Response of wheat to saline irrigation and drainage

The response of wheat (Triticum aestiuum L.) to varying depths of irrigation, quantity of water applied and to the drainage conditions was studied in 2 m × 2 m × 2 m size lysimeters filled in with a sandy loam soil. Saline water with an electrical conductivity of 8.6 dS m⁻¹ was used for irrigation. The treatments included four irrigations of 5 cm depth, four irrigations of 7 cm, and three irrigations of 9 cm, scheduled on the basis of cumulative pan evaporation, while the drainage conditions were represented by the drained and undrained lysimeters. Another treatment, using good quality water for irrigation, represented the potential yield of the crop. The growth parameters, as well as the yield, showed an improvement with larger irrigation depth increments in the drained lysimeters. But, in contrast, in the undrained lysimeters, the yield was reduced with larger irrigation depth increments, mainly due to a sharp rise in water table depth during the irrigation cycles. The rise and fall in water table showed a high sensitivity and were also highly disproportionate to the irrigation and evapotranspiration events. The yield tended to be higher with a smaller depth of water applied more frequently in the undrained lysimeters. But, in view of the limitations of conventional surface irrigation to apply water in smaller depth increments, an improved drainage is imperative for cropping in shallow saline water table conditions.     

Soil type effects on soybean crop water use in weighing lysimeters

Summary In a previous experiment, evaporation from soybeans (Glycine max L.) in two weighing lysimeters with different soil types was found to differ by up to 30%. This occurred despite good canopy development and maintenance of well watered conditions. The present experiment sought to repeat the previous observation and to define its cause. Soybeans were sown in and around the two weighing lysimeters on 9 December 1987 and were well watered through the entire season. The lysimeters, L1 and L2 contained undisturbed blocks of Hanwood loam and Mundiwa clay loam soils, respectively, both Rhodoxeralfs. Crop growth, radiant energy interception, soil heat flux, canopy temperature and root growth were monitored through the season. Plant growth in L2 was slower than in L1 such that by 46 days from sowing (DFS), L1 plants had one leaf more on average than those in L2 and by 76 DFS plants in L2 were about 0.1 m shorter than those in either L1 or in the area immediately surrounding it. The ratio of L2 to L1 daily evaporation was 0.76 during the period 75 to 84 DFS; this being very similar to the effect observed previoulsy. The crop canopy in a 100 m² area centred around L2 was reduced in height by removing the top 0.15 m at 85 DFS. This treatment caused the L2L1 evaporation ratio to increase to 1.07. The effect of reducing the height of plants surrounding L2 was to increase net radiant energy intercepted in the canopy of the L2 plants and to change the turbulent transfer processes over the L2 canopy. Shading from the taller surrounding plants was estimated to have reduced evaporation by 4% while increased aerodynamic resistance above the L2 canopy as the result of the height discontinuity accounted for a further 20% reduction. This study highlights limitations in the application of one dimensional energy balance theory to non-ideal canopy configurations and to the care needed to ensure plant growth within lysimeters is the same as the surrounds.Visiting scientist     

Short term effects of saline irrigation on evapotranspiration from lysimeter-grown citrus trees

Eight-year-old Murcott orange trees grown in greenhouse lysimeters filled with sandy soil were subjected to irrigation with saline water to investigate the influence of salinity on daily evapotranspiration (ET). The study was conducted in Japan from 1 August to 15 September 2000. The study duration was divided into three periods of about 2 weeks each. In period I, all lysimeters planted with a tree were irrigated with 60 mm of non-saline water at the water content of 70% of field capacity (FC). Salinity treatments for period II started on 14 August. The treatments during period II were as follows: Lysimeter 1 (L1) had 32 mm non-saline water with an electrical conductivity (EC_I) of 1.0 dS/m applied. At the same time Lysimeter 2 (L2) had 32 mm of saline water with an EC_I of 8.6 dS/m applied when the water content decreased to 70% of FC. Lysimeter 3 (L3) had 16 mm saline water (EC_I=8.6 dS/m) applied at 85% of FC. The irrigation amounts during period II were equal to those corresponding to 1.2 times of water required to reach FC. Treatments in period III were the same as in period I.Daily ET was similar for all weighing lysimeters during period I. The average relative ET for L2 and L3 with respect to L1 (L2/L1 and L3/L1) were similar during this period, with a mean value of 0.99. During period II, ET from L1 was consistently higher than that from L2 and L3. In addition, L3 with a higher irrigation frequency because of irrigation at higher soil water content resulted in higher ET than L2. The average relative ET of period II was 0.71 and 0.88 for both L2 and L3. During the last half of period III, reductions occurred in the ET differences between the saline treatments (L2 and L3) and non-saline control (L1).Evaporation rates from soil did not exceed 0.7 mm per day. Transpiration rates from L1, L2 and L3 during period II varied between 6.3 and 3.1 mm per day, 4.5 and 2.2 mm per day, and 5.8 and 3.0 mm per day, respectively. The results reflected a tangible difference of water extraction by roots from individual soil layers. Maximum water uptake by these trees was observed at layer of 30–60 cm. Nevertheless, no clear differences in water extraction pattern between trees were observed.Approximately, 95% of drainage occurred during the first 2 days following irrigation. The electrical conductivity of soil water (EC_S) and the electrical conductivity of drainage water (EC_D) for the saline water treatments (L2 and L3), compared to the control (L1) were significantly different during period II. EC_S values were 2–5 times higher in saline treatments compared to the control treatment. After irrigating trees with saline water, EC_S increased from 5 to 14 and 16 dS/m in L2 and L3, respectively. Similarly, in both saline treatments, EC_D values were greatly increased after irrigation. During period III, EC_D values increased from 5 to 8 dS/m in L2, and from 3 to 11 dS/m in L3. By contrast, EC_S declined from 14 to 5 dS/m in L2, and from 16 to 3 dS/m in L3 over the same period.     

Effect of shallow groundwater table on crop water requirements and crop yields

Due to the increasing demand for food and fiber by its ever-increasing population, the pressure on fresh water resources of Pakistan is increasing. Optimum utilization of surface and groundwater resources has become extremely important to fill the gap between water demand and supply. At Lahore, Pakistan 18 lysimeters, each 3.05 m × 3.05 m × 6.1 m deep were constructed to investigate the effect of shallow water tables on crop water requirements. The lysimeters were connected to bottles with Marriotte siphons to maintain the water tables at the desired levels and tensiometers were installed to measure soil water potential. The crops studied included wheat, sugarcane, maize, sorghum, berseem and sunflower. The results of these studies showed that the contribution of groundwater in meeting the crop water requirements varied with the water-table depth. With the water table at 0.5 m depth, wheat met its entire water requirement from the groundwater and sunflower absorbed more than 80% of its required water from groundwater. Maize and sorghum were found to be waterlogging sensitive crops whose yields were reduced with higher water table. However, maximum sugarcane yield was obtained with the water table at or below 2.0 m depth. Generally, the water-table depth of 1.5–2.0 m was found to be optimum for all the crops studied. In areas where the water table is shallow, the present system of irrigation supplies and water allowance needs adjustments to avoid over irrigation and in-efficient use of water.     

The effect of salinity on water productivity of wheat under deficit irrigation above shallow groundwater

Saline groundwater is often found at shallow depth in irrigated areas of arid and semi-arid regions and is associated with problems of soil salinisation and land degradation. The conventional solution is to maintain a deeper water-table through provision of engineered drainage disposal systems, but the sustainability of such systems is disputed. This shallow groundwater should, however, be seen as a valuable resource, which can be utilised via capillary rise (i.e. sub-irrigation). In this way, it is possible to meet part of the crop water requirement, even where the groundwater is saline, thus decreasing the need for irrigation water and simultaneously alleviating the problem of disposing of saline drainage effluent. Management of conditions within the root zone can be achieved by means of a controlled drainage system.A series of lysimeter experiments have permitted a detailed investigation of capillary upward flow from a water-table controlled at shallow depth (1.0 m) under conditions of moderately high (5 mm/day) evaporative demand and with different levels of salinity. Experiments were conducted on a wheat crop grown in a sandy loam soil. Groundwater salinity was held at values from 2 to 8 dS/m while supplementary (deficit) irrigation was applied at the surface with salinity in the range 1-4 dS/m.Our experiments show that increased salinity decreased total water uptake by the crop, but in most treatments wheat still extracted 40% of its requirement from the groundwater, similar to the proportion reported for non-saline conditions. Yield depression was limited to 30% of maximum when the irrigation water was of relatively good quality (1 and 2 dS/m) even with saline groundwater (up to 6 dS/m). Crop water productivity (grain yield basis) was around 0.35 kg/m³ over a wide range of salinity conditions when calculated conventionally on the basis of total water use, but was generally above 1.0 kg/m³ if calculated on the basis of irrigation input only.     

The contribution of shallow groundwater by safflower (Carthamus tinctorius L.) under high water table conditions, with and without supplementary irrigation

Lysimetric experiments were conducted to determine the contribution made by groundwater to the overall water requirements of safflower (Carthamus tinctorius L.). The plants were grown in 24 columns, each having a diameter of 0.40 m and packed with silty clay soil. The four replicate randomized complete block factorial experiments were carried out using different treatment combinations. Six treatments were applied during each experiment by maintaining groundwater, with an EC of 1 dS m^?1, at three different water table levels (0.6, 0.8 and 1.10 m) with and without supplementary irrigation. The uptake of groundwater as a part of crop evapotranspiration was measured by taking daily readings of the water levels found in Mariotte tubes. The supplementary irrigation requirement for each treatment was applied by adding water (EC of 1 dS m^?1). The average percentage contribution from groundwater for the treatments (with and without supplementary irrigation under water table levels of 0.6, 0.8 and 1.10 m) were found to be 65, 59, 38% and 72, 70, 47% of the average annual safflower water requirement (6,466 m³ ha^?1). The increase in groundwater depths under supplementary irrigation treatments from 0.6 to 0.80 and 1.10 m caused seed and oil yield reductions of (7, 23.10%) and (48.23, 65.40%), respectively.     

Effect of level and timing of moisture stress on soybean plant development and yield components

Field studies were conducted in 1984 and 1985 using non-weighing lysimeters of 1 m² to determine the effect of level of soil moisture stress (L) and timing of moisture stress (T) on reproductive phenology, plant characteristics, and yield components of Maple Amber soybean [Glycine max (L.) Merr.]. The study incorporated the most probable combinations of moisture stress in order to provide a basis for planning and managing irrigation for optimum soybean production. The effect of L was consistent over T and Year. Neither L nor T affected days to full bloom stage (R2). Increases in L linearly (r² = 0.71) increased days to beginning seed stage (R5), but decreased (r² = 0.93) days to full maturity stage (R8). L was related quadratically to plant height (r² = 0.88) and number of pod-bearing nodes (r² = 0.98), and linearly to number of pods (r² = 0.75) and total dry matter (r² = 0.91). No consistent effect of T on the timing of R5 to R8 stages was found, mainly because of the presence of a Year X T interaction (P = 0.05). Withholding irrigation during the R5 stage from previously adequately watered crops caused significant reduction in plant height (7%), number of pod-bearing nodes on the main stem (13%), number of pods (18%), number of seeds (20%), total seed weight (25%), and total dry matter (23%), indicating the importance of irrigation during this stage. It was concluded that, if irrigation water is limiting, with-holding irrigation at R2 may be acceptable, but irrigation at R5 is essential for optimizing yield components and thus yield.     

亏缺灌溉对成龄库尔勒香梨产量与根系生长的影响

研究了亏缺灌溉对成龄库尔勒香梨树营养生长、果实生长与吸收根系分布的影响。香梨的生育中期,设计了4种土壤水分亏缺(灌前土壤水势下限):轻度胁迫(-100 kPa)、中度胁迫(-200 kPa)、重度胁迫(-300 kPa)和对照(-50 kPa)。灌溉系统为地表滴灌。试验结果表明,亏缺灌溉显著地减少了夏季剪枝量,提高了果实产量。充分灌溉条件下,香梨树根长密度从树行由内向外呈递减趋势,滴灌促进了湿润体内的吸收根系的发育;距树行0.5 m,根长密度随着深度的增加而减少;距树行1 m至1.5 m,根长密度随着深度的增加而增加;距树行2 m的非灌溉区,根长密度在垂直方向上变化不显著。为适应滴灌湿润体内水分胁迫,香梨树在湿润体内及非灌溉区的吸收根系的分布均发生了显著的调整。适度水分胁迫下的根长密度增加对香梨产量有促进作用。     

Tomato yield, biomass accumulation, root distribution and irrigation water use efficiency on a sandy soil, as affected by nitrogen rate and irrigation scheduling

Florida is the largest producer of fresh-market tomatoes in the United States. Production areas are typically intensively managed with high inputs of fertilizer and irrigation. The objectives of this 3-year field study were to evaluate the interaction between N-fertilizer rates and irrigation scheduling on yield, irrigation water use efficiency (iWUE) and root distribution of tomato cultivated in a plastic mulched/drip irrigated production systems. Experimental treatments included three irrigation scheduling regimes and three N-rates (176, 220 and 230 kg ha⁻¹). Irrigation treatments included were: (1) SUR (surface drip irrigation) both irrigation and fertigation line placed right underneath the plastic mulch; (2) SDI (subsurface drip irrigation) where the irrigation line was placed 0.15 m below the fertigation line which was located on top of the bed; and (3) TIME (conventional control) with irrigation and fertigation lines placed as in SUR and irrigation being applied once a day. Except for the “TIME” treatment all irrigation treatments were controlled by soil moisture sensor (SMS)-based irrigation set at 10% volumetric water content which was allotted five irrigation windows daily and bypassed events if the soil water content exceeded the established threshold. Average marketable fruit yields were 28, 56 and 79 Mg ha⁻¹ for years 1-3, respectively. The SUR treatment required 15-51% less irrigation water when compared to TIME treatments, while the reductions in irrigation water use for SDI were 7-29%. Tomato yield was 11-80% higher for the SUR and SDI treatments than TIME where as N-rate did not affect yield. Root concentration was greatest in the vicinity of the irrigation and fertigation drip lines for all irrigation treatments. At the beginning of reproductive phase about 70-75% of the total root length density (RLD) was concentrated in the 0-15 cm soil layer while 15-20% of the roots were found in the 15-30 cm layer. Corresponding RLD distribution values during the reproductive phase were 68% and 22%, respectively. Root distribution in the soil profile thus appears to be mainly driven by development stage, soil moisture and nutrient availability. It is concluded that use of SDI and SMS-based systems consistently increased tomato yields while greatly improving irrigation water use efficiency and thereby reduced both irrigation water use and potential N leaching.     

Effects of irrigation strategies and soils on field grown potatoes: Root distribution

Root distribution of field grown potatoes (cv. Folva) was studied in 4.32 m² lysimeters and subjected to full (FI), deficit (DI), and partial root-zone drying (PRD) irrigation strategies. Drip irrigation was applied for all irrigations. Irrigations were run in three different soils: coarse sand, loamy sand, and sandy loam. Irrigation treatments started after tuber bulking and lasted until final harvest with PRD and DI receiving 65% of FI. Potatoes irrigated with water-saving irrigation techniques (PRD and DI) did not show statistically different dry root mass and root length density (RLD, cm root per cm³ soil) compared with root development in fully irrigated (FI) potatoes. Highest RLD existed in the top 30-40 cm of the ridge below which it decreased sharply. The RLD was distributed homogenously along the ridge and furrow but heterogeneously across the ridge and furrow with highest root density in the furrow. Most roots accumulated in the surface layers of coarse sand as compared to the other soil types. In the deep soil profile (30-70 cm) a higher root density was found in loamy sand compared with the sandy loam and coarse sand. Approximately twice the amounts of roots were found below the furrows compared with the corresponding layers below the ridges. The RLD values in the soil profile of the ridges and the furrows followed the Gerwitz and Page model: RLD = α × exp(−β × z). The highest value of surface root density (α) and rate of change in density (β) was found in coarse sand while the lowest values of α and β were found in the sandy loam and loamy sand. The model estimated the effective rooting depth in coarse sand and sandy loam quite well but did slightly overestimate it in the loamy sand. Statistical analysis showed that one α and β value can be used for each soil irrespective of the irrigation treatment. Thus, the effective rooting depths corresponding to root length densities of 0.1 and 0.25 cm cm⁻³ for sandy loam, loamy sand, and coarse sand soils were 99, 141, and 94 cm, and 80, 115, and 78 cm, respectively, calculated from top of the ridge. The findings of this study can be used in practice for efficient use of water and nutrients in the field.     

Roots in irrigated clay soils: Measurement techniques and responses to rootzone conditions

Summary This paper reviews research carried out at the Griffith Laboratory in Australia over the last decade on techniques for, and results of, observations of roots in irrigated clay soils. Our results emphasise the adaptability of root systems to rootzone conditions. Experiences with techniques for observing roots non-destructively in the field and both non-destructively and destructively in lysimeters are described. We concluded that the minirhizotron technique, applied in the field, was unreliable under our conditions. Horizontal root observation tubes were used in lysimeters to measure root length density (RLD) and to assess whether roots were clumped together or randomly distributed. Destructive sampling and measurement of RLD was used to establish a theoretical relationship between root intercept counts along the tubes and RLD. The application of image analysis to both destructive and non-destructive sampling in the lysimeters is outlined. The non-destructive lysimeter studies showed that roots were significantly clumped. Analysis of root intercept and root hole counts on the faces of sample cubes taken from the lysimeters showed root distribution was anisotropic over the whole soil profile for both safflower and wheat. There were many more roots and root holes present in the sampled soil cubes than was indicated by independent sampling for washed out RLD. Safflower appeared to have a faster turnover of roots than did wheat or maize. Lysimeters, equipped with horizontal root observation tubes, enabled studies to be made of many factors affecting root growth. Soils affect where and how fast roots grow, although there is also a strong species interaction. For example, soybean roots proliferated above a fresh water table in one soil but not in another; wheat had little tendency to proliferate above the water table in either soil. In wet soils, roots cease to grow once soil oxygen levels decrease below 10 mg O₂ l _soil ^-1 . This level should form the basis for soil drainage criteria. In drying soils, roots will grow successively into soil regions containing soil water: the level of adaptation being determined by soil conditions, crop growth stage and level of evaporative demand. The methods of root observation used in our studies have given quantitative assessment of root distribution. However, further research is needed to link horizontal and vertical root distribution and root adaptation more strongly to crop development and soil conditions.     

Water balance of flooded rice paddies

Rice (Oryza sativa L., var. Labelle) was grown in 300 m² paddies of Beaumont clay soil (Typic pelludert) and subjected to two management schemes of flooded rice culture. These schemes were continuous irrigation and intermittent irrigation. Careful measurements of irrigation, precipitation, evapotranspiration, deep percolation and runoff were made, and the total water balance for the two water management schemes was calculated.The results show continuous irrigation to be very wasteful of water with slightly over 1 m of irrigation water applied to supply an evapotranspirational need of 0.5–0.6 m. The intermittent irrigation management is less wasteful but still could be improved upon. Suggestions are presented for techniques to help improve the water use efficiency and reduce runoff losses.     

Soil water balance trial involving capacitance and neutron probe measurements

The objective of this study was to compare soil water measurements made using capacitance and neutron probes by means of a water balance experiment in a drainage lysimeter. The experiment was conducted in a 5-year-old drip-irrigated peach orchard (Prunus persica L. Batsch, cv. Flordastar, on GF-677 peach rootstock) planted in a clay loam textured soil located in southern Spain. Four drainage lysimeters (5 m × 5 m × 1.5 m), each containing one tree, were constructed and equipped with one lateral line containing eight drippers per tree, with a discharge rate of 2 L h⁻¹. Three access tubes for the neutron probe (NP), symmetrically facing three PVC access tubes containing the multi-depth capacitance probes (MDCP) were located perpendicularly to the drip line (0.2, 0.6 and 1 m). The results demonstrated that both the capacitance and neutron probes gave similar soil water content values under steady state hydraulic gradient conditions (0.2 m from the emitter) although some discrepancies were found in heterogeneous soil water distribution conditions (1 m from the emitter), which might be attributed to the smaller soil volume explored by the MDCP compared with the NP. Explanations for the discrepancies between both devised are presented. When water inputs and outputs were fairly constant, the volumetric soil water content could be considered to represent field saturation (θ_sat = 0.36 m³ m⁻³). When drainage was zero, there were 2 days when the soil water content was constant and could be considered as field capacity (θ_fc = 0.31 m³ m⁻³). The findings suggest that: (i) capacitance probes can be used for continuous real-time soil water content monitoring unlike the manual measurements obtained with the neutron probe; (ii) the location of the sensors is critical when used for drip irrigation scheduling and our recommendations for practical agricultural purposes would be to place MDCP sensors in the place representing the highest root density, leading the sensors to become biological sensors rather than mere soil moisture sensors; and (iii) on average, the water balance values determined by lysimeter match those calculated using the data from both probes. However, due to the smaller soil volume explored by MDCP, more of these sensors must be used to characterize the soil water status in water balance studies.     

Leaching irrigated saline sandy to sandy loam apedal soils with water of a constant salinity

With the optimization of irrigation, more salts accumulate in the root zone of soils, due to less over-irrigation. On-farm irrigation management requires a certain amount of leaching to ensure sustainability. The objective is to quantify the pore volume of water required to efficiently leach excess salts from two saline soils, widely irrigated in central South Africa. A total of 30 lysimeters, 15 per soil type arranged in two parallel rows under a moveable rain shelter, were used. Five different salinity profiles per soil type, replicated three times, were leached using irrigation water with a 75 mS m⁻¹ electrical conductivity. During irrigation the residual more saline pore water was displaced from the top downward through the root zone. The mean salinity of the soil profiles approached an equilibrium concentration equal to that of the irrigation water after 0.9 pore volume of soil was displaced by drainage water. For the sandy soil 0.2 and for the sandy loam soil 0.3 pore volumes were required to efficiently remove 70% of the excess salts. The remainder of the water was needed to leach the remaining 20% of the excess salts. This, however, was not efficient in terms of the amount of water required.     

Wheat yield response to line source sprinkler irrigation and soil management practices on medium-textured shallow soils of arid environment

A field experiment was conducted for 3 years to evaluate the effect of deficit irrigation under different soil management practices on biomass production, grain yield, yield components and water productivity of spring wheat (Triticum estivum L.). Soil management practices consisted of tillage (conventional and deep tillage) and Farmyard manure (0 and 10 t ha^?1 FYM). Line source sprinkler laterals were used to generate one full- (ETm) and four deficit irrigation treatments that were 88, 75, 62 and 46 % of ETm, and designated as ETd₁, ETd₂, ETd₃, and ETd₄. Deep tillage significantly enhanced grain yield (14–18 %) and water productivity (1.27–1.34 kg m^?3) over conventional tillage. Similarly, application of FYM at 10 t ha^?1 significantly improved grain yield (10–13 %) and water productivity (1.25–1.31 kg m^?3) in comparison with no FYM. Grain yield response to irrigation varied significantly (5,281–2,704 kg ha^?1) due to differences in soil water contents. Water productivity varied from 1.05 to 1.34 kg m^?3, among the treatments in 3 years. The interactive effect of irrigation × tillage practices and irrigation × FYM on grain yield was significant. Yield performance proved that deficit irrigation (ETd₂) subjected to 75 % soil water deficit had the smallest yield decline with significant water saving would be the most appropriate irrigation level for wheat production in arid regions.     

Trickle irrigation of cotton: Effect on soil chemical properties

The management of soil salinity and sodicity in the root zone (0–150 cm) of Panoche clay loam soil was studied during three consecutive growing seasons in a field experiment designed to determine the water requirement of Acala SJ-2 cotton (Gossypium hirsutum L.) under trickle irrigation in the western San Joaquin Valley of California. The trickle irrigation treatments (20, 40, 60, 80 and 100% of the previous day's pan evaporation, PE) were imposed on each of three preplant furrow irrigation depths of 0, 190 and 380 mm. The electrical conductivity, EC, of the irrigation water ranged from 0.4 to 0.5 dS m⁻¹ unless groundwater had to be mixed with the surface supply; this practice raised the EC to 0.6–0.9 dS m⁻¹ for short time periods. The unadjusted sodium adsorption ratio, SAR, of the irrigation water averaged < 4.Soil salinity, as measured by EC_e, was < 5 dS m⁻¹ and sodicity, as measured by SAR_e, was < 15 in the root zone. Changes in these soil chemical properties were more related to the amount of preplant irrigation at the lower (< 40% PE) trickle-irrigation levels than at the higher levels. Results suggest that soil salinity and sodicity can be maintained at acceptably low levels by appropriate preplant irrigation with consideration to amount of winter rainfall; even when during the season only sufficient trickle irrigation is given to meet crop water requirements without regards to leaching needs. The lint cotton yields over the 3-year period ranged from 357 to 1542 kg ha⁻¹; the corresponding applied water ranged from 175 to 744 mm.     

Determination of the crop coefficient for grafted ‘Tahiti’ lime trees and soil evaporation coefficient of Rhodic Kandiudalf clay soil in Sao Paulo,Brazil

The expansion of permanent trickle irrigation systems in Sao Paulo (Brazil) citrus has changed the focus of irrigation scheduling from determining irrigation timing to quantifying irrigation amounts. The water requirements of citrus orchards are difficult to estimate, since they are influenced by heterogeneous factors such as age, planting density and irrigation system. In this study, we estimated the water requirements of young ‘Tahiti’ lime orchards, considering the independent contributions from soil evaporation and crop transpiration by splitting the crop coefficient (Kc = ETc/ETo) into two separate coefficients; Ke, a soil evaporation coefficient and Kcb, a crop transpiration coefficient. Hence, the water requirement in young ‘Tahiti’ lime (ET_y) is ET_y = (Ke + Kcb) · ETo, where ETo is the reference crop evapotranspiration. Mature tree water requirement (ET_m) is ET_m = Kcb · ETo, assuming no soil water evaporation. Two lysimeters were used; one was 1.6 m in diameter and 0.7 m deep, and the other was 2.7 m in diameter and 0.8-m deep. The first one was used to calculate evaporation and the second one was used for transpiration. ETo was estimated by the Penman–Monteith method (FAO-56). The measurements were conducted during a period between August 2002 and April 2005 in Piracicaba, Sao Paulo state, Brazil. The lysimeters were installed at the center of a 1.0-ha plot planted with ‘Tahiti’ lime trees grafted on ‘Swingle’ citrumelo rootstock. The trees were 1-year old at planting, spaced 7 × 4 m, and were irrigated by a drip irrigation system. During the study period, Kc varied between 0.6 and 1.22, and Kcb varied between 0.4 and 1.0. The results suggested that for young lime trees, the volume of water per tree calculated by Ke + Kcb is about 80% higher than the volume calculated using Kc. For mature trees, the volume of water per tree calculated using just Kcb can be 10% less than using Kc. The independent influence of soil evaporation and transpiration is important to better understand the water consumption of young lime trees during growth compared to mature lime trees.     

Effects of irrigation strategies and soils on field grown potatoes: Yield and water productivity

Yield and water productivity of potatoes grown in 4.32 m² lysimeters were measured in coarse sand, loamy sand, and sandy loam and imposed to full (FI), deficit (DI), and partial root-zone drying (PRD) irrigation strategies. PRD and DI as water-saving irrigation treatments received 65% of FI after tuber bulking and lasted for 6 weeks until final harvest. Analysis across the soil textures showed that fresh yields were not significant between the irrigation treatments. However, the same analysis across the irrigation treatments revealed that the effect of soil texture was significant on the fresh yield and loamy sand produced significantly higher fresh yield than the other two soils, probably because of higher leaf area index, higher photosynthesis rates, and “stay-green” effect late in the growing season. More analysis showed that there was a significant interaction between the irrigation treatments and soil textures that the highest fresh yield was obtained under FI in loamy sand. Furthermore, analysis across the soil textures showed that water productivities, WP (kg ha⁻¹ fresh tuber yield mm⁻¹ ET) were not significantly different between the irrigation treatments. However, across the irrigation treatments, the soil textures were significantly different. This showed that the interaction between irrigation treatments and soil textures was significant that the highest significant WP was obtained under DI in sandy loam. While PRD and DI treatments increased WP by, respectively, 11 and 5% in coarse sand and 28 and 36% in sandy loam relative to FI, they decreased WP in loamy sand by 15 and 13%. The reduced WP in loamy sand was due to nearly 28% fresh tuber yield loss in PRD and DI relative to FI even though ET was reduced by 9 and 11% in these irrigation treatments. This study showed that different soils will affect water-saving irrigation strategies that are worth knowing for suitable agricultural water management. So, under non-limited water resources conditions, loamy sand produces the highest yield under full irrigation but water-saving irrigations (PRD and DI) are not recommended due to considerable loss (28%) in yield. However, under restricted water resources, it is recommended to apply water-saving irrigations in sandy loam and coarse sand to achieve the highest water productivity.     

Effects of water quality and frequency of irrigation on growth and yield of barley (Hordeum vulgare L.)

A field trial on a loamy sand soil was carried out to study the effect of three irrigation waters with different qualities on growth and yield of ‘Gesto’, a barley (Hordeum vulgare L.) cultivar. Three irrigation water quality treatments (canal irrigation water, drainage water, and mixed canal and drainage waters at 1:1 ratio) were imposed with two irrigation frequencies (I and 2 week intervals). In addition, nitrogen and phosphorus fertilizers were applied at different rates. Barley grain and straw yields were significantly decreased under the use of drainage water (EC 10.7–16.7 dS m⁻¹), attributed mainly to reduction in the number of spikes per plant and grain weight. The mixed irrigation water (EC 6.8–9.9 dS m⁻¹) produced high seedling emergence and good vegetative growth, which was followed by high grain and straw yields. These yields were not significantly different from those under fresh canal irrigation water (EC 2.8–3.9 dS m⁻¹). Thus, mixed water could be another alternative for irrigation under similar experimental conditions especially with high rates of nitrogen (250–350 kg ha⁻¹) and phosphorus (90 kg ha⁻¹) fertilization at weekly irrigation intervals, which could eventually save more fresh irrigation canal water for other cultivated crops.     

A modeling-GIS approach for assessing irrigation effects on soil salinisation under global warming conditions

Soil salinisation is very often due to excessive irrigation. However, irrigation is absolutely essential for obtaining reliable crop yields, particularly under predicted global warming conditions. A simple methodology for assessing the salinisation risk for any water management situation and under predicted global warming conditions is presented. The methodology is illustrated by the assessment of irrigation effects on soil salinity at San Antonio del Sur Valley, in the southeast of Cuba. Irrigation from a new dam will support agriculture in the Valley, but at the same time soil salinity is expected to increase. Soil electrical conductivity at several depths and topographical altitudes were used to create raster layers in a Geographic Information System (GIS), thus, determining the border of the saline-affected zones by a GIS analysis. Water-table depth at the border of the saline zones was assumed to be 2 m. The physically based SWAP model was used to predict future water-table depths after irrigation begins and under global warming conditions. Future temperature and precipitation daily values were calculated from a linear increase/decrease of the daily values corresponding to a typical year, according to a global-change forecast for the zone. Soil hydraulic properties were estimated from pedotransfer function and published soil data. Simulated results predict a fast water-table raise of 1 m, due to the increase of irrigation water. Borders of the new saline zones under these conditions (i.e. the places where the water-table is at a 2 m depth) were calculated using a digital terrain model, assuming that the water-table rose 1 m over the whole valley. According to the simulation results, the original saline zones of the valley will be enlarged from 31.4 to 96.8 ha 15 years after the scheduled start of irrigation. The methodology could be used by farmers and decision-makers to select the most suitable water management solution considering both economical and environmental criteria.