Effects of soil type on soybean crop water use in weighing lysimeters

Summary Different soils are known to affect the amount and distribution of both available water and roots. Optimising irrigation water use, especially when shallow water-tables are present requires accurate knowledge of the root zone dynamics. This study was conducted to determine the effect of two soil types on root growth, soil water extraction patterns, and contributions of a water-table to crop evaporation (E). Two weighing lysimeters (L1 and L2) with undisturbed blocks of soil were used. The soil in L1 had higher hydraulic conductivity and lower bulk density than that in L2. Well watered conditions were maintained by irrigation for the first 110 days from sowing (DFS). Root length density (RLD) was calculated from observations made in clear acrylic tubes installed into the sides of the lysimeters. Volumetric soil water contents were measured with a neutron probe. A water-table (EC = 0.01 S m^-1) was established 1 m below the soil surface 18 DFS. RLD values were greater in L1 than L2 at any depth. In L1, maximum RLD values (3 × 10⁴ m m^-3) were measured immediately above the water-table at physiological maturity (133 DFS). In L2, maximum RLD values (1.5 × 10⁴ m m^-3) were measured at 0.42 m on 120 DFS and few roots were present above the water-table. From 71 to 74 DFS, 55 and 64% of E was extracted from above 0.2 m for L1 and L2, respectively. In L2, extraction was essentially limited to the upper 0.4 m, while L1 extraction was to 0.8 m depth. Around 100 DFS the water-table contributed 29% (L1) and 7% (L2) of the water evaporated. This proportion increased rapidly as the upper soil layers dried following the last substantial irrigation 106 DFS. Over the whole season the water-table contributed 24% in L1 and 6.5% in L2 of total E.     

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

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

Effect of initial soil salinity and subirrigation water salinity on potato tuber yield and size

A field lysimeter study was conducted to investigate the effect of initial soil salinity and salinity level of brackish subirrigation water on tuber weight and tuber size of three potato (Solanum tuberosum L.) cultivars (Kennebec, Norland and Russet Burbank) under simulated arid conditions. Both saline and non-saline initial soil conditions were simulated in a total of 36 lysimeters. Eighteen lysimeters were flushed with fresh water (0.2 dS/m), while the remaining 18 lysimeters were flushed with brackish water (2 dS/m). For each soil condition, two subirrigation water concentrations, 1 and 9 dS/m, were used in nine lysimeters each. For each subirrigation water treatment, three potato cultivars were grown. In all lysimeters, water table was maintained at 0.4 m from the soil surface. Arid conditions were simulated by covering the lysimeter top with plastic mulch, allowing the potato shoots to grow through a cut in the mulch. The average root zone salinities (EC_w) were found to be 1.2 and 1.5 dS/m in non-saline lysimeters subirrigated with 1 and 9 dS/m waters, respectively. The corresponding salinities were 3.2 and 3.7 dS/m in the saline lysimeters. Across cultivars, there was no significant effect of either initial soil salinity or subirrigation water salinity on total tuber weight. However, the weight of Grade A tubers was higher in non-saline soil than in saline soil. Kennebec and Russet Burbank Grade A tuber weights were not affected by the initial soil salinity. On the contrary, a significant reduction in Grade A and total tuber weight under initially saline soil was evident for the Norland cultivar.     

污水灌溉对冬小麦根长密度和根系吸水速率分布的影响

由于淡水资源的紧缺 ,已有越来越多的污水被用于农业灌溉。设置田间冬小麦栽培试验 ,分析比较了污水、淡水灌溉条件下冬小麦根长密度分布和相对根长密度分布的变化规律 ,并应用平均根系吸水速率的反求方法对各处理冬小麦不同生育阶段的平均根系吸水速率分布进行了估算与分析。结果表明 :与淡水灌溉相比 ,若采用二级处理污水 (电导率 1.14m S/cm)对冬小麦实施灌溉 ,将使得近地表处的根长密度有所增加 ,而下部土层中的根长密度分布则变化不大 ;尽管灌溉水质不同、生育阶段各异 ,但冬小麦相对根长密度在相对深度上的分布却差异不大 ;污水灌溉能显著降低冬小麦的平均根系吸水速率 ,影响作物对土壤水分的吸收利用。     

Prediction of Trafficability and Workability using Tensiometers

The relationship between the strength of field soils, under grass ley or winter wheat, and soil water suction was monitored over an 18 month period. The soil types selected for this investigation represent a range of agricultural soils commonly found in central and eastern England. Results are presented which show that a strong correlation exists between soil strength and soil water suction. This relationship was found to be dependent on soil type, soil bulk density and type of vegetative cover. Critical soil water suction limits for predicting the trafficability and workability of the soils are presented. These can be used in conjunction with data from tensiometers to assess the status of field soils, repeatedly and non-destructively, at fixed points within fields.     

矿区复垦土壤种植小麦的重金属安全性

为分析矿区充填复垦土壤种植农作物的生态安全性,以中国徐州市柳新矿区复垦场地为例,对照当地农田土壤,对充填场地种植小麦不同部位As、Cd、Cr、Cu、Hg、Pb、Zn 7种重金属元素含量进行检测,分析了Cd、Cr在成熟小麦不同器官中的分布、富集情况,并利用单项污染指数法和综合指数法对小麦籽粒进行了风险评价。结果表明,三场地中Cd 和Hg在未成熟苗中的含量小于成熟茎中的含量,Cr、Cu和Zn则与之相反,Pb和As的含量规律性不太明显;复垦场地小麦未成熟根的重金属含量都小于成熟根中的含量;比照地中除Cr 和Zn之外,其余元素在未成熟根中的含量均大于成熟根的含量;参照粮食污染限量标准,小麦籽粒中Cd和Cr含量均超标。风险评价显示三场地小麦籽粒中Cd、Cr单项污染指数达到轻度至重度水平,综合污染指数处于轻度污染至中度污染水平之间。该研究对矿区粮食安全的监控具有重要的实际应用价值。     

亏缺灌溉对成龄库尔勒香梨产量与根系生长的影响

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

Avocado root distribution in fine and coarse-textured soils under drip and microsprinkler irrigation

A common irrigation-scheduling problem in orchards is the proper location of instruments for monitoring soil water content within the active root zone. Given the high spatial variability of soils in the field, and seasonal changes in root distribution and frequency, both within the orchard and around the trees, the accuracy and representativeness of soil water measurements can be strongly affected. Adequate soil water monitoring in orchards thus requires assessment of the variability and location of the active roots in a given location over an extended period of time. We examined the root systems of 12-year-old ‘Hass’ avocado (Persea americana Mill.) trees grafted on ‘Mexicola’ seedling rootstocks, growing in fine or coarse-textured soils, under either drip or microsprinkler irrigation systems in Central Chile. We dug 3 m long and 0.75 m deep trenches within the tree rows in spring, summer and autumn, and counted the active roots (white, diameter ≤2 mm) found on the walls. Over the three growing seasons of our study, season had the most significant effect on root distribution, as autumn root frequencies accounted for about half of the cumulative average. Also, the location of the highest concentration of roots under microsprinklers in autumn clearly differed between the fine soils, at about 200 cm from the trunk and 50–60 cm deep, and coarse soils, where they were found within 30 cm from the trunk, and within the first 25 cm of soil. Trees in fine soil had 25% more roots than those in coarse soil, and drip irrigation produced about 30% more roots than microsprinkler, although both of these figures are mainly due to the high number of roots found in the fine soil-drip irrigation combination. Overall, we found the highest root frequency within the first meter from the tree trunk, for all combinations, with some differences between irrigation types. Throughout the growing season in semi-arid regions, some changes in both the quantity of tree roots and the location of the zones of the greatest root activity should be expected, which will vary according to the seasonal soil temperatures, soil texture, and type of irrigation used.     

Effects of Treflan injection on winter wheat growth and root clogging of subsurface drippers

The clogging of drippers caused by crop root intrusion has been a great concern of subsurface drip irrigation (SDI) systems. To attempt to solve the problem of root clogging of drippers, a series of field experiments were conducted in the growing seasons of 2006-2008. The goal was to investigate the effects of Treflan injection on dripper clogging by roots, and on root distribution, yield, and the quality of winter wheat (Triticum aestivum L.) under SDI. For each growing season, two Treflan injection dates (March 6 and April 15 for the 2006-2007 growing season, and March 6 and April 15 for the 2007-2008 growing season) and three injection concentrations of 0, 3, and 7 mg/l were arranged in a randomized block experimental design. During harvest, root length density (RLD) and Treflan concentration at different soil layers were measured using the auger-sampling method. Thirty-five drippers from each treatment were randomly chosen to observe evidence of root intrusion into the dripper flow passage in order to estimate root clogging. The experimental results showed that Treflan injection could effectively reduce root density in areas adjacent to drippers, thereby significantly decreasing the potential of root clogging. In 2007, 4 out of the 35 drippers were found with root intrusion problems in the control (without Treflan injection), while no root clogging existed any dripper in Treflan application treatments. In 2008, 6 drippers from the control but only 1 dripper from those treated with Treflan application showed root clogging. In addition, within the range of concentration used by the current experiment, Treflan concentrations had no significant effects on winter wheat root distribution, yield, and quality. Injection date, however, influenced the vertical root distribution significantly. Injection of Treflan late in the growing season influenced the root distribution only in the areas close to the drippers; the influenced areas increased if Treflan was injected early in the growing season.     

Macroscopic scale soil moisture dynamics model for a wheat crop

Summary A numerical soil moisture dynamics model was developed for; wheat crop using either observed or generated root length densities with root sink incorporating diminishing rate of water uptake by plant roots due to decreasing soil moisture in drying cycles and loss of absorptive power of roots due to ageing. The simulated soil moisture contents were overestimated by 6.0 and 9.6% on an overall basis by the model when observed and generated root length densities were used, respectively, in comparison to observed moisture contents. The model using generated root length densities simulated less water uptake in comparison with the model which utilized observed root length densities.     

Effects of drip irrigation frequency on soil wetting pattern and potato growth in North China Plain

The effect of irrigation frequency on soil water distribution, potato root distribution, potato tuber yield and water use efficiency was studied in 2001 and 2002 field experiments. Treatments consisted of six different drip irrigation frequencies: N1 (once every day), N2 (once every 2 days), N3 (once every 3 days), N4 (once every 4 days), N6 (once every 6 days) and N8 (once every 8 days), with total drip irrigation water equal for the different frequencies. The results indicated that drip irrigation frequency did affect soil water distribution, depending on potato growing stage, soil depth and distance from the emitter. Under treatment N1, soil matric potential (ψ_m) Variations at depths of 70 and 90 cm showed a larger wetted soil range than was initially expected. Potato root growth was also affected by drip irrigation frequency to some extent: the higher the frequency, the higher was the root length density (RLD) in 0–60 cm soil layer and the lower was the root length density (RWD) in 0–10 cm soil layer. On the other hand, potato roots were not limited in wetted soil volume even when the crop was irrigated at the highest frequency. High frequency irrigation enhanced potato tuber growth and water use efficiency (WUE). Reducing irrigation frequency from N1 to N8 resulted in significant yield reductions by 33.4 and 29.1% in 2001 and 2002, respectively. For total ET, little difference was found among the different irrigation frequency treatments.     

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

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

Irrigation and tillage effects on root development,water use and yield of wheat on coarse textured soils

Summary Rapid drying of surface layers of coarse-textured soils early in the growth season increases soil strength and restricts root growth. This constraint on root growth may be countered by deep tillage and/or early irrigation. We investigated tillage and irrigation effects on root growth, water use, dry matter and grain yield of wheat on loamy sand and sandy loam soils for three years. Treatments included all combinations of two tillage systems i) conventional tillage (CT) — stirring the soil to 10 cm depth, ii) deep tillage (DT) — subsoiling with a single-tine chisel down to 35–40 cm, 40 cm apart followed by CT; and four irrigation regimes, i) I₀ — no post-seeding irrigation, ii) I₁ — 50 mm irrigation 30 days after seeding (DAS), iii) I₂ — 50 mm irrigation 30 DAS and subsequent irrigations of 75 mm each when net evaporation from USWB class A open pan (PAN-E) since previous irrigation accumulated to 82 mm, and iv) I₃ — same as in I² but irrigation applied when PAN-E accumulated to 62 mm. The crop of wheat (Triticum aestivum L. HD 2329) was fertilized with 20kg P, 10kg K and 5kg Zn ha^–1 at seeding. The rate of nitrogen fertilization was 60 kg ha^–1 in the unirrigated and 120 kg ha^–1 in the irrigated treatments. Tillage decreased soil strength and so did the early post-seeding irrigation. Both deep tillage and early irrigation shortened the time needed for the root system to reach a specified depth. Subsequent wetting through rain/irrigation reduced the rate of root penetration down the profile and also negated deep tillage effects on rooting depth. However, tillage/irrigation increased root length density in the rooted profile even in a wet year. Better rooting resulted in greater profile water depletion, more favourable plant water status and higher dry matter and grain yields. In a dry year, the wheat in the DT plots used 46 mm more water, remained 3.3 °C cooler at grain-fill and yielded 68% more grain than in CT when unirrigated and grown in the loamy sand. Early irrigation also increased profile water depletion, more so in CT than DT. Averaged over three years, grain yield in DT was 12 and 9% higher than in CT on loamy sand and sandy loam, respectively. Benefits of DT decreased with increase in rainfall and irrigation. Irrigation significantly increased grain yield on both soils, but the response was greatly influenced by soil type, tillage system and year. The study shows that soil related constraints on root growth may be alleviated through deep tillage and/or early irrigation.     

Yield and water relations of wheat as influenced by irrigation and depth of tillage

Summary Development of a ploughpan has been reported in Bangladesh for almost all ploughed soils which are puddled for transplanted rice cultivation. Field information on the water requirement of dryland crops such as wheat and the effects of loosening the dense layer on crop yield and water use efficiency are very limited. Field experiments were, therefore, conducted in the grey floodplain soil of Sonatala series (Aeric Haplaquept) to study the irrigation and tillage effects on the yield and water relations of wheat (Triticum aestivum L. cv. Sonalika). The split plot design experiment comprised four irrigation treatments in the mainplots viz. W₀ = no irrigation, W₁ = irrigation of 5 cm at 4 weeks after planting, W₂-W₁ + irrigation(s) of 5 cm each at irrigation water to cummulative pan evaporation (IW/CPE) ratio of 0.75 and W₃- W₁ + irrigation(s) of 5 cm eacht at IW/CPE ratio of 0.50. The sub-plot tillage depth treatments were: A-7.5 cm (traditional), B-15 cm, C-22.5 cm, D-22.5 cm practised in alternate wheat seasons. Measurements were made of grain and straw yield, soil water depletion and water expense efficiency.Irrigation had no effect on grain or straw yield. Tillage to 15 cm increased wheat yield by about 15% over traditional depth to ploughing. In general, deep tillage coupled with one irrigation at four weeks after planting produced the largest wheat yield.Soil water depletion (SWD) in the 0–90 cm profile was greatest in the treatment receiving two irrigations, one at 4 weeks and again at IW/CPE ratio of 0.50. The average SWD in this treatment was 113 in 1982–83 and 82 mm in 1983–84. Plots receiving traditional tillage (7.5 cm) had the greatest SWD. Total water expense were the greatest in treatments receiving three irrigations. The maximum water expense efficiency (WEE) of wheat was observed in the non-irrigated plots in 1982–83 and 1983–84, respectively. Deep tillage treatments, in general, had significantly greater WEE than those under traditional ploughing. Intensive irrigation and efficient soil and water management are important factors in enhancing crop productivity. The former not only permits judicious water use but also better utilization of other production factors thereby leading to increased crop yield which, in turn, helps stabilize the farming economy. The best way to meet increasing demand for water is to adopt efficient water management practices to increase water use efficiency.Irrigation should aim at restoring the soil water in the root zone to a level at which the crop can fully meet its evapo-transpiration (ET) requirement. The amount of water to be applied at each irrigation and how often a soil should be irrigated depend, however, on several factors such as the degree of soil water deficit before irrigation, soil types, crops, and climatic conditions (Chaudhury and Gupta 1980).Knowledge of movement of water through the soil is imperative to efficient water management and utilization. The presence of a dense pan impedes water movement into the sub-soil. As a result, the top soil becomes saturated by irrigation and sensitive dryland crops can fail as this plough layer impedes the penetration of roots into deeper soil layers and decreases water extraction. Crops growing in these soils often undergo severe water stress within 5–8 days after rainfall or irrigation (Lowry et al. 1970). Due to decrease rates of water flow, the lower soil layer may remain unsaturated and as a result, the recharge and soil water storage in the profile are considerably decreased (Sur et al. 1981).In Bangladesh, ploughpans develop to varying degree in almost all ploughed soils (Brammer 1980). They are particularly marked in soils which are puddled for transplanted rice cultivation where the pan is usually only 8–10 cm below the soil surface and 3–5 cm thick. Its presence is generally regarded as advantageous for cultivation of transplanted rice in that it prevents excessive deep percolation losses of water. But in the same soil this cultivation for a subsequent dryland crop would adversely affect yield. A slight modification of the plough layer could enable good yields of both rice and a dryland crop to be obtained in the same soil in different seasons (Brammer 1980). The sub soils have a good bearing capacity, both when wet and dry and the pan can easily be reformed, if desired, for cultivating transplanted rice after a dryland crop like wheat.Professor of Soil Science, Dhaka University, Dhaka, Bangladesh     

Effect of root and water distribution in lysimeters and in the field on the onset of crop water stress

Summary Lysimeters have been frequently used to study crop response to the onset of water stress. To test the representativeness of lysimeter derived criteria for the onset of crop water stress, spring wheat (Triticum aestivum L.) was grown in two field plots with 1.0 m deep lysimeters in the center of each plot. One plot was well-watered while the second was subjected to a drying period with no irrigation. Crop water stress was assessed by monitoring leaf water potential ( _l ), stomatal diffusive resistance (r _s ), canopy temperature (CT), evapotranspiration (ET), and soil water content in both plots and lysimeters. The rate of change of all these measured parameters, when compared to the well-watered field control-plot revealed that the field-grown plants showed signs of water stress long before the lysimeter-grown plants. Water stress developed gradually for the field crop, but the transition from the well-watered to the stressed condition happened abruptly for the lysimeter-grown plants. Once this transition occurred, the lysimeter-grown plants were more drought stressed than the field-grown plant. Water profiles measured inside the lysimeter were different from those measured in the adjacent plots. An increase in root length density with depths below 0.6 m was observed in the lysimeters as opposed to a quasimonotonic decrease with depth in the field. The response of the lysimeter-grown plants was a result of the anomalous water content and root distribution. We conclude that threshold values of ET, _l , r _s , and CT for the onset of water stress obtained when deep-rooted crops grown in a shallow lysimeter are subjected to drought periods may not be directly applicable to field situations.     

外源物质对镉胁迫下不同品种冬小麦苗期镉吸收特征的影响

[目的]分析不同外源物质对不同品种冬小麦苗期Cd吸收和迁移特征的影响及其差异性.[方法]通过向含不同质量浓度Cd(10、30 mg/L)的营养液中添加不同质量浓度的Si(28、56 mg/L)、Ca(50、100 mg/L)、Mg(50、100 mg/L)和腐殖酸(5、15 mg/L),在水培条件下研究了籽粒Cd高积累...     

Prediction of the soil-depth salinity-trend in a vineyard after sustained irrigation with saline water

Remote sensing combined with an ability to look deeper than the soil surface is currently high in demand. This study was conducted through scaling down the amount of soil data from a saline irrigation water experiment to see if one can still capture the essential soil salinity depth trends within the data, to a level that can enhance the ability of remote methods. A saline irrigation experiment with 6 water qualities was conducted for 8 years on 1.2 ha of vineyard land near Robertson in the Western Cape Province of South Africa. Soil water was sampled at regular intervals at 5 depths between 0.15 and 1.2 m with suction cup lysimeters at a fixed time following each irrigation. Electrical conductivity of the soil water (EC_sw) was determined after sampling. Data collected over the full 8-year period were investigated for depth trends in EC_sw, seeking trend lines with lowest polynomial order that were still significantly predict the salinity profile. At all treatment levels a first order polynomial equation, fitted to the salinity profiles, significantly predicted the salinity trends. The EC_sw value at only two depths could therefore be used to calculate total salt accumulation and soil water quality below the root zone. The implication is that considerable value can be obtained from minimal measurements both in estimating salt accumulation in the soil profile and predicting water quality in return flow from saline irrigation.     

Distribution of roots and root length density in a maize/soybean strip intercropping system

In a field experiment in the Yellow River Basin conducted in 2007 and 2008, it was found that, under full irrigation, the roots of maize not only penetrated deeper than those of soybean but also extended into soybean stands underneath the space between inner rows of soybean. The roots of soybean, however, were confined mainly to the zone near the plants. Horizontal growth of the roots of both the crops was confined mainly to the soil layer 16-22 cm below the surface, a layer that lay above an existing plough pan. Root length density (RLD) was much higher in the top layer (0-30 cm deep) and in the zone closer to the plants. The exponential model proved suitable to describe the RLD vertically and horizontally in both sole cropping and in intercropping.     

A simulation model of water dynamics in winter wheat field and its application in a semiarid region

Many models for water flow in cropped soil contain parameters such as rooting density, root permeability, and root water potential. Usually these parameters are chosen by trial-and-error method and direct measurements are difficult and impractical in some cases. This study presents a simulation model capable of analyzing water transport dynamics in a soil–plant–atmosphere continuum (SPAC). This model is developed by combining an existing mathematical model for soil water flow, a modified transpiration model taking into account of the air pressure and diurnal changes of the extinction coefficient of crop canopies, and a new simple model for root water uptake. Using data from lysimeters in a field experiment carried out on a wheat crop, we also developed two new empirical equations for the estimation of total canopy resistance and soil evaporation.We then applied the model for 2 years (1990–1991, 1991–1992) on winter wheat in a semiarid area of northwest China. Required parameters, particularly soil hydraulic and crop parameters, were determined by field and laboratory tests. Outputs from the simulation were in good agreement with the independent field measurements of seasonal changes in soil water content, canopy transpiration, surface evaporation, and root water uptake along the soil profile. In addition, this simulation agreed well with the actual measurements of seasonal crop water consumption and soil water balance among the treatments for different irrigation amounts.     

Soil type effects on soybean crop water use in weighing lysimeters

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