Soil solution and exchange complex response to repeated wetting–drying with modestly saline–sodic water

Coal bed natural gas (CBNG) extraction in the Powder River (PR) Basin of Wyoming and Montana produces modestly saline-sodic wastewater, which may have electrical conductivity (EC) and sodium adsorption ratios (SAR) exceeding accepted thresholds for irrigation (EC = 3 dS m⁻¹, SAR = 12 (mmol_c l⁻¹)^1/2. As an approach to managing large volumes of CBNG-produced water, treatment processes have been developed to adjust produced water salinity and sodicity to published irrigation guidelines and legislated in-stream standards. The objective of this laboratory study was to assess acute and chronic soil solution EC and SAR responses to various wetting regimes simulating repeated flood irrigation with treated CBNG product water, followed by single rainfall events. Fifty-four soil samples from irrigated fields in southeast Montana were subjected to simulated PR water or CBNG water treated to EC and SAR values accepted as thresholds for designation of saline × sodic water, in a single wetting event, five wetting–drying events, or five wetting–drying events, followed by leaching with distilled water. Resultant saturated paste extract EC (ECe) and SAR of soils having <33% clay did not differ from one another, but resulting ECe and SAR were all less than those for soil having >33% clay. Repeated wetting with PR water having EC of 1.56 dS m⁻¹ and SAR of 4.54 led to SAR <12, but brought ECe near 3 dS m⁻¹. Repeated wetting with water having salinity = 3.12 dS m⁻¹ and SAR = 13.09 led to ECe >3 dS m⁻¹ and SAR near 12. Subsequent inundation and drainage with distilled water, simulating rainfall-quality leaching, reduced ECe and SAR more often in coarse-textured, high salt content soils than in finer-textured, lower salt content soils. Decreases in ECe upon leaching with distilled water were of greater magnitude than corresponding decreases in SAR, reinforcing supposition of sodium-induced dispersion of fine-textured soils as a consequence of rainfall following irrigation with water having salinity and sodicity levels equal to previously published thresholds.     

Changes in spatial and temporal variability of SAR affected by shallow groundwater management of an irrigated field, California

In the irrigated western U.S. disposal of drainage water has become a significant economic and environmental liability. Development of irrigation water management practices that reduce drainage water volumes is essential. One strategy combines restricted drainage outflow (by plugging the drains) with deficit irrigation to maximize shallow groundwater consumption by crops, thus reducing drainage that needs disposal. This approach is not without potential pitfalls; upward movement of groundwater in response to crop water uptake may increase salt and sodium concentrations in the root zone. The purposes for this study were: to observe changes in the spatial and temporal distributions of SAR (sodium adsorption ratio) and salt in a field managed to minimize drainage discharge; to determine if in situ drainage reduction strategy affects SAR distribution in the soil profile; and to identify soil or management factors that can help explain field wide variability. We measured SAR, soil salinity (EC_1:1) and soil texture over 3 years in a 60-ha irrigated field on the west side of the San Joaquin Valley, California. At the time we started our measurements, the field was beginning to be managed according to a shallow groundwater/drainage reduction strategy. Soil salinity and SAR were found to be highly correlated in the field. The observed spatial and temporal variability in SAR was largely a product of soil textural variations within the field and their associated variations in apparent leaching fraction. During the 3-year study period, the percentage of the field in which the lower profile (90-180 cm) depth averaged SAR was above 10, increased from 20 to 40%. Since salinity was increasing concomitantly with SAR, and because the soil contained gypsum, sodium hazard was not expected to become a limiting factor for long term shallow groundwater management by drain control. It is anticipated that the technology will be viable for future seasons.     

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

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

不同淋洗条件下黄河三角洲盐渍土脱盐规律研究

【目的】探索黄河三角洲地区盐渍土在不同淋洗条件下土壤脱盐规律。【方法】通过室内土柱淋洗脱盐模拟试验,设置2种淋洗方式(连续淋洗和间歇淋洗),分析了在连续淋洗和间歇淋洗条件下土壤淋洗耗水量、淋洗滤液的矿化度随时间、滤液累积量的变化规律和滤液的脱盐速率,同时分析了0～20、20～40、40～60、60～80、80～100 cm土层的电导率、SAR(钠吸附比)的变化过程。【结果】①不同淋洗方式条件下,土体脱盐共有3个过程,分别为盐分峰值初步形成过程、盐分峰值向下移动过程和土柱底层土体盐分峰值消失过程;连续淋洗和间歇淋洗土柱为达到一般农作物(计划湿润层为0～60 cm)生长所需淋洗水量为472.70 mm和411.60 mm,间歇淋洗较连续淋洗省水14.8%。②连续淋洗和间歇淋洗滤液矿化度随时间均表现为幂函数关系;连续淋洗和间歇淋洗的滤液脱盐效率分别为18.45 g/L2和28.49 g/L2,连续淋洗的滤液脱盐效率为间歇淋洗的64.7%。③连续淋洗土柱和间歇淋洗土柱淋洗后含盐量是淋洗脱盐前的11.89%和8.39%(以40～60 cm土层为例),间歇淋洗土柱中各层pH值增量均小于在连续淋洗土柱中pH值的增量,并且在间歇淋洗后各层土壤pH值虽有增加,但是还在一般植物生长的允许范围内;间歇淋洗土柱中SAR减小量大于连续淋洗土柱中SAR的减小量,RSC增量小于连续淋洗土柱中RSC的增量,SAR和RSC均在一般植物生长的允许范围内。【结论】盐渍土经过淋洗脱盐可以达到植物生长的要求,同时,间歇淋洗明显比连续淋洗节约水,在生产实践中采用间歇淋洗土壤脱盐效果更好。     

Salt movement,leaching efficiency,and leaching requirement

To study the salt movement in a soil profile, experiments were conducted on sandy loam and silty clay loam in tanks. The chloride concentration and electrical conductivity of the soil water were found from soil water samplers and salinity sensors. The standard deviation of the chloride concentration at each depth was small at the beginning of the percolation process when the soil was uniformly non-saline or highly saline; it increased strongly during the process and returned to its original value at the end. This points to a very heterogeneous water and salt movement through the soil profile. The chloride concentration, when increasing or decreasing rapidly, shows a large scattering. The effective mixing length in the tank experiments appears to be much greater than in laboratory soil columns. It varies between 10 and 15 cm in sandy loam and between 15 and 30 cm in silty clay loam. Irrigation water and soil moisture do not mix completely. An increasing part of the irrigation water moves through the soil without contributing to the leaching process. The results of the tank experiments agree with those of field experiments on similar soils. The consequences for the calculation of the leaching requirement in practice are discussed.     

Hydraulic conductivity and clay dispersion as affected by application sequence of saline and simulated rain water

Summary Saturated hydraulic conductivity HC, and degree of clay dispersion DD, were determined for a sandy loam and a clay loam soil with waters of different combinations of sodium adsorption ratios SAR (5, 15, 30 and 45 mmol^1/2l^–1/2) and total electrolyte concentration TEC (15, 30, 60 and 90 me l^–1) followed by distilled water to simulate rainwater. Increase in SAR and decrease in TEC of leaching water increased DD and decreased HC of soils. The HC values were more highly correlated with SAR than TEC. The critical ratio of TEC/SAR of water below which the relative HC is less than the hreshold value (i.e. 0.75) was 3.82 and 2.01 for clay loam and sandy loam, respectively taking the HC of initial soil with good quality water (SAR = 0.5, EC = 0.3dS m^–1) as the reference. Drastic reductions in conductivity were observed even at SAR = 5 (60–83%) when saline water was displaced by rainwater, sensitivity being greater for the sandy loam than for the clay loam; recovery was negligible when the saline water was again applied. Data of EC and clay content of the effluent on application of distilled water suggested that clay dispersion, its movement and lodgement into conducting pores, may be the major cause of HC reduction in sandy loam, whereas in clay loam, surface sealing is the major cause. With distilled water application HC values were governed by SAR rather than TEC of initial water used. The study thus suggests that existing water quality criterion may underestimate the real soil permeability hazards from saline-sodic waters during rainfall infiltration in monsoon season.     

Effect of sustained saline irrigation on soil salinity and crop yields

Summary Field studies were conducted for a period of ten years (1974 to 1984) on Typic Ustochrept to determine the sustained effects of saline irrigation water electrical conductivity (EC _iw ) 3.2 dS/m, sodium adsorption ratio (SAR) 21 (mmol/1)^1/2 and residual sodium carbonate (RSC) 4me/1, on the build up of salinity in the soil profile and yield of crops grown under fixed rice-wheat and maize/millet-wheat rotations. Saline waters were continuously used with and without the addition of gypsum (at the rate needed to reduce RSC to zero) applied at each irrigation. In maize/millet-wheat rotation, two additional treatments viz. (i) irrigation with 50% extra water over and above the normal 6 cm irrigation, and (ii) irrigation with good water and saline water alternately, were also kept. The results showed that salinity increased rapidly in the profile during the initial years but after five years (1979–1984) the average soluble salt concentration in 0–90 cm soil profile did not appreciably vary and the mean EC _e values under saline water treatment remained almost similar to EC _iw , under both the crop rotations.Saline water irrigation increased pH and Na saturation of the soil, reduced water infiltration rate and decreased yields of maize, rice and wheat. The differences in the build up of salinity and ESP of the soil under the two cropping sequences seemed to be related with the differences in leaching that occurred under rice-wheat and maize/millet-wheat rotations. Application of gypsum increased the removal of Na from the profile, appreciably decreased the pH and Na saturation and improved water infiltration rate and raised crop yields. Application of non-saline and saline waters alternately was found to be a useful practice but irrigation with 50% extra water to meet the leaching requirement did not control salinity and hence lowered crop yields.     

Reclamation of saline organic soil

Summary Reclamation of saline, organic soils in the Sacramento-San Joaquin Delta of California was accomplished by both sprinkling and continuously ponding water on the soil surface. The reclamation data support the generalized guideline established for saline, organic soil.A 70% reduction in the average root zone salinity required 3 months under ponding, compared to 4 months under sprinkling. Although accurate measures of water application on the ponded trials were not possible, the limited data indicate that the amount of water required is about the same per unit depth of soil reclaimed for both ponding and sprinkling. Reclamation proceeded more quickly under the second ponding trial than for sprinkling or the first ponding trial because of improved subsurface drainage. With sprinklers, 70% of the salt was removed from the soil profile to a depth of 1.2 m after 850 mm of leaching water entered the profile. Reclamation by ponding required about the same quantity of water but the water required for leaching could be reduced significantly by improved drainage.     

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     

Average concentration of soluble salts in leached soils inferred from the convective–dispersive equation

The convective–dispersive, or advective–dispersive, equation (CDE) has long been the model of choice for solute transport in soils. Using the average concentration of soluble salts in soil profile to evaluate changes in salinity due to irrigation can be beneficial when spatial variability of soil salinity at selected depths is larger than spatial variability of soil salinity in the layer encompassing these depths, and when soil salinity is evaluated with electric conductivity measurements that give layer-average rather than depth-specific salinity values. The objective of this work was to present analytical solutions of the CDE that express the average soluble salt content in soil profile as the function of time, water flux, and solute dispersion parameter. The solutions were developed for both semiinfinite and finite domain and implemented in a computer code. Examples are presented of using these solutions to develop a nomogram for the dispersion coefficient estimation and to evaluate the applicability of the semiinfinite domain solution to soil monolith leaching experiments. In cases when the CDE application is justified, the analysis of the salt leaching based on the average salt concentrations in soil profile provides estimates of the effective salt dispersion parameter useful in land evaluation and soil reclamation.     

Comparative effects of chemical amendments on salt and NA leaching

Summary Efficiency of sodic soil reclamation is thought to vary with types of chemicals used. This study examined the effects of five inorganic (H₂SO₄, CaCl₂ · 2H₂O, CaSO₄ · 2H₂O, FeSO₄, Al₂(SO₄)₃) and two organic compounds (polyacrylamide, and trihydroxy glutaric acid) on the rate and the extent of salt and Na leaching in moderately Na-affected saline soils: Saneli silty clay loam (Vertic Torrifluvents, ESP=17.5%) and Glendale silty clay (Typic Torrifluvents, ESP=13.5%). Air-dry soil samples (<2mm) were packed in columns, and chemicals, except H₂SO₄, were incorporated into the surface 5 cm of the soils, and in selected cases, to 30 cm. H₂SO₄ was surface-applied. Application rates of the inorganic chemicals were 3.57 and 10.7 mmol(+) kg^-1 (2.5 and 7.5 Mg ha^-1 in gypsum equivalent weight) in the silty clay loam, and 8 and 24 mmol(+) kg^-1 in the silty clay, and the organic compounds were applied at rates of less than 620 kg ha^-1. The soils were then leached with simulated Rio Grande water (EC = 1.1 dS m^-1, SAR = 3.5) under continuous ponding. The tested inorganic compounds removed approximately equivalent amounts of exchangeable Na after approximately 35 cm of water application. However, the rate of water percolation (consequently the rate of salt leaching) from CaCl₂ treated columns, became progressively slow after about 20 cm of water intake. The combined effect of rapid electrolyte leaching and insufficient replacement of Na in the surface layer seemed to be responsible for the flow reduction. Gypsum and H₂SO₄ treatments provided lower ratios of sodicity to salinity in percolating solutions and relatively uniform hydraulic gradients throughout the soil depth. Incorporation of chemicals to the surface 30 cm did not alter performance, except in CaCl₂ treatments where water intake rates became even slower. The tested organic amendments improved initial water infiltration, but neither increased subsequent percolation rates nor improved salt and Na leaching. The fastest reclamation may be attained when chemicals are chosen and applied to yield an electrolyte concentration that is high enough to overcome Na effects at any depth of soil profiles throughout the leaching period.Contribution from Texas Agr. Expt. Sta. Texas A & M University System. This project was supported in part by the Binational Agricultural Research and Development (BARD) Fund and the Expanded Research Area Fund of the Texas Agricultural Experiment Station.     

农田土壤中土壤水渗漏与硝态氮淋失的模拟研究

应用HYDRUS-1D模型对黄淮海平原的主要土壤(黄潮土和风沙土)中水分与硝态氮的垂直运移规律进行了模拟分析。对模型参数的敏感性分析表明:饱和水力传导度是最敏感的参数,饱和含水量的敏感性次之。数值模拟结果表明:该地区在传统水氮管理制度下,土壤水渗漏和硝态氮淋失非常严重;全耕作年风沙土的土壤水渗漏大于黄潮土,分别为34.3cm和22.7cm,占灌水量的42.1%和74.6%;风沙土的硝态氮淋失大于黄潮土,分别为108.0kg/hm和76.6kg/hm,占总输入氮量的25.3%、14.3%。     

Seasonal movement of salts in naturally structured saline-sodic clay soils

Seasonal changes in the distribution of salt and water in fields of both arable and grassland saline sodic clay soils were studied under temperate rainfed conditions. Leaching of the topsoils during winter rains was further investigated in soil columns. The field studies indicated the cyclical nature of leaching. During winter rains the water moving through the macropores uniformly leached salt from the soil profile to a depth of 1.2 m, but in late summer the salt content of the grassland and arable soils had increased again by 11% and 35% respectively compared with their early spring salinity levels. The results indicated that the salt leached in winter was mainly not lost, but leached below 1.2 m, only to rise again as the soil profile dried in the summer. The implications for managing and reclaiming these soils with gypsum are discussed.Undisturbed grassland topsoils were slow to release salt into the leaching water, maximum salt concentration in the leachate only being reached well into the winters rains. In disturbed arable soils the maximum leachate concentration was achieved shortly after leaching commenced. The changes in surface structure brought about by rainfall impact on bare restructured ploughlayer soils caused a significant decline in leaching efficiency (up to 40%).The observed pattern of leaching questions the validity of the basic assumptions used in most of the mathematical leaching models.     

Influence of leaching with different amounts of water on desalinization and permeability behaviour of chloride and sulphate-dominated saline soils

Desalinization studies were carried out in Cl-dominated (Cl:S0₄ = 7:3) and SO₄-dominated (Cl:S0₄ = 3:7), saline (ECe = 9 dS m⁻¹, ESP = 10) sandy loam soils, filled in plexiglass columns up to a height of 60 cm at uniform bulk density. Both soils were subjected to leaching with three amounts, i.e. 20, 40 and 60 cm of distilled water, 5 cm at a time until the three leaching cycles were completed. The S0₄ profiles had 20–75% faster percolation than their Cl counterparts, when 40 cm of water was leached. With higher amounts of leaching water, the differences in permeability between the two soils narrowed and became indistinguishable when 60 cm of water was leached. Desalinization of the Cl-profiles was more efficient than the S0₄-profiles, but the reverse was the case with desodification. When the depth of water (dW) leached was less than or equal to the depth of soil WS), desalinization was less efficient (10–25%) under S0₄ than under Cl-salinity. However, when dW was greater than dS, the differences in the magnitude of leaching of salts under the two salinities narrowed and became equal at a dW/dS ratio of 2. An increase in the soil ESP from 10 to 35 resulted in a 75% decrease in soil permeability under Cl compared with a 64% decrease in S0₄-salinity. At ESP of 10, application of farmyard manure (FYM) as an amendment increased soil permeability for both Cl- and S0₄-dominated soils. However, at higher ESP levels (30–35), FYM application decreased soil permeability under Cl-dominated soil, but increased it under S0₄-dominated soil.     

Reclaiming a saline-sodic,sandy loam soil under rice production

A field-plot reclamation experiment was conducted on a virgin saline-sodic, sandy loam, permeable soil while growing rice with pre- and post-planting leaching under conditions of continuous and intermittent submergence. The soil studied contained very high amounts of soluble salts and exchangeable sodium throughout the profile. The chief salts were Cl^? and SO^2?₄ of Na⁺, Ca²⁺ and Mg²⁺. The data obtained showed that post-transplanting leaching under intermittent submergence alone progressively decreased salinity and sodicity throughout the top 100 cm of the soil to levels safe for cultivation of relatively deep-rooted crops. The surface few centimetres of soil were essentially reclaimed within a few hours after leaching so that young rice seedlings established and survived to give good yield. It was concluded, therefore, that reclamation of these types of soil in arid and semi-arid regions, where good quality water is not available for leaching prior to transplanting rice, would not require any such pre-planting leaching. The results further indicated that there is no need to apply an amendment such as gypsum, mainly because Ca²⁺ and Mg²⁺ present in such soils are adequate to replace the initially high exchangeable sodium during leaching. Leaching efficiency was high under conditions of intermittent submergence. It was shown that leaching curves could be useful in determining the amount of leaching water required for a given mode of application in order to decrease harmful levels of salinity and sodicity to safer levels for a particular crop.     

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

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

Effect of water quality on soil structure and infiltration under furrow irrigation

The quality of irrigation water has the potential to significantly affect soil structural properties, infiltration and irrigation application efficiency. While the effect of electrolyte concentration (as indicated by the electrical conductivity EC) and sodium adsorption ratio (SAR) have been studied under laboratory conditions, the effect on soil profile structural properties and irrigation performance have not been widely investigated under field conditions. In this paper, water with three different levels of sodium (SAR = 0.9, 10 and 30) was applied as alternative treatments to a clay loam soil. The application of 238–261 mm of medium- to high-SAR water was found to reduce aggregate stability, increase the bulk density of both the surface crust and underlying soil, and reduce the total depth of infiltration and final infiltration rate. Where low-SAR water was used, there was no significant (P<0.05) difference in final infiltration rate after the first four irrigations. However, where moderate- and high-SAR water was applied after the first four irrigations with the low EC-SAR water, the final infiltration rate was found to decrease on each of the successive irrigation events. For the moderate- and high-SAR treatments, this suggests that a steady-state equilibrium had not been reached within that part of the soil profile impeding infiltration. It is proposed that the initial reduction in infiltration is related to the physical processes of slaking leading to the development of an apedal, hardsetting surface soil layer. Similarly, it is proposed that the subsequent increase in bulk density and decline in infiltration where moderate and high EC-SAR water is applied is due to an increase in clay tactoid swelling reducing the size of the conducting micropores, dispersion blocking pores, and/or an increase in the thickness of the apedal surface layer. The reduction in infiltration associated with the use of high-SAR irrigation water was found to reduce the performance of the irrigations, with the application efficiency of the final irrigation decreasing from 40% where the low-SAR water was used, to 21% where the high-SAR water was applied. The implications for surface irrigating with water containing high sodium levels are discussed.Communicated by A. Kassam     

Seasonal variations in nitrate leaching in structured clay soils under mixed land use

Nitrate leaching was studied for 2 years in a structured clay soil (Evesham series) under grass, winter wheat and spring barley at N fertilizer inputs of 135–144 kg ha^?1 year^?1. Measurements of soil water to 2 m depth by neutron probe showed that the year could be divided into well defined periods of deficit, separated by a period when the soil was at its winter mean water content. Soil water potentials showed very little gradient for water flow below 1 m, and a persistent convergent zero flux plane at 40–60 cm depth during the autumn wetting-up period (September—November).Nitrate concentration in the drainage increased with discharge rates up to 3–6 mm day^?1. Mean nitrate concentrations were generally highest during intermittent drain-flow in the autumn. Of the total N leached over the 2 years, 23 to 28% (5–7 kg N ha^?1) was lost during this period. The remainder (13–25 kg N ha^?1) was leached during winter and virtually no N was lost in the following spring-early summer. This seasonal pattern of N leaching was interpreted in terms of intermittent flow during rainfall of nitrate-rich water from surface layers, which bypassed the relatively dry soil matrix at 40–60 cm, but was intercepted by natural and artificial drainage channels. Implications for the prediction of N leaching loss based on the concept of excess winter rainfall are discussed. When predicting the start of N leaching in structured clay soils, the soil water status should be assessed from measurements of water potential rather than water content.     

Leaching requirement for salinity control I. Wheat,sorghum, and lettuce

Crop productivity and water use efficiency when saline irrigation water is used are highest when efficient irrigation systems are managed to meet the crop's leaching requirement. The objective of this experiment was to establish the leaching requirement. The objective of this experiment was to establish the leaching requirements for frequently irrigated wheat, grain sorghum, and head lettuce. The 4-year study in field plots consisted of six replicated leaching fraction treatments. The plots were pulse-irrigated daily with water having a total dissolved salts concentration of 1350 mg/l.The leaching requirements are 0.08 for wheat and sorghum, and 0.26 for lettuce. The respective evapotranspiration during each crop's growing season coincident with the leaching requirements was 440, 550, and 245 mm. A pan factor of 0.7 was consistent among these three crops at their respective leaching requirements. With daily irrigation, 90% of the crop's water uptake occurred above a soil depth of 0.6 m, independent of leaching fraction.     

Modeling an irrigation management strategy for minimizing the leaching of atrazine

Possible contamination of water resources by applied pesticides (including insecticides and herbicides) is a problem currently confronting irrigated agricultural production. Best management practices have to be adopted to minimize pesticide transport and leaching under irrigated conditions. A field capacity/mixing-cell model (IRRSCHM) and a model that uses Richard’s equation and the convection–dispersion equation to describe water and contaminant dynamics in soils (LEACHP) were used to assess the leaching of atrazine (a herbicide) under corn receiving different levels of early-season irrigation. The early-season irrigation levels were 11.1, 16.8, 23.3, and 28.8 cm out of corresponding seasonal irrigation levels of 31.2, 39.6, 45.5, and 53.1 cm. The objectives were to (a) use a modeling approach to evaluate water management effects on atrazine leaching, and (b) assess the feasibility of using IRRSCHM and LEACHP to guide irrigation for minimizing atrazine leaching.IRRSCHM and LEACHP simulations deviated from the measured atrazine profile, but both models predicted reasonably well the progression in atrazine leaching with increasing water application. Additionally, atrazine pulses predicted by IRRSCHM were ahead of those by LEACHP but lagged behind those observed under the different irrigation levels. Similarly, both models underestimated atrazine leaching, with IRRSCHM leaching estimates being closer to the observed than the LEACHP estimates. For example, the atrazine profile’s center of mass position at 143 days after application, ranged from 34.2 to 49.4 cm for IRRSCHM, 23.8 to 34.7 cm for LEACHP, and 40.6 to 60.9 cm for the measured atrazine profile under irrigation levels that ranged from 31.2 to 53.1 cm of water. Based on accurate predictions of the trends in atrazine leaching in relation to different irrigation levels, IRRSCHM and LEACHP could be used for preliminary assessment of the likely amount of atrazine leaching, resulting from targeted irrigation management strategies.