Francisco J. Villalobos

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.

A Simulation Model for Irrigation Scheduling Under Variable Rainfall

ABSTRACT Asimulation model was developed by coupling a simple daily rainfall generator to a water balance model that determines irrigation dates and amounts. The rainfall model uses average monthly data to generate daily precipitation and the water balance uses average ETQ values and estimates separately evaporation and transpiration as well as allowable depletion. The results of 50 years of simulating irrigation schedules for corn and sugarbeets at Cordoba and Badajoz, Spain and Davis, CA were analyzed to evaluate the effects of planting date, soil storage capacity and irrigation system constraints. Probability distribution functions of the date of first irrigation were developed for ideal and practical irrigation situations. A method is proposed based on the simulation to develop a predictive irrigation schedule at selected probability levels for optimal and suboptimal irrigation dates.

Effects of shading on dry matter partitioning and yield of field-grown sunflower

Abstract Crop simulation models require quantitative descriptions of the effects of irradiance on dry matter partition and yield. The objective of this work was to quantify the effects of reduced radiation intensity during different phenological stages on the growth, dry matter partitioning and grain numbers of sunflower (Helianthus annuus, L.). A field experiment was carried out in 1990 with 50 per cent shading treatments. The earliest treatment began at crop emergence while the latest ended at first anthesis. Shading had little effect on plant leaf area growth but reduced biomass and yield. The dry matter: radiation quotient and specific leaf area increased with shading. Grain number per head was decreased by shading, with the greatest effect occurring when shading was applied prior to anthesis. All shading treatments increased dry matter partitioning to stems, decreased assimilate partitioning to the heads and had no effect on the partitioning to leaves.

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.

Estimation of evapotranspiration on discontinuous crop canopies using high resolution thermal imagery

Efficient water management in agriculture requires accurate estimation of the evapotranspiration (ET) of crops. This work presents the progress made on assessing the water status and ET estimation on discontinuous crop canopies where soil and shadow components affect the remote sensing imagery used. Two orchards used to conduct regulated deficit irrigation (RDI) experiments were monitored between 2004 and 2006 and at three different levels: i) near-field point thermal sensors monitoring single crowns continuously; ii) airborne level using high-resolution thermal and multispectral imagery collected at different times of day; and iii) satellite level using TERRA-ASTER and Quickbird for estimating surface temperature and multispectral imagery, respectively.