Root growth, water potential and yield of irrigated wheat

Root length density (LV), mid-day leaf water potential (Ψ _leaf) and yield of wheat were studied in 1983 – 1984 and 1984 – 1985 on a Phoolbagh clay loam (Typic Haplaquoll) and on a Beni silty clay loam (Aquic Hapludoll) in the Tarai region of Uttar Pradesh under naturally fluctuating shallow (0.4 – 0.9 m, SWT) and medium-depth (0.8 – 1.3 m, MWT) water table conditions with six water regimes: rainfed (I₀); irrigation at cown root initiation (I₁); at crown root initiation and milk (I₂); at crown root initiation, maximum tillering and milk (I₃); at crown root initiation, maximum tillering, flowering and milk (I₄); and at crown root initiation, maximum tillering, flowering, milk and dough (I₅). Maximum rooting depth (0.8 m under SWT and 1.05 m under MWT conditions) was attained at the dough stage (115 days after sowing, DAS) and was more strongly influenced by fluctuations in water table depth than by the water regime. For wet regimes (I₂– I₅), roots were concentrated at and above the water table interface and had greater horizontal development, whereas in dry regimens (I₀ and I₁), due to deficient moisture conditions in the upper soil layer (0.45 m) they invaded lower horizons and had a greater vertical distribution Ψ _leaf was not significantly affected by water regime (I₁– I₅) up to 94 DAS during a wet year (1983 – 1984) and up to 74 DAS during a dry year (1984 – 1985), but was significantly affected thereafter. Grain yields with water regimens I₁– I₅ during a wet year and for the I₂– I₅ treatments during a dry year at either water table depth were not significantly different, but there was a (non-significant) trend to lower yield with increasing soil water deficit. Under SWT in I₂, the average grain yield wsa 5130 kg ha^–1 and under the I₃ regime, 5200 kg ha^–1. Likewise, under MWT in I₃, it was 5188 kg ha^–1 and under the I₄ regime, 5218 kg ha^–1. The results indicate that application of irrigation of more than 120 and 180 mm under SWT and MWT conditions, respectively, did not raise yield. Irrigation given as per schedule I₂ under SWT and I₃ under MWT conditions in the Tarai situation, appears to be more effective than a very wet regime (I₅). Received: 9 December 1997     

Irrigation effects on the growth, yield, and water use efficiency of alfalfa

A field study was conducted in the semiarid region of northern Sudan to investigate the effects of variable irrigation on the growth, yield, and water use efficiency (WUE) of alfalfa (Medicago sativa L.). Treatments were 65 mm of water applied every 7 days, 80 mm of water applied every 10 days, or 104 mm of water applied every 13 days. The heavy, infrequent irrigation reduced stem height, stem density, leaf area index (LAI), total biomass production, and the WUE of alfalfa plants. Maximum yields for six harvests were 15.3, 12.9, and 11.2 ton ha^–1 and the WUE values were 0.12, 0.10, and 0.08 ton ha^–1 cm^–1 for the frequent, less-frequent, and infrequent irrigation regimes, respectively. In all the treatments, alfalfa dry matter yield was positively correlated with stem height and LAI. The relationship between dry matter yield and total water use was a linear function (R ²=0.99), regardless of the irrigation treatment. Alfalfa growth, yield, and WUE remained high during the relatively cool months and declined during the hot period under the three water regimes. It was concluded that alfalfa grown under semiarid conditions should be watered lightly and frequently to attain high yields and high WUE. Received: 19 February 1996     

Water use efficiency of irrigated wheat in the Tarai region of India

Experiments were conducted during the winter seasons of 1983–1984 and 1984–1985 to identify suitable irrigation regimes s for wheat grown after rice in soils with naturally fluctuating shallow water table (SWT) at a depth of 0.4 to 0.9 m and medium water table (MWT) at a depth of 0.8 to 1.3 m. Based on physiological stages, the crop was subjected to six irrigation regimes viz., rainfed (I₀); irrigation only at crown root initiation (I₁); at only crown root initiation and milk (I₂); at crown root initiation, maximum tillering and milk (I₃); at crown root initiation, maximum tillering, flowering and milk (I₄); and at crown root initiation, maximum tillering, flowering milk and dough (I₅). Tube-well water with an EC <0.4 dsm^–1 was used for irrigation. Based on 166 mm effective precipitation during the cropping season, 1983–1984 was designated as a wet year and 1984–1985 with 51 mm as a dry year. The change in profile soil water content W (depletion) in the wet year was less (23%) under SWT and 10% under MWT as compared to the dry year. The ground water contribution (GWC) to evapotranspiration (ET) was 58% under SWT and 42% under MWT conditions in both the years. The GWC in the wet year was 20% under SWT and 23% under MWT. Of the total net water use (NWU), about 85% was ET and 15% drainage losses. The NWU was highest (641 and 586 mm) in I₅ under SWT and MWT conditions, respectively, but not the yield (5069 kg ha^–1). Compared to I₅, NWU in I₂ treatment decreased by 10% in the wet and 25% in the dry year. A similar trend was observed in the I₃ treatment under MWT condition. However, there was no statistically significant difference between yields of the I₁ to I₅ treatments of either water table depth during the wet year. This was also true during the dry year for the I₂ to I₅ treatments. Under SWT, in I₂, the grain yield was 5130 kg ha^–1 and under I₃ regime, 5200 kg ha^–1. Under MWT in I₃, the yield was 5188 kg ha^–1 and under I₄ regime, 5218 kg ha^–1. Thus it appears that in the Tarai region where the water table remains shallow (<0.9 m) and medium (<1.3 m) for most of the wheat growing season applications of more than 120 and 180 mm irrigation under SWT and MWT conditions, respectively were not necessary. Irrigation given only at crown root initiation and milk stages under shallow water table conditions, and at crown root initiation, maximum tillering and milk stages under medium water table conditions, appears to be as effective as frequent irrigations.     

Forage sorghum yield and water use efficiency under variable irrigation

The response of forage sorghum [Sorghum bicolor (L.) Moench] to three irrigation treatments in a semiarid environment was studied in the field for two seasons. Treatments were light frequent, moderate less frequent, and heavy infrequent irrigation, where irriga-tion water at 8 mm day^–1 was delivered every 7, 10, and 13 days, respectively. These irrigation regimes meant heavier water inputs with increasing irrigation frequency. Plant heights and leaf area indices of forage sorghum were higher in the frequently watered plots than in plots where irrigation water was delivered less frequently. Averaged over the two seasons, maximum dry matter (DM) yields were 16.3, 11.8, and 10.5 tonnes ha^–1 for frequent, intermediate, and infrequent irrigation regimes, respectively. Light, frequent irrigation resulted in a significantly higher water use efficiency (WUE) compared to the other two regimes, thus increasing the return from irrigation. These results suggest that in such semiarid environments, DM yields and WUE of forage sorghum could be increased by combining light irrigation with a short interval. Received: 6 February 1997     

Effect of water-saving irrigation on rice yield and water use in typical lowland conditions in Asia

With decreasing water availability for agriculture and increasing demand for rice, water use in rice production systems has to be reduced and water productivity increased. Alternately submerged–nonsubmerged (ASNS) systems save water compared with continuous submergence (CS). However, the reported effect on yield varies widely and detailed characterizations of the hydrological conditions of ASNS experiments are often lacking so that generalizations are difficult to make. We compared the effects of ASNS and CS on crop performance and water use, at different levels of N input, in field experiments in China and the Philippines, while recording in detail the hydrological dynamics during the experiment. The experiments were conducted in irrigated lowlands and followed ASNS practices as recommended to farmers in China. The sites had silty clay loam soils, shallow groundwater tables and percolation rates of 1–4.5 mm per day.Grain yields were 4.1–5.0 t ha⁻¹ with 0 kg N ha⁻¹ and 6.8–9.2 t ha⁻¹ with 180 kg N ha⁻¹. Biomass and yield did not significantly differ between ASNS and CS, but water productivity was significantly higher under ASNS than under CS in two out of three experiments. There was no significant water×N interaction on yield, biomass, and water productivity. Combined rainfall plus irrigation water inputs were 600–960 mm under CS, and 6–14% lower under ASNS. Irrigation water input was 15–18% lower under ASNS than under CS, but only significantly so in one experiment. Under ASNS, the soils had no ponded water for 40–60% of the total time of crop growth. During the nonsubmerged periods, ponded water depths or shallow groundwater tables never went deeper than −35 cm and remained most of the time within the rooted depth of the soil. Soil water potentials did not drop below −10 kPa. We argue that our results are typical for poorly-drained irrigated lowlands in Asia, and that ASNS can reduce water use up to 15% without affecting yield when the shallow groundwater stays within about 0–30 cm. A hydrological characterization and mapping of Asia’s rice area is needed to assess the extent and magnitude of potential water savings.     

Efficiency of controlled drainage and subirrigation in reducing nitrogen losses from agricultural fields

In northeast Italy, a regimen of controlled drainage in winter and subirrigation in summer was tested as a strategy for continuous water table management with the benefits of optimizing water use and reducing unnecessary drainage and nitrogen losses from agricultural fields.To study the feasibility and performance of water table management, an experimental facility was set up in 1996 to reproduce a hypothetical 6-ha agricultural basin with different land drainage systems existing in the region. Four treatments were compared: open ditches with free drainage and no irrigation (O), open ditches with controlled drainage and subirrigation (O-CI), subsurface corrugated drains with free drainage and no irrigation (S), subsurface corrugated drains with controlled drainage and subirrigation (S-CI). As typically in the region free drainage ditches were spaced 30 m apart, and subsurface corrugated drains were spaced 8 m apart.Data were collected from 1997 to 2003 on water table depth, drained volume, nitrate-nitrogen concentration in the drainage water, and nitrate-nitrogen concentration in the groundwater at various depths up to 3 m.Subsurface corrugated drains with free drainage (S) gave the highest measured drainage volume of the four regimes, discharging, on average, more than 50% of annual rainfall, the second-highest concentration of nitrate-nitrogen in the drainage water, and the highest nitrate-nitrogen losses at 236 k ha⁻¹.Open ditches with free drainage (O) showed 18% drainage return of rainfall, relatively low concentration of nitrate-nitrogen in the drainage water, the highest nitrate-nitrogen concentration in the shallow groundwater, and 51 kg ha⁻¹ nitrate-nitrogen losses.Both treatments with controlled drainage and subirrigation (O-CI and S-CI) showed annual rainfall drainage of approximately 10%. O-CI showed the lowest nitrate-nitrogen concentration in the drainage water, and the lowest nitrogen losses (15 kg ha⁻¹). S-CI showed the highest nitrate-nitrogen concentration in the drainage water, and 70 kg ha⁻¹ nitrate-nitrogen losses. Reduced drained volumes resulted from the combined effects of reduced peak flow and reduced number of days with drainage.A linear relationship between daily cumulative nitrate-nitrogen losses and daily cumulative drainage volumes was found, with slopes of 0.16, 0.12, 0.07, and 0.04 kg ha⁻¹ of nitrate-nitrogen lost per mm of drained water in S-CI, S, O, and O-CI respectively.These data suggest that controlled drainage and subirrigation can be applied at farm scale in northeast Italy, with advantages for water conservation.     

Runoff nitrogen from a large sized paddy field during a crop period

Nutrient load management is an important environmental issue because nutrient loads from farmlands degrade surface waters as a result of anthropogenic eutrophication. Nitrogen load from a large sized paddy field during the crop period was examined from the results of field measurements carried out in 2004. The 1.5 ha paddy field was located east of Biwa Lake. Irrigation water volume and ponded water depth were continuously observed. Field measurements were carried out at least once a week to analyze total nitrogen (TN) concentration in the irrigation water and ponded water. Daily inflow and outflow of nitrogen was obtained by multiplication of the nitrogen concentration and transported water volume, consisting of irrigation, precipitation, evapotranspiration, percolation and surface discharge. Water outflow volume was calculated by a tank model that consisted of three small tanks connected to represent ponded water depth differences in the large paddy field. The calculated nitrogen load was 18.8 kg ha⁻¹, with 7.2 kg ha⁻¹ from surface drainage and 11.6 kg ha⁻¹ from percolation loss. The runoff nitrogen value of 18.8 kg ha⁻¹ was within the range of the reported values investigated in a medium-sized paddy field. The observed value was close to the value for a low percolation flux paddy field where less irrigation water has been applied. These results suggest that less irrigation water keep runoff nitrogen low. This also indicates that irrigation water management can reduce nitrogen load from large sized paddy fields.     

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.     

Optimizing yield,water requirements,and water productivity of aerobic rice for the North China Plain

Water resources for agriculture are rapidly declining in the North China Plain because of increasing industrial and domestic use and because of decreasing rainfall resulting from climate change. Water-efficient agricultural technologies need to be developed. Aerobic rice is a new crop production system in which rice is grown in nonflooded and nonsaturated aerobic soil, just like wheat and maize. Although an estimated 80,000 ha are cultivated with aerobic rice in the plain, there is little knowledge on obtainable yields and water requirements to assist farmers in improving their management. We present results from field experiments with aerobic rice variety HD297 near Beijing, from 2002 to 2004. The crop growth simulation model ORYZA2000 was used to extrapolate the experimental results to different weather conditions, irrigation management, and soil types. We quantified yields, water inputs, water use, and water productivities. On typical freely draining soils of the North China Plain, aerobic rice yields can reach 6–6.8 t ha⁻¹, with a total water input ranging between 589 and 797 (rainfall = 477 m and water application = 112–320 mm). For efficient water use, the irrigation water can be supplied in 2–4 applications and should aim at keeping the soil water tension in the rootzone below 100–200 kPa. Under those conditions, the amount of water use by evapotranspiration was 458–483 mm. The water productivity with respect to total water input (irrigation plus rainfall) was 0.89–1.05 g grain kg⁻¹ water, and with respect to evapotranspiration, 1.28–1.42 g grain kg⁻¹ water. Drought around flowering should be avoided to minimize the risk of spikelet sterility and low grain yields. The simulations suggest that, theoretically, yields can go up to 7.5 t ha⁻¹ and beyond. Further research is needed to determine whether the panicle (sink) size is large enough to support such yields and/or whether improved management is needed.     

The effect of drip line spacing, irrigation regimes and planting geometries of tomato on yield, irrigation water use efficiency and net return

This study was conducted in order to determine the effect of drip line spacing, irrigation regimes and planting geometries of tomato on yield, irrigation water use efficiency (IWUE) and net return. The experiments were carried out in the conditions of Eskisehir in Central Anatolian part of Turkey, between 2003 and 2005, with cv. Dual Large F1 tomatoes (Lycopercion esculentum L). The maximum yield of 121.1 t ha⁻¹ was obtained from the treatment in which both the lateral and row spacing were 1 m, and irrigated with water amount based on the percentage of canopy cover. The seasonal irrigation water amount of the treatment was 551 mm. Tomatoes yield of 109.9 t ha⁻¹ was obtained under conditions of 491 mm seasonal irrigation water applied for the 2-m lateral spacing in which two plant rows (twin rows) were planted 0.35 m on either side of the lateral with a row spacing of 0.70 m across the drip lateral and 1.30 m in the interrow between each set of twin rows. Although water saving of 60 mm and investments economy of 40% were provided from the twin-row design, the yearly return of the design including one lateral for each row was US$ 1590 ha⁻¹ higher than that the return of the twin-row design. The method of determination of irrigation water amount based on the percentage of canopy cover appeared to be the most reasonable and effective one in terms of the yield and IWUE. On the other hand, the maximum irrigation water use efficiency (22.3 kg m³) was obtained from 2-m lateral spacing and the percentage of canopy cover for calculation of the amount of irrigation water applied. Thus, canopy cover may be used successfully at any lateral design conditions.     

In-season wheat root growth and soil water extraction in the Mediterranean environment of northern Syria

Wheat is the most important cereal crop in the semi-arid eastern Mediterranean region that includes northern Syria. Knowledge of wheat root depth and the vertical distribution during the winter growing season is needed for sound scheduling of irrigation and efficient use of water. This article reports evaluation of root development for three winter-grown bread (Triticum aestivum L.) and durum (Triticum turgidum L.) wheat under four soil water regimes (rainfed and full irrigation with two intermediate levels of 33 and 66% of full irrigation). Roots were sampled by soil coring to a depth of 0.75 m at four occasions during 2005-2006 growing season. Two distinct phases of root development were identified, a rapid downward penetration from emergence to end tillering phase, followed by a substantial root mass growth along the profile from tillering to mid-stem-elongation phase. Roots were detected as deep as 0.75 m during the initial rapid penetration, yet only 29% of the total seasonal root mass was developed. This downward penetration rate averaged 7 mm d⁻¹ and produced 10.8 kg ha⁻¹ d⁻¹ of root dry-biomass. The bulging of root mass from tillering to mid-stem-elongation coincided with vigorous shoot growth, doubling root dry-biomass at a rate of 52 kg ha⁻¹ d⁻¹, compared to the seasonal root growth rate of 18.3 kg ha⁻¹ d⁻¹. A second-degree equation described the total root dry-biomass as a function of days after emergence (r² = 0.85), whereas a simpler equation predicted it as a function of cumulative growing degree days (r² = 0.85). The final grain yield was a strong function of irrigation regimes, varying from 3.0 to 6.5 t ha⁻¹, but showed no correlation with root biomass which remained similar as soil water regimes changed. This observation must be viewed with care as it lacks statistical evidence. Results showed 90% of root mass at first irrigation (15 April) confined in the top 0.60-0.75 m soil in bread wheat. Presence of shallow restricting soil layers limited root depth of durum wheat to 0.45 m, yet total seasonal root mass and grain yield were comparable with non-restricted bread wheat. Most root growth occurred during the cool rainy season and prior to the late irrigation season. The root sampling is short of rigorous, but results complement the limited field data in literature collectively suggesting that irrigation following the rainy season may best be scheduled assuming a well developed root zone as deep as the effective soil depth within the top meter of soil.     

Remote sensing and in situ-based estimates of evapotranspiration for subirrigated meadow,dry valley,and upland dune ecosystems in the semi-arid sand hills of Nebraska,USA

Water consumed through evapotranspiration (ET) impacts local and regional hydrologic regimes on various spatial and temporal scales. Estimating ET in the Great Plains is a prerequisite for effective regional water resource management of the Ogallala (High Plains) Aquifer, which supplies vital water resources in the form of irrigation for extensive agricultural production. The Sand Hills region of Nebraska is one of the largest grass-stabilized eolian (windblown) sand dune formations in the world, with an area of roughly 50,000–60,000 km² that supports a system of five major land cover types: (1) lakes, (2) wetlands (with lakes, ~5%), (3) subirrigated meadows (water table is within ~1 m of surface; ~10%), (4) dry valleys (water table is 1–10 m below surface; ~20%), and (5) upland dunes (water table is more than 10 m below surface; ~65%). Fully understanding the hydrologic regime of these different ecosystems is a fundamental challenge in regional water resource assessment. The surface energy and water balances were analyzed using Bowen Ratio Energy Balance Systems (BREBS) at three locations: (1) a meadow, (2) a valley, and (3) an upland dune. Measurement of the energy budget by BREBS, in concert with Landsat remote sensing image processing for 2004 reveals strong spatial gradients between sites in latent heat flux that are associated with undulating topographic relief. We find that daily estimates of ET from BREBS measurements and remote sensing agree well, with an uncertainty within 1 mm, which is encouraging when applying remote sensing results across such a broad spatial scale and undulating topography.     

Modeling irrigated cotton with shallow groundwater in the Aral Sea Basin of Uzbekistan: II. Soil salinity dynamics

Years of ill-managed irrigation have triggered secondary soil salinization in the Khorezm region of Uzbekistan located in the Aral Sea basin. To assess the magnitude and dynamics of secondary soil salinization, to quantify improved management strategies and to derive updated irrigation standards, the soil water model Hydrus-1D was used. Water and soil salinity dynamics in three cotton fields with different soil textures were monitored and simulated for the years 2003 and 2005. Until now in Khorezm, overall soil salinity could only be controlled by pre-season salt leaching using high amounts of water. This water, however, may not be available anymore in the near future because of global climate change and shrinking fresh water resources. Simulations confirmed that the present leaching practice is barely effective. At two out of the three locations within a sandy loam field, leaching did not remove salts from the 2 m profile. Instead, salts were only shifted from the upper (0–0.8 m) to the lower (0.8–2 m) soil layer. Strong groundwater contribution to evapotranspiration triggered secondary (re)-salinization of the topsoil during the cropping season. As a consequence, salt amounts in the top 0.8 m of soil increased from 9 to 22 Mg ha⁻¹ in the field with loamy texture, and from 4 to 12 Mg ha⁻¹ in the field with sandy texture. Management strategy analyses revealed that reducing soil evaporation by a surface residue layer would notably decrease secondary soil salinization. Here, owing to the reduced capillary rise of groundwater, post-season salt contents of the three fields were reduced by between 12 and 19% when compared with residue-free conditions. Even more effective would be improving the efficiency of the drainage system so as to lower the groundwater table. This would require a revision of the current irrigation management schemes, but could, as simulations revealed, reduce the post-season salt content in the 2 m soil profile of the three fields by between 36 and 59% when compared with unaltered conditions. For the revised irrigation management in total not more water than already foreseen by national irrigation recommendations would be needed. Increasing leaching and irrigation efficiency would help sustaining the present cotton production levels while reducing future leaching demands.     

Water use, crop coefficients, and irrigation management criteria for camelina production in arid regions

Camelina sativa (L.) Crantz is an oilseed crop touted as being suitable for production in the arid southwestern USA. However, because any significant development of the crop has been limited to cooler, rain-fed climate-areas, information and guidance for managing irrigated-camelina are lacking. This study measured the crop water use of a November-through-April camelina crop in Arizona using frequent measurements of soil water contents. The crop was grown under surface irrigation using five treatment levels of soil water depletion. The seed yields of treatments averaged 1,142 kg ha⁻¹ (8.0% seed moisture) and were generally comparable with camelina yields reported in other parts of the USA. Varying total irrigation water amounts to treatments (295–330 mm) did not significantly affect yield, whereas total crop evapotranspiration (ET_c) was increased for the most frequently irrigated treatment. However, total ET_c for the camelina treatments (332–371 mm) was markedly less than that typically needed by grain and vegetable crops (600–655 mm), which are commonly grown during the same timeframe in Arizona. The camelina water-use data were used to develop crop coefficients based on days past planting, growing degree days, and canopy spectral reflectance. The crop coefficient curves, along with information presented on camelina soil water depletion and root zone water extraction characteristics will provide camelina growers in arid regions with practical tools for managing irrigations.     

A modified checkbook irrigation method based on GIS-coupled model for regional irrigation scheduling

In this study, a regional irrigation schedule optimization method was proposed and applied in Fengqiu County in the North China Plain, which often suffers serious soil water drainage and nitrogen (N) leaching problems caused by excessive irrigation. The irrigation scheduling method was established by integrating the ‘checkbook irrigation method’ into a GIS-coupled soil water and nitrogen management model (WNMM) as an extension. The soil water and crop information required by the checkbook method, and previously collected from field observations, was estimated by the WNMM. By replacing manually observed data with simulated data from WNMM, the application range of the checkbook method could be extended from field scale to regional scale. The WNMM and the checkbook irrigation method were both validated by field experiments in the study region. The irrigation experiment in fluvo–aquic soil showed that the checkbook method had excellent performance; soil water drainage and N leaching were reduced by 83.1 and 85.6%, respectively, when compared with local farmers’ flood irrigation. Using the validated WNMM, the performance of checkbook irrigation in an entire winter wheat and summer maize rotation was also validated: the average soil water drainage and N leaching in four types of soils decreased from 331 to 75 mm year⁻¹ and 47.7 to 9.3 kg ha⁻¹ year⁻¹, respectively; and average irrigation water use efficiency increased from 26.5 to 57.2 kg ha⁻¹ mm⁻¹. The regional irrigation schedule optimization method based on WNMM was applied in Fengqiu County. The results showed a good effect on saving irrigation water, decreasing soil water drainage and then saving agricultural inputs. In a typical meteorological year, it could save >110 mm of irrigation water on average, translating to >7.26 × 10⁷ m³ of agricultural water saved each year within the county. Annual soil water drainage was reduced to <143 mm and N leaching to <27 kg ha⁻¹ in most soils, all of which were significantly lower than local farmers’ flood irrigation. In the mean time, crop yield also had an average increase of 2,890 kg ha⁻¹ when checkbook irrigation was applied.     

Loquat as a crop model for successful deficit irrigation

During three consecutive seasons, two different deficit irrigation strategies were compared with control fully irrigated trees regarding their capacity to induce early bloom and harvest in “Algerie” loquat. The first strategy, a continuous deficit irrigation strategy, consisted in a uniform reduction of 20% water needs through the entire season; the second strategy, a regulated deficit irrigation approach, while accounting for the same global reduction of 20% loquat water needs, concentrated water shortages after harvest from mid-May through the end of August. Regulated deficit irrigation resulted more successful. Postharvest regulated deficit irrigation advanced full bloom 10–20 days depending on the season. Such enhancement led to more precocious and valuable yield, with an average increase of fruit value of 0.21 € kg⁻¹. The effects of continuous deficit irrigation were less noticeable and average fruit value was increased 0.08 € kg⁻¹. Yield and fruit quality were not affected for the different deficit irrigation strategies. Water savings established around 1450 m³ ha⁻¹ year⁻¹. Deficit irrigation rose water use efficiency up to more than a 40%.     

Spatial and temporal variability of nitrate in irrigated salad crops

The objective of this study was to analyze the spatial and seasonal variations in NO₃ ^–-N concentration in soil samples and solution samplers and the N leaching of an irrigated crop cultivated intensively in the Mediterranean zone. Although much information is available from controlled field experiments concerning N concentration and its spatial variability, quantitative estimates of nitrate fluxes under normal farming conditions and when the field is directly managed by farmers are rare. This is particularly true for gardening crops in the Mediterranean zone, where high evapotranspiration rates lead to intensive irrigation and may be responsible for N leaching. A field experiment was conducted in the Departement du Gard under agricultural conditions. Salads (Cichorium endivia, Lactuca sativa) were planted in three consecutive periods. The field was irrigated with sprinklers. Local measurements with a neutron probe were made at two sites (row, interrow), and an experimental plot (95 m×25 m) was surveyed at 36 points located on a 10 m×10 m equilateral grid to analyze the spatial variability of water and NO₃ ^–-N balances. To analyze the basic statistical properties of our sampling scheme, random fields of soil concentration were simulated with the turning-bands method. Sampling strategy simulations indicated that when a spatial structure exists, sampling according to a regular grid was more efficient than a purely random sampling strategy. Global trends indicated high spatial variability for nitrate leaching with differences between periods of different irrigation intensity (97 kg ha^–1 NO₃ ^–-N leaching during the spring and summer, and 199 kg ha^–1 NO₃ ^–-N leaching during autumn and winter). Leaching caused temporal variations in the spatial distributions of NO₃ ^–-N. The origin of the spatial variability of N leaching was explained by first, the variability in NO₃ ^–-N concentration in the soil profile, and second, by spatial variability in irrigation. Furthermore, the spatial distribution of the NO₃ ^–-N concentration was time dependent, and NO₃ ^–-N spatial distributions became independent after approximately 2 or 3 months under our conditions. Our results show that better management of irrigation and fertilizer in spring and summer may reduce N leaching and, thus, improve ground water quality. Received: 15 March 1996     

Deficit irrigation of sunflower (Helianthus annuus L.) in a sub-humid climate

The response of sunflower (Helianthus annuus L.) to 14 irrigation treatments in a sub-humid environment (Bursa, Turkey) was studied in the field for two seasons. A rainfed (non-irrigated) treatment as the control and 13 irrigation treatments with full and 12 different deficit irrigations were applied to the hybrid Sanbro (Novartis Seed Company) planted on clay soil, at three critical development stages: heading (H), flowering (F) and milk ripening (M). The yield increased with irrigation water amount, and the highest seed yield (3.95 t ha⁻¹) and oil yield (1.78 t ha⁻¹) were obtained from the HFM treatment (full irrigation at three stages); 82.9 and 85.4% increases, respectively, compared to the control. Evapotranspiration (ET) increased with increased amounts of irrigation water supplied. The highest seasonal ET (average of 652 mm) was estimated at the HFM treatment. Additionally, yield response factor (k _y) was separately calculated for each, two and total growth stages, and k _y was found to be 0.8382, 0.9159 (the highest value) and 0.7708 (the lowest value) for the total growing season, heading, and flowering-milk ripening combination stages, respectively. It is concluded that HFM irrigation is the best choice for maximum yield under the local conditions, but these irrigation schemes must be re-considered in areas where water resources are more limited. In the case of more restricted irrigation, the limitation of irrigation water at the flowering period should be avoided; as the highest water use efficiency (WUE) (7.80 kg ha⁻¹ mm⁻¹) and irrigation water use efficiency (IWUE) (10.19 kg ha⁻¹ mm⁻¹) were obtained from the F treatment.     

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.     

Cotton yields under different furrow irrigation management techniques in the Southeastern Anatolia Project (GAP) area,Turkey

The continuous flow furrow irrigation (COFFI), surge flow furrow irrigation (SUFFI), cutback flow furrow irrigation (CUFFI), variable alternate flow furrow irrigation (VAFFI), and tail water reuse system furrow (TWRSF) techniques with the same inflow rate of 0.072 m³ min⁻¹ were compared in relation to the cotton yield and water use efficiency at a 3-year field study conducted on cotton (Gossypium spp.) in the Southeastern Anatolia Project (GAP) area of Turkey. Yields revealed significant statistical differences between the furrow management techniques (P < 0.05). The maximum yield was obtained from the COFFI treatment (2,630 and 2,920 kg ha⁻¹) in the first 2 years, and from SUFFI and CUFFI treatments (3,690 and 3,780 kg ha⁻¹, respectively) in the last year. There were significant yield reductions, which varied from 10 to 35% in TWRSF and from 11 to 19% in VAFFI treatments although 43 and 28% more water was applied to the TWRSF than to CUFFI and SUFFI treatments, respectively. The average total water use efficiencies (WUE_ET) varied from 4.14 (VAFFI) to 2.59 (COFFI). The corresponding values were 0.37 and 0.36 kg ha⁻¹ m⁻³ for CUFFI and SUFFI, respectively. The average irrigation water use efficiency (WUE_IR) for CUFFI and SUFFI treatments were 0.30 and 0.23 kg ha⁻¹ m⁻³, respectively.