Francisco Orgaz

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.

Non-destructive measurement of leaf area in olive (Olea europaea L.) trees using a gap inversion method

Leaf Area Index (LAI) data are required to characterise evaporation and assimilation rates from canopies. The LAI of a canopy of trees can be estimated from the transmittance of radiation at various angles. A commercial sensor for LAI determination (Plant Canopy Analyzer LI-COR LAI-2000) was tested for olive trees (Olea europaea L.) during 1992 and 1993 in Cordoba, Spain. Plant leaf area of single olive trees was measured destructively and compared with indirect measurements with the PCA. A simulation model was constructed to test sampling strategies to determine plant area index (PAI) with the PCA in tree orchards. Plant areas and area densities of isolated olive trees in the field were accurately determined using the PCA. The PAI in arrays of young potted olive trees was also measured adequately. Measurements in an olive orchard and results of the simulation model of the PCA indicated that the PCA alone will underpredict PAI by around 30% in olive orchards or arrays of trees with similar geometry. Accurate PAI determination may be obtained in some cases by using the PCA to calculate the PA of single trees. Alternatively, PAI may be calculated as the weighted average of the maximum and minimum values of PAI in the orchard using the ground cover as a weighting factor. PAI estimates will be very close to LAI in olive orchards as the fraction of total area corresponding to leaves exceeds 90% in olive trees grown in the field.

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.

Reflections on food security under water scarcity

Forecasts on population growth and economic development indicate that there will be substantial increases in food demand for the forthcoming decades. We focus here on the water requirements of food production, on the issue of whether there would be enough water to produce sufficient food in the future, and we offer options to face this challenge based on recent trends observed in some agricultural systems. Given the competition for water faced by the agricultural sector, and the uncertainties associated with climate change, improving the efficiency of water use in both rain-fed and irrigated systems is the main avenue to face the challenge. In rain-fed agriculture, managing the risk associated with rainfall variability is a promising option to increase productivity. In irrigated systems, a case study on the improvements in water productivity in Andalusia, Spain, is used to illustrate some of the opportunities to make progress. Progress in reducing irrigation water use in recent decades has been substantial, but decreasing the consumptive use of crops is a much more difficult challenge. The need for more research and technology transfer on improving water-limited crop production is highlighted, and emphasis is placed on interdisciplinary approaches to gain the insight needed to achieve new breakthroughs that would help in tackling this complex problem.

Measurement and simulation of evaporation from soil in olive orchards

Abstract Evaporation from the soil (Es) beneath an olive orchard was characterised in a semi-arid Mediterranean climate (Córdoba, Spain). First, the microlysimeter method was modified to measure accurately Es beneath tree orchards. The variability in irradiance reaching the soil beneath the orchard caused spatial variations in Es during both evaporation stages. In the first days of the drying cycle, Es was higher for high irradiance locations but the opposite occurred the subsequent days, although daily differences in Es between locations progressively declined. For the energy-limiting stage, linear relationships between Es values and incident photosynthetically active radiation were found for different times throughout the season. The slopes of the relationships were similar, but their intercepts differed substantially, showing the importance of a variable aerodynamic component in determining Es. A simple functional model was formulated to estimate Es at daily time steps. During the energy-limiting stage, Es is calculated as the sum of the equilibrium evaporation at the soil surface and an aerodynamic term, derived from the Penman equation. For the falling rate stage, Ritchies (1972) approach is adopted for the Es calculations. The model was successfully tested in an orchard of 6×6 m spacing, typical of intensive olive orchards, under a wide range of evaporative demand conditions. Trees covered around 36% of the soil surface. The model predicted an average seasonal Es of 286 mm, which represents around one third of the estimated olive evapotranspiration and about 50% of the average seasonal rainfall of the area.

A comparison between drip and furrow irrigation in cotton at two levels of water supply

Abstract Optimal soil water status improves cotton yields by increasing flower production and boll retention and by controlling the time to maturity. Drip irrigation presumably improves the soil water regime thus leading to higher crop yields but the extent of its potential in cotton is unclear. A two-year experiment was carried out on a sandy loam soil at Cordoba, Spain, aimed at comparing drip against furrow irrigation in cotton ( Gossypium hirsutum L.). The irrigation methods were tested using a split-split-plot design which included two irrigation amounts and two or three cultivars each year. The advantage of one method over the other varied from year to year. The furrow irrigation treatments yielded significantly more than drip irrigation yields in 1985 but less in 1986. The differences in yield were explained by the crop water relations and the seasonal evapotranspiration. The 1986 results pointed to an advantage for drip irrigation under deficit irrigation conditions. Water application efficiency was 30% higher in the drip irrigation treatments indicating a definitive advantage of this method under limited water supply. Midday leaf water potential fluctuated along the season in both drip and furrow irrigation. The fluctuations were related to the irrigation events but not to the soil water deficit.

Effect of genetic characteristics and environmental factors on organosulfur compounds in garlic ( Allium sativum L.) grown in Andalusia, Spain.

The content of organosulfur compounds was determined in selected garlic cultivars grown at four locations in Andalusia, Spain. The organosulfur compounds studied were three γ-glutamyl peptides, namely, γ-l-glutamyl-S-(2-propenyl)-l-cysteine (GSAC), γ-l-glutamyl-S-(trans-1-propenyl)-l-cysteine (GSPC), and γ-l-glutamyl-S-methyl-l-cysteine (GSMC), and four cysteine sulfoxides (alliin, isoalliin, methiin, and cycloalliin). There was a significant effect of the location, cultivar, and garlic ecotype on individual organosulfur compound contents. Purple-type cultivars showed on average the highest contents of GSMC, GSAC, alliin, and methiin but the lowest isoalliin content. The impact of genotype was relatively high for GSAC, whereas this factor hardly contributed to the total variability in alliin and isoalliin content. Planting date had a significant effect on the content of alliin and isoalliin. Discriminant analysis evidenced the ability of organosulfur compounds to distinguish among garlic bulbs from different locations or ecotypes with 81 or 86% accuracy, respectively.

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.

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.