approach with the soil multi-compartment solution provides a useful tool to explore tree Ep for different degrees of water availability and distribution.

Low winter temperatures induce a disturbance of water relations in field olive trees

Key messageLow winter temperatures induce an increase in the soil-to-trunk hydraulic resistance of field-grown olive trees resulting in a significant disturbance of their water relations.AbstractA disturbance of water relations in response to chilling have long been observed in potted plants growing under controlled conditions, but information is lacking for field plants. The aim of this study was to assess the effects of winter low temperatures on the water relations of mature olive trees. To this end, water potential, sap flux density, soil temperature and meteorological data were monitored in a hedgerow olive orchard near Córdoba, southern Spain, throughout two consecutive winters. Water stress symptoms were found in terms of midday Ψ, despite adequate water supply and low evaporative demand. These effects were associated with changes in the soil-to-trunk hydraulic resistance (Rroot), which increased by December–January to much higher values than those previously reported in the literature, particularly in the year of higher fruit load. The contribution of viscosity (η) to the observed Rroot dynamics was almost negligible as deduced from measurements of soil temperature, so the high winter values of Rroot were likely to have originated from other causes such as reductions in membrane permeability and root growth. The findings of this work raise new major issues that deserve further research such as the impact of the winter water stress on stomatal conductance and photosynthesis rates in mature olive trees.

Assessment of a Remote Sensing Energy Balance Methodology (SEBAL) Using Different Interpolation Methods to Determine Evapotranspiration in a Citrus Orchard

A surface energy balance algorithm for land (SEBAL) for estimating evapotranspiration (ET) has been parameterized and tested in a 400-ha drip irrigated citrus orchard. Simultaneously, during three growing seasons, energy fluxes were measured using Eddy Covariance. Instantaneous fluxes obtained with SEBAL using 10 images from Landsat-5 were compared with the measured fluxes. The Perrier function was the best method for properly estimating the roughness momentum length for discontinuous canopies, as in citrus orchards. Crop height was estimated using LIDAR data. In general, SEBAL performed well for net radiation estimation but failed in soil heat flux estimation. Latent heat estimations from the SEBAL model had a relative root mean square error (rRMSE) of 0.06 when compared with measurements obtained by Eddy Covariance. Three procedures were tested for up-scaling the instantaneous ET estimates from SEBAL to daily ET values: 1) assuming the fraction between the actual ET and the reference ET is constant throughout the day; 2) using actual local crop coefficient curves; and 3) using an up-scaling factor where the fraction of hourly ET to daily ET equals the ratio of hourly to daily global solar radiation. This last method gave acceptable results for daily ET estimations (rRMSE = 0.09) and for 15day ET (rRMSE = 0.19), and its main advantage is that no local data are required. It is concluded that the SEBAL methodology can be successfully applied for determining actual ET, even in discontinuous citrus canopies. However, additional parameterizations of momentum roughness length were needed in order to obtain reliable ET determinations.

The Energy Balance

The main components of the energy balance are net radiation, latent heat flux (LE), sensible heat flux (H) and soil heat flux (G). These can be manipulated through changes in net radiation, LE, H or G. The relative importance of the components depends mainly on the availability of water for evaporation. The extreme cases will be the humid environment (LE approaches Rn) and the desert environment (Rn is partitioned between H and G). The energy balance of farming (energy produced per unit energy consumed in all farming operations) may be also analyzed in terms of inputs and outputs which can be estimated by assessing the energy embodied in the amount of materials employed (fertilizers, water, seeds) and in the operations performed.

A single‐probe heat pulse method for estimating sap velocity in trees

Available sap flow methods are still far from being simple, cheap and reliable enough to be used beyond very specific research purposes. This study presents and tests a new single-probe heat pulse (SPHP) method for monitoring sap velocity in trees using a single-probe sensor, rather than the multi-probe arrangements used up to now. Based on the fundamental conduction-convection principles of heat transport in sapwood, convective velocity (Vh ) is estimated from the temperature increase in the heater after the application of a heat pulse (ΔT). The method was validated against measurements performed with the compensation heat pulse (CHP) technique in field trees of six different species. To do so, a dedicated three-probe sensor capable of simultaneously applying both methods was produced and used. Experimental measurements in the six species showed an excellent agreement between SPHP and CHP outputs for moderate to high flow rates, confirming the applicability of the method. In relation to other sap flow methods, SPHP presents several significant advantages: it requires low power inputs, it uses technically simpler and potentially cheaper instrumentation, the physical damage to the tree is minimal and artefacts caused by incorrect probe spacing and alignment are removed.

Stomatal oscillations in olive trees: analysis and methodological implications

Stomatal oscillations have long been disregarded in the literature despite the fact that the phenomenon has been described for a variety of plant species. This study aims to characterize the occurrence of oscillations in olive trees (Olea europaea L.) under different growing conditions and its methodological implications. Three experiments with young potted olives and one with large field-grown trees were performed. Sap flow measurements were always used to monitor the occurrence of oscillations, with additional determinations of trunk diameter variations and leaf-level stomatal conductance, photosynthesis and water potential also conducted in some cases. Strong oscillations with periods of 30-60 min were generally observed for young trees, while large field trees rarely showed significant oscillations. Severe water stress led to the disappearance of oscillations, but moderate water deficits occasionally promoted them. Simultaneous oscillations were also found for leaf stomatal conductance, leaf photosynthesis and trunk diameter, with the former presenting the highest amplitudes. The strong oscillations found in young potted olive trees preclude the use of infrequent measurements of stomatal conductance and related variables to characterize differences between trees of different cultivars or subjected to different experimental treatments. Under these circumstances, our results suggest that reliable estimates could be obtained using measurement intervals below 15 min.