Water use and distribution profile under pulse and oilseed crops in semiarid northern high latitude areas

Oilseed and pulse crops have been increasingly used to replace conventional summer fallow and diversify cropping systems in northern high latitude areas. The knowledge of water use (WU) and its distribution profile in the soil is essential for optimizing cropping systems aimed at improving water use efficiency (WUE). This study characterized water use and distribution profile for pulse and oilseed crops compared to spring wheat (Triticum aestivum L.) in a semiarid environment. Three oilseeds [canola (Brassica napus L.), mustard (Brassica juncea L.) and flax (Linum usitatissimum L.)], three pulses [chickpea (Cicer arietinum L.), dry pea (Pisum sativum L.) and lentil (Lens culinaris Medik.)], and spring wheat were seeded in removable 100 cm deep × 15 cm diameter lysimeters placed in an Aridic Haploboroll soil, in southwest Saskatchewan in 2006 and 2007. Crops were studied under rainfed and irrigated conditions where lysimeters were removed and sampled for plant biomass and WU at various soil depths. Wheat yields were greater than pulse crop yields which were greater than oilseed yields, and WUE averaged 4.08 kg ha⁻¹ mm⁻¹ for pulse crops, 3.64 kg ha⁻¹ mm⁻¹ for oilseeds, and ranged between 5.5 and 7.0 kg ha⁻¹ mm⁻¹ for wheat. Wheat used water faster than pulse and oilseed crops with crop growth. Pulse crops extracted water mostly from the upper 60 cm soil depths, and left more water unused in the profile at maturity compared to oilseeds or wheat. Among the three pulses, lentil used the least amount of water and appeared to have a shallower rooting depth than chickpea and dry pea. Soil WU and distribution profile under canola and mustard were generally similar; both using more water than flax. Differences in WU and distribution profile were similar for crops grown under rainfall and irrigation conditions. A deep rooting crop grown after pulses may receive more benefits from water conservation in the soil profile than when grown after oilseed or wheat. Alternating pulse crops with oilseeds or wheat in a well-planned crop sequence may improve WUE for the entire cropping systems in semiarid environments.     

Crop rotations in Argentina: Analysis of water balance and yield using crop models

Cropping schemes have developed in east-central Argentina for rainfed soybean (Glycine max Merr.) production that invariably employ no-tillage management. Often these schemes include growing soybean in a sequence of crops including wheat (Triticum aestivum L.) and maize (Zea mays L.). The full impact of various rotation schemes on soil water balance through a sequence of seasons has not been explored, although the value of these rotations has been studied experimentally. The objective of this work was to investigate through simulations, potential differences in temporal soil water status among rotations over five years. In this study, mechanistic models of soybean (Soy), maize (Maz), and wheat (Wht) were linked over a five-years period at Marcos Juárez, Argentina to simulate soil water status, crop growth, and yield of four no-till rotations (Soy/Soy, Soy/Wht, Soy/Maz, and Soy/Maz/Wht). Published data on sowing dates and initial soil water contents in the first year from a no-till rotation experiment were used as inputs to the model. After the first year, soil water status output from the model was used to initiate the next crop simulation in the sequence. The results of these simulations indicated a positive impact on soil water balance resulting from crop residue on the soil surface under no-till management. Continuous soybean and the two-year soybean/maize rotation did not efficiently use the available water from rainfall. Residue from maize was simulated to be especially effective in suppressing soil evaporation. Thus, the Soy/Maz simulation results indicated that this rotation resulted in enhanced soil water retention, increased deep water percolation, and increased soybean yields compared with continuous soybean crops. The simulated results matched well with experimental observations. The three-crop rotation of Soy/Maz/Wht did not increase simulated soybean yields, but the additional water retained as a result of decreased soil evaporation resulting from the maize residue allowed the addition of a wheat crop in this two-year rotation. Simulated soybean yields were poorly correlated with both the amount of soil water at sowing and the rainfall during the cropping period. These results highlight the importance of temporal distribution of rainfall on final yield. These models proved a valuable tool for assessing the consequences of various rotation schemes now being employed in Argentina on temporal soil water status, and ultimately crop yield.     

Crop yield and soil water restoration on 9-year-old alfalfa pasture in the semiarid Loess Plateau of China

The alfalfa pastureland in the semiarid Loess Plateau region of Northwest China usually has dry soil layers. A field experiment was conducted from October 2000 to October 2004 to examine soil water recovery and crop productivity on a 9-year-old alfalfa pasture. This experiment included six treatments: alfalfa pasture for 10-14 years, a conventional farming system without prior alfalfa planting, and four alfalfa-crop rotation treatments. For the rotation treatments, after 9 years of alfalfa selected crops were planted from 2001 to 2004 in the following sequence: (1) millet, spring wheat, potatoes, peas; (2) millet, corn, corn, spring wheat; (3) millet, potatoes, spring wheat, corn; (4) millet, fallow, peas, potatoes. The results showed that dry soil layers occurred in alfalfa pasture. We then plowed the alfalfa pasture and planted different crops. The soil water gradually increased during crop growth in the experimental period. The degree of soil water recovery in the four alfalfa-crop rotation treatments was derived from comparison with the soil water in the conventional system. After 4 years, the soil water recovery from the alfalfa-crop rotation systems at 0-500 cm soil depth was 90.5%, 89.8%, 92.2% and 96.7%, respectively. Soil total N content and soil respiration rate were high in the alfalfa-crop rotation systems. The yields of spring wheat in 2002, peas in 2003 and potatoes in 2004 in the alfalfa-crop rotation systems were not significantly different from yields in the conventional system. In the alfalfa-crop rotation systems, the yields of spring wheat and peas were greatly influenced by rainfall and were lowest in the dry year of 2004; the yields of corn and potatoes had a direct relationship with water use and were lowest in 2003. In summary, soil water in dry soil layers can recover, and crop yields in the alfalfa-crop systems were equal to those of the conventional system.     

Recent changes in the climatic yield potential of various crops in Europe

Recent changes in the simulated potential crop yield and biomass production caused by changes in the temperature and global radiation patterns are examined, using the Crop Growth Monitoring System. The investigated crops are winter wheat, spring barley, maize, winter rapeseed, potato, sugar beet, pulses and sunflower. The period considered is 1976-2005. The research was executed at NUTS2 level. Maize and sugar beet were the crops least affected by changing temperature and global radiation patterns. For the other crops the simulated potential yield remained stable in the majority of regions, while decreasing trends in simulated potential yields prevailed in the remaining regions. The changes appear in a geographical pattern. In Italy and southern central Europe, temperature and radiation change effects are more severe than elsewhere, in these areas potential crop yields of more than three crops significantly decreased. In the UK and some regions in northern Europe the yield potential of various crops increased.In a next step the national yield statistics were analyzed. For a large majority of the countries the yield increases of wheat, barley and to a lesser extent rapeseed are leveling off. Several explanations could be given, however, as the simulated yield potential for these crops decreased in various regions, the changing temperature and radiation patterns may also contribute to the diminishing yield increases or to the stagnation. In more than 50% of the investigated countries the maize, potato and sugar beet yields continue to increase. This can be attributed to improving production techniques, new crop varieties, sometimes in combination with an improving climatic potential. In some regions in northern Europe, yields continue to increase.     

Salinity effect on crop development and yield,analysis of salt tolerance according to several classification methods

The publication is a synthesis of previous publications on the results of a long-term lysimeter experiment. From 1989 to 1998, the experimental variables were soil salinity and soil type, from 1999 onwards, soil salinity and crop variety. The plant was studied during the whole growing period by measuring the saline stress and analyzing its effect on leaf area and dry matter development and on crop yield. Salinity affected the pre-dawn leaf water potential, stomatal conductance, evapotranspiration, leaf area and yield.The following criteria were used for crop salt tolerance classification: soil salinity, evapotranspiration deficit, water stress day index. The classification according to soil salinity distinguished the salt tolerant group of sugar beet and wheat, the moderately salt sensitive group comprising broadbean, maize, potato, soybean, sunflower and tomato, and the salt sensitive group of chickpea and lentil. The results for the salt tolerant and the moderately salt sensitive groups correspond with the classification of Maas and Hoffman, excepted for soybean.The evapotranspiration deficit criterion was used, because for certain crops the relation between yield and evapotranspiration remains the same in case of drought and salinity. This criterion, however, did not appear useful for salt tolerance classification.The water stress day index, based on the pre-dawn leaf water potential, distinguished a tolerant group, comprising sugar beet, wheat, maize, sunflower and potato, and a sensitive group, comprising tomato, soybean, broadbean, chickpea and lentil. The classification corresponds with a difference in water use efficiency. The tolerant crops show a more or less constant water use efficiency. The sensitive crops show a decrease of the water use efficiency with increasing salinity, as their yield decreases stronger than the evapotranspiration. No correlation could be found between osmotic adjustment, leaf area and yield reduction. As the flowering period is a sensitive period for grain and fruit formation and the sensitive crops are all of indeterminate flowering, their longer flowering period could be a cause of their greater sensitivity.The tolerant group according to water stress day index can be divided according to soil salinity in a salt tolerant group of sugar beet and wheat and a moderately sensitive group, comprising maize, sunflower and potato. The difference in classification can be attributed to the difference in evaporative demand during the growing period.The sensitive group according to water stress day index can be divided according to soil salinity in a moderately sensitive group, comprising tomato, soybean and broadbean, and a salt sensitive group of chickpea and lentil. The difference in classification can be attributed to the greater salt sensitivity of the symbiosis between rhizobia and grain legume in the case of chickpea and lentil.     

河西绿洲灌区主要作物需水量及作物系数试验研究

利用Penman-Monteith公式计算了甘肃张掖绿洲主要作物各生育期参考作物蒸散量,利用农田水量平衡方程及土壤水分胁迫系数计算了作物实际蒸发蒸腾量,并计算比较了充分灌溉和非充分灌溉条件下不同生育期作物需水特征,确定了非充分灌溉条件下主要作物的作物系数。结果表明,非充分灌溉条件下,主要作物各生育期需水规律和充分灌溉具有一致变化趋势。非充分灌溉条件下,小麦、玉米、马铃薯全生育期作物系数平均值分别为0.81、0.7和0.73。在全生育期当中,随生育期的延续,主要作物叶面蒸腾比例逐渐增大,棵间蒸发逐渐减少。     

内蒙古河套灌区主要粮油作物系数的确定

根据FAO-56作物需水量指南推荐的作物系数计算方法,分别计算内蒙古河套灌区磴口试验站小麦、玉米(覆膜与未覆膜)、油料向日葵3种作物的单、双值作物系数。结果表明,磴口试验站单值作物系数较双值作物系数值计算的ET值偏低,双值作物系数计算的ET值与实测结果更为接近,验证了双值作物系数在河套灌区的适用性。     

A new look at an old practice: Benefits from soil water accumulation in long fallows under Mediterranean conditions

The practice of long fallowing, by omitting a year of cropping, is gaining renewed focus in the low rainfall zone of the northern agriculture region of Western Australia. The impetus behind this practice change has been a reduced use of pasture breaks in cereal crop rotations, and the belief that a fallow can improve soil water accumulation and thus buffer the negative effects of dry seasons on crop yields. We evaluated the benefits of long fallowing (full stubble retention, no weed growth allowed) in a continuous wheat sequence via simulation modelling with APSIM at two rainfall locations and five soil types. The simulated benefits to long fallowing were attributable to soil water accumulation only, as the effects on soil nitrogen, diseases or weeds were not evaluated.The long-term (100 years) mean wheat yield benefit to fallowing was 0.36-0.43 t/ha in clay, 0.20-0.23 t/ha in sand and loam, and 0-0.03 t/ha in shallow sand and shallow loams. Over the range of seasons simulated the response varied from −0.20 to 3.87 t/ha in the clay and −0.48 to 2.0 t/ha for the other soils. The accumulation of soil water and associated yield benefits occurred in 30-40% of years on better soils and only 10-20% on poorer soils. For the loam soil, the majority of the yield increases occurred when the growing-season (May-September) rainfall following the fallow was low (<210 mm) and the difference in plant available soil water at sowing between fallowed and continuously cropped soil was high (>30 mm), although yield increase did occur with other combinations of growing-season rainfall and soil water. Over several years of a crop sequence involving fallow and wheat, the benefits from long fallowing due to greater soil water accumulation did not offset yield lost from omitting years from crop production, although the coefficient of variation for inter-annual farm grain production was reduced, particularly on clay soils during the 1998-2007 decade of below-average rainfall. We conclude that under future drying climates in Western Australia, fallowing may have a role to play in buffering the effects of enhanced inter-annual variability in rainfall. Investigations are required on the management of fallows, and management of subsequent crops (i.e. sowing earlier and crop density) so as to maximise yield benefits to subsequent crops while maintaining groundcover to prevent soil erosion.     

Response of wheat to irrigation with small amounts of water applied in various ways

In dry land agriculture, crop failure is often due to dry seed beds which hamper crop stand establishment and root growth. Irrigation with small amounts of water may make the difference between success and failure of the crop. A 2-year field study on a sandy loam soil evaluated the response of wheat to irrigation with 4—60 mm water applied in various ways. An amount of 4 or 6 mm water applied in the furrow enhanced above-ground plant growth, root growth, grain yield and water use considerably compared to the same amount of water surface applied to seeded rows. With surface irrigation of 40 or 60 mm water, crop growth and grain yield increased further; however, a significantly higher grain yield was obtained with post-seeding rather than pre-seeding irrigation.     

微咸水灌溉对作物生长影响的试验研究

在田间试验条件下,采用正常灌溉定额和淋洗灌溉定额2种灌溉水平,对小麦、玉米、葵花3种作物进行了不同灌溉水矿化度处理的灌溉试验,对3种作物的生长过程、地上部干物质积累规律、作物产量等进行了研究。结果表明:小麦在2种灌溉定额下的耐盐能力为4.5 g/L。正常定额下玉米的耐盐能力为3 g/L,葵花为5 g/L。淋洗定额下玉米的耐盐能力为3.5 g/L,葵花为7 g/L。     

Effects of soil tillage and fallow management on soil water storage and sunflower production in a semi-arid Mediterranean climate

Normally sown in March in the region of Meknès (Morocco), rainfed sunflower suffers from a severe water deficit from anthesis which seriously affects grain filling. Increasing the stored soil water by appropriate management during the long period of bare soil preceding sunflower planting could be an opportunity which has not been explored for this spring-sown crop.Five methods for autumn soil tillage (mouldboard ploughing, chiselling, paraploughing, disc harrowing, no tillage) and four fallowing methods (chemical weed control, mechanical weeding, allowing weeds and volunteer crops, sowing barley) were compared in Meknès between 1994 and 1998 on calcimagnesic soils with vertic behaviour. Two additional experiments were carried out in 1997 and 1998 to create a range of leaf area indexes and transpiration requirements for sunflower. This was obtained (i) in 1997, by four levels of plant density (2.5–10.0 plants/m²) and three levels of soil water at planting (89, 37, and 29% of total available soil water); (ii) in 1998, by six levels of sunflower defoliation at star bud stage. Simulations with the EPIC-Phase model were performed to explore a wider range of weather conditions (1960–1998) than experienced.The differences in water storage at planting were explained partly by the mode of action of each of the implements tested and partly by the weather conditions which prevailed during the fallow period. After a very dry fallow period (with a frequency less than 1 year in 10), water storage was maximal after disc harrowing and paraploughing (including straw mulching) because soil layers were only marginally exposed to evaporation. Conversely, in a year with a wet fallow period (with a frequency of 4 years in 10), mouldboard and chisel ploughing gave the largest water reserves at planting because of better infiltration at depth with increased porosity. When the fallow period was initially wet, but dry in early spring (with a frequency of 2 years in 10), minimum and no tillage gave the best water storage but the differences between tillage methods were small. In spite of differences in soil water content at planting and clear differences in rooting systems, sunflower yield and seasonal water use were not significantly affected by soil tillage provided that the plant population was the same and weed control was adequate in reduced tillage systems. However, chisel ploughing was a good compromise for maximising stored water at sunflower planting on the clay soils of Meknès.Surprisingly, maximizing soil water content at sunflower planting was not systematically the best solution for maximizing sunflower yield and water use efficiency under the semi-arid conditions of Meknès. A high soil water content at planting leads to excessive leaf area index at the bud stage and consequently to rapid water depletion and yield reduction, especially when seasonal precipitation is low. A 50% refilling of the soil water reserve is sufficient for spring-sown sunflower as was confirmed by the simulation study. Soil moisture in the uppermost layer which governs seedling establishment is a more limiting factor for sunflower yield than total soil water content at planting.     

Conjunctive use of saline and non-saline irrigation waters in semi-arid regions

In arid and semi-arid regions, effluent from sub-surface drainage systems is often saline and during the dry season its disposal poses an environmental problem. A field experiment was conducted from 1989 to 1992 using saline drainage water (EC=10.5–15.0 dS/m) together with fresh canal water (EC=0.4 dS/m) for irrigation during the dry winter season. The aim was to find if crop production would still be feasible and soil salinity would not be increased unacceptably by this practice. The experimental crops were a winter crop, wheat, and pearl-millet and sorghum, the rainy season crops, grown on a sandy loam soil. All crops were given a pre-plant irrigation with fresh canal water. Subsequently, the wheat crop was irrigated four times with different sequences of saline drainage water and canal water. The rainy season crops received no further irrigation as they were rainfed. Taking the wheat yield obtained with fresh canal water as the potential value (100%), the mean relative yield of wheat irrigated with only saline drainage water was 74%. Substitution of canal water at first post-plant irrigation and applying thereafter only saline drainage water, increased the yield to 84%. Cyclic irrigations with canal and drainage water in different treatments resulted in yields of 88% to 94% of the potential. Pearl-millet and sorghum yields decreased significantly where 3 or 4 post-plant irrigations were applied with saline drainage water to previous wheat crop, but cyclic irrigations did not cause yield reduction. The high salinity and sodicity of the drainage water increased the soil salinity and sodicity in the soil profile during the winter season, but these hazards were eliminated by the sub-surface drainage system during the ensuing monsoon periods. The results obtained provide a promising option for the use of poor quality drainage water in conjunction with fresh canal water without undue yield reduction and soil degradation. This will save the scarce canal water, reduce the drainage water disposal needs and associated environmental problems.     

不同作物种植条件下微咸水灌溉的土壤环境效应试验研究

在田间试验条件下,采用正常灌溉定额和淋洗灌溉定额2种灌溉水平,对小麦、葵花、玉米3种作物进行了不同灌溉水浓度处理的灌溉试验,对3种作物种植条件下土壤水盐运移规律进行了研究。研究发现灌溉水浓度达到某一临界值时,盐分将在土壤剖面一定深度的土层内聚集明显增大。且不同作物的临界灌水浓度值不同。     

Studies on some land management practices for increasing agricultural production in Vertisols of Central India

In Central India a large area of Vertisols is kept fallow during the rainy season and only one crop is raised in post-rainy season on profile stored soil moisture. It shows under utilization of potential cropping in these soils which are inherently very productive. Thus, to develop land and water management practices for raising two crops in a year, a field experimentation was carried out in deep Vertisols of high rainfall areas at J.N. Agricultural University Experiment Station, Jabalpur during 1979–1991.Soybean (Glycine max. L. Merrill) was grown in rainy season in the raised beds of 6, 9, 12 and 15 m width and 0.30 to 0.35 m in height, whereas, paddy (Oryza sativa L.) was grown in the sunken beds of 6 m width running in parallel to the raised beds. In post-rainy season, chickpea (Cicer arietinum L.), linseed (Linum usitatissimum L.) and safflower (Carthamus tinctorius L.) were grown in raised beds and wheat (Triticum aestivum L.) in sunken beds.Performance of soybean improved significantly when planted in raised beds instead of using the normal flat planting. Maximum average seed yield of soybean was recorded in 6 m wide raised bed followed by 9 m raised bed and minimum in flat plots. Besides providing adequate surface drainage to soybean crop, the land configurations were also useful during prolonged dry-spell thereby, minimizing any adverse effect of soil moisture stress at flowering and seed development stages of rainy season crops. Post-rainy season crops were successfully grown with sufficiently high yields. Soybeansafflower and soybean-chickpea cropping sequences proved to be most economic than the other sequences. Economic viability analysis of raised-sunken bed (RSB) system indicated that the net return was Rs.13363 ha⁻¹ annum⁻¹ as against Rs.1003 ha⁻¹ from soybean grown under farmer's practice.The field evaluation of RSB system clearly suggests that the system fulfils the aims of optimizing the physical environment in the field by minimizing run-off and erosion, and ensures adequate water storage in sunken beds and aeration around the plant roots of upland crops like soybean. This provides the favourable condition to grow crops in rainy as well as in post-rainy season. For Vertisols of dependable high rainfall ( > 1000 mm) areas, the raised beds of 6 to 9 m width and 0.35 m height along with sunken beds of 6 m width are recommended. Field operations like tillage by bullock/tractor drawn implements are feasible. A good performance of soybean and paddy in rainy season and chickpea, safflower and linseed in post-rainy season has demonstrated the potential of RSB system for raising agricultural production in Vertisols of high rainfall areas.     

小麦免耕播种机驱动双向螺旋刀开沟防堵装置

针对河北省一年两熟地区小麦免耕播种机存在的问题,提出利用驱动双向螺旋刀作为向种行两侧推开秸秆和切断种行上玉米根茬的开沟装置,通过分析确定了双向螺旋刀、弧形种肥开沟器的主要参数和双向螺旋刀在刀轴上的排列方案。田间试验表明,驱动双向螺旋刀具有较强的向种行两侧推开秸秆的能力,能够有效地切断或切碎种行上的玉米根茬,安装双向螺旋刀与弧形种肥开沟器的小麦免耕播种机通过性能较好;施肥深度为8．56cm,播种深度为4．3cm;出苗情况表明作业质量能够满足小麦播种要求。     

Postharvest residual soil nutrients and yield of spring wheat under water deficit in arid northwest China

In areas where two crops are grown per year or three crops every 2 years, the status of residual soil nutrients after the harvest of the first crop is critical to the crop to be grown immediately after, while the postharvest soil nutrient status can be influenced by irrigation applied to the test crop. This study determined the effect of various soil water treatments applied to the test crop on the status of postharvest residual soil nutrient pools in an arid environment. Spring wheat (Triticum aestivum L.) was grown as test crop under conditions of full- (as control), high-, moderate-, and low-water conditions during jointing, booting-heading, and grain filling stages, in 2003 and 2004. Compared to the control, grain yield and water use efficiency (WUE) were significantly increased by subjecting the wheat crop to moderate-water conditions during various growth stages, and low-water conditions at jointing stage in both years. Soil C at harvest decreased linearly with increased grain yield of the test crop. Moderate- to high-water conditions during jointing stage resulted in 12-24% greater soil C in the top 40 cm depth in 2003, with a marginal difference in 2004. Water treatments impacted the status of residual soil nutrients in 2003; soil total N and available soil P in the top 40 cm depth were significantly higher in low- to moderate-water treatments compared to the control, while in 2004 significantly higher total N and P, available N, P and K were found only in the top 20 cm depth. Increased yield of wheat test crop with moderate-water resulted in increased postharvest residual soil nutrients, whereas the ratios of C/N, C/P, and C/K were largely influenced by years and were less related to water treatments. We conclude that the determination of postharvest soil C and nutrient elements may provide useful information in monitoring potential changes of soil nutrient status over time in the intensified cropping systems, and that the recommendation of fertilization for the crop to be grown immediately following the first crop can be established by simply analyzing the productivity of the first crop without intensive measurements of soil nutrients.     

AquaCrop作物模型在松嫩平原春麦区的校正和验证

为了研究AquaCrop作物模型在松嫩平原春麦区的适用性,利用实测的土壤水分、春小麦生长和产量数据,结合气象数据,获得AquaCrop模拟土壤水分和春小麦生长的模型参数,并用往年的作物生长数据对模型进行验证。结果表明,春小麦的产量和生物量的实测值与模拟值的绝对平均误差(MAE)为0.058和0.109、均方根误差(RMSE)为0.06和0.11t/hm2、模拟性能指数(EF)为0.795和0.822、残差聚集系数(CRM)为-0.006 96和0.005 87、一致性系数(IoA)为0.959和0.966;对10cm和20cm土壤体积含水率的实测值与模拟值的MAE为5.23和2.53、RMSE为6.47%和7.95%、EF为-0.277和-0.069、CRM为0.097和0.212、IoA为0.585和0.741。说明AquaCrop模型对春小麦的生物量和产量及生育期土壤体积含水率的模拟结果总体较好,对松嫩平原春麦生产有一定的参考意义。     

Application of a simulation based optimization model for winter wheat irrigation scheduling in North China

Irrigation needs to be scheduled properly for winter wheat, the main food crop in North China where the water resources are limited. We optimized the irrigation timing of crops under limited water supply by integrating a soil water balance model, dated water production function with cumulative function of water sensitivity index, and a nonlinear search method. The optimization produced the optimal irrigation date series with the predetermined irrigation quota for each application, which aims to obtain higher crop yield with limited irrigation water and be convenient for irrigation management. This simulation–optimization model was used to investigate the irrigation scheduling of winter wheat in Xiaohe irrigation Area in North China. Results show that optimal irrigation date series, corresponding relative yield and relative evapotranspiration are all closely related to the irrigation quota and initial soil water conditions. For rich and medium initial soil water conditions in medium precipitation year, it takes four times of irrigation (60 mm each time) after greening in order to obtain higher crop yield. But it increases to five times for poor initial condition. With limited irrigation water, irrigation should generally be applied in the preferential sequence of early May or late April (in the jointing stage), then mid and late May (in the heading stage), and finally March (in the greening stage). Irrigation should be applied earlier with lower initial soil water storage. Higher irrigation quota increases the crop yield but tends to decrease the marginal value, especially when irrigation quota exceeds 180 mm. The study also indicates that the optimized relative yield is generally higher than that obtained in field experiment. Based on the optimization, we proposed to use the quadratic polynomial function to describe the frontier water production function, which shows the mathematical relationship between optimized relative yield and relative evapotranspiration.     

Modeling nitrogen cycling in tomato–safflower and tomato–wheat rotations

The use of crop models to simulate the nitrogen (N) cycle in crop rotations is of major interest because of the complexity of processes that simultaneously interact. We studied the performance of the Erosion Productivity Impact Calculator (EPIC) model in simulating the N cycle in two different rotations under irrigation: tomato (Lycopersicon esculentum Mill.)–safflower (Carthamus tinctorius L.) and tomato–wheat (Triticum aestivum L.). Processing tomatoes were grown on raised beds and furrow irrigated in 1994 in the Sacramento Valley of California, USA. Safflower and wheat were grown in 1995 and 1994–95, respectively, after the previous tomato crop. A data set from safflower grown on different plots in 1994 was used to calibrate the model for this crop. The model accurately predicted the yield, biomass and N uptake of the crops in the rotation. Soil inorganic N was also accurately simulated in the two rotations. The model predicted important amounts of N leached during the winter period of 1994–95 due to the heavy rainfall. The model was used to explore the influence of rotation type (tomato–safflower or tomato–wheat) and irrigation type (fixed amounts and dates or flexible automatic irrigation). Simulation results of the two rotations during 10 years (1986–95) predicted average losses by leaching higher than 200 kg N ha⁻¹ for each rotation period, irrespective of the rotation type. Losses were more important during the fall–winter and increased as rainfall increased above a threshold rainfall of 300 mm. The flexible automatic irrigation resulted in lower N leached during the tomato crop season. Simulation results indicated that a fallow period during the fall–winter following processing tomatoes should be avoided because of the high risk of N leaching losses. The introduction in the rotation of a deep-rooted crop, such as safflower, grown with low irrigation, drastically reduced the risk of N leaching during the following fall–winter period, without substantial yield reductions.     

Water-use efficiency of wheat-based rotation systems in a Mediterranean environment

Crop production in Mediterranean-type environments is invariably limited by low and erratic rainfall (200-600 mm year⁻¹), and thus soil moisture, and by high evapotranspiration resulting from high temperature. Consequently, a major research challenge is to devise cropping systems that maximize water-use efficiency (WUE). In a long-term trial in northern Syria (1986-1998) we compared the effects of seven wheat-based rotations on soil water dynamics and WUE in both the wheat and non-wheat phase. The cropping systems were durum wheat (Triticum turgidum L.) in rotation with fallow, watermelon (Citrullus vulgaris), lentil (Lens culinaris), chickpea (Cicer arietinum), vetch (Vicia sativa), medic pasture (Medicago spp.), and wheat. Seasonal recharge/discharge were identified using the neutron probe. Depth of wetting varied with seasonal rainfall (233-503 mm). Based on crop yields, WUE was calculated for each cropping option in relation to the durum wheat crop.The greatest limitation to growth was the supply of water and not the soil moisture storage potential. Wheat grain yield was dictated by the extent to which the alternative crops in the rotation dried out the soil profile, in addition to seasonal rainfall and its distribution. Chickpea and medic extracted as much water as continuous wheat. Wheat after these crops was solely dependent on current seasonal rainfall, but fallow, lentil, watermelon, and vetch did not deplete soil moisture to the same extent, leaving some residual soil moisture for the succeeding wheat crop. This difference in soil water resulted in a significant difference in wheat yield and hence WUE, which decreased in the following crop rotation sequence: fallow, medic, lentil, chickpea, and continuous wheat. However, on the system basis, the wheat/lentil or wheat/vetch systems were most efficient at using rainfall, producing 27% more grain than the wheat/fallow, while the wheat/chickpea system was as efficient as wheat/fallow system, with continuous wheat being least efficient. With N added to the cereal phase, system WUE of the system increased, being least for continuous wheat and greatest for wheat/lentil. Wheat-legume rotation systems with additional N input in the wheat phase not only can maintain sustainable production system, but also are more efficient in utilizing limited rainfall.