Relationship between stable carbon isotope discrimination and water use efficiency under regulated deficit irrigation of pear-jujube tree

To investigate the relationship between stable carbon isotope discrimination (Δ) of different organs and water use efficiency (WUE) under different water deficit levels, severe, moderate and low water deficit levels were treated at bud burst to leafing, flowering to fruit set, fruit growth and fruit maturation stages of field grown pear-jujube tree, and leaf stable carbon isotope discrimination (Δ_L) at different growth stages and fruit stable carbon isotope discrimination (Δ_F) at fruit maturation stage were measured. The results indicated that water deficit had significant effect on Δ_L at different growth stages and Δ_F at fruit maturation stage. As compared with full irrigation, the average Δ_L at different growth stages and Δ_F at fruit maturation stage were decreased by 1.23% and 2.67% for different water deficit levels, respectively. Δ_L and Δ_F among different water deficit treatments had significant difference at the same growth stage (P < 0.05). Under different water deficit conditions, significant relationships between the Δ_L and WUE_i (photosynthesis rate/transpiration rate, P_n/T_r), WUE_n (photosynthesis rate/stomatal conductance of CO_2, P_n/g_s), WUE_y (yield/crop water consumption, Y/ET_c) and yield, or between the Δ_F and WUE_y and yield were found, respectively. There were significantly negative correlations of Δ_L with WUE_i, WUE_n, WUE_y and yield (P < 0.01) at the fruit maturation stage, or Δ_L with WUE_i and WUE_n (P < 0.01) over whole growth stage, respectively. Δ_F was negatively correlated with WUE_y, WUE_n and yield (P < 0.05), but positively correlated with ET_c (P < 0.01) over the whole growth stage. Thus Δ_L or Δ_F can compare WUE_n and WUE_y, so the stable carbon isotope discrimination method can be applied to evaluate the water use efficiency of pear-jujube tree under the regulated deficit irrigation.     

Response of vegetative growth and fruit development to regulated deficit irrigation at different growth stages of pear-jujube tree

In order to investigate the response of vegetative growth, fruit development and water use efficiency to regulated deficit irrigation at different growth stages of pear-jujube tree (Zizyphus jujube Mill.), different water deficit at single-stage were treated on field grown 7-year old pear-jujube trees in 2005 and 2006. Treatments included severe (SD), moderate (MD) and low (LD) water deficit treatments at bud-burst to leafing (I), flowering to fruit set (II), fruit growth (III) and fruit maturation (IV) stages. Compared to the full irrigation (control), different water deficit treatments at different growth stages reduced photosynthesis rate (P_n) slightly and transpiration rate (T_r) significantly, thus it improved leaf water use efficiency (WUE_L, defined as the ratio of P_n to T_r) by 2.7-26.1%. After the re-watering, P_n had significant compensatory effect, but T_r was not enhanced significantly, thus WUE_L was improved by 31.4-42.2%. I-SD, I-MD, II-SD and II-MD decreased new shoot length, new shoot diameter and panicle length by 8-28%, 13-23% and 10-31%, respectively. Simultaneously, they reduced leaf area index (LAI) and pruning amount significantly. Flowering of pear-jujube tree advanced by 3-8 days in the water deficit treatments at stage I, Furthermore, SD and MD at stage I increased flowers per panicle and final fruit set by 18.9-40.5% and 15.5-36.6%, respectively. After a period of re-watering, different water deficit treatments at different growth stages improved the fruit growth rate by 15-30% without reduction of the final fruit volume. Compared to the control, I-MD, I-SD, I-LD, I-MD and I-SD treatments increased fruit yield by 13.2-31.9%, but reduced water consumption by 9.7-17.5%, therefore, they enhanced water use efficiency at yield level (WUE_Y, defined as ratio of fruit yield to total water use) by 17.3-41.4%. Therefore, suitable period and degree of water deficit can reduce irrigation water and restrain growth redundancy significantly, and it optimize the relationship between vegetative growth and reproductive growth of pear-jujube trees, which maintained or slightly increased the fruit yield, thus water use efficiency was significantly increased.     

Effects of salinity on fruit yield and quality of tomato grown in soil-less culture in greenhouses in Mediterranean climatic conditions

There is increasing pressure to reduce water use and environmental impact associated with open system, soil-less production in simple, plastic greenhouses on the Mediterranean coast. This may force the adoption of re-circulation of nutrient solutions. In south-eastern Spain, irrigation water is mostly from aquifers and has moderate levels of salinity. The adoption of re-circulation using moderately saline water requires detailed information of crop response to salinity, in order to optimise management. The effect of salinity on fruit yield, yield components and fruit quality of tomato grown in soil-less culture in plastic greenhouses in Mediterranean climate conditions was evaluated. Two spring growing periods (experiments 1 and 2) and one long season, autumn to spring growing period (experiment 3) studies were conducted. Two cultivars, ‘Daniela’ (experiment 1) and ‘Boludo’ (experiments 2 and 3), were used. Seven levels of electrical conductivity (EC) in the nutrient solution were compared in experiment 1 (2.5–8.0 dS m⁻¹) and five levels in experiments 2 and 3 (2.5–8.5 dS m⁻¹). Total and marketable yield decreased linearly with increasing salinity above a threshold EC value (EC_t). There were only small effects of climate and cultivar on the EC_t value for yield. Average threshold EC values for total and marketable fruit yield were, respectively, 3.2 and 3.3 dS m⁻¹. The linear reductions of total and marketable yield with EC above EC_t showed significant differences between experiments, the slope varying from 7.2% (autumn to spring period, ‘Boludo’) to 9.9% (spring period, ‘Boludo’) decreases per dS m⁻¹ increase in EC for total yield, and from 8.1% (spring period, ‘Daniela’) to 11.8% (spring period, ‘Boludo’) for marketable yield. The decrease of fresh fruit yield with salinity was mostly due to a linear decrease of the fruit weight of 6.1% per dS m⁻¹ from an EC_t of 3.0 dS m⁻¹ for marketable fruits. Reduction in fruit number with salinity made a smaller relative contribution to reduced yield. Blossom-end rot (BER) increased with increasing salinity. There was a higher incidence of BER with spring grown crops, and ‘Boludo’ was more sensitive than ‘Daniela’. Increasing salinity improved various aspects of fruit quality, such as: (i) proportion of ‘Extra’ fruits (high visual quality), (ii) soluble solids content, and (iii) titratable acidity content. However, salinity decreased fruit size, which is a major determinant of price. An economic analysis indicated that the EC threshold value above which the value of fruit production decreased linearly with increasing salinity was 3.3 dS m⁻¹, which was the same as that for marketable yield. In the economic analysis, the value of increased visual fruit quality was offset by reduced yield and smaller fruit size.     

Economic analysis of navel orange cv. ‘Lane late’ grown on two different drought-tolerant rootstocks under deficit irrigation in South-eastern Spain

During a three-year period, we evaluated the profitability of a deficit-irrigation (DI) treatment in mature ‘Lane late’ navel orange (Citrus sinensis (L.) Osb.) trees grafted on two different drought-tolerant rootstocks, ‘Cleopatra’ mandarin (Citrus reshni Hort. ex Tanaka) and ‘Carrizo’ citrange (C. sinensis (L.) Osb. × Poncirus trifoliata L. Raf.). The irrigation strategies for each rootstock were a control treatment, irrigated at 100% crop evapotranspiration (ETc) during the entire season, and a DI, irrigated at 100% ETc except during phase I (fruit set) and phase III (fruit maturation) of fruit growth, when complete irrigation cut-off was applied. The main difference found was between rootstocks, orchards of ‘Carrizo’ being 39% more profitable than those of ‘Cleopatra’ due to the greater yield and fruit size and higher price (0.02 € kg⁻¹) for trees on ‘Carrizo’. The application of the DI treatment increased the profit for ‘Carrizo’ since the decrease in pruning costs was greater than the reduction of incomes. The profit of ‘Cleopatra’ under DI decreased due to yield reduction. The variable and fixed operating costs during the growth cycle were decreased by the DI treatment, with a reduction of fertiliser (40%), water applied (30%) and electricity consumed (30%) compared with the control. In addition, in ‘Carrizo’, DI decreased the pruning (16%), machinery (11%) and phytosanitary products (9%) costs as a result of the reduction of the canopy growth. From these results, we conclude that, with similar crop management, orchards of ‘Lane late’ navel orange on ‘Carrizo’ rootstock were more profitable than those on ‘Cleopatra’ under deficit-irrigation conditions.     

Yield and quality of two tomato (Solanum lycopersicum L.) cultivars as influenced by drip and furrow irrigation using waters having high residual sodium carbonate

Performance of tomato when irrigated with sodic waters particularly under drip irrigation is not well known. A field experiment was conducted for 3 years to study the response of tomato crop to sodic water irrigation on a sandy loam soil. Irrigation waters having 0, 5 and 10 mmol_c L⁻¹ residual sodium carbonate (RSC) were applied through drip and furrow irrigation to two tomato cultivars, Edkawi (a salt tolerant cultivar) and Punjab Chhuhara (PC). High RSC of irrigation water significantly increased soil pH, ECe and exchangeable sodium percentage progressively; the increases were higher in furrow compared to drip irrigation. Effect of high RSC on increasing bulk density and decreasing infiltration rate of soil was also pronounced in furrow-irrigated plots. Higher soil moisture and lower salinity near the plant was maintained under drip irrigation than under furrow irrigation. Performance of the two cultivars was significantly different; pooled over 2002–03 and 2003–04 seasons, PC yielded 38.8 and 30.0 Mg ha⁻¹ and Edkawi yielded 31.8 and 22.9 Mg ha⁻¹ under drip and furrow irrigation, respectively. At RSC10, cultivar PC produced 38 and 46% higher fruit yield than cultivar Edkawi under drip and furrow irrigation, respectively. Reduction in fruit yield at higher RSC was due to lower fruit weight under drip irrigation and due to reduced fruit number as well as fruit weight under furrow irrigation. Decrease in fruit weight was more pronounced in cultivar Edkawi than in cultivar PC. Increase in RSC lowered quality of the fruits except the ascorbic acid content. High RSC under drip irrigation, in general, had lesser deteriorating effect on the fruit quality particularly for cultivar PC than under furrow irrigation. For obtaining high tomato yield and better-quality fruits using high RSC sodic waters, drip irrigation should be preferred over furrow irrigation. Better performance of local cultivar PC compared to Edkawi at medium and high RSC suggests that cultivars categorized as tolerant to salinity should be evaluated in the sodic environment particularly when irrigated with high RSC sodic waters.     

Monitoring the pollution risk and water use in orchard terraces with mango and cherimoya trees by drainage lysimeters

Agricultural nonpoint-source pollution is the leading cause of water-quality degeneration of rivers and groundwater. In this context, the coast of Granada province (SE Spain) is economically an important area for the subtropical fruit cultivation. This intensively irrigated agriculture often uses excessive fertilizers, resulting to water pollution. Therefore, a 2-year experiment was conducted using drainage lysimeters to determine the potential risk of nutrient pollution in mango (Mangifera indica L. cv. Osteen) and cherimoya (Annona cherimola Mill. cv. Fino de Jete) orchards. These lysimeters were used to estimate the nutrient budgeting for each crop. NO₃-N, NH₄-N, PO₄-P and K losses according to lysimeters were, respectively, 55.1, 12.4, 3.7, and 0.6 for mango and 61.8, 17.8, 4.9, and 0.5 kg ha⁻¹ yr⁻¹, for cherimoya. NO₃, concentrations in the leachates ranged from 1.8 to 44.3 mg L⁻¹, and from 23.0 to 51.0 mg L⁻¹, for mango and cherimoya, respectively, in some cases exceeding the limits for safe drinking water. PO₄ also exceeded the permitted concentrations related to eutrophication of water, ranging from 0.07 to 0.5 mg L⁻¹ and from 0.12 to 0.68 mg L⁻¹ from mango and cherimoya lysimeters, respectively. With respect to the nutrient balance, N, P, and K removed by cherimoya fruits was 76.4, 5.5, and 22.6 kg ha⁻¹ yr⁻¹, and for mango fruits 30.2, 3.3 and 27.8 kg ha⁻¹ yr⁻¹, respectively. Nutrient losses in the leachates were surprisingly low, considering total N, P, and K applied during the year, in mango lysimeters 3.8, 0.11, and 12.6%, and in cherimoya lysimeters 7.7, 0.23 and 16.0%, respectively, indicating a potential soil accumulation and eventual loss risk, especially during torrential rains. Crop coefficient (Kc) values of mango trees varied within ranges of 0.35–0.67, 0.55–0.89, and 0.39–0.80 at flowering, fruit set, and fruit growth, respectively. Kc values for cherimoya trees had ranges of 0.58–0.67, 0.61–0.68, and 0.43–0.62 at flowering, fruit set and fruit growth, respectively. In this study, the Kc values of mango and cherimoya were significantly correlated to julian days. Therefore, the estimated WUE in the mango and cherimoya orchards reached 21.2 and 14.0 kg ha⁻¹ mm⁻¹, respectively. Thus, this study highlights the urgency to establish the optimal use of fertilizers and irrigation water with respect to crop requirements, to preserve surface-water and groundwater quality, thereby achieving more sustainable agriculture in orchard terraces.     

Effect of water shortage on yield, and protein and mineral compositions of drip-irrigated sweet corn in sustainable agricultural systems

Water is a natural resource of prime value that is very often limited and costly, particularly in semi-arid regions. While traditional irrigation methods increase the amount of surface runoff, drip irrigation provides efficient use of the limited water resources. This study was conducted to determine the water-yield relationship and the quality of sweet corn grown under deficit drip irrigation in 1998 and 1999 in Sanliurfa, Turkey. Irrigation treatments analysed in this study were full irrigation as well as 10, 20 and 30% deficiency of Class A pan evaporation. In both years, water use efficiency ranged between 1.18 and 1.36 kg m⁻³, and irrigation water use efficiency ranged between 1.36 and 1.62 kg m⁻³. The yield response factor (k_y) or the ratio of the decrease in relative yield to the decrease in relative water consumption varied from 0.82 to 1.43, and the water-saving rate ranged from 10.9 to 31.1%. The relationships between fresh ear yield and the irrigation treatments were statistically significant (P < 0.01), and the yield decreased with increasing deficit irrigation. Root dry matter increased with water shortage. Maximum values of leaf area index were obtained at full irrigation, whereas the lowest values were found at 30% water deficiency. The values of the deficit irrigation stress index increased with decreasing water application. Although the protein content increased, the Fe, Zn and Cu concentration of the kernels decreased with increasing water deficiency. Despite the reduction of fresh ear yield with deficit irrigation, the number of marketable ears at 10% water deficiency was still high and acceptable for sweet corn (var. Reward) in south-eastern Turkey.     

Irrigation rate and plant density effects on yield and water use efficiency of drip-irrigated corn

The efficient use of water by modern irrigation systems is becoming increasingly important in arid and semi-arid regions with limited water resources. This study was conducted for 2 years (2005 and 2006) to establish optimal irrigation rates and plant population densities for corn (Zea mays L.) in sandy soils using drip irrigation system. The study aimed at achieving high yield and efficient irrigation water use (IWUE) simultaneously. A field experiment was conducted using a randomized complete block split plot design with three drip irrigation rates (I₁: 1.00, I₂: 0.80, and I₃: 0.60 of the estimated evapotranspiration), and three plant population densities (D₁: 48,000, D₂: 71,000 and D₃: 95,000 plants ha⁻¹) as the main plot and split plot, respectively. Irrigation water applied at I₁, I₂ and I₃ were 5955, 4762 and 3572 m³ ha⁻¹, respectively. A 3-day irrigation interval and three-way cross 310 hybrid corn were used. Results indicated that corn yield, yield components, and IWUE increased with increasing irrigation rates and decreasing plant population densities. Significant interaction effects between irrigation rate and plant population density were detected in both seasons for yield, selected yield components, and IWUE. The highest grain yield, yield components, and IWUE were found for I₁D₁, I₁D₂, or I₂D_1, while the lowest were found for I₃D₂ or I₃D₃. Thus, a high irrigation rate with low or medium plant population densities or a medium irrigation rate with a low plant population density are recommended for drip-irrigated corn in sandy soil. Crop production functions with respect to irrigation rates, determined for grain yield and different yield components, enable the results from this study to be extrapolated to similar agro-climatic conditions.     

Production and oil quality in ‘Arbequina’ olive (Olea europaea, L.) trees under two deficit irrigation strategies

The effect of two deficit irrigation (DI) strategies on fruit and oil production and quality in a 12-year-old ‘Arbequina’ olive orchard with 238 trees ha^?1 was evaluated. The T1 treatment was a sustained DI regime (65% ETc, 2–3 irrigation events per week). The T2 treatment was a low-frequency DI (increasing stress/rewatering cycles, which consisted in withholding irrigation until fruit shrivelling and then applying a recovery irrigation providing the same amount of water that supplied in T1 for that period). As compared to full irrigation, both strategies reduced fruit production and increased the variability of fruit ripening, but favoured oil extraction. Free acidity, peroxide value, K₂₃₂, K₂₇₀ and sensory quality of oil were not affected by DI. Furthermore, carotenoid, chlorophyll, phenol, and oleic contents increased. The greatest phenol content and bitterness index were found in oil from T2 trees. Later harvesting caused sensory quality and tocopherol losses, although the oil synthesized in DI olives increased.     

Integrated management of irrigation water and fertilizers for wheat crop using field experiments and simulation modeling

The reported study aimed at developing an integrated management strategy for irrigation water and fertilizers in case of wheat crop in a sub-tropical sub-humid region. Field experiments were conducted on wheat crop (cultivar Sonalika) during the years 2002–2003, 2003–2004 and 2004–2005. Each experiment included four fertilizer treatments and three irrigation treatments during the wheat growth period. During the experiment, the irrigation treatments considered were I₁ = 10% maximum allowable depletion (MAD) of available soil water (ASW); I₂ = 40% MAD of ASW; I₃ = 60% MAD of ASW. The fertilizer treatments considered in the experiments were F₁ = control treatment with N:P₂O₅:K₂O as 0:0:0 kg ha⁻¹, F₂ = fertilizer application of N:P₂O₅:K₂O as 80:40:40 kg ha⁻¹; F₃ = fertilizer application of N:P₂O₅:K₂O as 120:60:60 kg ha⁻¹ and F₄ = fertilizer application of N:P₂O₅:K₂O as 160:80:80 kg ha⁻¹. In this study CERES-wheat crop growth model of the DSSAT v4.0 was used to simulate the growth, development and yield of wheat crop using soil, daily weather and management inputs, to aid farmers and decision makers in developing strategies for effective management of inputs. The results of the investigation revealed that magnitudes of grain yield, straw yield and maximum LAI of wheat crop were higher in low volume high frequency irrigation (I₁) than the high volume low frequency irrigation (I₃). The grain yield, straw yield and maximum LAI increased with increase in fertilization rate for the wheat crop. The results also revealed that increase in level of fertilization increased water use efficiency (WUE) considerably. However, WUE of the I₂ irrigation schedule was comparatively higher than the I₁ and I₃ irrigation schedules due to higher grain yield per unit use of water. Therefore, irrigation schedule with 40% maximum allowable depletion of available soil water (I₂) could safely be maintained during the non-critical stages to save water without sacrificing the crop yield. Increase in level of fertilization increases the WUE but it will cause environmental problem beyond certain limit. The calibrated CERES-wheat model could predict the grain yield, straw yield and maximum LAI of wheat crop with considerable accuracy and therefore can be recommended for decision-making in similar regions.     

Possible utilization of high-salinity waters and application of low amounts of water for production of the halophyte Kochia scoparia as alternative fodder in saline agroecosystems

Production of halophytes using saline waters and soils and feeding them to livestock is one of the most sustainable methods of conservation in desert ecosystems, in addition to accomplishing food production for the people living in these areas. Therefore, to study the possibility of irrigating Kochia (Kochia scoparia L. Schrad) with minimum quantities of highly saline water for use as a fodder crop in arid environments stretching across saline waters, two experiments were carried out in the Research Farm of the Ferdowsi University of Mashhad, Iran. In the salinity experiments, two populations of Kochia, including the Sabzevar and Indian genotypes, were irrigated with ground water having electrical conductivity (EC) of 5, 15, and 20 dS m⁻¹. In the irrigation-treatment experiments, two local populations of Kochia, including Sabzevar and Borujerd, were subjected to four irrigation regimes as follows: complete irrigation (100%), 80%, 60%, and 40% of the water requirements using a saline ground water with EC = 5 dS m⁻¹. Because, the Indian genotype is preferred as an ornamental plant, it is not suitable for increased dry-matter production under high-salinity irrigation water compared to the local genotype (Sabzevar), which is suitable for forage. The Sabzevar genotype produced a large amount of dry matter (7530 kg ha⁻¹), even when irrigated with 20 dS m⁻¹ saline water. The best time for harvesting Kochia for fresh feeding is at the end of flowering (88 days after sowing or DAS), when the biomass is relatively high (6500 kg ha⁻¹) and the leaf-to-shoot ratio, as a quality index, is approximately 50%. The highest green-area index was observed at 15 dS m⁻¹ and decreased at high levels of salinity. Photosynthesis and transpiration rate did not decline significantly with increasing external salinity four weeks after salinization, but increased in both genotypes at 15 dS m⁻¹, indicating that the salinity-tolerance threshold of Kochia for both photosynthesis and transpiration reduction is above this salinity level. The Indian genotype also showed a very low seed yield (210 kg ha⁻¹) at low levels of salinity, whereas Sabzevar produced 1120 kg ha⁻¹ seed under the same conditions. Different irrigation regimes had a significant effect on the biomass and seed production of Kochia. The highest forage yield was obtained from complete irrigation, with 11.1 Mg ha⁻¹ dry material. Sabzevar local population represented a better performance in terms of all characteristics, except accumulation of inflorescence dry matter, and no significant effects were recorded. In conclusion, Kochia's high foliage production capacity in the presence of salinity and limited irrigation make this plant suitable for use as an alternative forage crop in harsh environmental conditions. There is a wide range of intraspecific variation in K. scoparia, but more investigation is needed to introduce it as a cash crop.     

Yield and foliar injury responses of mature plum trees to salinity

Summary The salt tolerance of mature Santa Rosa plum trees was assessed on 20-year-old trees grown in the San Joaquin Valley of California. The experimental design consisted of six levels of irrigation water salinity (electrical conductivities of 0.3 to 8 dS/m) replicated five times with each replication consisting of ten trees. Salinity treatments imposed in March 1984 did not influence tree yields harvested in June 1984. In 1985, the second year of treatments, yield from the highest salt treatment (electrical conductivity of irrigation water, EC _i , of 8 dS/m) was reduced by half; the number of fruit harvested was reduced 40%, and fruit size was reduced significantly. Foliar damage was so severe by the end of 1985 that nonsaline water was applied to the two highest salt treatments (EC _i = 6 and 8 dS/m) in an attempt to restore tree vigor. In 1986 salt effects had become progressively worse on the continuing saline treatments. A linear piece-wise salt tolerance curve is presented for soil salinity values, expressed as the electrical conductivity of saturated extracts (EC _e ) integrated to a soil depth of 1.2 m over a 2-year period. The salt tolerance threshold for relative yield (Y _r ) based on 3 years of data was 2.6 dS/m and yield reduction at salinity levels beyond the threshold was 31% per dS/m (Y _ir=100 – 31 [EC _e – 2.6]j). Significant foliar damage occurred when leaf chloride concentrations surpassed 200 mmol/kg of leaf dry weight (0.7%). Sodium concentrations in the leaves remained below 10 mmol/kg (0.02%) until foliar damage became severe. This suggests that chloride was the dominant ion causing foliar damage.     

Towards using a thermal infrared index combined with water balance modelling to monitor sugarcane irrigation in a tropical environment

In humid regions, the timing and quantity of a complementary irrigation regime is challenging because of the irregularity of rainfalls events. In this study, we tested the use of a thermal infrared derived empirical crop water stress index (CWSI_e) as an in situ measurement of the water status of sugarcane, to better monitor the irrigation scheduling. To do this, we set up a 2-year experiment in Reunion Island, on a trial with plots under different water conditions (rainfed and irrigated). Crop surface temperature was measured daily with infrared radiometers (Apogee Instruments) installed above the canopy, and soil moisture and drainage measurements were used to derive the ratio between actual and maximum evapotranspiration (AET/MET) values that were then averaged on “hydrically homogeneous” time periods (between 7 and 25 days). Only the thermal data acquired on clear days and 1 h after noon in 2007 were used to define the empirical lower and upper baselines required for the calculation of empirical CWSI. The data set acquired in 2008 was used to test the robustness of the method as we used the upper and lower baselines defined in 2007 to calculate CWSI_e. The linear regression between AET/MET and (1 − CWSI_e) averaged on the same periods (values ranging between 0.4 and 1) showed a significant correlation for both experimental years (global R² = 0.75 and RMSE = 0.12). This result indicates the effectiveness of the CWSI_e to measure the water status of the sugarcane crop, even in humid conditions with a vapor pressure deficit (VPD) between 0.5 and 2.1. We conclude the study by discussing the complementarity of this remote water stress index (CWSI_e) with OSIRI water balance modelling tool currently used in Reunion Island for monitoring sugarcane crop irrigation.     

Effect of stage-specific saline irrigation on greenhouse tomato production

In many water scarce areas, saline water has been included as an important substitutable resource in agricultural irrigation. It would be of practical use to investigate the effect of stage-specific saline irrigation on yield, fruit quality, and other growth responses of greenhouse tomato, to establish a proper irrigation management strategy for tomato production in these regions. Here, saline irrigations (3.33, 8.33, and 16.67 dS m⁻¹ NaCl solution) were applied during four growth stages of greenhouse tomato (L. esculentum Mill. cv. Zhongza No. 9) grown in the North China Plain, respectively. These include flowering and fruit-bearing stage (stage 1), first cluster fruit expanding stage (stage 2), second cluster fruit expanding stage (stage 3), and harvesting stage (stage 4). Compared with the following three stages, yield loss was most remarkable in stage 1 under all three salinity levels. Under irrigation practices using 3.33 dS m⁻¹ saline water in all four stages, 8.33 dS m⁻¹ saline water in latter three stages, and 16.67 dS m⁻¹ saline water in stage 4, yield reduction was not significant while fruit quality was improved. In conclusion, it is feasible to use stage-specific saline irrigation for tomato production in water scarce areas like North China Plain.     

Subsurface drip irrigation and reclaimed water quality effects on phosphorus and salinity distribution and forage production

In the Canary Islands, water scarcity is one of the constraints for agricultural activity. Non-conventional water resources generally represent more water volume than conventional ones. The distribution of these resources frequently permits the possibility of a conjunctive use of desalinated (DW) water and reclaimed municipal wastewater (RW). Field testing with both water qualities and different irrigation systems is necessary for optimal site-specific management. The objective of this work was to evaluate soil salinity and phosphorus distribution, and alfalfa yield in a 20 month field experiment carried out in the island of Gran Canaria, using municipal RW and freshwater (FW) under subsurface drip irrigation (SDI). Phosphorus speciation was performed both in irrigation waters and in soils (Olsen's inorganic, organic, and microbial). RW had large EC values (2.4 dS m⁻¹) with a remarkable nutrient load contribution and an average total P around 3 mg L⁻¹, predominantly hydrolysable forms, while FW had very low salinity and negligible amounts of P. For the RW treatment a salt gradient was established, causing plant mortality between the irrigation lines. The study of P speciation allows describing P distribution and plant uptake in terms of P forms. Large values of microbial P were produced for the two irrigation waters around the emitters, especially for FW.A faster P-cycling could have contributed to the significantly larger inorganic P contents observed in FW irrigated soils, in spite no external sources were added by the irrigation water.     

Evaluation of CERES-Wheat and CropSyst models for water–nitrogen interactions in wheat crop

Crop simulation models can provide an alternative, less time-consuming and inexpensive means of determining the optimum crop N and irrigation requirements under varied soil and climatic conditions. In this context, two dynamic mechanistic models (CERES (Crop Environment REsource Synthesis)-Wheat and CropSyst (Cropping Systems Simulation Model)) were validated for predicting growth and yield of wheat (Triticum aestivum L) under different nitrogen and water management conditions. Their potential as N and water management tool was evaluated for New Delhi representing semi-arid irrigated ecosystems in the Indo-Gangetic Plains. The field experiment was carried out on a silty clay loam soil at the Research Farm of the Indian Agricultural Research Institute, New Delhi, India during 2000–2001 to collect the input data for the calibration and validation of both the models on wheat crop (variety HD 2687). The models were evaluated for three water regimes [I4 (4 irrigations within the growing season), I3 (3 irrigations within the growing season) and I2 (2 irrigations within the growing season)] and five N treatments (N₀, N₆₀, N₉₀, N₁₂₀ and N₁₅₀). Both the models were calibrated using data obtained from the treatments receiving maximum nitrogen and irrigations, i.e., N₁₅₀ and I4 treatments. The models were then validated against other water and nitrogen treatments. For performance evaluation, in addition to coefficient of determination (R²), root mean square error (RMSE), mean absolute error (MAE) and Wilmot's index of agreement (IoA) were estimated. Both CERES-Wheat and CropSyst provided very satisfactory estimates for the emergence, flowering and physiological maturity dates. For CERES-Wheat overall prediction (pooled result of the three water regimes) of grain yield was satisfactory with significant R² values (0.88). The model, however, under estimated the biomass under all water regimes and N levels except for N₀ level, under which biomass was overpredicted. CropSyst predicted yield and biomass of wheat more closely than CERES-Wheat. The combined RMSE for the three water regimes between predicted and observed grain yield was 0.36 Mg ha⁻¹ for CropSyst as compared to 0.63 Mg ha⁻¹ for CERES-Wheat. Similarly, RMSE between observed and predicted biomass by CropSyst was 1.27 Mg ha⁻¹ as compared to 1.94 Mg ha⁻¹ between observed and predicted biomass by CERES-Wheat. Wilmot's index of agreement (IoA) also indicated that CropSyst model is more appropriate than CERES-Wheat in predicting growth and yield of wheat under different N and irrigation application situations in this study.     

Use of CropSyst as a decision support system for scheduling regulated deficit irrigation in a pear orchard

Prediction of plant water status is necessary for the judicious application of regulated deficit irrigation. CropSyst, a generic crop growth model that is applicable to fruit trees, was used to forecast plant water potential for irrigation management recommendations in a pear orchard. Plant water potential is predicted along with tree transpiration using Ohm’s law analogy. The parameters of the model were adjusted by using field measured data on a lysimeter-grown pear tree. After adjustment, and using the same lysimeter data, a satisfactory agreement was found between simulated and measured tree transpiration, light interception, and stem water potential. Model simulations were also performed for other independent field data. These corresponded to eight different conditions of a deficit-irrigated field experiment in a pear orchard. Each condition differed in soil texture, time of irrigation cut-off, crop load, and tree leaf area. Deficit irrigation was managed first by withholding irrigation until reaching a threshold in midday stem water potential of −1.5 MPa. Subsequently, irrigation was applied at fixed proportions of full irrigation requirements. Simulations with CropSyst were used as decision support system that could work independently of stem water potential measurements. Simulations in all eight sites were satisfactory at providing adequate time without irrigation during the first part of the deficit period. A highly significant relationship (r ²  = 0.71) between predicted and measured stem water potentials was found for a simulation period of 40 days. Simulations for longer periods (i.e. 74 days) decreased the r ² to 0.61, and for this reason after resuming irrigation, slight deviations were found for the average stem water potential in two out of five sites. In conclusion, CropSyst produced relevant information for managing deficit irrigation in simulation periods shorter than 40 days.     

Response of sweet orange cv ‘Lane late’ to deficit irrigation in two rootstocks. I: water relations,leaf gas exchange and vegetative growth

The influence of a deficit-irrigation (DI) strategy on soil–plant water relations and gas exchange activity was analysed during a 3-year period in mature ‘Lane late’ (Citrus sinensis (L.) Osb.) citrus trees grafted on two different rootstocks, ‘Cleopatra’ mandarin (Citrus reshni Hort. ex Tanaka ) and ‘Carrizo’ citrange (C. sinensis L., Osbeck × Poncirus trifoliata L.). Two treatments were applied for each rootstock: a control treatment, irrigated at 100% ETc (crop evapotranspiration) during the entire season, and a DI treatment, irrigated at 100% ETc, except during Phase I (cell division) and Phase III (ripening and harvest) of fruit growth, when complete irrigation cut-off was applied. Under soil water deficit, the seasonal variations of soil water content suggested that ‘Cleopatra’ mandarin had a better root efficiency for soil water extraction than ‘Carrizo’ citrange. Moreover, in all years, trees on ‘Cleopatra’ reached a lower water-stress level (midday xylem water potential values (Ψ_md) > −2 MPa), maintaining a better plant water status during the water-stress periods than trees on ‘Carrizo’ (Ψ_md < −2 MPa). Similarly, net CO₂ assimilation rate (A) was higher in trees on ‘Cleopatra’ during the water-stress periods. In addition, the better plant water status in trees on ‘Cleopatra’ under DI conditions stimulated a greater vegetative growth compared to trees on ‘Carrizo’. From a physiological point of view, ‘Cleopatra’ mandarin was more tolerant of severe water stress (applied in Phases I and III of fruit growth) than ‘Carrizo’ citrange.     

Overland water and salt flows in a set of rice paddies

Cultivation of paddy rice in semiarid areas of the world faces problems related to water scarcity. This paper aims at characterizing water use in a set of paddies located in the central Ebro basin of Spain using experimentation and computer simulation. A commercial field with six interconnected paddies, with a total area of 5.31 ha, was instrumented to measure discharge and water quality at the inflow and at the runoff outlet. The soil was classified as a Typic Calcixerept, and was characterized by a mild salinity (2.5 dS m⁻¹) and an infiltration rate of 5.8 mm day⁻¹. The evolution of flow depth at all paddies was recorded. Data from the 2002 rice-growing season was elaborated using a mass balance approach to estimate the infiltration rate and the evolution of discharge between paddies. Seasonal crop evapotranspiration, estimated with the surface renewal method, was 731 mm (5.1 mm day⁻¹), very similar to that of other summer cereals grown in the area, like corn. The irrigation input was 1874 mm, deep percolation was 830 mm and surface runoff was 372 mm. Irrigation efficiency was estimated as 41%. The quality of surface runoff water was slightly degraded due to evapoconcentration and to the contact with the soil. During the period 2001–2003, the electrical conductivity of surface runoff water was 54% higher than that of irrigation water. However, the runoff water was suitable for irrigation. A mechanistic mass balance model of inter-paddy water flow permitted to conclude that improvements in irrigation efficiency cannot be easily obtained in the experimental conditions. Since deep percolation losses more than double surface runoff losses, a reduction in irrigation discharge would not have much room for efficiency improvement. Simulations also showed that rice irrigation performance was not negatively affected by the fluctuating inflow hydrograph. These hydrographs are typical of turnouts located at the tail end of tertiary irrigation ditches. In fact, these are the sites where rice has been historically cultivated in the study area, since local soils are often saline-sodic and can only grow paddy rice taking advantage of the low salinity of the irrigation water. The low infiltration rate characteristic of these saline-sodic soils (an experimental value of 3.2 mm day⁻¹ was obtained) combined with a reduced irrigation discharge resulted in a simulated irrigation efficiency of 60%. Paddy rice irrigation efficiency can attain reasonable values in the local saline-sodic soils, where the infiltration rate is clearly smaller than the average daily rice evapotranspiration.     

Modeling the potential for closing quinoa yield gaps under varying water availability in the Bolivian Altiplano

In the Bolivian Altiplano, the yields of rainfed quinoa are relatively low and highly unstable. We use a validated crop water productivity model to examine the potential of closing quinoa yield gaps in this region. We simulate the expectable yields under rainfed cultivation and under different deficit irrigation (DI) strategies using the AquaCrop model for the Northern, Central and Southern Bolivian Altiplano. Simulated DI scenarios include a reference strategy avoiding stomatal closure during all sensitive growth stages and allowing drought stress during the tolerant growth stages (DI₀) and various restrictive deficit irrigation strategies (DI_i) representing cases when water resources are limited. We obtain a logistic crop water production function for quinoa by plotting the seasonal actual evapotranspiration versus total grain yield. Due to the large scatter, this function only indicatively provides expectable yields. From the scenario analysis, we derive yield probability curves for the 3 agro-climatic regions. DI, without restriction in irrigation water during the drought sensitive growth stages, is able to close the yield gaps in the Northern, Central and Southern Bolivian Altiplano, and would guarantee a high and stable level of water productivity (WP). The yields of quinoa under rainfed cultivation during dry years are only 1.1, 0.5 and 0.2 Mg ha⁻¹ in the Northern, Central and Southern Bolivian Altiplano, whereas under DI₀ they are 2.2, 1.6 and 1.5 Mg ha⁻¹, respectively. Under limited water availability for irrigation, these stable yield levels decrease, most drastically in the Southern Bolivian Altiplano. Below a minimum water availability of 600 m³ per ha and 700 m³ per ha in the Central and Southern Bolivian Altiplano, respectively, the application of DI for quinoa is not significantly effective and should be avoided to save valuable resources. The yield probability curves we derive can serve as input for stochastic economic analysis of DI of quinoa in the Bolivian Altiplano.