The Use of Saline Water in Agriculture in the Near East and North Africa Region: Present and Future

SUMMARY

Salinity is a major problem that negatively impacts agricultural activities in many regions in the world, and especially the Near East and North Africa region. Generally, salinity problems increase with increasing salt concentration in irrigation water. Crop growth reduction due to salinity is generally related to the osmotic potential of the root-zone soil solution. This will lead to certain phenological changes and substantial reduction in productivity. Salinity also affects the soil physical properties. Sewage treated wastewater is an alternative water source for irrigation. Using such wastewater will provide a new water resource to expand agricultural activities as well as reduce the environmental pollution. Each country in the region has a unique system of rules and regulations to protect the quality of water resources. Important aspects that should be taken into account when using wastewater for irrigation are discussed, including some information on the different irrigation systems used in the region, and the factors leading to success of using saline water for economic crop production. Information on the use of saline water or marginal saline soils for wheat production and improvement of irrigation systems, including old land irrigation systems, under Egyptian conditions is also presented. The regional experiences and the future prospects of using saline water for crop production that vary greatly among countries in the Near East and North Africa region are summarized. This article also presents information on special cultivation methods, such as protected agriculture and soilless culture that can help in alleviating the salinity effects. Finally, the article includes some examples on the inherited knowledge for saline agriculture that conveys the grower's experience in the Near East and North Africa region where several living examples for unique and sustainable cultivation system are still in operation. One of the most impressive cultivation techniques for bio-saline crop management in Egypt is the Edkawy production system.     

Long-Term Reuse of Drainage Waters of Varying Salinities for Crop Irrigation in a Cotton-Safflower Rotation System in the San Joaquin Valley of California—A Nine Year Study: II. Safflower (Carthamus tinctorius L.)

Summary

This paper reports the results on safflower crops grown in a nine-year study, conducted on a 8 ha site, to determine the feasibility of using drainage water for irrigation in a 2-year cotton/1-year safflow-er rotation system. The cotton crops were irrigated with waters of 400, 1,500, 3,000, 4,500, 6,000, and 9,000 ppm total dissolved salts, and safflower was grown only with a preplant irrigation with nonsaline water. The use of drainage water for crop irrigation may be a means of decreasing its volume. Even though safflower was never irrigated with saline drainage water directly, the residual effect of using saline water for cotton irrigation adversely impacted safflower growth and development. Safflower seed yields were reduced in plots previously irrigated with waters of 4,500 ppm or higher salinity and even more severe effects on crop growth were seen as the numbers of years of irrigation with the saline water increased. After irrigating six cotton crops, the safflower seed yield in plots irrigated with 9,000 ppm waters was reduced to only 14% of the control. The safflower oil content and quality were not affected. Impacts on plant density, shoot height, shoot biomass, and leaf ionic content also are discussed.     

Field Crop Production in Areas with Saline Soils and Shallow Saline Groundwater in the San Joaquin Valley of California

SUMMARY

Salinity in soil and water is irrevocably associated with irrigated agriculture throughout the world and as a result requires that salt management becomes an integral part of the production system. With careful water management, it is possible to sustain irrigated agriculture in areas with saline soil and saline groundwater with and without subsurface drainage. The results from two field projects conducted in an area with saline soils and saline groundwater demonstrated the type of irrigation systems and management needed to sustain production of moderately salt tolerant and tolerant crops. During the first study at Murrieta farms, yields of cotton and sugar beet were maintained using both saline and non-saline water for irrigation when pre-plant irrigation and rainfall were adequate to maintain soil salinity at a tolerable level. Wheat production was reduced in areas that used saline water for irrigation. Use of saline water containing toxic elements such as boron for irrigation poses a threat to the sustainability of the system. The second study evaluated the management of furrow and subsurface drip irrigation in the presence of shallow saline groundwater. Careful management of the furrow system during pre-plant irrigation and the first irrigation of the growing season was required to prevent waterlogging. It was possible to manage the subsurface drip system to induce significant crop water use from shallow groundwater. Rainfall and pre-plant irrigation were adequate at this site to manage soil salinity.     

Long-Term Reuse of Drainage Waters of Varying Salinities for Crop Irrigation in a Cotton-Safflower Rotation System in the San Joaquin Valley of California—A Nine Year Study: I. Cotton (Gossypium hirsutum L.)

Summary

Use of saline drainage water for crop irrigation was evaluated as a means of decreasing its volume. Results of a nine-year crop rotation (cotton-cotton-safflower, × 3) in which only the cotton was irrigated with drainage water of 400, 1,500, 3,000, 4,500, 6,000, and 9,000 ppm total dissolved salts are presented. The different salinity levels of irrigation waters were achieved by mixing nonsaline canal water (400 ppm) and saline drainage water. Cotton lint yields were not affected by increased salinity level of the irrigation water for the first two years. Detrimental effects became evident in the third cotton crop with increasing severity in later years. In the fifth year of cotton (seventh year of the study), lint yields were adversely affected by waters of salinity greater than 3,000 ppm. However, fiber quality remained unaffected at all levels of irrigation water salinity. The reductions in lint yield appeared to be a function of time and the salinity level of applied water. Shoot height and biomass were reduced by the irrigation water salinity before lint yields. Stand establishment appeared to be the most sensitive to salinity and was perhaps the main reason for yield reduction. Increase in irrigation water salinity increased Na⁺ content of leaf blades and petioles and decreased K⁺/Na⁺ ratio of leaf blades and petioles. The study showed that irrigation waters of up to 3,000 ppm salinity may be used for four years without any yield reductions, as long as some leaching occurs through preplant irrigations with low salinity water. Data on crop growth and development and ionic content collected over the nine year period are presented.     

Response of Sunflowers to Quantities of Irrigation Water,Irrigation Regimes and Salinities in the Water and Soil

Summary

The production of sunflower grains for roasting was investigated in two soil types under different quantities of applied saline and non-saline irrigation water, different irrigation managements, soil salinity due to previous use of saline water or due to a raised water table. It was shown in one experiment, conducted in a loess type soil, that sunflowers extracted water at least to a soil depth of 120 cm, when the available water from the top layers was used up. The crop in this soil consumed all the available soil water from nearly the entire root zone, while in the clay soil limited water was consumed from deep layers, due to the high salinity and lack of aeration.

No decrease in yield was found in the loess soil when 75% of the full amount of water (which was 0.8 of Class A pan evaporation rate) was applied. When only 50% was applied a significant decrease in yield was obtained. In contrast, in the clay soil even 75% of the full amount of water decreased the yield remarkably. Under dry-land conditions approximately 65% of maximum yield was found in the loess soil but only 45% in the clay soil. These differences are all attributed to a shallow active root system in the clay soil. Residual soil salinity from previously use of saline water had no effect on grain production in the loess soil, while saline irrigation water applied during the irrigation season decreased production, but only when water supply was not rate limiting. The combination of saline water and residual soil salinity had a marked effect on the decrease of grain yield under limited irrigation. In both soils a reduction in the amount of water applied per single irrigation and maintaining the entire irrigation period caused a significantly smaller decrease in yield than shortening the irrigation period and applying the full demand.     

More Rice,Less Water—Integrated Approaches for Increasing Water Productivity in Irrigated Rice-Based Systems in Asia

Abstract

The water crisis is threatening the sustainability of the irrigated rice system and food security in Asia. Our challenge is to develop novel technologies and production systems that allow rice production to be maintained or increased in the face of declining water availability. This paper introduces principles that govern technologies and systems for reducing water inputs and increasing water productivity, and assesses the opportunities of such technologies and systems at spatial scale levels from plant to field, to irrigation system, and to agro-ecological zones. We concluded that, while increasing the productivity of irrigated rice with transpired water may require breakthroughs in breeding, many technologies can reduce water inputs at the field level and increase field-level water productivity with respect to irrigation and total water inputs. Most of them, however, come at the cost of decreased yield. More rice with less water can only be achieved when water management is integrated with (i) germplasm selection and other crop and resource management practices to increase yield, and (ii) system-level management such that the water saved at the field level is used more effectively to irrigate previously un-irrigated or low-productivity lands. The amount of water that can be saved at the system level could be far less than assumed from computations of field-level water savings because there is already a high degree of recycling and conjunctive use of water in many rice areas. The impact of reducing water inputs for rice production on weeds, nutrients, sustainability, and environmental services of rice ecosystems warrants further investigation.     

Evapotranspiration,irrigation water requirement,and water productivity of rice (<Emphasis Type="Italic">Oryza sativa</Emphasis> L.) in the Sahelian environment

Rice is the main crop produced in the Senegal River Valley under the semiarid Sahelian climate where water resource management is critical for the resource use sustainability. However, very limited data exit on rice water use and irrigation water requirement in this water scarcity environment under climate change conditions. Understanding crop water requirements is essential for better irrigation practices, scheduling and efficient use of water. The objectives of this study were to estimate crop water use and irrigation water requirement of rice in the Senegal River Valley at Fanaye. Field experiments were conducted during the 2013 hot and dry season and wet season, and 2014 hot and dry season and wet seasons. Three nitrogen fertilizer treatments were applied to rice variety Sahel 108: 60, 120, and 180 kg N ha^?1. Rice water use was estimated by the two-step approach. Results indicated that crop actual evapotranspiration (ETa) varied from 632 to 929 mm with the highest ETa obtained during the hot and dry seasons. Irrigation water requirement varied from 863 to 1198 mm per season. Rice grain yield was function of the growing season and varied from 4.1 to 10.7 tons ha^?1 and increased with nitrogen fertilizer rate. Rice water use efficiency relative to ETa and irrigation requirements increased with nitrogen fertilizer rate while rice nitrogen use efficiency decreased with the nitrogen fertilizer rates. The results of this study can be used as a guideline for rice water use and irrigation water requirement for the irrigation design projects, consultants, universities, producers, and other operators within rice value chain in the Senegal River Valley.     

Evapotranspiration Responses of Plants and Crops to Carbon Dioxide and Temperature

Summary

Atmospheric carbon dioxide (CO2) concentration has risen from about 270 mmol (CO2) mol^?1 (air) (i.e., mole fraction of dry atmospheric air basis) before 1700 to about 370 mmol mol ^?1 currently. General Circulation Models (GCM) have predicted a global temperature rise of 2.8 to 5.2°C for a doubling of CO2. This review examines evapotranspiration and water-use efficiency responses of plants to rising CO2 and climatic changes, especially temperature. Doubling of CO2 will decrease leaf stomatal conductance to water vapor about 40%. However, water use by C3 crop plants under field conditions has usually been decreased only 12% or less for two reasons. Firstly, feedbacks in the energy balance of plant foliage cause leaf temperatures to rise as stomatal conductance is decreased. Increases of leaf temperature raise the vapor pressure of water inside the leaf, which increases the leaf-to-air vapor pressure difference. This increased driving force for transpiration offsets in large part the decreased leaf conductance caused by elevated CO2. Secondly, CO2 enrichment tends to cause leaf area to increase more rapidly in many crops. This increased leaf surface area for transpiration also offsets part of the decreased stomatal conductance per unit leaf area on the whole canopy evapotranspiration, but the energy budget feedbacks are more important.

Experiments point to a yield enhancement of 30 to 35% for C3 crops for the direct effects a doubling of CO2 (without ancillary climate change). If temperature rises, this yield enhancement may be greater for vegetative growth but less for seed grain yield. Experiments on both ambient and elevated CO2 treatments in sunlit growth chambers showed that transpiration rates increased 20% when air temperature was changed from 28 to 33 °C and increased 30% when temperature was increased from 28 to 35 °C. Thus, under well-watered conditions, evapotranspiration will increase about 4 to 5% per 1°C rise in temperature.

Crop model predictions of yields of soybean and maize showed a reduction due to temperature increases by two GCM models. Under Southeastern USA conditions, doubling CO2 in the Goddard Institute for Space Studies (GISS) climate change scenario resulted in an 12% increase in yields, but yields decreased 50% in the Geophysical Fluids Dynamics Laboratory (GFDL) climate change scenario. Optimum irrigation for both models gave yield increases of about 10%. These model results illustrate the critical requirement of water for production of crops. Under rainfed conditions, crop yields could suffer tremendously if growing season precipitation is decreased, but yields could increase moderately if growing season precipitation is increased. Under the high growing season rainfall scenario (GISS), irrigation requirements for optimum soil water were increased 22%, but under the low rainfall scenario (GFDL), irrigation requirements were increased 111%.

Without the effects of climate change, rising CO2 will cause an increase in crop water-use efficiency (WUE). Most of the increases in WUE will be due to increases in dry matter, with little or no contribution from decreases in water use per unit land area. Growers could produce higher yields per unit land area with higher total production, or maintain the same total production with less land and less total water use. However, if temperatures rise, transpirational water use will increase, and WUE will decline. Higher temperatures, and especially less rainfall, would raise the irrigation requirements of crops. Competition for water resources from other uses could result in less water available for irrigation.     

Deficit irrigation management for rice using crop growth simulation model in an optimization framework

Optimization of irrigation water is an important issue in agricultural production for maximizing the return from the limited water availability. The current study proposes a simulation–optimization framework for developing optimal irrigation schedules for rice crop (Oryza sativa) under water deficit conditions. The framework utilizes a rice crop growth simulation model to identify the critical periods of growth that are highly sensitive to the reduction in final crop yield, and a genetic algorithm based optimizer develops the optimal water allocations during the crop growing period. The model ORYZA2000, which is employed as the crop growth simulation model, is calibrated and validated using field experimental data prior to incorporating in the proposed framework. The proposed framework was applied to a real world case study of a command area in southern India, and it was found that significant improvement in total yield can be achieved by the model compared to other water saving irrigation methods. The results of the study were highly encouraging and suggest that by employing a calibrated crop growth model combined with an optimization algorithm can lead to achieve maximum water use efficiency.     

Use and Management of Poor Quality Waters for the Rice-Wheat Based Production System

Summary

India's major improvements in food production have been made possible via a shift towards rice-wheat systems as a consequence of enhanced utilization of ground waters. Rice-wheat rotation covering 10.5 million ha contribute about 75 percent of total food production. But it is being observed now that yield increases in rice and wheat has slowed down and there is rather a decline in factor productivity. One of the major reasons for this decline is indiscriminate use of alkali waters constituting about 25-42 percent of ground waters surveyed especially in the northwestern states of the Indo-Gangetic Plains (IGP). Because of high water requirements of the system, sodication rates of soil being irrigated and their steady state pH and sodicity values are much more (about 1.8 times) than that of the low water requiring rotations like millet/maize-wheat. So there has been a dilemma on whether or not rice-wheat system should be advocated with alkali irrigation waters. Consistent research efforts have lead to the guidelines for irrigation with such waters with respect to their amendment needs (gypsum requirements, frequency and mode of application), conjunctive use with canal waters and use of organic materials and chemical fertilizers, etc. These results do show that subject to the following of specific soil-water-crop management systems, it is possible to control the build up of sodicity in soils and sustain crop yields. Options available in terms of management practices and some of the researchable issue are discussed in this paper.     

Genetic Opportunities to Improve Cereal Root Systems for Dryland Agriculture

Abstract

Understanding the major limitations to root growth is very important if we are to maximize water and nutrient use and increase yields. Limitations may be insufficient rooting depth, root diseases, nutrient deficiencies, toxicities and soil hardness. An understanding of these limitations will lead to more precisely identifying traits for which to select and breed. Examples of successfully overcoming limiting factors to improve crop performance by breeding and selection are given for cereal cyst nematodes in wheat, soil acidity and salinity. The importance of altered crop management practices to reduce limitations is also stressed. These have resulted in a more effective and healthier root system, which results in more water use and greater yields. Opportunities to genetically increase the size of the root system in dryland systems where water and nutrients are not all used by the crop are given.     

Effects of Irrigation and Straw Mulching on Microclimate Characteristics and Water Use Efficiency of Winter Wheat in North Chinas

Abstract

In North China, irrigation is required to obtain a high yield from winter wheat; this results in rapid aquifer depletion. The primary objective of this study was to investigate the influencing mechanisms of irrigation and straw mulching in preserving the soil moisture. Maize straw (3?5 cm) was mulched immediately after sowing winter wheat, and irrigation water was supplied at 60 mm, controlled by using a flow meter, during the jointing, heading, or milking stages of the crop. The results revealed that irrigation decreased the eddy thermal diffusivity, sensible heat flux, and soil heat flux, but increased the latent heat flux. In contrast, straw mulching enhanced the eddy thermal diffusivity and sensible heat flux, but decreased the latent heat flux. Straw mulching increased the soil temperature at 5 cm depth form January to February, but decreased the soil temperature before January and after February. There were no significant differences in the total evapotranspiration between mulched and non-mulched treatments, however, there was a statistically significant difference in the evapotranspiration among the different growing seasons. Straw mulching reduced the evapotranspiration from the seeding stage to the regrowing stage, and the evapotranspiration with mulching was less than that non-mulching 47.4 mm. Further, straw mulching significantly reduced the number of spikes in the crop. Both irrigation and straw mulching increased the number of kernels, but had no visible effects on the thousand kernel weight. These results indicate that straw mulching may decrease the yield and water use efficiency (WUE) of winter wheat in North China.     

The response of autumn and spring sown sugar beet (Beta vulgaris L.) to irrigation in Southern Italy: Water and radiation use efficiency

The Apulia region in Southern Italy is an important area for sugar beet cultivation. It is characterised by clay soils and a hot-arid and winter-temperate climate. The capability of sugar beet to exploit solar radiation, water use and irrigation supply in root yield, total dry matter and sucrose production was studied and analysed in relation to two experimental factors: sowing date – autumn (October–December) and spring (March) – and irrigation regime – optimal and reduced (respectively with 100 and 60% of actual evapotranspiration). Data sets from three experiments of spring sowing and three of autumn sowing were used to calculate: (1) water use efficiency in the conversion in dry matter (WUE_dm, plant dry matter at harvest versus seasonal water use ratio), in sucrose (WUE_suc, sucrose yield versus seasonal water use ratio); (2) irrigation water use efficiency in the conversion in dry matter (IRRWUE_dm), in sucrose (IRRWUE_suc) and fresh root yield (IRRWUE_fr); and (3) radiation use efficiency (RUE, plant dry matter during the crop cycle and at harvest versus intercepted solar radiation ratio).Autumnal beet was more productive than spring for fresh root, plant total dry matter, sucrose yield and concentration; also WUE_suc and IRRWUE_s were higher in the autumnal sugar beet, but no difference was observed in WUE_dm (on average, 2.83 g of dry matter kg⁻¹ of water used). An average saving of about 26% of seasonal irrigation supply (equivalent to about 100 mm) was measured in the three years with the earliest sowing time. The optimal irrigation regime produced higher root yield, plant total dry matter and sucrose yield than the reduced one; on the contrary the IRRWUE_fr and IRRWUE_dm were higher in the reduced irrigation strategy. WUE_s and IRRUWE_s correlated positively with the length of crop cycle, expressed in growth degree days and, in particular, to the length of the period from full soil cover canopy to crop harvest, the period when plant photosynthetic activity and sucrose accumulation are at maximum rates. Seasonal RUE was higher in the spring than in the autumn sowing (1.14 μg J⁻¹ versus 1.00 μg J⁻¹). The RUE values during the crop cycle reached the maximum in the period around complete canopy soil cover. The results showed the importance for better use of water and radiation resources of autumnal sowing time and of reduced irrigation regime in sugar beet cropped in a Mediterranean environment.     

Effects of irrigation system and green manure on yield and nicotine content of Virginia (flue-cured) Organic tobacco (Nicotiana tabaccum), under Mediterranean conditions

Organic tobacco consists of a new industrial crop product. Field experiments were conducted to determine the effects of irrigation system and fertilization on the growth (biomass and roots) of organic tobacco plants (Nicotiana tabaccum cv NC 71). The experiment was designed as a split plot design with four replicates, two main plots (drip and sprinkler irrigation) and three sub-plots (vetch as green manure, red clover as green manure and control; without fertilization). Drip irrigation was characterised by lower amount of water applied to the soil. Furthermore the tobacco yield was not affected by the reduced water application and the crops under drip irrigation were higher than those with sprinkler irrigation. Positive effect of green manure in the nicotine content of tobacco leaves was also observed, and that reported for first time under organic system. Equally the higher amount of nitrogen from green fertilization with vetch led to increase of nicotine concentration (0.91% max concentration). Agronomic water use efficiency (WUE) for drip irrigation was always higher than those for sprinkler irrigation. There were no significant differences between the drip and sprinkler irrigation concerning the root biomass. Moreover, green manures increased roots dry weight. Yield of tobacco crop was significantly increased by the green manures, with the lowest yield (1850 kg ha^?1) to be found in the control plots. Finally, green manures increased the SPAD values and number of leaves, with most significant impact the time when vetch applied to soil.     

The influence of seepage from canals and paddy fields on the groundwater level of neighboring rotation cropping fields: a case study from the lower Ili River Basin,Kazakhstan

In the large-scale irrigation schemes of the lower Ili River Basin of Kazakhstan, crop rotation combines paddy rice and non-rice crops. Continuous irrigation is practiced in paddy fields, whereas other crops are sustained from groundwater after only limited early irrigation. The water table in non-rice crops is raised by seepage from canals and the flooded paddy fields. We investigated the areal extent to which the groundwater level of non-irrigated fields is influenced by seepage from canals and paddy fields by examining the relationship between distance (from canal and paddy field) and groundwater level in upland fields. The groundwater level was influenced for up to 300 and 400 m from the canals and paddy fields, respectively. Geographic information system analysis of crop and canal patterns in the 11 selected years showed that if the zone of influence is 300 and 400 m from the canals and paddy fields, respectively, the groundwater level of most of the area of upland fields was raised by seepage. We conclude that the water supply to cropping fields by seepage from irrigation canals and paddy fields is adequate, but the spatial distribution of the paddy fields may be an important factor that needs more attention to help improve water use efficiency in this irrigation district.     

Growth, yield and water productivity of zero till wheat as affected by rice straw mulch and irrigation schedule

Intensive cultivation of rice and wheat in north-west India has resulted in air pollution from rice straw burning, soil degradation and declining groundwater resources. The retention of rice residues as a surface mulch could be beneficial for moisture conservation and yield, and for hence water productivity, in addition to reducing air pollution and loss of soil organic matter. Two field experiments were conducted in Punjab, India, to study the effects of rice straw mulch and irrigation scheduling on wheat growth, yield, water use and water productivity during 2006-2008. Mulching increased soil water content and this led to significant improvement in crop growth and yield determining attributes where water was limiting, but this only resulted in significant grain yield increase in two instances. There was no effect of irrigation treatment in the first year because of well-distributed rains. In the second year, yield decreased with decrease and delay in the number of irrigations between crown root initiation and grain filling. With soil matric potential (SMP)-based irrigation scheduling, the irrigation amount was reduced by 75 mm each year with mulch in comparison with no mulch, while maintaining grain yield. Total crop water use (ET) was not significantly affected by mulch in either year, but was significantly affected by irrigation treatment in the second year. Mulch had a positive or neutral effect on grain water productivity with respect to ET (WP_ET) and irrigation (WP_I). Maximum WP_I occurred in the treatment which received the least irrigation, but this was also the lowest yielding treatment. The current irrigation scheduling guidelines based on cumulative pan evaporation (CPE) resulted in sub-optimal irrigation (loss of yield) in one of the two years, and higher irrigation input and lower WP_I of the mulched treatment in comparison with SMP-based irrigation scheduling. The results from this and other studies suggest that farmers in Punjab greatly over-irrigate wheat. Further field and modelling studies are needed to extrapolate the findings to a wider range of seasonal and site conditions, and to develop simple tools and guidelines to assist farmers to better schedule irrigation to wheat.     

Agronomic Practices Influence Maize Grain Quality

Abstract

Maize grain yields have increased dramatically over the past fifty years, and concurrently end-uses have proliferated requiring special quality characteristics. Plant breeders have developed many specialty types of maize, all of which are influenced by the agronomic practices used to produce the crop. Grain yield increases have resulted in lower protein concentration except when the yield increase resulted from nitrogen fertilizer application. Irrigation improves the biological value of protein, while higher nitrogen application rates alter the amino acid balance thereby reducing the nutritional value. Kernel breakage susceptibility and kernel density increase with higher nitrogen fertilizer application rates, and are reduced by irrigation. Extractable starch and oil concentration are largely influenced by hybrid choice, but small production practice effects have been documented. Essential mineral nutrient levels are often influenced by soil or foliar fertilizer application. The production of aflatoxin can be reduced by irrigation or other strategies to avoid water stress during grain fill. Although genetics usually exerts the largest effect on maize grain quality, agronomic practices are also important.     

Effects of Soil Moisture Conditions before Heading on Growth of Wheat Plants under Drought Conditions in the Ripening Stage: Insufficient Soil Moisture Conditions before Heading Render Wheat Plants More Resistant to Drought during Ripening

Abstract

Plants growing on soil with insufficient moisture need deep and dense roots to avoid water stress. In crop plants, the production of dry matter during ripening of grains is critically important for grain yield. We postulated that shoot growth would be suppressed but root growth would continue under an insufficient soil moisture condition before heading, while shoot growth would be more vigorous than root growth under a sufficient soil moisture condition. We anticipated that the plants growing under an insufficient soil moisture condition before heading would produce more dry matter and grain under an insufficient soil moisture condition during ripening. In order to examine our hypotheses and to determine the fundamental conditions for improving grain yield and efficient use of irrigated water under limited irrigation, we grew wheat plants (Triticum aestivum L., cv. Ayahikari) in pots (30 cm in diameter, 150 cm in height) with insufficient soil moisture (PD-D pots) or sufficient soil moisture (PW-D pots) for six weeks before heading followed by full irrigation, and then insufficient soil moisture condition during ripening. The growth of shoots was suppressed significantly but that of roots was not before heading in PD-D plants, with a higher resultant ratio of root to shoot than in PW-D plants. The former retained a high leaf water potential and, therefore, were able to produce more dry matter and grain during soil moisture depletion during ripening as compared with the latter plants. We also obtained similar results with field-grown plants.     

Modeling of continental-scale crop water requirement and available water resources

The relationship between agricultural water demand and supply has been of interest to government decision makers and scientists because of its importance in water resources management. We developed a water cycle model for eastern Eurasia that can estimate water requirements for crop growth and evaluate the demand–supply relationships of agricultural water use on a continental scale. To produce an appropriate water cycle, the model was constructed based on small drainage basins. To validate the model performance with respect to simulated runoff, which is here considered as the available water resource, we compared our outputs with those of other models and with observed river discharges. The results show that this model is comparable to other models and that it is applicable for the evaluation of water cycles at continental scale. We defined two types of crop water deficits (CWDs) as indicators of agricultural water demand. These were formulated by considering the physical processes of crop water use; we did not include water consumption that is dependent on cultivation management practices, such as water losses in irrigation systems. We assessed the reliability of our indicators by comparison with indicators from other studies and with published statistics related to agricultural water use. These comparisons suggest that our indicators are consistent with independent data and can provide a reasonable representation of water requirements for crop growth.