地下水脆弱性评价研究进展

在分析地下水脆弱性内涵的基础上,评述了常见的地下水脆弱性评价方法以及典型的评价模型,从地下水本质脆弱性、特殊脆弱性以及脆弱性的定量研究方面归纳了地下水脆弱性的研究进展。针对目前地下水脆弱性理论研究中的不足以及评判方法的优劣,建议查明研究区的水文地质背景条件,选用合适的脆弱性评价方法,着力考虑地下水脆弱性评价内容的全面性,即根据生产的需要评价除了潜水含水层以外的半承压含水层以及承压含水层的脆弱性,力求评价的规范化,同时将信息技术融于地下水脆弱性评价当中,在地下水脆弱性评价方面开展深入的研究。期待一种集成工具,收集所需的各类评价参数并将评价工作最终定量化,旨在为地下水资源的可持续利用提供理论依据,同时为地下水风险的评估提供支撑。     

Coastal salinization: A case history from Oman

Nearly half the agriculture in the Sultanate of Oman is concentrated along the coastal strip of the Batinah Plain. Arid tropical conditions and the absence of surface water create total dependence upon groundwater withdrawal from wells. The initial equilibrium groundwater conditions were upset, first by the transition from animal bailed to pumped wells, and subsequently by agricultural expansion and increasing urban and industrial water demands. The consequent effects of saline upconing and intrusion have been monitored in selected areas by repeated electrical conductivity surveys over a 9-year period. Despite an apparent excess of fresh water in the Wadi Semail catchment, locally severe salinization has occurred, mainly due to heterogeneous aquifer conditions, thereby detracting from the benefits of agricultural expansion. In the Wadi Rusayl catchment excess of extraction over recharge has resulted in severe salinization. The situation will probably deteriorate further unless rigorous conservation measures and enhanced aquifer recharge are implemented. In such sensitive coastal areas, even if a catchment water balance has ‘excess’ groundwater flow seaward, the local subcatchment response may involve a high salinization risk. Therefore if groundwater extraction is to be increased, the water balance alone may be an insufficient basis for water resources management. The effects upon ‘high risk’/interfluvial areas should also be taken into account.     

Sensitivity analysis and field testing of the RISK-N model in the Central Valley of Chile

We present the results from a sensitivity analysis and a preliminary short-term, site-scale performance assessment of the analytical soil and groundwater nitrate transport RISK-N. The study was carried out in the Central Valley of Chile, on a 2.6 ha corn (Zea mays L.) field underlain by a shallow unconfined aquifer during the cropping season 2000–2001. Nitrogen levels in soils as well as NO₃⁻–N irrigation water and groundwater concentrations were monitored through the crop-growing period, the latter by a network of 16 monitoring wells. A sensitivity analysis shows that both the nitrate flux from the vadose zone and NO₃⁻–N groundwater concentration are mainly influenced by the initial soil nitrogen levels, water input, and soil porosity. Also, simulated groundwater NO₃⁻–N levels are sensitive to changes on the saturated zone denitrification constant. An additional analysis further reveals the significance of the latter parameter, in conjunction with the amount of applied nitrogen fertilizer. We obtained a good agreement between observed average and simulated values. While the model performs well when spatially averaged values are used (root mean square error, RMSE = 1.4 mg l⁻¹ of NO₃⁻–N), the prediction error increases (RMSE = 1.9 mg l⁻¹ of NO₃⁻–N) when the concentration in each well is considered. This fact could be explained by the time and space scale of the experiment and the characteristics of the RISK-N model. The model is easy to use and seems appropriate for mid- and long-term studies of nitrogen contamination in groundwater for agricultural conditions in the Central Valley of Chile and under limited field data availability conditions. However, it needs to be tested for longer periods and under different climatic conditions, soil types, and aquifer characteristics, before its range of applicability can be fully established and recognized.     

Assessment of a groundwater quality monitoring network using vulnerability mapping and geostatistics: A case study from Heretaunga Plains, New Zealand

A groundwater monitoring network can provide quantity and quality data necessary to make informed decisions regarding the state of the environment. A properly designed monitoring system provides a representative understanding of the state of the monitored area. The selection of the optimum number of monitoring sites and their spatial distribution is a major challenge for the hydrogeologist.On the one hand, improper distribution of monitoring sites or insufficient number of sites will not provide a representative view of the state of the environment. On the other hand, if the sampled sites are too many, the information obtained is redundant and the monitoring network is costly and inefficient.A new methodology combining vulnerability mapping and geostatistics is proposed to help define the most efficient groundwater quality monitoring network on a regional scale.Vulnerability mapping identifies areas with high pollution potential, and in turn, prioritises for monitoring. A geostatistics methodology is then used to interpret the obtained data and to examine the spatial distribution of monitored parameters at different sites. The accuracy of spatial mapping reflects the effectiveness of the distribution of the monitoring sites.The methodology was applied to assess the nitrate monitoring network in the Heretaunga basin, Hawke's Bay, New Zealand. The DRASTIC approach was used to prepare a vulnerability map for the area of study, and kriging variance was used to check the spatial distribution of the sites. Based on this study, it was found that some areas with high vulnerability are not covered within the existing network indicating the number of monitoring sites and their distribution is not efficient. Some sites should be dropped and some others need to be added to the existing network.     

基于DRASTIC的集对分析在地下水脆弱性评价中的应用

在传统的DRASTIC指标评价法的基础上,联合集对分析对地下水脆弱性进行评价。将7个不同评价指标分成5级,利用集对分析建立评价指标与评价等级之间的联系度U,从而确定地下水脆弱性的等级。将DRASTIC指标评价法与集对分析法联合应用的好处是:一定程度避免了原有评价方法中人为因素的干扰。新的评价体系具有计算简便,评价合理,结果较为客观等优越性。     

Assessment of groundwater quality and its variations in the capture zone of a pumping well in an agricultural area

Agricultural activities are frequently associated with water contamination. The spreading and storage of fertilizers, for instance, may result in groundwater contamination due to pollutants leaching into an aquifer. Nitrates and fecal bacteria are two important contaminants associated with agriculture. Thus, the development of efficient strategies for groundwater protection in agricultural areas requires an assessment of these two contaminants. Given this perspective, groundwater quality monitoring was carried out over the whole capture zone of a municipal well located in an agricultural area in the St.-Lawrence Lowlands in Québec. Thirty-eight piezometers were installed within the roughly 2 km² capture area of the well to measure physico-chemical parameters such as major ions, field measured parameters (pH, electrical conductivity, dissolved oxygen, water level, temperature), and isotopic ratios, bacteriological parameters (Heterotrophic Plate Count—HPC, enterococci, total coliforms, Escherichia coli) and their variations in space and time. Groundwater was sampled from the pumping well and the piezometers during 25 field campaigns in 2005, 2006 and 2007. The results demonstrate the impact of agricultural activities on nitrate contamination. They indicate high spatial and temporal variations in nitrate concentrations, from 6 to 125 mgNO₃⁻/L within the capture area, with 40% of the samples exceeding the Québec drinking water limit of 45 mgNO₃⁻/L. Nitrate pollution in the municipal well exceeded 45 mgNO₃⁻/L during 2005, but no bacteriological contamination was observed. The results also show a high variability of nitrate concentration with depth within the capture zone. Electrical conductivity appears as a good indicator of the presence of nitrate and calcium ions in this capture zone. Correlations between nitrate, calcium and chloride suggest that these ions come from the same source of fertilizer. Nitrate isotopic composition suggests that nitrate in groundwater originates from both chemical and organic fertilizers. The bacteriological results show that the extracted volume of water during sampling of a piezometer has a significant impact on the bacteria count. The variability of bacteriological pollution is important in space and time, showing a higher contamination during summer. Only 2% of the raw water samples exhibit contamination exceeding the drinking water standard for treated water. Total coliforms seem to be a good precursor of E. coli or enterococci contamination. Globally, the physico-chemical and bacteriological groundwater quality within the studied capture area and the pumping well shows contamination by nitrates, but low contamination levels by fecal bacteria.     

基于熵权模糊理论的地下水有机污染健康风险评价

健康风险评价过程中由于多方面的不确定使得评价结果的可信度下降,熵权模糊理论的应用可以显著提高评价结果的可靠度。文中针对地下水有机物的特性,对U．S．EPA推荐模型进行了改进,考虑有机物自然衰减作用。本次研究以沈阳细河周边地区为例,在分析测试该地区地下水有机污染特征的基础上,采用熵权模糊综合评价方法,考虑各有机物对其致癌风险的贡献率计算致癌风险和非致癌危害指数以及评价标准,确定致癌风险等级,进行地下水有机污染健康风险评价。结果表明：细河及其周边地区地下水有机污染致癌风险和非致癌危害指数都很高,对人体健康产生一定的影响,应该加强重视有机污染造成的危害,减少有机污染,为公众和环境管理者提供相对准确和科学的信息。     

A GIS-based susceptibility indexing method for irrigation and drinking water management planning: Application to Chebba-Mellouleche Aquifer, Tunisia

Locating high quality groundwater resources in semi-arid regions with growing population and agricultural development is an expensive undertaking. Simple susceptibility indexing method, based on vulnerability and quality index, can be used to facilitate this application. The GIS technique provides an efficient environment to reach this objective. The contamination susceptibility index was calculated by taking the product of the vulnerability index (VI) and the quality index (QI). The VI index was calculated using DRASTIC method to evaluate the hydrogeological characteristics of the Aquifer. The quality index calculation procedure, based on the water classification, was introduced to evaluate hydrochemical data. The susceptibility indexing method was applied in the Chebba-Mellouleche Aquifer, located in Eastern Tunisia. The results show a clear degradation of the water quality throughout the Aquifer. In addition, the susceptibility indexing map which incorporates hydrogeological and hydrochemical datasets reveals a similarity with areas of high anthropogenic activities. Thus, it is more realistic to estimate the groundwater contamination. The main aim of this study is to give an overview of the drinking and irrigation water quality in relation to the hydrogeological characteristics of the Chebba-Mellouleche Aquifer. This overview can form the basis for further investigations.     

地下水亚硝酸盐污染风险分析：以沈阳冲洪积扇农业灌区为例

为验证亚硝酸盐污染地下水的风险评价方法,针对农业非点源污染引起的地下水亚硝酸盐污染问题,以沈阳冲洪积扇区为例,进行亚硝酸盐污染地下水风险分析。采用SPSS18.0软件的因子分析法,对包气带岩性样品的X衍射结果以及相应的有机质、亚硝酸盐含量等进行因子分析,确定土壤中亚硝酸盐主要来源于氮肥。在此基础上,针对传统DRASTIC模型的不足,结合研究区实际特征提出地下水脆弱性DRSICLN模型。基于GIS软件,确定沈阳冲洪积扇区地下水亚硝酸盐污染风险的3个等级,并结合地下水中NO2-N等值线图进行对比,结果表明,DRSICLN模型适用于研究区地下水污染风险评价。     

Estimating potential nitrate leaching in nitrogen fertilized and irrigated tomato using the computer model NLEAP

Groundwater pollution caused by leaching of NO₃-N from agricultural systems has caused public concern for decades. To preserve the groundwater and reduce economic losses for the farmers, a rapid and accurate estimation of NO₃-N moving below the root zone is crucial. In this study, the value of the computer program NLEAP (Nitrate Leaching and Economic Analysis Package) to simulate nitrate leaching was evaluated using data from an experiment conducted with 12 lysimeters (1.25 m i.d. and 2 m deep) in 1996 and 1997. Three tomato (H2274 variety) seedlings were planted in each lysimeter and nitrogen rates of 0, 80, 160, and 240 kg N ha⁻¹, as ammonium nitrate and ammonium sulphate, were applied to the lysimeters under a fixed irrigation program. Effluent was collected from the outlets of the lysimeters and analyzed for NO₃. The model adequately simulated nitrogen leaching for each year (R²=0.93 and P<0.03 for 1996, and R²=0.87 and P<0.06 for 1997). The high coefficients of determination, between observed and simulated values, revealed that the model can be successfully used to estimate the amount of the NO₃ leaching under the experimental conditions. The results also showed that the NO₃ available for leaching (NAL) values were important background information for determining an optimum N rate for groundwater quality and maximum gain, and NO₃ available for leaching (NAL), amount of NO₃ leached (NL), movement risk index (MRI), and annual leaching risk potential (ALRP) parameters should be considered together to estimate the nitrogen pollution risk.     

Identification of nitrate leaching hot spots in a large area with contrasting soil texture and management

Identification of nitrate (NO₃) leaching hot spots is important in mitigating environmental effect of NO₃. Once identified, the hot spots can be further analyzed in detail for evaluating appropriate alternative management techniques to reduce impact of nitrate on groundwater. This study was conducted to identify NO₃ leaching hot spots in an approximately 36,000 ha area in Serik plain, which is used intensively for agriculture in the Antalya region of Southern Turkey. Geo-referenced water samples were taken from 161 wells and from the representative soils around the wells during the period from late May to early June of 2009. The data were analyzed by classical statistics and geostatistics. Both soil and groundwater NO₃-N concentrations demonstrated a considerably high variation, with a mean of 10.2 mg kg⁻¹ and 2.1 mg L⁻¹ NO₃-N for soil and groundwater, respectively. The NO₃-N concentrations ranged from 0.01 to 102.5 mg L⁻¹ in well waters and from 1.89 to 106.4 mg kg⁻¹ in soils. Nitrate leaching was spatially dependent in the study area. Six hot spots were identified in the plain, and in general, the hot spots coincided with high water table, high sand content, and irrigated wheat and cotton. The adverse effects of NO₃ can be mitigated by switching the surface and furrow irrigation methods to sprinkler irrigation, which results in a more efficient N and water use. Computer models such as NLEAP can be used to analyze alternative management practices together with soil, aquifer, and climate characteristics to determine a set of management alternatives to mitigate NO₃ effect in these hot spot areas.     

Optimization of groundwater abstraction system and distribution pipe in pressurized irrigation systems for minimum cost

A tool named DOPIR (Dimensioning Of Pressurized IRrigation) was developed to optimize the process of water abstraction from an aquifer for pressurized irrigation systems. This tool integrates the main factors throughout the irrigation process, from the water source to the emitter. The objective is to minimize the total cost of water abstraction and application (C _T) (investment (C _a) + operation (C _op) per unit of irrigated area according to the type of aquifer, crop water requirement and electricity rate periods. To highlight the usefulness of this tool, DOPIR has been applied to a corn crop in Spain with a permanent sprinkler irrigation system, considering two types of aquifer: confined and unconfined. The effects of parameters such as the static water table in the aquifer (SWT), irrigated area (S), number of subunits in the plot (NS), sprinkler and lateral pipe spacing, and average application rate (ARa) on C _T have been analyzed. Results show that energy cost (C _e) is the most important component of C _T (50–72 % in the case studies). Thus, it is very important to adapt the design and management of the irrigation and pumping system throughout the irrigation season to the energy rate periods.     

Simulation of nitrate leaching under potato crops in a Mediterranean area. Influence of frost prevention irrigation on nitrogen transport

The Sa Pobla area (Majorca Island, Spain) heavily depends on the use of groundwater resources for irrigation and urban water supply and is characterised by the presence of intensive potato farming activities. The Plioquaternary aquifer is unconfined and contains high levels of nitrate concentrations. To analyse the risk of contamination to the aquifer arising from agricultural practices, the amount of water and nitrogen leached below the root zone was simulated by the GLEAMS code. Data for model calibration and validation were obtained from field experiments on six potato crops for the years 2004-2007.When air temperatures drop below 1 °C irrigation water is applied to prevent crops from frost damage. During times of anomalously low air temperatures, the risk of nitrate leaching is increased by as much as 318% from frost prevention irrigation under normal local conditions.The GLEAMS simulation model was successfully calibrated for Sa Pobla conditions under potato cropping as shown by RMSE values for the water transport module of 0.19, 0.14 and 0.13 for the calibration period and 0.20, 0.25 and 0.15 for the validation period at depths of 0.3, 0.6, and 0.9 m respectively; and for the chemical transport module the R² value was 0.82 for the calibration period and 0.60 for the validation period. Consequently, for Sa Pobla conditions, GLEAMS can be used to assess the effectiveness of different agricultural management practices to reduce nitrate leaching. It was concluded that additional irrigation water applied for frost prevention plays a very important role in nitrate leaching below the root zone, which enhances the nitrogen loading to the aquifer.     

Geochemical conditions in groundwater systems: Implications for the attenuation of agricultural nitrate

Nitrate resulting from nitrogen fertilisers used in Agriculture is a widespread contaminant of shallow groundwater and causes adverse effects on human, animal and ecosystem health. In order to evaluate the full extent of groundwater nitrate contamination, and how it might evolve in time, it is essential to understand controls on aquifer assimilative capacity. This level of understanding will also help to better target policies and incentives aimed at controlling the amount of nitrate entering downstream water systems.The potential for nitrate attenuation in groundwater was assessed by examining the concentration and distribution pattern of electron donors such as dissolved organic carbon (DOC), ferrous iron, and redox indicators such as dissolved oxygen (DO) and Eh in 57 monitoring bores on the lower Burdekin coastal floodplain, one of Queensland's and Australia's premier irrigation districts. Nitrate concentrations ranged from 0.1 to 14.4 mg/L NO₃-N but were mostly undetectable in bores close to the coast. Groundwater age dates suggest that while there are nitrate ‘hot spots’ in certain areas, some or most of the nitrate is being consumed on its way to the ocean. Low nitrate concentrations were coupled with high ferrous concentrations. The low DO concentrations (<2 mg/L) and high ferrous concentrations found in 55% of the bores indicate that redox conditions are suitable for nitrate attenuation by either denitrification or dissimilatory nitrate reduction to ammonium. The reducing environment may be associated with the high DOC concentrations (up to 82 mg C/L) found in these groundwaters. Furthermore, high levels of ferrous iron found in the Ayr area combined with the wide spread geographical distribution of DOC indicate that these areas have a high potential for sustaining geochemical processes that reduces nitrate levels. The distribution of geochemical indicators also suggests that the shallower depths (<15 m) of the groundwater systems have more potential for nitrate reduction than the deeper depths. The map identifying areas within the lower Burdekin with most potential for denitrification is a valuable first step in helping to understand and manage the fate of nitrate entering the groundwater.     

数值法与解析法在地下水环境影响评价中的应用研究

【目的】探究数值法与解析法在进行地下水环境影响评价时的差异,以及在进行地下水环境评价时,选择更为合适的计算方法。【方法】以四川省宜宾市某氯化法钛白粉项目为例,利用瞬时点源一维对流扩散模型、瞬时点源二维扩散模型和GMS数值模型研究非正常工况下苯储罐泄漏时污染物对地下水的影响,预测了污染物在潜水含水层中的中心运移距离、最大浓度和污染范围,并对3种方法的计算结果进行比较分析。【结果】一维对流模型只考虑地下水流向方向上的运移情况,计算结果最大,方法简单方便,参数要求较少,但精度较低,无法预测污染面积;二维扩散模型考虑了平面上的污染物分布情况,可以预测污染物范围及浓度变化,但预测运移距离较大,与实际的边界条件矛盾;GMS数值模型是三维模拟,计算精度高,更符合实际情况,但建模过程复杂,水文地质条件限制较多,所需水文地质参数较多,耗时较长。【结论】综上考虑,水文地质条件简单、地下水环境评价等级不高的工程项目使用解析法预测较为方便;在水文地质资料充足、评价等级高的区域使用数值法预测精度更高。     

Shallow groundwater contribution to pistachio water use

Pistachio can be grown in the central desert of Islamic Republic (I.R.) of Iran with adverse conditions such as shallow saline groundwater tables. The contribution of water from shallow, saline groundwater to crop water use may be important in such conditions. The objectives of this study were to determine the contributions from shallow, saline groundwater to water use of pistachio seedlings, and how this contribution was affected by groundwater depth, salinitiy, and irrigation conditions. The results indicated that an increase in groundwater depth resulted in significant increase in root depth and significant decrease in seasonal evapotranspiration (ET), transpiration, and groundwater contribution to the plant water use. Non-saline shallow (30–120 cm depth) groundwater under irrigated and non-irrigated conditions contributed 72.4–89.7% and 90.7–100.0% of plant water use, respectively. However, these contributions were 57.2–74.8% and 79.3–100.0% for irrigated and non-irrigated conditions, respectively for saline shallow (30–120 cm depth) groundwater. The effect of groundwater depths (D, cm) on groundwater contributions (q, %) was found to be influenced by the salinity levels of the groundwater (EC, dS m⁻¹). The linear multiple regression equations were q = 97.5 − 1.24(EC) − 0.194(D) and q = 105.9 − 0.48(EC) − 0.154(D) for irrigated and non-irrigated conditions, respectively. The maximum reductions in relative plant dry weight of 80.3% and 44.8% were occurred under non-irrigated condition and saline groundwater depth of 30 cm and non-saline water depth of 60 cm, respectively. Root depth analysis indicated that vertical root growth caused the root to reach a moist layer near the groundwater. A very close to 1:1 relationship between relative reduction in top dry weight (1 − y/y_m) and relative reduction in transpiration (1 − T/T_m) was obtained.     

Performance assessment of irrigation water management in old lands of the Nile delta of Egypt

This paper provides the methodology and results of a cross-scale diagnostic performance assessment program of the irrigation water management in the old lands of the Nile Delta of Egypt. The analysis was done at three levels; main canal level, branch canals level and on-farm level of the Meet Yazid command (82,740 ha) for the year 2008?C2009 to highlight areas for improvement. At the main canal level the annual average percentage of irrigation water returning to drains and groundwater was 53% of the total water supplied. Since Meet Yazid lies at tail end of the delta, and there is groundwater salinity, opportunities for reuse are increasingly limited moving north to Lake Burullus. This would indicate opportunities for real water savings. The results of monthly relative water supply of the main canal indicated mismatch between demand and supply especially during the winter months, and when supply is low farmers do reuse drainage or groundwater. Also, the assessment of the three branch canals showed non-uniformity of water distribution and mismatch between demand and supply even when comparing improved and non-improved canals. At the on-farm level in paddy fields, the amount of irrigation flows to drains and saline sinks varied from 0.46 to 0.71 of inflow. In spite of these values of non-uniformity and low depleted fraction, the relative evapotranspiration (ratio of actual to potential) evaporation was uniformly high, indicating most crops of most farmers were not water stressed, which is also confirmed by the high yield values. The average values of productivity per unit water depleted by ET_act were 1.04 and 1.05 kg/m³ for rice and wheat fields, respectively, with yields of rice and wheat at 8 and 6 t per ha respectively. On farm and tertiary improvements alone will not yield real water savings, as excess water in the main canal and drains will continue to flow out of the system. Rather the focus should first be on supplies to the main canal, accompanied by more precise on farm and water delivery practices at branch and tertiary levels, and ensuring that environmental flows are met. There is an added advantage of focusing on this tail end region of Egypt that this response would lessen vulnerability to reuse of polluted and saline water.     

Decision support framework for assessment of non-point-source pollution of groundwater in large irrigation projects

Agriculture is the main non-point polluter of groundwater in irrigated areas as fertilizers and other agrochemicals are the main contaminants in the water that drains out of the root zone to recharge the aquifer. Nitrates from fertilizers, dissolved in percolation losses from rice fields, are the source of pollution considered. The concentration of nitrates in the percolated water depends on the distributed field water and nitrogen balances over the area. Its concentration in the groundwater depends on the total recharge, pollution loading, groundwater flow and solute transport within the aquifer. The development and application of a GIS based decision support framework that integrates field scale models of these processes for assessment of non-point-source pollution of groundwater in canal irrigation project areas is presented. The GIS is used for representing the spatial variations in input data over the area and map the output of the recharge and nitrogen balance models. The latter are used to provide the spatially distributed recharge and pollutant load inputs to the distributed groundwater flow and transport models, respectively. Alternate strategies for water and fertilizer use can be evaluated using this framework to ensure long-term sustainability of productive agriculture in large irrigation projects. The development and application of the framework is illustrated by taking a case study of a large canal irrigation system in India.     

Conjunctive use of canal and groundwater in Punjab,Pakistan: management and policy options

Data from 41 watercourses commands in Pakistan show that, as expected, farmers in head end reaches of canals receive more canal water than those in tail end reaches. Contrary to conventional wisdom, however, these head end farmers also use more groundwater than those at the tail end. Overall, groundwater plays a more important role in irrigation than surface water, ranging from 65% dependence on pumped water in head end areas to over 90% in tail end areas. This means that groundwater is no longer supplemental to canal water, but is an integral part of the irrigated agricultural environment. However, the cropping choices of farmers appear to reflect the amount of good quality canal water they receive: head end farmers are able to grow more high value basmati rice in the summer and more vegetables in the winter, leaving tail enders to rely on less valuable crops such as fodder and wheat.Tail end areas are not only deprived of their fair share of surface water: they have to pump proportionately more groundwater which shows decreasing quality towards the tail. Typically, head end areas have groundwater with EC values of less than 1.0 dS/m, rising to over 2.0 dS/m in tail end areas. When the quality of both surface and groundwater used by farmers is examined, only the top 40% of the distributary gets water of adequate quality, the next 40% get below average quality, while the tail 20% of farmers irrigate with water that is classified as saline.Because of higher dependence on more expensive groundwater tail enders use less water per unit area, thereby reducing the leaching requirement. The result is a clear increase in soil salinity from head to tail along distributary canals, and there is some evidence of land abandonment in tail end watercourses due to excess salinity.The implications of these results are far reaching. Government policy includes plans to divert significant quantities of fresh canal water to areas underlain by saline groundwater on the basis that farmers already have adapted to pumping fresh groundwater. The results reported suggest that if this policy were implemented, there is a risk that over-dependence on fresh groundwater could lead to an intensification of the rate of soil salinization and deterioration of quality in areas currently classified as fresh groundwater zones.At present, the location and utilization of privately owned shallow tubewells is not monitored, and thus it is not possible for government agencies to determine just how much water of different qualities is being used. Further, canal water deliveries, public deep well monitoring, watercourse monitoring programs, soil salinity measurements, and agricultural performance monitoring are all scattered among different agencies and organizations, making the task of effective conjunctive management of surface and groundwater even more difficult.Conventional wisdom: Groundwater in Pakistan ... where it exists within the canal system ... is used to supplement surface water supplies to meet peaks in demand. (WAPDA, 1990)     

The effect of organic manuring and water quality on water transmission parameters and sodication of a sandy loam soil

Water transmission characteristics under saturated and unsaturated conditions were studied in a sandy loam soil with (F₁) and without (F₀) long-term farmyard manure (FYM) treatments, in relation to sodium adsorption ratios (SAR) and electrolyte concentrations of water. The effect of FYM and ratios of Ca²⁺ : Mg²⁺ in water at a given SAR on sodication of the soil was also studied.Saturated hydraulic conductivity (k) and weighted mean diffusivity ($\text{D}\text{?}$) were slightly higher for F₁ than for F₀, whereas sodication indices like Gapon constant (K_G), Krishnamoorthy-Davis-Overstreet constant (K_KDO) and Vanselow constant (K_V) were slightly smaller. The k and $\text{D}\text{?}$ decreased with an increase of SAR and decrease of electrolyte concentration, the effect of SAR being more pronounced. There was proportionately a sharper decrease in the k and $\text{D}\text{?}$ values at SAR 10 with total electrolyte concentrations of 10–40 meq 1^?1. However, with a total electrolyte concentration of 80 meq 1^?1, there was a smaller drop at SAR 10.A small difference in the build-up of exchangeable sodium percentage (ESP) in F₁ and F₀ treatments at a given SAR suggests that, apart from slightly improving water transmission parameters, the use of FYM also reduces the sodication hazard in a soil irrigated with sodic waters. An increase in the Ca²⁺ : Mg²⁺ ratio from 25:75 to 75:25 slightly decreased the values of K_G, K_KDO and K_V, thus indicating somewhat more preference for Ca²⁺ to Mg²⁺ at a given SAR, which was more so in F₁ soil. This fact could also be expressed in terms of a slight shift of thermodynamic exchange constant (K) and standard free energy change of the exchange reaction (ΔG⁰_r). The presence of some unidentified Na⁺ releasing minerals in the soils studied was observed and correction for exchangeable Na⁺ determination applied.