Evaluating the effectiveness of a hydrophobic polymer for conserving water and reducing weed infection in a sandy loam soil

In this work we tested the influence of different solutions of a hydrophobic polymer named Guilspare^®, applied to the soil surface to reduce soil evaporation, on the soil water status, soil temperature, crop performance and weed emergence. Two tests were carried out on a farm of the Guadalquivir river valley, southwest Spain, one with a maize crop and the other with bare soil. In the test with maize, we evaluated the effect of applying a solution of 2% v/v of Guilspare^® in water, at the rate of 3 l m⁻², on the crop performance and weed emergence. On both the treated and the untreated control plots, three rates of irrigation were applied, namely 100, 75 and 50% of the locally determined optimal irrigation depth to cover the crop needs for an optimum development and yield. For the case of 50% of the irrigation dose, the performance of the crop treated with the polymer (T50) was much better than that of the untreated control plot (C50). The crop height and green leaf area index for T50 were nearly as good as for the C100 control plants receiving 100% of the irrigation dose. The T50 crop was 73% of the yield of the treated and fully irrigated T100 crop, while the C50 yield was only 38% of the C100 yield. The treated crop reached the different phenological stages quicker than the untreated crop. The polymer was effective in reducing weed emergence. In the test with bare soil, 0.8% v/v of Guilspare^® in water, at the rate of 1 l m⁻², kept levels of water content in the soil as high as other solutions with greater amounts both of polymer and water. The average soil water content during the irrigation period in this lower treatment was 34 and 53% higher at depths of 0.15 and 0.25 m, respectively, than in the untreated plots. No influence of the polymer on soil temperature was observed. Results from additional measurements on weed emergence and hydraulic conductivity of the soil surface showed that the polymer was still effective 7 months after application. In fact, the hydraulic conductivity in the range near saturation was 44% greater in the treated plots than in the untreated ones, and the number of weeds was 27% lower.     

Field calibration of a neutron moisture meter in a cracking grey clay

Summary A neutron moisture meter was field calibrated in a cracking grey clay prepared for furrow irrigation at Narrabri, N.S.W. Neutron counts were taken in successive 0.1 m increments between the 0 and 1.5 m depths. Concomitant measurements using undisturbed soil cores provided independent estimates of volumetric water content. Separate linear calibrations were required for the 0–0.1 m, 0.1–0.2 m and 0.2–1.5 m depth increments. Correction for bias due to cracking and changes in bulk density slightly improved the calibrations. The accuracy of predicted soil water content was improved relative to previous calibrations. A precision of ±0.01 m³m^–3 required 3 samples per mean by the neutron method or 11 samples per mean by the core sampling method.     

Dynamics and modeling of soil water under subsurface drip irrigated onion

Subsurface drip irrigation provides water to the plants around the root zone while maintaining a dry soil surface. A problem associated with the subsurface drip irrigation is the formation of cavity at the soil surface above the water emission points. This can be resolved through matching dripper flow rates to the soil hydraulic properties. Such a matching can be obtained either by the field experiments supplemented by modeling. Simulation model (Hydrus-2D) was used and tested in onion crop (Allium cepa L.) irrigated through subsurface drip system during 2002-2003, 2003-2004 and 2004-2005. Onion was transplanted at a plant to plant and row to row spacing of 10 cm × 15 cm with 3 irrigation levels and 6 depths of placement of drip lateral. The specific objective of this study was to assess the effect of depth of placement of drip laterals on crop yield and application of Hydrus-2D model for the simulation of soil water. In sandy loam soils, it was observed that operating pressures of up to 1.0 kg cm⁻² did not lead to the formation of cavity above the subsurface dripper having drippers of 2.0 l h⁻¹ discharge at depths up to 30 cm. Wetted soil area of 60 cm wide and up to a depth of 30 cm had more than 18% soil water content, which was conducive for good growth of crop resulting in higher onion yields when drip laterals were placed either on soil surface or placed up to depths of 15 cm. In deeper placement of drip lateral (20 and 30 cm below surface), adequate soil water was found at 30, 45 and 60 cm soil depth. Maximum drainage occurred when drip lateral was placed at 30 cm depth. Maximum onion yield was recorded at 10 cm depth of drip lateral (25.7 t ha⁻¹). The application of Hydrus-2D confirmed the movement of soil water at 20 and 30 cm depth of placement of drip laterals. The model performance in simulating soil water was evaluated by comparing the measured and predicted values using three parameters namely, AE, RMSE and model efficiency. Distribution of soil water under field experiment and by model simulation at different growth stages agreed closely and the differences were statistically insignificant. The use of Hydrus-2D enabled corroborating the conclusions derived from the field experimentation made on soil water distribution at different depths of placement of drip laterals. This model helped in designing the subsurface drip system for efficient use of water with minimum drainage.     

Simulation of putting-green soil water dynamics: Implications for turfgrass water use

Results from a field experiment examining soil water fate within U.S. Golf Association (USGA) putting greens were used to examine the validity of a water flow simulation model. The experiment used six different sandy root zones each with depths of 300 mm overlying a 100 mm thick gravel layer. Data collected over two growing seasons consisted of measured rainfall, irrigation, drainage volume, and soil water contents; and calculated turfgrass evapotranspiration (ET). Turfgrass rooting was measured at the end of each growing season, and water retention curve and saturated hydraulic conductivity measurements were conducted at the end of the study. For each root zone treatment, HYDRUS-2D (H2D) was calibrated using a subset of the experimental data and then validated by comparing observed and predicted water contents at 76, 152 and 229 mm depth and over both growing seasons. Model efficiency (E) ranged from 0.33 to 0.78; Mean Absolute Error (MAE) ranged from 0.012 to 0.024 m³ m⁻³; and Root Mean Square Error (RMSE) ranged from 0.015 to 0.028 m³ m⁻³, for the six treatments and both years. Also, RMSE values were at best slightly larger than and at worst twice as large as the mean standard deviation values of replicate measurements. Thus, H2D simulation performed reasonably well in describing the water content results of the field study. The calibration results provide evidence of hysteresis in water retention where water retention properties from the field appear to follow the sorption or wetting curve as compared with the laboratory measurements following the desorption or drying curve. This suggests that standard laboratory measurements of water release would not precisely predict water retention behavior in the field and over estimate water storage of these capillary barrier soils. The validation results provide evidence for turfgrass use of perched water held within these profiles, even though turfgrass rooting is shallow and water storage principally occurs deep within the root zone. Thus, the perched water of USGA putting greens should serve reasonably well as a water reservoir for subsequent turfgrass use, allowing for water conserving irrigation practices that makes use of this stored water.     

Production and water use in lettuces under variable water supply

The effects of a variable water supply on the water use, growth and yield of two crisphead and one romaine (i.e., Cos) lettuce cultivar were examined in a field experiment using a line source sprinkler system that produced a range of water regimes that occur in growers fields. Four locations at increasing distances from the main line were monitored through the season (i.e., from thinning to harvest, 28–63 days after planting (DAP)). These locations at the end of the season corresponded to: (1) rewatering to field capacity (FC); (2) watering with a volume 13% below that required in the field capacity treatment (0.87*FC); (3) 30% below FC (0.70*FC); and (4) 55% below FC (0.45*FC). A linear production function for dry matter accumulation and fresh weight vs. crop evapotranspiration (ET_c) was determined for lettuce during this period, giving a water use efficiency for dry matter of 1.86 g m^–2 mm^–1 and for fresh weight of 48 g m^–2 mm^–1 . For lettuce irrigated to field capacity, ET_c between thinning and harvest was 146 mm; maximum crop coefficients of 0.81–1.02 were obtained at maturity (55–63 DAP). For the three irrigation treatments receiving the largest water application, ET_c was higher in the Cos culivar than in the two crisphead lettuce cultivars which had similar ET_c. Plant fresh weight was more sensitive than dry weight to reduction in water supply. In the FC treatment, root length density and soil water extraction were greatest in the top 0–45 cm, and decreased rapidly below 45 cm depth. Soil water extraction by roots increased at lower depths when irrigation was reduced. Instantaneous rates of leaf photosynthesis and leaf water potential showed no response to the irrigation treatments in this study, despite differences in biomass production. Evaporation was determined to be the major component of ET_c for 45 of the 63 days of the growing season. The large loss of water by evaporation during mid-season and the apparent insensitivity of lettuce to the volume of irrigation during this period may provide an opportunity for reducing irrigation applications.     

Mapping the total volumetric irrigation water requirements in England and Wales

The net volumetric (m³) irrigation water requirements for the main crop categories currently irrigated in England and Wales have been calculated and mapped within a geographic information system (GIS). The procedure developed by Knox et al. (1996, Agric. Water Manage., 31: 1–15) for maincrop potatoes (Solanum tuberosum) was extended to cater for the other crops currently irrigated. The annual irrigation needs (mm) for the eight major irrigated crop categories, grown on three contrasting soil types at 11 representative weather stations, were determined using a daily water balance irrigation scheduling model. The results were correlated with existing national datasets of climate, current land use, soils and irrigation practice, to generate volumetric (m³) irrigation water requirement maps at 2 km resolution.The total net volumetric irrigation water requirements for a UK ‘design’ dry year (defined as the requirement with a 20% probability of exceedance) are estimated to be 140 × 10⁶ m³ for the eight main crop categories currently irrigated and the 1994 cropping pattern. Previous theoretical dry year demand estimates, using scheduling models and large agroclimatic areas, were 109 × 10⁶ m³ and 222 × 10⁶ m³. The irrigation demand for other crops grown in the open would typically add another 4%.The procedure has been validated nationally, by comparing the calculated dry year demand for 1990 against government irrigation survey returns for 1990, for each crop category, and regionally against National Rivers Authority (NRA) abstraction records for 1990, for each NRA Region. The estimates obtained agree well with the reported distribution between crops and between regions.The most recent actual ‘dry’ year for which comparative data are available is 1990. It is estimated that the dry year requirements for the 1990 land use would have been 148 × 10⁶ m³. Although farmer demand, actual abstractions and crop requirements are not necessarily the same, irrigation survey returns to the Government indicated that 134 × 10⁶ m³ were actually applied, and the NRA estimated from meter returns that 138 × 10⁶ m³ were abstracted. It is noted, however, that some abstraction restrictions were in force, the scope of the data is slightly different and all figures contain inaccuracies. Potential applications for improving irrigation demand management and water conservation at regional and catchment levels are discussed with reference to two contrasting regions.     

Impact of soil water stress on Nigellone oil content of black cumin seeds grown in calcareous-gypsifereous soils

Nigellone (dithymoquinone) is the main active constituent of volatile oil of black cumin (Nigella sativa) seeds. It is presently used in traditional medicines, for culinary as ornamentals, and is also considered for its abundant nectar secretion. While black cumin, investigated recently (for the oil, essential oil, and other biologically active constituents of their seeds) the effects of deficit irrigation on seeds Nigellone content produced on gypsifereous soils are not known. Randomized complete block design experiments were conducted with three replications and four irrigation treatments on soils with five different gypsum contents over two growing seasons (2008-2009 and 2009-2010). These experiments aim to monitor and quantify water stress and Nigellone volatile oil content of black cumin as a function of crop water stress index and soil gypsum content. The soil gypsum content treatments were 60.0 (G₁), 137.6 (G₂), 275.2 (G₃), 314.2 (G₄) and 486.0 (G₅) g kg⁻¹. Three irrigation treatments were based on replenishing the 0.60 m deep root zone to field capacity when the maximum allowable depletion (MAD) of the available soil water holding capacity of 25% (I₁), 50% (I₂) and 75% (I₃) were maintained in the crop experiments. A dryland treatment (fully stressed, I₄) was also included. The lower (non-stressed) and upper (stressed) baselines were measured to calculate crop water stress index. The crop water stress index behaved as expected, dropping to near zero following an irrigation and increasing gradually as black cumin plants depleted soil water reserves. The seasonal mean values of crop water stress index for the irrigation treatments; I₁, I₂, and I₃ were increased from 0.189, 0.287, 0.380 to 0.239, 0.366, 0.467, respectively when the soil gypsum content increased from 60.0 to 486.0 g kg⁻¹. The highest Nigellone volatile oil content of black cumin seeds was obtained for G₁I₁ treatment (5.1 g kg⁻¹) while the lowest content (3.5 g kg⁻¹) was obtained for G₅I₁ treatment. Equations that can be used to predict the Nigellone volatile oil content of black cumin seeds were developed for the three irrigation schedules of different maximum allowable depletion of available soil water holding capacity using the relationships between the Nigellone volatile oil content and the seasonal mean crop water stress index for different soil gypsum contents. The relationships between black cumin seed yield, Nigellone volatile oil content and seasonal mean crop water stress index values were primarily linear. These relations can be used to predict the yield of black cumin seeds, seeds Nigellone volatile oil content, and irrigation timing in soils with different soil gypsum contents. Thus, the obtained data will be beneficial for further research.     

Effects of applied water and sprinkler irrigation uniformity on alfalfa growth and hay yield

This study was conducted to investigate the effects of applied water and sprinkler irrigation uniformity on alfalfa (Medicago sativa L.) growth and hay yield in a semi-arid region. Field experiments were carried out in 2006 in Varamin, Iran, on three plots of 25 m × 30 m. Each plot was subdivided into 25 subplots of 5 m × 6 m. Different irrigation depths and sprinkler water uniformities were obtained by various scenarios of sprinkler nozzle pressure. In each plot, applied water was measured at 250 points (125 points above and 125 points below canopy) and the soil water content of 40 cm deep below soil surface was monitored at 25 points, each in the center of a subplot, throughout the irrigation season. The results showed that sprinkler water and soil water content uniformity varied between 66-78 and 88-91%, respectively. The findings revealed that soil water content uniformity was around 20% higher than sprinkler water uniformity. The irrigation uniformity below the canopy was estimated to be 2.5% greater than above the canopy, and canopy-intercepted water could account for 11-15% of the total seasonal applied water. Evaluation showed that alfalfa leaf area index relies more heavily on farm water application uniformity than hay yield and crop height. The experimental results illustrated that water distribution in sprinkler irrigation systems has a direct effect on alfalfa growth, hay yield and water productivity such that the applied water reduction and the increased sprinkler water uniformity led to an increased alfalfa water productivity of 2.41 kg m⁻³.     

Irrigation of lucerne with saline groundwater on a slowly permeable,duplex soil

Summary Lucerne was irrigated for three years on a slowly permeable, duplex soil, with saline water up to 2.4 dS m^–1 without significant yield decline. Irrigation water of 4.5 dS m^–1 significantly reduced yield. Lucerne yield was most closely related to the soil ECe of the 0–15 cm depth, rather than the total rootzone, and was described by; Relative yield=100–6.5 (ECe-2.1). While lucerne roots reached depths of at least 150 cm, approximately 80% of total root length was located in the 0–60 cm depth.Increasing salinity increased the plant concentrations of sodium and chloride, however, these changes were not closely related to changes in yield.Soil salinity increased with increasing salinity of the applied water. However, during the irrigation season water penetration and the accumulation of salt within the profile was predominantly restricted to the 0–60 cm depth. No portion of the applied irrigation water was available as a leaching fraction. Any leaching of salts to the watertable, particularly below 120 cm, was due to winter rainfall rather than the application of summer irrigation water.Ripping the soil to a depth of 75 cm increased water infiltration and resulted in increased crop yields, but did not significantly affect the crop relative yield-soil ECe relationship.From the results it is proposed that on the slowly permeable duplex soils, when watertable depth is controlled, management strategies for lucerne irrigated with saline water should be based on controlling the salinity of the shallow soil depths, to 60 cm.     

Soil water content and water balance in rainfed fields in Northeast Thailand

Northeast Thailand has a semi-humid tropical climate which is characterized by dry and rainy seasons. In order to stabilize crop production, it may be necessary to develop new water resources, such as soil moisture and groundwater, instead of rainfed resources. This is because rainfed agriculture has already been unsuccessfully tried in many areas of this region. In this study, we investigate the soil water content in rainfed fields in Khon Kaen in Northeast Thailand, where rice and sugarcane were planted, over a 1-year period that contained both dry and rainy seasons, and estimate the actual evapotranspiration (ET_a) using micrometeorological data. In addition, we assess the water balance from the results of the soil water content investigation and the actual evapotranspiration. Although the soil water content at depths above 0.6 m in both the lower and the sloping fields gradually decreased during the dry season, the soil water content at a depth of 1.0 m was under almost constant wet conditions. Two-dimensional profiles of the soil water content demonstrated that at the end of the dry season, the soil layers below a depth of 0.4 m showed a soil water content of more than 0.10-0.15 m³ m⁻³, thus suggesting that water was supplied to the sugarcane from those layers. The range in ET_a rates was almost the same as that in the previous study. The average ET_a rates were 3.7 mm d⁻¹ for the lower field and 4.2 mm d⁻¹ for the sloping field. In the dry season, an upward water flow of 373 mm (equivalent to a flux of 1.9 mm d⁻¹) was estimated from outside the profile. The source of this upward water flow was the sandy clay (SC) layer below a depth of 1 m. It was this soil water supply from the SC layer that allowed the sugarcane to grow without irrigation.     

Effects of salinity and fertigation practice on cotton yield and N recovery

The purpose of optimal water and nutrient management is to maximize water and fertilizer use efficiency and crop production, and to minimize groundwater pollution. In this study, field experiments were conducted to investigate the effect of soil salinity and N fertigation strategy on plant growth, N uptake, as well as plant and soil ¹⁵N recovery. The experimental design was a 3 × 3 factorial with three soil salinity levels (2.5, 6.3, and 10.8 dS m⁻¹) and three N fertigation strategies (N applied at the beginning, end, and in the middle of an irrigation cycle). Seed cotton yield, dry matter, N uptake, and plant ¹⁵N recovery significantly increased as soil salinity level increased from 2.5 to 6.3 dS m⁻¹, but they decreased markedly at higher soil salinity of 10.8 dS m⁻¹. Soil ¹⁵N recovery was higher under soil salinity of 10.8 dS m⁻¹ than those under soil salinity of 6.3 dS m⁻¹, but was not significantly different from that under soil salinity of 2.5 dS m⁻¹. The fertigation strategy that nitrogen applied at the beginning of an irrigation cycle had the highest seed cotton yield and plant ¹⁵N recovery, but showed higher potential loss of fertilizer N from the root zone. While the fertigation strategy of applying N at the end of an irrigation cycle tended to avoid potential N loss from the root zone, it had the lowest cotton yield and nitrogen use efficiency. Total ¹⁵N recovery was not significantly affected by soil salinity, fertigation strategy, and their interaction. These results suggest that applying nitrogen at the beginning of an irrigation cycle has an advantage on promoting yield and fertilizer use efficiency, therefore, is an agronomically efficient way to provide cotton with fertilizer N under the given production conditions.     

Effect of subsurface drip irrigation on onion yield

Subsurface drip system is the latest method of irrigation. The design of subsurface drip system involves consideration of structure and texture of soil, and crop’s root development pattern. A 3-year experiment was conducted on onion (Allium Cepa L., cv. Creole Red) in a sandy loam soil from October to May in 2002–2003, 2003–2004 and 2004–2005 to study the effect of depth of placement of drip lateral and different levels of irrigation on yield. Tests for uniformity of water application through the system were carried out in December of each year. Three different irrigation levels of 60, 80 and 100% of the crop evapotranspiration and six placement depths of the drip laterals (surface (0), 5, 10, 15, 20 and 30 cm) were maintained in the study. Onion yield was significantly affected by the placement depth of the drip lateral. Maximum yield (25.7 t ha⁻¹) was obtained by applying the 60.7 cm of irrigation water and by placing the drip lateral at 10 cm soil depth. Maximum irrigation water use efficiency (IWUE) (0.55 t ha⁻¹ cm⁻¹) was obtained by placing the drip lateral at 10 cm depth. The greater vertical movement of water in the sandy-loam soil took place because of the predominant role of gravity rather than that of the capillary forces. Therefore, placement of drip lateral at shallow depths is recommended in onion crop to get higher yield.     

Evaluation of crop water stress index for LEPA irrigated corn

This study was designed to evaluate the crop water stress index (CWSI) for low-energy precision application (LEPA) irrigated corn (Zea mays L.) grown on slowly-permeable Pullman clay loam soil (fine, mixed, Torrertic Paleustoll) during the 1992 growing season at Bushland, Tex. The effects of six different irrigation levels (100%, 80%, 60%, 40%, 20%, and 0% replenishment of soil water depleted from the 1.5-m soil profile depth) on corn yields and the resulting CWSI were investigated. Irrigations were applied in 25 mm increments to maintain the soil water in the 100% treatment within 60–80% of the “plant extractable soil water” using LEPA technology, which wets alternate furrows only. The 1992 growing season was slightly wetter than normal. Thus, irrigation water use was less than normal, but the corn dry matter and grain yield were still significantly increased by irrigation. The yield, water use, and water use efficiency of fully irrigated corn were 1.246 kg/m², 786 mm, and 1.34 kg/m³, respectively. CWSI was calculated from measurements of infrared canopy temperatures, ambient air temperatures, and vapor pressure deficit values for the six irrigation levels. A “non-water-stressed baseline” equation for corn was developed using the diurnal infrared canopy temperature measurements as T _c–T _a = 1.06–2.56 VPD, where T _c was the canopy temperature (°C), Ta was the air temperature (°C) and VPD was the vapor pressure deficit (kPa). Trends in CWSI values were consistent with the soil water contents induced by the deficit irrigations. Both the dry matter and grain yields decreased with increased soil water deficit. Minimal yield reductions were observed at a threshold CWSI value of 0.33 or less for corn. The CWSI was useful for evaluating crop water stress in corn and should be a valuable tool to assist irrigation decision making together with soil water measurements and/or evapotranspiration models. Received: 19 May 1998     

Effect of brackish water irrigation and straw mulching on soil salinity and crop yields under monsoonal climatic conditions

Fresh water shortages are severally restricting sustainable agriculture development in the North China Plain. The scarcity of fresh water has forced farmers to use brackish water from shallow underground sources, which helps to overcome drought and increase crop yields but also increases the risk of soil salinization. To identify safe and effective ways of using brackish water in this region, field experiments were conducted to evaluate the effect of brackish water irrigation and straw mulching on soil salinity and crop yield in a winter wheat-summer maize double cropping system. The experiment was in a split-plot design. Six rates of straw mulching (0, 4.5, 6.0, 7.5, 15.0 and 30.0 Mg/ha) were assigned to the main plots and two irrigation water qualities (i.e. brackish water with salt content of 3.0-5.0 g/L and fresh water with only 1.27 g salt/L) were applied to subplots. The brackish water irrigation significantly increased the salt content at different soil depths in the upper 1 m soil layer during the two growing seasons. Straw mulching affected the vertical distribution of salt in the brackish water irrigation plots and the average salt content of straw mulch treatments (4.5, 6.0, 7.5, 15.0 and 30.0 Mg/ha) within the 0-20, 20-40 and 0-100 cm soil depths was 10.2, 14.0 and 1.8% lower than that without straw mulch (A₀). No salt accumulation occurred to a depth of 1 m in the brackish water irrigation plots and there was no correlation between the value of SAS (salt accumulated in 1 m of soil) and straw mulch rate. In 2000 and 2001, the salt content within the 0-40 cm soil layer in brackish water irrigation plots increased due to high evaporation rates during April-June, and then decreased up to September as salts were leached by rain. For the fresh water irrigation plots, the salt content remained relatively stable. Straw mulching affected the salt content in the 0-40 cm soil layer in brackish water irrigation plots in different periods of 2000 and 2001, but no correlation between salt content and straw mulch rates was observed except in September of 2000. Unlike for wheat, the yield of maize increased as the straw mulch rate increased according to the equation, y = 0.1589x + 5.3432 (R² = 0.6506). Our results would be helpful in adopting brackish water irrigation and straw mulching in ways that enhance crop yields and reduce the risk of soil salinization. However, long-term effects of brackish water irrigation and straw mulching on soil salinity and crop yield need to be further evaluated for sustainability of the system.     

Nitrate leaching in a silage maize field under different irrigation and nitrogen fertilizer rates

Quantification of the interactive effects of nitrogen (N) and water on nitrate (NO₃) loss provides an important insight for more effective N and water management. The goal of this study was to evaluate the effect of different irrigation and nitrogen fertilizer levels on nitrate-nitrogen (NO₃-N) leaching in a silage maize field. The experiment included four irrigation levels (0.7, 0.85, 1.0, and 1.13 of soil moisture depletion, SMD) and three N fertilization levels (0, 142, and 189 kg N ha⁻¹), with three replications. Ceramic suction cups were used to extract soil solution at 30 and 60 cm soil depths for all 36 experimental plots. Soil NO₃-N content of 0-30 and 30-60-cm layers were evaluated at planting and harvest maturity. Total N uptake (NU) by the crop was also determined. Maximum NO₃-N leaching out of the 60-cm soil layer was 8.43 kg N ha⁻¹, for the 142 kg N ha⁻¹ and over irrigation (1.13 SMD) treatment. The minimum and maximum seasonal average NO₃ concentration at the 60 cm depth was 46 and 138 mg l⁻¹, respectively. Based on our findings, it is possible to control NO₃ leaching out of the root zone during the growing season with a proper combination of irrigation and fertilizer management.     

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.     

Modelling soil water and salt dynamics under pulsed and continuous surface drip irrigation of almond and implications of system design

The HYDRUS-2D model was experimentally verified for water and salinity distribution during the profile establishment stage (33?days) of almond under pulsed and continuous drip irrigation. The model simulated values of water content obtained at different lateral distances (0, 20, 40, 60, 100?cm) from a dripper at 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 120, 140 and 160?cm soil depths at different times (5, 12, 19, 26 and 33?days of profile establishment) were compared with neutron probe measured values under both irrigation scenarios. The model closely predicted water content distribution at all distances, times and soil depths as RMSE values ranged between 0.017 and 0.049. The measured mean soil water salinity (ECsw) at 25?cm from the dripper at 30, 60, 90 and 150?cm soil depth also matched well with the predicted values. A correlation of 0.97 in pulsed and 0.98 in continuous drip systems with measured values indicated the model closely predicted total salts in the root zone. Thus, HYDRUS-2D successfully simulated the change in soil water content and soil water salinity in both the wetting pattern and in the flow domain. The initial mean ECsw below the dripper in pulsed (5.25?dSm^?1) and continuous (6.07?dSm^?1) irrigations decreased to 1.31 and 1.36?dSm^?1, respectively, showing a respective 75.1 and 77.6% decrease in the initial salinity. The power function [y?=?ax ^?b ] best described the mathematical relationship between salt removal from the soil profile as a function of irrigation time under both irrigation scenarios. Contrary to other studies, higher leaching fraction (6.4–43.1%) was recorded in pulsed than continuous (1.1–35.1%) irrigation with the same amount of applied water which was brought about by the variation in initial soil water content and time of irrigation application. It was pertinent to note that a small (0.012) increase in mean antecedent water content (θ _i ) brought about 8.25–9.06% increase in the leaching fraction during the profile establishment irrespective of the emitter geometry, discharge rate, and irrigation scenario. Under similar θ _i , water applied at a higher discharge rate (3.876?Lh^?1) has resulted in slightly higher leaching fraction than at a low discharge rate (1.91?Lh^?1) under pulsing only owing to the variation in time of irrigation application. The influence of pulsing on soil water content, salinity distribution, and drainage flux vanished completely when irrigation was applied daily on the basis of crop evapotranspiration (ETc) with a suitable leaching fraction. Therefore, antecedent soil water content and scheduling or duration of water application play a significant role in the design of drip irrigation systems for light textured soils. These factors are the major driving force to move water and solutes within the soil profile and may influence the off-site impacts such as drainage flux and pollution of the groundwater.     

Distribution and dynamics of soil water and salt under different drip irrigation regimes in northwest China

A 2-year experiment was carried out to investigate the effects of different drip irrigation regimes on distribution and dynamics of soil water and salt in north Xinjiang, China. Five treatments—F7 (0.24 dS m^?1 + Once every 7 days), B7 (4.68 dS m^?1 + Once every 7 days), S7 (7.42 dS m^?1 + Once every 7 days), F10 (0.24 dS m^?1 + Once every 10 days) and F3 (0.24 dS m^?1 + Once every 3 days)—were designed. For all treatments, additional 150-mm fresh water was applied on 10th November in 2009 (winter irrigation) to leach the accumulated salt. The results revealed that irrigation frequency and water quality had significant effects on the spatial distribution and change of soil water content, soil salt and the crop water consumption rate, but had a limited impact on the seasonal accumulative water consumption, and the cotton yield decreased with the decrease in irrigation frequency and water quality on the whole. During the cotton growing season, results showed that the salt mainly accumulated in the 0- to 60-cm soil layer, while the soil salt in 60- to 100-cm layer changed slightly, indicating that the drip irrigation could not leach the soil salt out of the root zone under the irrigation regimes. Therefore, salt leaching was necessary to maintain the soil water–salt balance and to prevent excessive salt accumulation in the root zone. After the 150-mm winter irrigation and subsequent thawing, soil salts were leached into the deeper layers (below 60 cm), and the soil salt content (SSC) (EC_1:5) in root zone in the next year was about 0.2 dS m^?1. Moreover, compared to 2009 season, the SSC within the root zone did not increase even the EC of the irrigation water was up to 7.42 dS m^?1. Additionally, it is important to note that the results were concluded based on the data of the 2-year experiment; further studies are need to optimize winter irrigation amount and assess the sustainability of saline water irrigation since long-term utilization of saline water may lead to soil degradation.     

Evapotranspiration and water use of full and deficit irrigated cotton in the Mediterranean environment in northern Syria

Cotton (Gossypium hirsutum L.) is the most important industrial and summer cash crop in Syria and many other countries in the arid areas but there are concerns about future production levels, given the high water requirements and the decline in water availability. Most farmers in Syria aim to maximize yield per unit of land regardless of the quantity of water applied. Water losses can be reduced and water productivity (yield per unit of water consumed) improved by applying deficit irrigation, but this requires a better understanding of crop response to various levels of water stress. This paper presents results from a 3-year study (2004-2006) conducted in northern Syria to quantify cotton yield response to different levels of water and fertilizer. The experiment included four irrigation levels and three levels of nitrogen (N) fertilizer under drip irrigation. The overall mean cotton (lint plus seed, or lintseed) yield was 2502 kg ha⁻¹, ranging from 1520 kg ha⁻¹ under 40% irrigation to 3460 kg ha⁻¹ under 100% irrigation. Mean water productivity (WP_ET) was 0.36 kg lintseed per m³ of crop actual evapotranspiration (ET_c), ranging from 0.32 kg m⁻³ under 40% irrigation to 0.39 kg m⁻³ under the 100% treatment. Results suggest that deficit irrigation does not improve biological water productivity of drip-irrigated cotton. Water and fertilizer levels (especially the former) have significant effects on yield, crop growth and WP_ET. Water, but not N level, has a highly significant effect on crop ET_c. The study provides production functions relating cotton yield to ET_c as well as soil water content at planting. These functions are useful for irrigation optimization and for forecasting the impact of water rationing and drought on regional water budgets and agricultural economies. The WP_ET values obtained in this study compare well with those reported from the southwestern USA, Argentina and other developed cotton producing regions. Most importantly, these WP_ET values are double the current values in Syria, suggesting that improved irrigation water and system management can improve WP_ET, and thus enhance conservation and sustainability in this water-scarce region.     

Assessment and management of poor quality waters for crop production: A simulation model (SWAM)

A simulation model has been prepared for assessing water quality to judge its suitability for irrigation. When water is classified as poor quality water (saline/sodic/saline-sodic) utilizing standard norms for Indian agro-climatic conditions, the model determines the potential of the water for direct application. Further, it also evaluates management strategies based on conjunctive use of fresh and saline waters. For this purpose, the model requires water quality data, crop data, soil data and rules established in the expert system rule-base. Data are compiled in data files which can be updated. For conjunctive use of saline and fresh waters, an irrigation scheduling sub-model has been modified to include a soil salinization-desalinization module based on layer-wise equilibrium theory. The module was independently tested using field data. The model SWAM has been successfully tested using data from a number of field experiments. Sodic waters of 16.2 meq l⁻¹ residual sodium carbonate would require 2.73 t ha⁻¹ of gypsum for each 20 cm of water applied to the soil. Field observations usually attest to this requirement. Likewise, saline water of 16 dS m⁻¹, when applied to a wheat crop in conjunction with fresh water of 0.5 dS m⁻¹, would yield optimally in case two saline water irrigations are followed by one fresh water irrigation in a normal rainfall year with an initial soil salinity of 2.98 dS m⁻¹. Some more useful data sets are analyzed and compared with results from field experiments. In our opinion, the model which is based upon recent guidelines can be applied to the classification of waters and their management. The minor changes necessary to apply the model to other conditions can be easily carried out.