Hydraulic and hydro-salinity behavior of skimming wells under different pumping regimes

Skimming wells are meant to extract top fresh water layer in the fresh-saline aquifer. Their development in the Indus basin occurred through private sector in a technological vacuum. As a result, these wells have some technical, environmental and social constraints, which hinder the sustainability of these wells. As an initial step to improve the well technology, the hydraulic and hydro-salinity responses of the fresh-saline aquifer under different pumping regimes need to be monitored. The present paper reports the hydraulic and hydro-salinity behavior of the Indus basin aquifer in Pakistan under field conditions at farmers’ wells. Two sites, having 6- and 16-strainer wells were monitored during July 2000–December 2001. The 6-strainer well was operated for 4 h with single-, 4- and 6-strainer arrangements and the spatial behaviors of specific drawdown were observed under these arrangements. The 16-strainer well was monitored continuously for the above period. The well discharge, pumped water quality and pumping duration was recorded of every pumping event under farmer’s practice to extract groundwater. The rainfall and temporal water table fluctuation was also recorded at this site. The impact of 24 years of well operation on groundwater quality was observed by comparing the hydro-salinity profiles of 1974 and 1998 under 3-strainer well. The results showed that the specific drawdown was higher for single-strainer and it decreased with the increase in number of strainers in skimming wells and hence reduced the chances of saline-upconing. Each strainer in multi-strainer well contributes equally in well discharge provided the horizontal distances among the strainers are equal. The pumped water quality in fresh-saline aquifer was a very sensitive function of fresh water recharge and pumping duration. It was observed that with the increase in daily operation from 2 to 12 h per day, the pumped quality deteriorated three-folds and there was also 30% reduction in well discharge due to high suction lift. It was observed that continuous operation of a 3-strainer well having discharge of 14 lps over the 24 years had raised the fresh-saline interface (iso-concentration line of 1.5 dS/m) to 9 m. Keeping in review the observations, it is recommended that the daily operation of 4–6 h keep the water quality within marginal limit (<1.5 dS/m) and the pumping operation is also cost-effective with only 15–20% reduction in well discharge for the study area.     

Feasible design and operational guidelines for skimming wells in the Indus basin,Pakistan

This paper reports work done to assess the status of groundwater extraction technologies and practices in the Indus basin of Pakistan and hence to improve these technologies for sustainable groundwater extraction. A socio-technical approach was used which involved a field survey using participatory rural appraisal (PRA), monitoring of existing farmers’ wells for hydraulic and hydrosalinity behavior of these wells, and simulating hydrosalinity behavior under skimming wells using appropriate groundwater and solute transport models. The data collected in PRA shows a variety of wells designs, which reflects the absence of design code for these wells. Consequently, farmers have to choose one of the design options provided by the local drillers.We monitored a farmer's operation of a multi-borehole skimming well to extract groundwater to supplement canal water. Most of the time the well was operated daily, and occasionally on alternate days. The number of operating hours varied from 2 to 12 h per pumping event. We also conducted pumping tests in two wells, one with a single-borehole, and another with six boreholes. The data obtained at these two wells were used to develop guidelines for well design and operation using a flow model, MODFLOW and a solute transport model, MT3D.The parameters considered in the model studies were as follows: perforated well depth with respect to depth of the freshwater layer in the aquifer (i.e. well penetration ratio), number of boreholes and spatial distance between them in a multi-borehole well system, well discharge rate and daily operational hours. The results indicate that a single borehole well operated at a discharge rate of ranging from 32 to 180 m³/h (9–50 l/s) can be operated successfully with a 30–60% well penetration ratio for an operating time of 4–8 h/day where the thickness of the freshwater layer ranges from 20 to 30 m. Multi-borehole wells consisting of four to eight boreholes at a spacing of 3 m can be installed where the thickness of the freshwater layer ranges from 10 to 20 m without compromising the quantity and quality of pumped water.     

Simulation of Hydrosalinity Behavior Under Skimming Wells

The movement of underlying saline water inresponse to freshwater abstraction infresh-saline aquifers is dependent uponaquifer properties, well design andoperational plans. The responses of thesefactors under various well configurationsand pumping modes are important forsustainable freshwater abstraction. In thepresent study, three shallow skimming wellswith different well configurations weremonitored under different pumping modes inthe Indus basin of Pakistan. The field datawere used to calibrate MODFLOW and MT3Dmodels for each well system. The resultsshowed that the selected models have thecapability to simulate both the continuousand intermittent pumping operations inskimming wells. The calibrated models wereused to evaluate the sensitivity of variousaquifers, well design, and operationalparameters. Hydrosalinity of the pumpedwater increased with the increase invertical hydraulic conductivity,longitudinal dispersivity, well abstractionrate, well penetration ratio, and pumpingduration; while increase in horizontalhydraulic conductivity, effective porosity,specific yield, freshwater recharge,transverse dispersivity, and number ofboreholes in multi-borehole skimming wellsdecreased the salinity of the pumpedwater.     

Simulating skimming well techniques for sustainable exploitation of groundwater

The problems of saline water upconing and intrusion are of major concern in the Indus Basin of Pakistan. The withdrawals of fresh water overlying saline groundwater have resulted in inferior quality of pumped water and degradation of the aquifer. The continuous use of such qualitatively inferior water for irrigation has severely affected fertile lands causing agro-economic declination. Nearly 197 billion cubic meters (bm3) of fresh water has accumulated in the form of a thin layer over saline water, and over 20 bm3 of fresh water is being recharged annually in saline zones of the basin. In these situations skimming wells can play an important role in augmenting irrigation supplies and watertable control, as well as alleviating degradation of the aquifer. However, the working efficiency of skimming wells is not fully known. To investigate the effectiveness of various skimming wells, a density dependent finite element numerical model was modified and applied. In the present study an analytical equation was developed by gauging the effects of temperature and viscosity on the hydraulic conductivity of the porous media subjected to the saline water. The new equation was incorporated into the model. Simulations of flow and solute transport towards single, two-strainer, radial, compound and recirculation wells were performed. Physical modelling of such skimming well configurations was undertaken to calibrate the numerical model. The double-strainer well method performed better than the other skimming configurations tested in this study.     

机井和地下水位对坎儿井水量影响的数值模拟

长期以来坎儿井和机井是吐鲁番地区2种重要的灌溉水源工程,但由于吐鲁番地区机井布局和水源开采缺乏科学规划,造成区域地下水位的下降,直接威胁到了坎儿井的存在.为了探究机井抽水和地下水位对坎儿井水量的影响规律,在研究火焰山山前洪积扇水文地质条件的基础上,结合坎儿井独特的运行机制,建立了坎儿井出水模型,通过施加不同位置和数量的机井,变换抽水量和地下水位等方式以控制影响因素.结果表明:坎儿井水量与机井的距离和抽水量呈负相关,但会因其位置和排布形式的不同而产生不同程度的影响;单井对坎儿井水量的影响十分有限,群井对坎儿井水量的影响较大,其中又以集水段附近设置机井的影响最大;而在所有对坎儿井水量影响的因素中,地下水位起到了决定性作用.因此坎儿井的保护工作应以控制地下水位为主,同时严格控制坎儿井周围的机井数量和抽水量.     

Design of a farm layout for irrigation with limited discharges

In order to improve the irrigation efficiencies of small farms employing cavity wells for their water supply, an experimental study was conducted at the Central Soil Salinity Research Institute, Karnal. The cavity wells of the Karnal region do not have any discharge regulating devices for improving the irrigation efficiencies. The only way of improving these efficiencies is by designing an efficient irrigation layout, so that uniform water application is accomplished. The present study involves field determination of the opportunity time at each point along the border from advance and recession curves and computing the depth of cumulative infiltration from the infiltration rate curve. The irrigation efficiencies are also calculated from soil moisture measurements made before and after each irrigation.The results of this study show that a realistic field assessment of the irrigated border efficiencies is obtained through a soil moisture measurement procedure. The procedure, based on opportunity time and infiltration, overestimates the irrigation efficiencies due to the empirical nature of the infiltration equation. For small farms, with a limited discharge of 10 l/s, an irrigation layout of borders of 50–70 m in length and 6–8 m in width is recommended.     

Modeling shallow water table evaporation in irrigated regions

Groundwater discharge through evaporation due to a shallow water table can be an important component of a regional scale water balance. Modeling this phenomenon in irrigated regions where soil moisture varies on short time scales is most accurately accomplished using variably saturated modeling codes. However, the computational demands of these models limit their application to field scale problems. The MODFLOW groundwater modeling code is applicable to regional scale problems and it has an evapotranspiration package that can be used to estimate this form of discharge, however, the use of time-invariant parameters in this module result in evaporation rates that are a function of water table depth only. This paper presents a calibration and validation of the previously developed MOD-HMS model code using lysimeter data. The model is then used to illustrate the dependence of bare soil evaporation rates on water table depth and soil moisture conditions. Finally, an approach for estimating the time varying parameters for the MODFLOW evapotranspiration package using a 1-D variably saturated MOD-HMS model is presented.     

辐射井在银北灌区开发浅层地下水中的应用

辐射井是开发浅层地下水的理想井型。针对银北灌区的水文地质条件。阐述了在银北灌区利用辐射井技术开发浅层地下水的设计方法，如竖井结构、井深设计、水平辐射管设计及涌水量计算等。并对成井技术、试验井的水量和水质等进行了探讨。     

基于Modflow-Hydrus耦合模型的改良盐碱土水平井排水效果分析

为了提高盐碱土改良水平,确定科学合理的灌排工程设计方案,建立饱和-非饱和土壤水分运移Modflow-Hydrus耦合模型,对试验区不同情景水平井的排水效果进行模拟分析.结果表明:当地下水位埋深较小时,季节变化对地下水综合补给强度有明显影响,当地下水位埋深大于6 m,不同时段地下水综合补给强度均趋于常数,当地下水位埋深较大时,通过厚层包气带的降水补给均匀;断续排水时单个水平井400 m控制范围内,潜水位显著下降,具有良好的疏干效果,降深与水平井的年抽水强度成正相关;天然环境条件下,区内降水入渗补给和潜水蒸发保持平衡,以试验区中心向外扩展,排水试验有效改善8倍试验区面积范围内降水入渗-潜水蒸发环境,使包气带土壤水分进入饱水带,潜水水位呈下降趋势.研究成果为盐碱土壤改良的水平井施工设计提供理论参考.     

Observations on soil and groundwater contamination with nitrate: A case study from Lebanon-East Mediterranean

The impact of agricultural practices on soil-groundwater quality in the sub-humid Bekaa plain of Lebanon-East Mediterranean was monitored in four fields (F) between July 2007 and July 2009. These were occupied by continuous mint (F1), summer potato/wheat/potato (F2), lettuce/lettuce/potato/wheat/summer potato (F3) and table grapes (F4). N input calculated on a two-year basis, was in the following ascending order F4, F2, F3 and F1. Soil samples, analyzed down to 200 cm depth, showed high nitrate and chloride concentrations at the end of the 2007 and 2008 seasons. Soil chloride and nitrate peaks recorded in October 2007 and 2008 disappeared below 200 cm overwinter. The calculated N biannual discharge ranged from 130 (F4), to 516 (F2), to 778 (F1), to 879 kg ha⁻¹ (F3). Groundwater quality was studied in 21 wells distributed along a sequence stretching from the Litani River to the eastern water dividing line. Based on the nitrate concentrations, the well located at the top of the water dividing line was the only one suitable for drinking purposes. Eight wells were mildly contaminated, therefore suitable for irrigation purposes except for sensitive crops. Twelve wells, positioned in the plain, showed a nitrate level exceeding 200 mg L⁻¹. Protecting the soil and groundwater quality is a top priority to maintain the ecological and agricultural functions of water.     

Verification of drainage design criteria in the Nile Delta,Egypt

A monitoring programme to verify the design criteria of subsurface drainage systems was conducted in a pilot area in the Nile Delta in Egypt. The programme, which covered a 9-year period, included the monitoring of the cropping pattern, crop yield, soil salinity, watertable, discharge and salinity of the drainage water and overpressure in the subsurface drainage system. The results showed that the yield of all crops (wheat, berseem, maize, rice and cotton) increased significantly after the installation of the subsurface drainage system. Optimum growing conditions for the combination of crops that are cultivated in rotation in the area required that the watertable midway between the drains had a average depth of 0.80 m. A corresponding drain discharge of 0.4 mm/d was sufficient to cope with the prevailing percolation losses of irrigation water and to maintain favourable soil-salinity levels. The additional natural drainage rate in the area was estimated at 0.5 mm/d. The most effective way to attain these favourable drainage conditions is to install drains at a depth between 1.20 to 1.40 m. For drain-pipe capacity the Manning equation can be used with a design rate of 1.2 mm/d, for collector drains this rate should be increased to 1.8 mm/d to compensate for the higher discharge rates from rice fields. These rates should be used in combination with a roughness coefficient (n) of 0.028 to take sedimentation and irregularities in the alignment into account. When this value of the roughness coefficient is used, no additional safety has to be incorporated in the other design factors (e.g. the design rate).     

薄层水流水力特性试验研究

对粘砂床面的薄层水流进行定床试验,研究了不同流量、坡度下薄层水流的水力学参数(流态、水深、流速及阻力系数)随坡度和流量的变化规律。结果表明:水流雷诺数Re随着坡度和流量的增大而增大,但Re主要是由流量决定的,坡度对Re的影响不大,水流流态基本上在过渡流区和紊流区;平均水深-单宽流量,流速-单宽流量,水流阻力系数-单宽流量均成很好的幂函数形式,平均水深和流速均随单宽流量的增大而增大,阻力系数随着单宽流量的增大而减小。     

Testing and statistical analysis of the performance of a pipe drainage system: A case study in north-eastern Italy

A survey program was carried out from June 1988 to august 1989 in North-eastern Italy in a pipe drainage area of 61 ha in order to verify if the year of installation (one part of the system has been installed in 1984 and another one year later) and the cover material of drains (pipes were covered with cocofibre for 2/5 of their length and without envelope for 3/5) could influence the functioning of the system. Collected data of drain discharge and water table depth were subjected to an elaborate statistical analysis.A methodological approach to determine the sample size (how many measurements of discharge and watertable depth are required, in space and time, from a statistical stand-point) in drainage experiment is proposed. For drainage systems similar to the considered one, a sample size of 10–12 drains and 6–8 observation wells can be recommended in order to obtain a standard error lower than 10–15% of the mean.     

沼蛤的生态水力学特性研究

我国南方地区的取水工程常受一种贝类--沼蛤的困扰,沼蛤在一些引水设施,如取水泵站、输水管道、电站冷凝系统、水厂集水井、沉淀池等水域大量繁殖、附着,堵塞过水通道,它们排出的粪便会对周围水体产生二次污染。同时沼蛤的生长,尤其是大量生长,会消耗水体中的溶解氧,使水质进一步恶化, 从而带来严重经济损失和环境危害。生态水力学的近年发展起来的新兴学科,属流体力学、生物学、生态学、环境科学与工程科学的交叉学科,主要研究水中生命体的扩散输移规律及其属力学范畴的控制技术。采用河流动力学的方法对沼蛤生态水力学特性进行研究,分别对沼蛤的几何形状特征、对温度、水深和水流改变的敏感性等方面进行研究。     

Generic unified rating for Cutthroat flumes

Computational fluid dynamics (CFD) software was used to generate water depth and discharge data for the 24 standard-size Cutthroat flumes (CTF), as well as an additional 27 intermediate sizes. The hydraulic model was validated using information from several sources, including physical model test data for 16 standard sizes and 3 intermediate sizes. Grid-size independence was also confirmed for a range of computational cell sizes, and the calibration results for 16 of the standard-size flumes were compared with previously published calibration parameters. Through the CFD simulations, a full set of calibration and validation data was generated for each of the 51 tested flume sizes, filling the gaps in the experimental data from physical models and permitting a full analysis of flume hydraulic performance. The study resulted in a “generic” (any CTF in the dimensional range of the 24 standard sizes and specified dimensional ratios according to the original design criteria) and “unified” (for both free- and submerged-flow conditions) calibrations. From these results, a single algebraic equation was developed for free and submerged flow, relating upstream water depth and submergence to discharge. The error in the CFD-predicted discharge for all flume sizes is less than 3 % of the full-scale discharge, and the equation is applicable to both standard and nonstandard sizes.     

沟灌条件下灌水沟入渗特性研究

为探明沟灌时灌水沟的水分入渗规律,从沟灌二维入渗过程、入渗湿润锋运移特性、累计入渗水量变化过程、土壤含水量分布等方面研究了沟灌的入渗特征及其影响,研究表明：灌水沟中水深、沟底宽、湿周对沟灌入渗过程均有明显影响。沟中水深增大,有利于加大侧向入渗,垂向入渗减少,而水深减小,会加大垂向入渗,增加深层渗漏。灌水沟底宽不影响灌水沟的侧向入渗,仅影响垂向入渗,底宽减小,垂向入渗深度相应减小,且土壤表面以下40 cm深以内水平向入渗深度平均值与最大垂向入渗深度的比值在沟底宽小时均大于沟底宽大时。合理的断面形式和大小有利于减小垂向入渗,加大水平侧向入渗,灌水沟断面形式为梯形断面时,宽深比近似为2效果最优。研究结果可为改进沟灌灌水技术提供参考。     

塑料大棚控制排水系统设计及水管理研究

采用水管理软件DRAINMOD,以SEW30为指标,确定塑料大棚暗管控制排水系统的间距和埋深。然后根据淋洗土壤盐分的需要,选取不同降雨水平年,采用不同的暗管控制排水出口深度及不同的灌水量,共组合成9种方案,以SEW30、土壤0~60 cm土层盐分脱减率、排水量作为评价指标,分析出研究区不同降雨水平年的水管理方案：丰水年...     

基于样本流量偏差率的微灌灌水均匀度评价方法

为了探索一种简便有效评价大田微灌系统灌水均匀度的新方法,定义了样本流量偏差率.基于灌水器流量服从正态分布的假设,建立了样本流量偏差率与现有灌水均匀度评价指标之间的关系,推导了抽样误差的计算公式,提出了合理的样本容量和样本分组数.建立了基于样本流量偏差率的灌水均匀度评价标准,最终提出了基于样本流量偏差率评价大田微灌灌水均匀度的新方法.案例应用表明:相比图解法,新方法根据数值计算结果评价灌水均匀度,可避免人为画线误差,提高评价结果的可靠度;相比传统数值法,新方法计算量少,可作为快速评价大田微灌系统灌水均匀度的有效方法.     

Groundwater quality in CR-V irrigation district (Bardenas I,Spain): Alternative scenarios to reduce off-site salt and nitrate contamination

Irrigated agriculture may negatively affect groundwater quality and increase off-site salt and nitrate contamination. Management alternatives aimed at reducing these potential problems were analysed in the 15498 ha CR-V Irrigation District (Spain) by monitoring 49 wells and modelling the hydrological regime in a representative well of the Miralbueno Aquifer. Groundwaters presented low to moderate electrical conductivity (EC) (mean = 0.89 dS/m) and high [NO₃⁻] (mean = 94 mg/L). The groundwater depth (GWD) during the 2001 hydrological year responded to the annual cycles of precipitation and irrigation as well as to the secondary cycles derived from irrigation scheduling. GWD were consistently simulated by the groundwater BAS-A model. Model results indicate that an increase in irrigation efficiency and the pumping of groundwater for irrigation will decrease GWD and aquifer's discharge by 56–70%, depending on scenarios. These recommendations will save good-quality water in the reservoir, will be beneficially economical to farmers, and will minimize off-site salt and nitrogen contamination.     

Assessing the well water pollution problem by nitrates in the small scale farming systems of the Niayes region, Senegal

Human activities exert many pressures on the quality of groundwater, and advanced assessment programmes are needed to design sustainable water management strategies. To contribute to this challenge, the nitrate pollution problem of groundwater wells in the small scale farming systems of the Niayes region in Senegal is assessed and explained in terms of well characteristics and land use properties. A field campaign was performed in 2007 to collect basic background data of the small scale farming systems and well water was analysed in 131 wells for nitrate content, pH and electrical conductivity. For a subset of wells, soil analyses were made of the well environment for assessing the attenuation properties of the protecting soil. Cluster analysis was used to define a well typology, while principal component and multiple correspondence analyses were used to relate the nitrate pollution to well characteristics and land use properties.The study confirms that the wells are seriously affected by the nitrate pollution problem. Wells can be classified in three well classes which exhibit highest nitrate content in the southern part, while the salinity affects particularly the wells in the northern area of the region. The nitrate levels exceeding 50 mg/L are more common in residential areas than in the horticultural fields. The results further show that, even if direct pollution problem of wells cannot be excluded, pollution from leaching out of the root zone of vegetable crops to the groundwater is likely. This is confirmed by the assessment of the farming activities in this area and this is consistent with the low attenuation properties of the soils characterizing this region.Considering the nitrate pollution threshold, very few non-polluting crops can be distinguished in this region, which calls for an urgent adoption of agriculture management to protect water resources from further deterioration.