Leaching requirement for soil salinity control: Steady-state versus transient models

Water scarcity and increased frequency of drought conditions, resulting from erratic weather attributable to climatic change or alterations in historical weather patterns, have caused greater scrutiny of irrigated agriculture's demand on water resources. The traditional guidelines for the calculation of the crop-specific leaching requirement (LR) of irrigated soils have fallen under the microscope of scrutiny and criticism because the commonly used traditional method is believed to erroneously estimate LR due to its assumption of steady-state flow and disregard for processes such as salt precipitation and preferential flow. An over-estimation of the LR would result in the application of excessive amounts of irrigation water and increased salt loads in drainage systems, which can detrimentally impact the environment and reduce water supplies. The objectives of this study are (i) to evaluate the appropriateness of the traditional steady-state method for estimating LR in comparison to the transient method and (ii) to discuss the implications these findings could have on irrigation guidelines and recommendations, particularly with respect to California's Imperial Valley. Steady-state models for calculating LR including the traditional model, which is an extension of the original U.S. Salinity Laboratory LR model, WATSUIT model, and water-production-function model were compared to transient models including TETrans and UNSATCHEM. The calculated LR was lower when determined using a transient approach than when using a steady-state approach. Transient conditions and the influence of preferential flow did not have as significant an effect on lowering the LR as salt precipitation for a representative study of the Imperial Valley using Colorado River water (EC = 1.23 dS/m) for irrigation. A valley-wide LR of 0.08 for a crop rotation of alfalfa/alfalfa/alfalfa/alfalfa/wheat/lettuce, as calculated by both WATSUIT and UNSATCHEM, was concluded to be the most reasonable estimate for the entire Imperial Valley as compared to a LR of 0.13 by the commonly used traditional method. The reduced LR for the Imperial Valley would result in a diminished drainage volume of approximately 1.23 × 10⁸ m³ (i.e., 100,000 ac-ft). The most significant conclusion derived from the comparison is that the use of the traditional steady-state model for estimating LR needs to be reevaluated.     

Determination of evapotranspiration from an alfalfa crop irrigated with saline waste water from an electrical power plant

Summary Investigations were carried out in 1989 to determine the evapotranspiration (ET) of alfalfa when irrigated with saline waste water coming from the evaporation of fresh water in the cooling towers of Utah Power and Light Company Electrical Power Plant at Huntington in central Utah, U.S.A. The primary goal is to dispose of the waste water from the power plant by irrigation and to maximize salt deposition in the soil, maximize crop ET, minimize runoff from the soil surface, and minimize leaching to the ground water. Using the Bowen ratio-energy balance method, alfalfa evapotranspiration was measured at an experimental site for each 20-minute period during the 1989 irrigation season. Using a simplified seasonal water balance, the results showed that cumulative irrigation plus rain was less than evapotranspiration for the 1989 irrigation season. This means that for the long term in addition to irrigation and precipitation some water was withdrawn from the soil for alfalfa crop water requirements (ET_a). Short term evaluations showed that because of unforeseen heavy rain (thunder showers) in this mountainous area between irrigations, ET_a was occasionally less than irrigation plus rain. This means the excess water was stored in the soil for later use. The average value for ET_a/ET_p (potential ET) for the 1989 irrigation season was 0.47 but occasionally the ratio was greater than unity. Short-term studies (Hanks et al. 1990 a) indicate that yield and ET_a are likely to decrease only slightly for the coming years if saline irrigation water is applied. This method of investigation can be applied to any industrial processes which produce waste water.     

Drip Irrigation of Tomato and Cotton Under Shallow Saline Ground Water Conditions

Artificial subsurface drainage is not an option for addressing the saline, shallow ground water conditions along the west side of the San Joaquin Valley because of the lack of drainage water disposal facilities. Thus, the salinity/drainage problem of the valley must be addressed through improved irrigation practices. One option is to use drip irrigation in the salt affected soil.A study evaluated the response of processing tomato and cotton to drip irrigation under shallow, saline ground water at depths less than 1 m. A randomized block experiment with four irrigation treatments of different water applications was used for both crops. Measurements included crop yield and quality, soil salinity, soil water content, soil water potential, and canopy coverage. Results showed drip irrigation of processing tomato to be highly profitable under these conditions due to the yield obtained for the highest water application. Water applications for drip-irrigated tomato should be about equal to seasonal crop evapotranspiration because yield decreased as applied water decreased. No yield response of cotton to applied water occurred indicating that as applied water decreased, cotton uptake of the shallow ground water increased. While a water balance showed no field-wide leaching, salinity data clearly showed salt leaching around the drip lines.     

Adapting the CROPGRO model for saline soils: the case for a common bean crop

Water scarcity and severe environmental degradation are causing water managers in the Fergana Valley, Uzbekistan to re-evaluate irrigation water use. Crop models could play an important role in helping farmers decide which systems (crops and irrigation technologies) are feasible. CROPGRO is a physiologically robust agronomic model, although the current version does not consider the effects of soil salinity on crop water use or growth. CROPGRO was modified to include a salinity response function and was tested for gypsiferous soils. A qualitative analysis of the model indicated the model performed as expected under a range of atmospheric, irrigation and crop tolerance scenarios. Model simulations compared very favourably for common bean (Phaseolus vulgaris) to results obtained in the greenhouse for yield and seasonal crop evapotranspiration with values of the Willmott agreement index (i) of 0.98 for both variables evaluated at different levels of salinity and deficit irrigation. Final biomass predictions were less satisfactory, although the modified model performed as well as the original model. The modified model was successfully tested with field data on common bean from an experiment in the Fergana Valley (i of 0.75 for ET and 0.74 for final yield), although the sensitivity of the model to a soil fertility function and relative nodule number made it difficult to assess the model performance.     

Modeling irrigated cotton with shallow groundwater in the Aral Sea Basin of Uzbekistan: I. Water dynamics

In Khorezm, a region located in the Aral Sea basin of Uzbekistan, water use for irrigation of predominantly cotton is high whereas water use efficiency is low. To quantify the seasonal water and salt balance, water application, crop growth, soil water, and groundwater dynamics were studied on a sandy, sandy loam and loamy cotton field in the years 2003 and 2005. To simulate and quantify improved management strategies and update irrigation standards, the soil water model Hydrus-1D was applied. Results showed that shallow groundwater contributed a substantial share (up to 399 mm) to actual evapotranspiration of cotton (estimated at 488–727 mm), which alleviated water stress in response to suboptimal quantities of water applied for irrigation, but enhanced concurrently secondary soil salinization. Thus, pre-season salt leaching becomes a necessity. Nevertheless, as long as farmers face high uncertainty in irrigation water supply, maintaining shallow groundwater tables can be considered as a safety-net against unreliable water delivery. Simulations showed that in 2003 around 200 mm would have been sufficient during pre-season leaching, whereas up to 300 mm of water was applied in reality amounting to an overuse of almost 33%. Using some of this water during the irrigation season would have alleviated season crop-water stress such as in June 2003. Management strategy analyses revealed that crop water uptake would only marginally benefit from a permanent crop residue layer, often recommended as part of conservation agriculture. Such a mulch layer, however, would substantially reduce soil evaporation, capillary rise of groundwater, and consequently secondary soil salinization. The simulations furthermore demonstrated that not relying on the contribution of shallow groundwater to satisfy crop water demand is possible by implementing timely and soil-specific irrigation scheduling. Water use would then not be higher than the current Uzbek irrigation standards. It is argued that if furrow irrigation is to be continued, pure sandy soils, which constitute <5% of the agricultural soils in Khorezm, are best to be taken out of annual cotton production.     

Twenty years of research on reclamation of salt-affected soils in Romanian rice fields

The physiological particularities of rice enable this plant to obtain its necessary oxygen for the roots through stems. This characteristic permits the use of rice during the reclamation of salt-affected soils with ponded flooding. This paper presents some results of twenty years of research in Romania, on the reclamation of saline and sodic soils under rice, including requirements for soil drainage, soil amendments, and salt leaching, at different stages of the reclamation period.Specific data are provided on the tolerance of rice to salinity, crop rotation in rice fields, fertilization, and irrigation practices.     

微咸水灌溉与土壤水盐调控研究进展

随着淡水资源短缺的日益加剧,合理开发利用微咸水已成为缓解水资源供需矛盾的重要途径之一。由于微咸水中含有大量盐分,用其灌溉必然增加土壤盐分,影响作物生长和土地质量。因此,采取有效措施调控土壤水盐状况成为微咸水安全利用的基础。本文较详细地回顾了微咸水灌溉条件下土壤水盐运移特征、微咸水入渗模型和水盐运移模型、微咸水灌溉方法、微咸水灌溉对作物生长的影响、土壤水盐调控方法等方面的研究进展,并结合目前研究中关注的核心问题,提出了微咸水安全利用方面需要重点研究的科学和技术问题,为进一步研究微咸水灌溉对土壤和作物生长的影响和其内在机制,以及构建合理利用微咸水灌溉模式提供参考。     

Simulations on direct and cyclic use of saline waters for sustaining cotton–wheat in a semi-arid area of north-west India

A validated agro-hydrological model soil water atmosphere plant (SWAP) was applied to formulate guidelines for irrigation planning in cotton–wheat crop rotation using saline ground water as such and in alternation with canal water for sustainable crop production. Six ground water qualities (4, 6, 8, 10, 12 and 14 dS/m), four irrigation schedules with different irrigation depths (4, 6, 8 and 10  cm) and two soil types (sandy loam and loamy sand) were considered for each simulation. The impact of the each irrigation scenario on crop performance, and salinization/desalinisation processes occurring in the soil profile (0–2 m) was evaluated through Water Management Response Indicators (WMRIs). The criterion adopted for sustainable crop production was a minimum of pre-specified values of ET_rel (≥0.75 and ≥0.65 for wheat and cotton, respectively) at the end of the 5th year of simulation corresponding to minimum deep percolation loss of applied water. The extended simulation study revealed that it was possible to use the saline water upto 14 dS/m alternatively with canal water for cotton–wheat rotation in both sandy loam and loamy sand soils. In all situations pre-sown irrigation must be accomplished with canal water (0.3–0.4 dS/m). Also when the quality of ground water deteriorates beyond 10 dS/m, it was suggested to use groundwater for post-sown irrigations alternately with canal water. Generally, percolation losses increased with the increase in level of salinity of ground water to account for leaching and thus maintain a favourable salt balance in the root zone to achieve pre-specified values of ET_rel.     

A proposal for reclamation by dilution of irrigation water

Summary A proposal is developed for reclamation of a soil which has become salinized from irrigation with saline water. The procedure permits continued irrigation after dilution of the saline irrigation water with higher quality water, which achieves partial reclamation without taking the land out of production, and without creating a large flux of drainage.Model calculations using a salt transport-chemical equilibrium model are performed for the case of a saline irrigation water derived from power plant cooling water residual (EC=4.2 mmho/cm), which is diluted by a high quality well water (EC=0.5 mmho/cm) to produce a water which is alternated with the saline water in cycles of two years. Soil salinity and drainage salt loads resulting under this system are compared with the simulated results of a more traditional method of salt leaching.Associate Professor of Soil Physics, Associate Research Agricultural Engineer, and Professor of Soil Physics, respectively     

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.     

Field water supply:yield relationships of grain sorghum grown in three USA Southern Great Plains soils

Field water supply (FWS) combines the three sources of water used by a crop for evapotranspiration (ET), and consists of available soil water at planting (ASWP), rainfall, and irrigation. Examining the grain yield and FWS relationship (Y_g:FWS) may provide insight into the reported variability in crop water production functions such as water productivity (WP) and irrigation water productivity (IWP). Since water is most productive when entirely consumed in ET, diversion of FWS into non-ET losses such as drainage and excessive soil water evaporation results in declines in WP and IWP. The objective of this experiment was to examine the Y_g:FWS and Y_g:ET relationships of grain sorghum grown under a range of irrigation treatments (0, 25, 50, and 100% replacement of ET), beginning soil water contents, evaporative demands, in the Amarillo, Pullman, and Ulysses soils of the Great Plains. The purpose was to determine the amount of FWS beyond which declines in WP and IWP began to occur due to non-ET losses as indicated by a change in the slope and intercept of the Y_g:FWS and Y_g:ET relationships. Large amounts of non-ET irrigation application losses occurred in the finer-textured soils in the T-100 irrigation treatment. In both years, the T-100 irrigation application amounts and ASWP resulted in a FWS ranging from 750 to 870 mm which exceeded the maximum ET requirement of 530-630 mm and which reduced WP and IWP. Piecewise regression analysis of the Y_g:FWS and Y_g:ET relationships for the crops in the Pullman and Ulysses soils identified the knot point, or change in slope and intercept, in the FWS where both WP and IWP tended to be optimized. This was about 500 mm in both soils, and involved the utilization of about 250 mm in ASWP, irrigation applications averaging about 250 mm, and about 60-130 mm remaining in the soil at harvest. For the coarser-textured Amarillo soil, the yield response to increasing FWS was linear, because non-ET application losses such as drainage gradually increased with the irrigation application amount. The linear Y_g response in the sandy Amarillo soil and the piecewise Y_g responses in the clay and silt loams of the Pullman and Ulysses soils to FWS also reflected the difference in water-holding capacities of the soils that affected the amount of available water as irrigation increased. Irrigating without considering FWS resulted in non-ET irrigation application losses and declines in WP and IWP.     

Water and salt regulation and its effects on Leymus chinensis growth under drip irrigation in saline-sodic soils of the Songnen Plain

Field experiments were carried out to investigate water and salt management and its effects on Leymus chinensis growth under drip irrigation on saline-sodic soils of the Songnen Plain, China. The ECe of the experiment soil here is 15.2 dS/m and SAR_e is 14.6 (mmol_c L⁻¹)^1/2. The threshold of soil matric potential (SMP) was preset in different treatments (−5, −10, −15, −20 and −25 kPa) to control the timing of the irrigation cycle using vacuum tensiometers buried at 0.2 m depth immediately under drip emitters. Drip irrigation frequency and soil matric potential significantly influenced water and salt distributions and L. chinensis growth. In the root zone, the soil water content increased with the SMP, but at deeper layers there were no significant differences in soil water content due to the effect of groundwater. Electrical conductivity showed that there was a low-salt zone near the emitters and that drip irrigation inhibited the buildup of salts in the root zone. There was more leaching of salts for −5 and −10 kPa treatments than for the −15, −20 and −25 kPa treatments. After two years of drip irrigation, the surface salts were well leached, and had moved down with the water to depths below 40 cm. The pH of each treatment was a little decreased and the soil nutrient of S1-S5 were all increased after reclamation, but there were no obvious differences of the five treatments. The best growth was achieved with soil matric potentials of −5 and −10 kPa: the plant height, number and length of spikes, number of tillers, coverage and aboveground biomass all attained their maximum values during the growth periods of L. chinensis, with no significant differences between those two treatments. Thus, in the Songnen Plain, drip irrigation can be used on transplanted L. chinensis for restoration of saline-sodic soils. The results provide theoretical and technological guidance for sustainable reclamation salt-affected soil and the quick restoration and reconstruction of saline-sodic grassland.     

咸淡轮灌和生物炭对滨海盐渍土水盐运移特征的影响

为利用滨海地区微咸水改良盐渍土,进行了不同咸淡水轮灌(淡淡、淡咸、咸淡、咸咸)和施用生物炭(0、15、30 t/hm2)的室内入渗试验,探讨了咸淡轮灌和生物炭施用下滨海盐渍土水盐运移过程.结果表明:滨海盐渍土水分运动主要受初始入渗水质的影响,先咸后淡的轮灌方式更有利于土壤水分入渗,入渗速率增加了8.2％～46.9％,并...     

Influence of different water quantities and qualities on lemon trees and soil salt distribution at the Jordan Valley

The influences of water quantity and quality on young lemon trees (Eureka) were studied at the University of Jordan Research Station at the Jordan Valley for 5 years (1996–2000). Five water levels and three water qualities were imposed via trickle irrigation system on clay loam soil. The primary effect of excess salinity is that it renders less water available to plants although some is still present in the root zone. Lemon trees water requirements should be modified year by year since planting according to the percentage shaded area, and this will lead into substantial water saving. Both evaporation from class A pan and the percentage shaded area can be used to give a satisfactory estimate of the lemon trees water requirement at the different growth stages. The highest lemon fruit yield was at irrigation water depth equal to evaporation depth from class A pan when corrected for tree canopy percentage area. Increasing irrigation water salinity 3.7 times increased average crop root zone salinity by about 3.8–4.1 times.The high salt concentration at the soil surface is due to high evaporation rate from wetted areas and the nature of soil water distribution associated with drip irrigation system. Then, the salt concentration decreased until the second depth, thereafter, salt concentration followed the bulb shape of the wetted soil volume under trickle irrigation. Irrigation water salinity is very important factor that should be managed with limited (deficit) irrigation. But increasing amount of applied saline water could result in a negative effect on crop yield and environment such as increasing average crop root zone salinity, nutrient leaching, water logging, increasing the drainage water load of salinity which might pollute ground water and other water sources.     

宁、陕大型灌区地面灌溉存在问题与对策

讨论了陕西与宁夏二省 (区 )大型灌区骨干工程、田间配套工程情况 ;土壤盐渍化发展与治理措施效果 ;井渠结合灌溉、管理体制改革经验等。分析了灌溉定额偏大及土壤盐渍化等问题的成因与危害。针对各灌区存在的实际问题 ,提出了解决的对策与措施     

Soil salinity and sodicity effects of wastewater irrigation in South East Australia

Although ‘sewage farming’ or wastewater irrigation started in Australia in the latter parts of the 19th century, it was in the late 1960s that a considerable interest was revived in arid and semi-arid parts of the world due to scarcity of alternative water sources and the urgency to increase local food production. The practice has manifold benefits in the form of water conservation, nutrient recycling, surface and ground water pollution prevention. But for arid and semi-arid regions like many parts of Australia, while wastewater irrigation can be an attractive solution to irrigation water problems, it might not be the ideal solution for the common soil types encountered in these regions. Due to characteristic low rainfall, high evaporation and low leaching, these soils tend to have higher salt accumulations. This paper examines the soil salinity and sodicity effects of wastewater irrigation in soil types typical to South Eastern Australia and takes the soils of Western Treatment Plant (WTP) as a case study to highlight these issues.     

Two-dimensional modeling of nitrate leaching for various fertigation scenarios under micro-irrigation

The regular application of nitrogen fertilizers by irrigation is likely responsible for the increase in nitrate concentrations of groundwater in areas dominated by irrigated agriculture. Consequently, sustainable agricultural systems must include environmentally sound irrigation practices. To reduce the harmful effects of irrigated agriculture on the environment, the evaluation of alternative irrigation water management practices is essential. Micro-irrigation offers a large degree of control, enabling accurate application according to crop water requirements, thereby minimize leaching. Furthermore, fertigation allows the controlled placement of nutrients near the plant roots, reducing fertilizer losses through leaching into the groundwater. The presented two-dimensional modeling approach provides information to improve fertigation practices. The specific objective of this project was to assess the effect of fertigation strategy and soil type on nitrate leaching potential for four different micro-irrigation systems. We found that seasonal leaching was the highest for coarse-textured soils, and conclude that fertigation at the beginning of the irrigation cycle tends to increase seasonal nitrate leaching. In contrast, fertigation events at the end of the irrigation cycle reduced the potential for nitrate leaching. For all surface-applied irrigation systems on finer-textured soils, lateral spreading of water and nitrates was enhanced by surface water ponding, causing the water to spread across the surface with subsequent infiltration downwards and horizontal spreading of soil nitrate near the soil surface. Leaching potential increased as the difference between the extent of the wetted soil volume and rooting zone increased.     

Drip irrigation water and salt flow model for table grapes in Coachella Valley,California

Unlike annual crops where reclamation leaching of salts can be readily conducted between cropping, leaching of salts in permanent crops that are drip irrigated pose challenges. A need exists to formulate and test a management-type of salinity model for drip irrigation of table grapes. The model reported herein predicts the distribution of salts along the vine row and between the rows during the growing season, as affected by reactivity of salts of the applied irrigation water as well as rate and duration of drip application. The calibrated model reproduced the initial field salinity profiles after repeated irrigation cycles by adjusting only the routing factor α which is the ratio of horizontal to vertical water flow. After eight cycles the profiles stabilized and the calibrated horizontal to vertical flow routing ratio was 0.6. There is remarkable agreement between measured and simulated salinity. Corresponding soil moisture profiles show the expected high water content with depth at the emitter, the decrease in surface water content with radial distance and the increase with depth, at the distal end of the wedge. Although the model is location specific it can be applied knowing soil, initial and boundary conditions, as well as irrigation application quantity and quality and as such can be applied location by location in order to assess flow and quality of deep percolation recharging the groundwater system. With this capacity the model can predict soil water quantity and quality outcomes for possible land and water management scenarios.     

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.     

Sugarcane water use from shallow water tables: implications for improving irrigation water use efficiency

The resource potential of shallow water tables for cropping systems has been investigated using the Australian sugar industry as a case study. Literature concerning shallow water table contributions to sugarcane crops has been summarised, and an assessment of required irrigation for water tables to depths of 2 m investigated using the SWIMv2.1 soil water balance model for three different soils. The study was undertaken because water availability is a major limitation for sugarcane and other crop production systems in Australia and knowledge on how best to incorporate upflow from water tables in irrigation scheduling is limited. Our results showed that for the three soils studied (representing a range of permeabilities as defined by near-saturated hydraulic conductivities), no irrigation would be required for static water tables within 1 m of the soil surface. Irrigation requirements when static water tables exceeded 1 m depth were dependent on the soil type and rooting characteristics (root depth and density). Our results also show that the near-saturated hydraulic conductivities are a better indicator of the ability of water tables below 1 m to supply sufficient upflow as opposed to soil textural classifications. We conclude that there is potential for reductions in irrigation and hence improvements in irrigation water use efficiency in areas where shallow water tables are a low salinity risk: either fresh, or the local hydrology results in net recharge.