The impact of controlled tidal exchange on drainage water quality in acid sulphate soil backswamps

Periodic opening of one-way tidal floodgates was undertaken on two coastal flood mitigation drains to promote exchange with estuarine water and improve drain water quality. The drains were located in areas with acid sulphate soils and their drainage water frequently had high acidity and low dissolved oxygen (DO). Tidal exchange via floodgate opening generally raised drain water pH levels through dilution and/or neutralisation of acidity. Increases in DO and moderation of extreme diurnal DO fluctuations were also observed. The magnitude and stability of the improved physico-chemical conditions was highly dependant on the volume and quality of tidal ingress water. Relatively rapid reversion (hours to days) in drain water pH and DO was observed once floodgates were closed again. The rate of reversion following floodgate closure was strongly related to outflow volumes, antecedent drain water quality conditions and groundwater levels. Floodgate opening caused changes in longitudinal drain water gradients and has potential to slow net drainage rates during non-flood periods. However, complex site specific interactions between drain water and adjacent groundwater can also occur. At one location, a 4-day floodgate opening event caused recharge of adjacent acid groundwater during the opening phase, raising the potentiometric groundwater level above local low tide minima. This was followed by tidally modulated draw down of acid groundwater and enhanced acid export in the period immediately following floodgate closure. There are also practical considerations, which limit the efficacy of floodgate opening as an acid management strategy. The low elevation (close to mean sea level) of some acid sulphate soil backswamps, combined with seasonal migration of the estuarine salt wedge, means there is considerable potential for saline overtopping of what is currently agricultural land. This constrains the magnitude and duration of controlled tidal exchange. Also, it is during wet periods that acid drainage outflow to the estuary is greatest. At such times the salinity and acid buffering capacity of estuarine water is often low, thus reducing the capacity of tidal exchange waters to neutralise acidity.     

Drainage water salinity of tubewells and pipe drains:: A case study from Pakistan

Drainage water salinity data from 71 public deep tubewells and 79 pipe drainage units near Faisalabad, Pakistan, were studied. Drainage water salinity of the tubewells and the pipe drains remained approximately constant with time. This was attributed to the deep, highly conductive, unconfined aquifer underlying the area, which facilitates lateral groundwater inflow into the drained areas. Tubewells alongside surface drains showed average electrical conductivity, sodium adsorption ratio, and residual sodium carbonate values of 3.2 dS m⁻¹, 17.2 (meq l⁻¹)^0.5, and 6.4 meq l⁻¹, respectively. For pipe drains, which are situated in areas with comparable conditions, the corresponding values were 2.5 dS m⁻¹, 12.2 (meq l⁻¹)^0.5, and 3.7 meq l⁻¹, respectively. Tubewells have an inferior drainage water quality because they attract water from greater depths, where the water is more saline.     

The effects of a weir on reducing acid flux from a drained coastal acid sulphate soil backswamp

A trial was conducted to examine the effects of retaining drain water with a weir on reducing acid flux from a drained coastal acid sulphate soil backswamp. Prior to weir construction, groundwater seepage to the ditch drain was the main hydrological pathway for acid flux. High hydraulic conductivity (>120 m per day) in the sulphuric horizons due to extensive macropores, combined with tidal modulation of drain water levels encouraged rapid seepage of acid groundwater. Most seepage occurred while the backswamp groundwater table was in a narrow elevation range, referred to as an ‘acid export window’. The acidity of drainage water was highly sensitive to the hydraulic gradient between the groundwater table and the adjacent drain water level. Acid flux rates from groundwater seepage were strongly positively correlated to effluent groundwater hydraulic gradients. The constructed weir was designed to reduce the magnitude of effluent groundwater gradients and retain shallow groundwater by maintaining high and stable drain water levels. This reduced groundwater seepage to the drain and increased the proportion of shallow groundwater lost from the system via evapotranspiration. The weir affected 60% of drainage network and observed and modelled data suggest acid flux from groundwater seepage was reduced by about 65–70%. Effluent groundwater gradients behind the weir were reduced by about 80%. The main effect of the weir was to reduce discharge volumes, although reductions in H⁺ and acidic metal cation concentrations were also observed. This study demonstrates that a weir can be an effective means of reducing acid flux in coastal acid sulphate soils where main hydrological pathway of acid export is groundwater seepage. However, this strategy may not prevent continued sulphide oxidation. Reduced acid export, but continued acid generation, combined with enhanced evaporative flux has the potential over the longer term to increase the net accumulation of acidic products in the backswamp soil and groundwater. Further monitoring is required to assess this possibility.     

Effect of water-saving irrigation on rice yield and water use in typical lowland conditions in Asia

With decreasing water availability for agriculture and increasing demand for rice, water use in rice production systems has to be reduced and water productivity increased. Alternately submerged–nonsubmerged (ASNS) systems save water compared with continuous submergence (CS). However, the reported effect on yield varies widely and detailed characterizations of the hydrological conditions of ASNS experiments are often lacking so that generalizations are difficult to make. We compared the effects of ASNS and CS on crop performance and water use, at different levels of N input, in field experiments in China and the Philippines, while recording in detail the hydrological dynamics during the experiment. The experiments were conducted in irrigated lowlands and followed ASNS practices as recommended to farmers in China. The sites had silty clay loam soils, shallow groundwater tables and percolation rates of 1–4.5 mm per day.Grain yields were 4.1–5.0 t ha⁻¹ with 0 kg N ha⁻¹ and 6.8–9.2 t ha⁻¹ with 180 kg N ha⁻¹. Biomass and yield did not significantly differ between ASNS and CS, but water productivity was significantly higher under ASNS than under CS in two out of three experiments. There was no significant water×N interaction on yield, biomass, and water productivity. Combined rainfall plus irrigation water inputs were 600–960 mm under CS, and 6–14% lower under ASNS. Irrigation water input was 15–18% lower under ASNS than under CS, but only significantly so in one experiment. Under ASNS, the soils had no ponded water for 40–60% of the total time of crop growth. During the nonsubmerged periods, ponded water depths or shallow groundwater tables never went deeper than −35 cm and remained most of the time within the rooted depth of the soil. Soil water potentials did not drop below −10 kPa. We argue that our results are typical for poorly-drained irrigated lowlands in Asia, and that ASNS can reduce water use up to 15% without affecting yield when the shallow groundwater stays within about 0–30 cm. A hydrological characterization and mapping of Asia’s rice area is needed to assess the extent and magnitude of potential water savings.     

Modelling the effect of groundwater depth on yield-increasing interventions in rainfed lowland rice in Central Java,Indonesia

Because of drought and nutrient stress, the yields of rainfed lowland rice in Central Java, Indonesia, are generally low and unstable. Variation in groundwater depth can contribute to experimental variability in results of yield-increasing interventions. To test this hypothesis, we used the crop growth simulation model ORYZA2000 to explore the impacts of groundwater depth on the effect of sowing date, tillage, fertiliser-N application and supplementary irrigation on the yield of lowland rice at Jakenan, Central Java, Indonesia. ORYZA2000 was first parameterized and evaluated using data from eight seasons of field experiments between 1995 and 2000. The model adequately simulated the soil water balance, crop growth and grain yield. With shallow to medium groundwater depth (less than 0.5 m deep), rainfed rice yields are close to potential yields with timely sowing in the wet season. With groundwater tables fluctuating mostly between 0.5 and 1.5 m, rainfed yields are 0.5–1 Mg ha⁻¹ lower than potential yields with timely sowing. The decrease in yield with late sowing sets in earlier and proceeds faster with deeper groundwater depths. Deep tillage and supplementary irrigation increase yield more with deep groundwater tables than with shallow groundwater tables, but N fertilisation increases yield more with shallow than with deep groundwater tables. Groundwater depth should be taken into account in the selection of yield-increasing interventions.     

Nitrogen loss through subsurface drainage effluent in coastal rice field from India

Experiments were conducted to estimate nitrogen loss through drainage effluent in subsurface drained farmers’ field at a coastal site near Machilipatnam, Andhra Pradesh, India. The concentration of three forms of nitrogen, namely, NH₄–N, NO₂–N and NO₃–N in the subsurface drainage effluent from 15, 35 and 55 m drain spacing areas were measured in 1999 and 2000. The area with 15 m spacing was already reclaimed during 1986–1998 by the subsurface drainage system. The soil salinity of the root zone was brought down from an initial high of 35 to 4 dS m⁻¹. The subsurface drainage system with 35 and 55 m drain spacing was laid in the adjoining area and commissioned in 1998. Earlier raising of any crop in the area with 35 and 55 m spacings was not possible due to very high salinity, sodicity and poor drainage conditions. The nitrate-nitrogen loss dominated in reclaimed land with 15 m spacing whereas ammonium-nitrogen loss dominated in the land that was highly saline and in the initial stage of reclamation by the subsurface drainage technology with 35 and 55 m drain spacing. The total nitrogen loss of 3.75 kg per ha per year in 15 m drain spacing area was minimum and 23.53 kg per ha per year in 35 m drain spacing area was maximum. The nitrate-nitrogen loss contributed the maximum of 82% and ammonium- and nitrite-nitrogen contributed 11 and 7%, respectively, in 15 m drain spacing area whereas the ammonium losses contributed 93 and 82% in 35 and 55 m drain spacing areas, respectively. The losses in the form of nitrite and nitrate remained negligible in 35 m drain spacing area, but the losses to the tune of 8 and 15% in the form of nitrite and nitrate, respectively, occurred in 55 m drain spacing area.     

Effect of shallow groundwater table on crop water requirements and crop yields

Due to the increasing demand for food and fiber by its ever-increasing population, the pressure on fresh water resources of Pakistan is increasing. Optimum utilization of surface and groundwater resources has become extremely important to fill the gap between water demand and supply. At Lahore, Pakistan 18 lysimeters, each 3.05 m × 3.05 m × 6.1 m deep were constructed to investigate the effect of shallow water tables on crop water requirements. The lysimeters were connected to bottles with Marriotte siphons to maintain the water tables at the desired levels and tensiometers were installed to measure soil water potential. The crops studied included wheat, sugarcane, maize, sorghum, berseem and sunflower. The results of these studies showed that the contribution of groundwater in meeting the crop water requirements varied with the water-table depth. With the water table at 0.5 m depth, wheat met its entire water requirement from the groundwater and sunflower absorbed more than 80% of its required water from groundwater. Maize and sorghum were found to be waterlogging sensitive crops whose yields were reduced with higher water table. However, maximum sugarcane yield was obtained with the water table at or below 2.0 m depth. Generally, the water-table depth of 1.5–2.0 m was found to be optimum for all the crops studied. In areas where the water table is shallow, the present system of irrigation supplies and water allowance needs adjustments to avoid over irrigation and in-efficient use of water.     

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.     

Irrigation with saline water in the reclaimed marsh soils of south-west Spain: impact on soil properties and cotton and sugar beet crops

The drained and irrigated marshes in south-west Spain are formed on soils of alluvial origin from the ancient Guadalquivir river estuary. The most important characteristics of these soils are the high clay content (about 70%), high salinity, and a shallow, extremely saline, water table. The reclaimed area near Lebrija, called Sector B-XII (about 15,000 ha), has been under cultivation since 1978. Some years, however, water supply for irrigation is limited due to drought periods. The objective of this work was to evaluate the effects of irrigation with high and moderately saline waters on soil properties and growth and yield of cotton and sugar beet crops. The experiments were carried out during 1997 and 1998 in a farm plot of 12.5 ha (250 m×500 m) in which a drainage system had been installed, consisting of cylindrical ceramic sections (0.3 m long) forming pipes 250 m long, buried at a depth of 1 m and spaced at intervals of 10 m. These drains discharge into a collecting channel perpendicular to the drains. Two subplots of 0.5 ha (20 m×250 m) each were selected. In 1997 cotton was growing in both subplots, and irrigation was applied by furrows. One subplot (A) was irrigated with fresh water (0.9 dS m⁻¹) during the whole season, while in the other subplot (B) one of the irrigations (at flowering stage) was with water of high salinity (22.7 dS m⁻¹). During 1998 both subplots were cropped with sugar beet. Subplot A was irrigated with fresh water (1.7 dS m⁻¹) during the whole season, while in subplot B two of the irrigations were with moderately saline water (5.9–7.0 dS m⁻¹). Several measurement sites were established in each subplot. Water content profile, tensiometric profile, water table level, drainage water flow, soil salinity, and crop development and yield were monitored. The results showed that after the irrigation with high saline water (subplot B) in 1997 (cotton), the soil salinity increased. This increase was more noticeable in the top layer (0–0.3 m depth). In contrast, for the same dates, the soil of subplot A showed no changes. After five irrigations with fresh water, the salinity of the soil in the subplot B reached values similar to those before the application of saline water. In 1998 (sugar beet) the application of moderately saline water in subplot B also increased soil salinity, but this increase was lower than in 1997. The irrigation with high saline water affected crop development. Cotton growth was reduced in comparison with that in the subplot irrigated only with fresh water. Despite this negative effect on crop development, the crop yield was the same as in the subplot A. Sugar beet development did not show differences between subplots, but yield was higher in subplot B than in subplot A.     

Management of declining groundwater in the Trans Indo-Gangetic Plain (India): Some options

The average productivity of rice–wheat sequence is quite impressive in the Trans Indo-Gangetic Plain (India) but these gains are over-shadowed due to declining groundwater, particularly in the areas, where groundwater quality is either good or marginal. The groundwater decline can be reversed through artificial groundwater recharge and by adopting suitable land and water management practices. Groundwater recharge is found technically feasible through vertical shafts conducting water from the ground surface directly to aquifers, after it has been passed through a sand-gravel filter. The recharge rate through this system is almost equal to a shallow cavity/filter well yield (about 11 l/s) and its cost is estimated at about INR 10/100 m³ (1 US$ = 45 INR). Further study in the Kaithal and Karnal districts of Haryana for stabilizing watertable within 6–7 m, which permits continuous use of shallow tubewell technology, indicated that the rice area could be supported at 60% of cultivable command area (CCA) and wheat between 65 and 80% of CCA with the existing management practices. The cultivation of wheat crop is sustainable in larger area, mainly due to its medium water requirement, salt resistance characteristics and consistent market demand resulting in assured returns. There is a possibility of supporting rice at a higher level, if part of the area (up to 10%) is left fallow and used for rainwater conservation and recharge. The fallow area may be subsequently put under early rabi (winter) crops like mustard, gram and other pulses. The effect of varying irrigation and fallowing would increase 23% equivalent wheat yield by changing land and water management practices. The analysis further indicated that the adoption of proposed irrigation management practices might stabilize watertable at desired level of 6–7 m in 10–15 years in high (3–4 m), 5 years in medium (5–10 m) and 40 years in deep (>10 m) watertable areas.     

Groundwater uptake and sustainability of farm plantations on saline sites in Punjab province,Pakistan

Productive tree plantations on degraded land within Pakistan’s irrigation areas may help control salinity by extracting shallow groundwater, but their adoption has been limited by a lack of information on tree–water–salt interactions. Tree growth, water use, climate and soil conditions were monitored between 1994 and 1998 in young plantations of Eucalyptus, Acacia and Prosopis at two locations in Punjab province. Eucalyptus camaldulensis on an irrigated, non-saline site near Lahore showed best growth till the age of 5 years, and an annual water use of 1393 mm. Irrigated Eucalyptus microtheca at this site and unirrigated E. camaldulensis dependent on saline groundwater at Pacca Anna also transpired over 1000 mm of water per year. Basal area growth of Acacia ampliceps at the latter site was similar to E. camaldulensis, but its water use was less. Lowest annual water use of 235 mm was shown by an understocked stand of Prosopis juliflora. Canopy conductance decreased with increasing vapour pressure deficit to a species-dependent minimum value. Results of soil sampling, chloride balance modelling and intensive monitoring of soil solution salinity demonstrated accumulation of salt in the root zone of plantations using saline groundwater. The concentration of stored salt varied seasonally as a result of water table fluctuations and redistribution processes within the unsaturated zone. The apparent limitation of salt accumulation by these processes and the continuing satisfactory growth of the plantations justify cautious support of tree growing as a control measure for shallow water tables and salinisation in Pakistan.     

Potential nitrate losses under different agricultural practices in the pampas region,Argentina

Fertilization is an important cause of groundwater contamination with nitrate in agricultural soils. The objectives of the present work were: (i) to quantify the nitrate leaching in two fertilized and irrigated soils of the Pampas Region, Argentina; (ii) to test the ability of the NLEAP model to predict residual and leached nitrate in those soils. The soils were a Typic Hapludoll and a Typic Argiudoll. The treatments were: natural grassland never ploughed or fertilized; maize with a short history of fertilization; maize with a long history of fertilization; irrigated maize with a long history of fertilization. Both sites were sampled after harvest in two consecutive years to a 3 m depth. Residual nitrate and potential losses below 150 cm were estimated by NLEAP model. The average amount of nitrate (NO₃-N), including values of all treatments, in the upper layer (0–1.5 m) was 128 kg NO₃-N ha⁻¹ in the first sampling date and was consistently lower in the second sampling date (38 kg NO₃-N ha⁻¹). In the deeper layer (1.5–3 m) these values were 80 and 28 kg NO₃-N ha⁻¹ for the first and second sampling date, respectively. Differences between the non-fertilized and the fertilized treatments were significantly smaller in the second sampling date. Obtained results suggest that the rainfall previous to the first sampling was not enough to displace nitrate below 3 m depth. The afterwards heavy rainfall leached nitrate previously accumulated in the soil. Complementary irrigation did not affect nitrate movements. Simulated residual and leached nitrate showed a high correlation with observed values. Nitrate leaching was more associated to rainfall regime and crop yields than to soil type. Simulated residual and leached nitrate showed a high correlation with measured values in both soils, which suggests that NLEAP was appropriate to predict soil nitrate leaching under the studied conditions.     

Evaluation of DRAINMOD for predicting water table heights in irrigated fields at the Jordan Valley

A detailed field experiment was carried out in the Jordan Valley, south of Lake Kinneret, Israel for evaluation of the water management model DRAINMOD. This field was chosen to represent the local agro-climate conditions of that zone. Banana crop was grown and was irrigated daily with about 3200 mm/year and 0.5 leaching fraction. Subsurface drainage system with 2.5 m drain depth and 160 m drain spacing existed in the field. The water table depth was measured with about 100 piezometers, in which most of them were observed weekly, and four were continuosly recording piezometers. Five identical drainage plots were selected, out of 10 existing, as replicates for the evaluation of DRAINMOD. Deviations in a range of 0.3–1.7 m between observed water table depth and that simulated by DRAINMOD were found in four out of the five replicates. A reasonable agreement was found only in one drainage plot out of the five tested. These findings contradict the world wide convention that DRAINMOD simulation is in a good agreement with observed field data. An additional study was therefore conducted to explore the reasons for these large deviations. Three reasons were suggested: (i) a strong side effect by the Jordan River, which flows some 350 m west to the test field; a very steep 4.6% gradient was found toward the Jordan River; (ii) presence of sandy permeable layers below the depth of the drains which magnifies the boundary condition effect of the Jordan River; (iii) a very significant component of deep and lateral seepage (more than 50% of the yearly irrigation plus rainfall). A combination of these three reasons was suggested as an explanation to the apparent large disagreement. It was therefore recommended not to use DRAINMOD or similar vertical flow models for simulation of water table depths in irrigated fields with subsurface drain pipe systems in the Jordan Valley.     

Managing subsurface drip irrigation in the presence of shallow ground water

A 3-year project compared the operation of a subsurface drip irrigation (SDI) and a furrow irrigation system in the presence of shallow saline ground water. We evaluated five types of drip irrigation tubing installed at a depth of 0.4 m with lateral spacings of 1.6 and 2 m on 2.4 ha plots of both cotton and tomato. Approximately 40% of the cotton water requirement and 10% of the tomato water requirement were obtained from shallow (<2 m) saline (5 dS/m) ground water. Yields of the drip-irrigated cotton improved during the 3-year study, while that of the furrow-irrigated cotton remained constant. Tomato yields were greater under drip than under furrow in both the years in which tomatoes were grown. Salt accumulation in the soil profile was managed through rainfall and pre-plant irrigation. Both drip tape and hard hose drip tubing are suitable for use in our subsurface drip system. Maximum shallow ground water use for cotton was obtained when the crop was irrigated only after a leaf water potential (LWP) of −1.4 MPa was reached. Drip irrigation was controlled automatically with a maximum application frequency of twice daily. Furrow irrigation was controlled by the calendar.     

Subsurface drain flow and crop yield predictions for different drain spacings using DRAINMOD

DRAINMOD was run for 15 years to predict and compare drain flow for three drain spacings and crop yield for four drain spacings at the Southeastern Purdue Agricultural Center (SEPAC). Data from two continuous years of daily drain flow from one spacing were used to calibrate the eight most uncertain parameters using a multi-objective calibration function and an automatic calibration method. The model was tested using the remaining field data for the 5, 10, and 20 m drain spacings for drain flow and the additional 40 m spacing for yield predictions. Nash–Sutcliffe efficiency (EF) for daily drain flow simulations for the calibration years and drain spacing ranged from 0.62 to 0.79. The daily EF for model testing ranged from −0.66 to 0.81, with the average deviations of 0.01 to 0.07 cm/day and standard errors of 0.03–0.17 cm/day. On a monthly basis, 91% of plot years had EF values over 0.5 and 76% over 0.6 for years with on-site rainfall data. The total yearly drain flow was predicted within ±25% in 71% of plot years, and within ±50% in 93% of plot years with on-site rainfall data. Statistical tests of daily drain flow EF values for three spacings and percent errors of crop relative yield for four spacings indicated that the reliability of the model is not significantly different among different spacings, supporting the use of DRAINMOD to study the efficiencies of different drain spacings and to guide the drain spacing design for specific soils. In general, the model correctly predicted the pattern of yearly relative yield change. The relative corn (Zea mays L.) and soybean (Glycine max L.) yields were well predicted on average, with percent errors ranging from 1.3 to 9.7% for corn and from −3.3 to 10.3% for soybean.     

Response of safflower (Carthamus tinctorius L.) to saline soils and irrigation: I. Consumptive water use

Salt-tolerant crops can be grown with saline water from tile drains and shallow wells as a practical strategy to manage salts and sustain agricultural production in the San Joaquin Valley (SJV) of California. Safflower (Carthamus tinctorius L.) was grown in previously salinized plots that varied in average electrical conductivity (EC_e) from 1.8 to 7.2 dS m⁻¹ (0–2.7 m depth) and irrigated with either high quality (EC_i<1 dS m⁻¹) or saline (EC_i=6.7 dS m⁻¹) water. One response of safflower to increasing root zone salinity was decreased water use and root growth. Plants in less saline plots recovered more water on average (515 mm) and at a greater depth than in more salinized plots (435 mm). With greater effective salinity, drainage increased with equivalent water application rates. Seed yield was not correlated with consumptive water use over the range of 400–580 mm. Total biomass and plant height at harvest were proportional to water use over the same range. Safflower tolerated greater levels of salinity than previously reported. Low temperatures and higher than average relative humidity in spring likely moderated the water use of safflower grown under saline conditions.     

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.     

Controlled drainage — effects on drain outflow and water quality

A field experimental project was set up in southern Sweden to assess the effects of controlled drainage on hydrology and environment. Controlled drainage makes it possible to vary the drainage intensity with the variation in drainage requirement during season by controlling the height of a riser in the drain outlet and thus to a certain degree control the amount of outflow of solutes via the drainage system. During periods with low drainage demand, the riser in the drain outlet can be raised and the groundwater level in field will rise up to the level of the riser before the discharge takes place. Three plots, each with an area of 0.2 ha (40 m×50 m) were installed on a loamy sand. One plot was drained by conventional subsurface drainage (CD) and two plots were drained by controlled drainage (CWT). The plots contained four lateral drain tubes, at 10 m spacing and placed at 1 m depth. Each plot was isolated by a double layer of plastic sheeting placed in the back-filled trenches to a depth of 1.6 m to prevent lateral leakage and subsurface interactions. Measurements of precipitation, drain outflow and soil and air temperatures were carried out hourly. Groundwater levels were measured and samples of drain outflow were collected twice a month for nitrogen and phosphorous analyses. Mineral nitrogen contents in soil were measured three times a year.Controlled drainage had a significant hydrological and environmental effect during the 2 years of measurement (1996–1998). Compared with CD, the total drain outflow from CWT was 79% less in Year 1 and 94% in Year 2. The total reduction in nitrate losses with CWT corresponded to the reduced outflow rates. Compared with CD, the total amounts of nitrate in drain outflow were 78% less in Year 1 and 94% in Year 2. The highest concentrations of nitrate were measured at the time of the largest outflow rates. The phosphorous losses were 58% less for CWT as compared to the CD values in Year 1 and 85% less in Year 2. The reduction in nitrogen content in the soil profile during the winter season was 60–70% less in CWT than in CD.     

Fate of gypsum surface — applied to an Appalachian hill pasture watershed

Low pH soils leached with gypsum solutions have been shown to sorb gypsum thereby increasing calcium and improving root growth of some crops. However, in some situations, exchangeable aluminum is leached which could enter waterways and potentially cause adverse off-site impacts. Due to relatively high solubility, gypsum itself has potential to impact stream water chemistry. In this study, 8250 kg ha⁻¹ gypsum was applied to plots in a steep 4.3 ha-pasture watershed in central Appalachia. Changes in the 0–40 cm soil profile and changes in water chemistry at a flume below a seep which drains the watershed were measured in response to this application. After the end of the second growing season, and a total rainfall of 1.7 m, 46% of the applied gypsum calcium was still in the top 40 cm of soil. The rainfall had a 11.4% efficiency rate of transporting gypsum out of the top 40 cm compared to movement of a saturated solution through an inert medium. Of the applied gypsum sulfate, 3.4% exited the watershed through stream flow. Maximum gypsum concentrations in stream flow, which occurred during a storm event, were 34 ppm or <1.5% of saturation levels. No detectable aluminum was measured in stream flow at the flume. These findings indicate that as long as a minor part of watersheds such as this one has gypsum applied at rates less than 10,000 kg ha⁻¹, off-site environment impacts should be minimal.     

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.