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The main objectives of the present study were to assess the water demand for heavy fruit load of ‘Hass’ avocado throughout the growth periods and to investigate the effects of deficit irrigation during sensitive phenological phases on yield. The experimental set-up allowed the comparison between trees responses to three irrigation strategies during the entire growth period (no water stress; excessive irrigation; constant water stress) as well as the comparison between regulated deficit irrigation (RDI) managements applied during the early or the late growth period. The yield of no water stress treatments during three experimental years was very high (25–31 t ha−1) while the yields of water-stressed trees were significantly lower (16–21 t ha−1). More importantly, the yield of no water stress trees was not susceptible to alternate bearing while the yield of water-stressed trees was considerably reduced during off-crop years. Irrigation rates and the actual evapotranspiration coefficient KL = ET/ET0 for the no water stress treatment may serve as a reasonable guide for irrigation management. Fruit load should be taken into account while planning irrigation and fertilization management and plant-based methods should be used for controlling the irrigation management (scheduling and quantities). Analyses of trunk diameter variation data that lead to evaluation of trunk growth rate and maximum daily shrinkage reflect phenological stages and periodicity of shoot, fruit and root growth, and also may provide an integrative, “holistic viewpoint” of overall tree status.

The main objective of this study was to assess the seasonal nutrients requirement of ‘Hass’ avocado trees grown in lysimeters, especially during flowering and the early period of fruit development that may affect later on the fruitlet abscission and determine crop yield. The experimental design included three fertigation treatments applying a fixed nutrient solution at three different starting dates of fertigation: (a) T1 – continuous fertigation, including macro nutrients (N-P -K) and micronutrients application, all over the year; (b) T2 – no fertilization (only irrigation) until 15 March, and fertigation as T1 since then; (c) T3 - no fertilization (only irrigation) until 15 May, and fertigation as T1 since then. Absence of fertilization during the winter period induced leaf-chlorosis while healthy, dense and plenteously green leaves characterized the fertilized trees (T1). The beneficial effect of early fertigation on fruit yield was statistically significant, mostly because of higher fruit number. Leaf analyses are commonly used in the avocado industry as a guide for fertilization yet; fruits rather than leaves are the main products of avocado orchards. Consequently, fruit rather than leaf analyses should determine fertilization management. Based on fruit growth data and nutrient concentration in the fruit, the N, P and K quantities removed by ‘Hass’ avocado fruit yield of 30 t ha−1 were 120, 25 and 240 kg ha−1. Taking into account common efficiency consideration (nutrient quantities removed by fruit yield divided by quantities added), the annual quantities of N, P and K required for attaining high quality avocado yield are 250–300, 80–120 and 500–600 kg ha−1, respectively. Thus, fertilization rate together with nutrient combination should be modified in order to insure optimal fruit development.

Treated wastewater (TWW) is a major source of water for agriculture in Israel; however, recent reports indicate a marked yield loss in TWW-irrigated avocado and citrus orchards planted in clayey soils. The association of the yield loss with clayey soils rather than sandy soils suggests that it is associated with conditions in the root zone, and specifically poor aeration. A three-year study (2012–2015) was conducted in an avocado orchard planted in clayey soil, comparing the oxygen and redox conditions in the root zone of TWW-irrigated plots with fresh water (FW)-irrigated plots, together with the physiological status of the trees. Soil parameters included: continuous in-situ measurement of soil-water tension (SWT), soil oxygen, and soil redox potential, and periodic measurements of soil solution composition. Physiological parameters included: mineral composition of plant tissue from the leaves, trunk xylem and roots, root growth, yield, fruit setting, plant volume, and yield. TWW-irrigated plots were found to endure longer periods of low SWT indicating higher water content, accompanied by lower oxygen levels and more reduced conditions in comparison to FW-irrigated plots. The differences in these soil parameters between treatments were greater during the irrigation season than during the rainy period. The more reduced conditions in the TWW plots did not lead to significant differences in Fe or Mn concentrations in the soil solution or in plant leaves. TWW soil solution had significantly higher Na levels compared with FW. This did not affect the leaf Na content, but was expressed in substantially higher Na content in the root and trunk xylem, with up to seven times more trunk xylem Na in TWW-irrigated plants compared with FW-irrigated plants. Root growth was significantly hindered in TWW-irrigated plots compared with FW-irrigated plots. A negative correlation was found between root growth and the duration of hypoxic conditions, and similarly between root growth and the Na levels in the roots. TWW-irrigated plants had greater fruitlet numbers at the initial fruit-setting stage, but had a smaller number of fruit and a lower yield at harvest. The yield (kg/tree) negatively correlated with the duration of hypoxic conditions in the root zone but not with the Na levels in the roots or xylem. Our findings point towards a substantial role of oxygen deprivation as a major factor leading to the damage to TWW-irrigated orchards in clayey soils. Based on the assimilation of data, we suggest that a downward cascade is instigated in the TWW-irrigated orchards by increased input of Na into the soil, leading to degradation of soil hydraulic properties and reduced aeration. Impaired physiological functioning of the roots due to limited oxygen supply results in less roots growth, lower water uptake and impaired selectivity against Na uptake, thus imposing a negative feedback to increase soil water content, reduce aeration and root-zone oxygen availability for the roots, and further impair plant resistance to the high Na levels.

The water content dynamics in the upper soil surface during evaporation is a key element in land–atmosphere exchanges. Previous experimental studies have suggested that the soil water content increases at the depth of 5 to 15 cm below the soil surface during evaporation, while the layer in the immediate vicinity of the soil surface is drying. In this study, the dynamics of water content profiles exposed to solar radiative forcing was monitored at a high temporal resolution using dielectric methods both in the presence and absence of evaporation. A 4-d comparison of reported moisture content in coarse sand in covered and uncovered buckets using a commercial dielectric-based probe (70 MHz ECH2O-5TE, Decagon Devices, Pullman, WA) and the standard 1-GHz time domain reflectometry method. Both sensors reported a positive correlation between temperature and water content in the 5- to 10-cm depth, most pronounced in the morning during heating and in the afternoon during cooling. Such positive correlation might have a physical origin induced by evaporation at the surface and redistribution due to liquid water fluxes resulting from the temperature-gradient dynamics within the sand profile at those depths. Our experimental data suggest that the combined effect of surface evaporation and temperature-gradient dynamics should be considered to analyze experimental soil water profiles. Additional effects related to the frequency of operation and to protocols for temperature compensation of the dielectric sensors may also affect the probes response during large temperature changes.

High-frequency drip irrigation supplies water and nutrients at a rate that is close to plant uptake, thus enhancing growth and production. In light of water scarcity in arid regions, marginal water is increasingly considered as a resource for agricultural production. The objective of this study was to investigate the combined effects of pulsed irrigation and water salinity on the response of the soil–plant system. As a test crop, bell pepper (Capsicum annuum L.) was cultivated in a screenhouse and drip irrigated daily (D) and at high frequency (P) with saline (S) and fresh (F) water. Simultaneous monitoring of meteorological, physiological, soil physical, plant and soil chemical, and yield data was performed during the experiment. Most physiological parameters were negatively affected by high water salinity. No consistent effect of the irrigation frequency was found on the overall season, although pulsed irrigation led to higher plant weight and leaf area at the early stages of plant growth. The distinct patterns of soil water content for the two irrigation frequencies are presented. Salinity in the root zone was higher under pulsed irrigation, an observation that is supported by measured leaf chloride content and tensiometer readings indicating that the once daily application may have more efficiently removed salts from the top soil. Yield, fruit weight, and irrigation water use efficiency (IWUE) were highest under once daily irrigation with fresh water. High-frequency irrigation led to higher Mn concentrations in leaves and fruits and increased concentrations of Cl, N, and P in leaves, confirming earlier conclusions on improved P mobilization and uptake under pulsed irrigation.

International audience; The hydraulic properties of the layer at the vicinity of the soil surface have significant impact on evaporation, and could be harnessed to reduce water losses. The effect of the properties of the upper layer on the evolution of phase distribution during the evaporation process is first illustrated from three-dimensional pore network simulations. This effect is then studied from experiments carried out on soil columns under laboratory conditions. Comparisons between homogeneous columns packed with coarse (sand) and fine (sandy loam) materials, and heterogeneous columns packed with layers of fine overlying coarse material and coarse overlying fine material of different thicknesses are performed to assess the impact of upper layer properties on evaporation. Experiments are analyzed using the classical approach based on the numerical solution of Richards's equation and semi-analytical theoretical predictions. The theoretical analysis is based on the clear distinction between two drying regimes, namely the capillary regime and the gravity-capillary regime, which are the prevailing regimes in our experiments. Simple relationships enabling to estimate the duration of stage-1 evaporation (S1) for both regimes are proposed. In particular, this led to defining the characteristic length for the gravity-capillary regime from the consideration of viscous effects at low water content differently from available expressions. The duration of S1, during which most of the water losses occur, for both the homogeneous and two-layer columns are presented and discussed. Finally, the impact of liquid films and it consequences on the soil hydraulic conductivity function are briefly discussed.

Heterogeneity in soil hydraulic properties has a significant impact on evaporation, and could be harnessed to reduce water losses and improve soil water conservation. This is illustrated through the consideration of the effect of Darcy scale heterogeneities resulting from horizontal layering. The impact of permeability gradient and thickness of layers has been investigated from evaporation experiments performed from homogeneous as well as horizontally multi-layered soil columns. Two main cases are distinguished depending on the sign of the permeability gradient, the unstable case when the permeability increases with depth and the stable case when, on the contrary, the permeability decreases with depth. The results indicate an interesting competition between stabilizing gravity effects and destabilizing or stabilizing permeability gradient effects and lead to the emergence of the concept of two-scale evaporation process.

The increasing reuse of treated wastewater (WW) for irrigation brings with it a need to reconsider irrigation water quality criteria because of the expected lower quality of WW. In particular, the impacts of higher sodium and dissolved organic carbon (DOC) concentrations on soil permeability must be evaluated in practical field settings over long periods of WW reuse. Here we report the long-term impact of WW reuse for irrigation on soils at three different semiarid-zone field sites under row crop or orchard agriculture. The soils contain about 60% clay, dominated by smectite, and present an order of magnitude variation in calcite content (1–11%). In two of the sites, parcels irrigated with freshwater (FW) are available for comparison. Our results show an increasing sodicity hazard and a decreasing saturated soil hydraulic conductivity ( K s ) from WW irrigation, although the depth profiles of soil chemical and physical properties were highly site-specific. Despite this spatial variability, all of the data on sodicity hazard, represented by the relationship between exchangeable sodium percentage (ESP) and the soil sodium adsorption ratio (SAR), could be incorporated into a single Gapon constant for calcium–sodium exchange whose values depended uniformly on the ratio of soil DOC concentration to calcite content. Moreover, all of the data on K s , for both FW and WW irrigation, could be incorporated into a single power-law relationship involving the ratio of ESP to soil DOC. These two relationships unify complex interactions between sodicity and organic matter (OM) that influence soil permeability to yield simple correlations with predictive power. The main detrimental effect of WW application was related to sodicity hazard. Therefore, effort should be invested in reducing the SAR of WW for irrigation. This could be achieved, for example, by mixing WW with FW, including desalinized water, when and if available.

Global water resources quantities and qualities are declining, but at the same time, a strong demand for higher agricultural productivity continues to emerge due to population growth. This calls for a significant increase of irrigation and fertilization efficiencies and requires improving our understanding of the interactions between plants and their physical environment. The main objective of this study is to analyze the combined effect of varying drip irrigation management techniques and growing conditions (media properties and container volumes) on soil–plant interactions. In a series of experiments, irrigation flow rates and intervals ranging from 2 d to 10 min were applied to the vegetative stage of a test crop (bell pepper [ Capsicum annuum L. ‘Selika’]) cultivated under different growing conditions—sand and perlite in buckets, perlite in containers, and loamy sand under field conditions. Data on soil water regime, plant water uptake, and plant development were monitored in each setup. Large differences were observed both in terms of root and canopy development in response to the different application rates and frequencies. The prevailing irrigation management reflects on the soil water content dynamics, and consequently, on the plant water uptake and growth. Sap flow rates measurements indicated that higher irrigation frequency or lower water application rates increased plant water uptake rates. However, in most of the cases (except for the sand) it also led to a lower root mass and a smaller root mass/leaf area ratio. Interestingly, in the single plant per bucket experiments, a larger leaf area seemed conditioned on a larger root mass, while the opposite was the case in those two experiments where plants were grown in rows (perlite in containers and loamy sand field), where most prolific canopy development was supported by the smallest root mass. Integrating findings across the different experiments, we introduce the concept of mean daily available water volume per plant as the product of container/bucket volume and mean daily water content in the medium to express the joint effect of constraints imposed by the physical volume of growing medium and their specific hydraulic properties. Mean daily available water volume per plant was found to be positively correlated with the dry root mass to leaf area ratio.

[1] The increasing demand for freshwater (FW) for domestic use turns treated wastewater (WW) into an attractive source of water for irrigated agriculture. The main goal of this study was to evaluate the impact of 15 yrs of irrigation with WW on hydraulic properties and flow processes in a clayey soil, compared to FW use. It also quantitatively addressed the distribution with depth along the soil profile of that impact on soil hydraulic properties. Standard methods used in soil physics at the laboratory scale, and numerical solutions of the flow equations on the basis of HYDRUS, were applied to define fundamental soil hydraulic properties of disturbed soil samples from 0–20, 20–40, and 40–60 cm layers in the root zone. Results showed that saturated hydraulic conductivity, sorptivity, and infiltration rates are consistently lower in the WW irrigated soil samples at all depths. Water retention and hydraulic conductivity functions were affected by the use of WW, leading to a smaller, simulated-wetted volume below a dripper for the WW-irrigated soil case. These results illustrate the combined and complex effect of WW use on soil-exchangeable sodium percentage, and suggest changes in contact angle and pore size distribution. They also suggest that WW application will affect differently different zones in the soil profile, depending on irrigation management parameters and plant uptake characteristics.

[1] Reservoirs enhance availability and temporal stability of water resources, however, water losses due to evaporation may significantly reduce their operational efficiency. Studies have evaluated the efficacy of various types of chemical or physical evaporation barriers often on a qualitative basis. Certain design and maintenance advantages are offered by self-assembling modular floating covers especially for large reservoir surfaces. Evidence suggests that evaporation suppression efficiency of these floating covers is not proportional to the covered surface fraction. Capitalizing on recent experimental results carried out on small vessels, we show that evaporation suppression efficiency of floating covers in much larger reservoirs (3 to 5 order of magnitude in volume) is proportional to the square root of open area. Thermal effects of evaporation from open and partially covered surfaces on surface water temperature are proportional to evaporation rate and thus provide a potential mean for real-time monitoring of cover performance. Floating covers seem to suppress evaporation more effectively than other suspended evaporation barriers. For a fixed covered surface fraction, larger “holes” suppress evaporation more efficiently than many smaller and distributed openings. The concepts and relationships developed in this study provide quantitative means for engineering design and economical evaluation of evaporative barriers for water reservoirs in terms of cover geometry and water saving efficiency.

The increasing demand for freshwater (FW) for domestic use turns treated wastewater (WW) into an attractive source of water for irrigated agriculture. The main goal of this study was to evaluate the impact of 16 yr of irrigation with WW on the conditions that developed in the root zone of avocado trees planted on clayey soil and compare with FW use. High-resolution field sampling determined the spatial distribution of chloride, exchangeable sodium percentage, and dissolved organic content below the dripper, revealing higher salinity and sodicity, lower hydraulic conductivity, and possible preferential flow pattern linked to wettability in WW-irrigated soils. Laboratory measurements on disturbed samples showed that higher swelling pressure developed in the 20- to 40-cm and 40- to 60-cm layers of the WW-irrigated soil. Finally, continuous monitoring of oxygen concentration at the 10-, 20-, and 30-cm depths in the root zone near the trees and halfway between adjacent trees revealed that the oxygen level at the 20-cm depth was the most affected by WW irrigation. During the rainfall season, this layer could experience relatively long periods with minimal oxygen concentrations. During the irrigation season, less oxygen is available in that layer than in the FW-irrigated one. Dynamics of oxygen concentration at the 30-cm depth show a clear event of wetting and drainage in the FW-irrigated plots, while the relatively stable high oxygen level in that depth in the WW-irrigated plots might reveal nonuniform wetting, insufficient water percolation due to low hydraulic conductivity, and related low leaching efficiency of the soil profile.