Impact of irrigated agriculture on groundwater-recharge salinity: a major sustainability concern in semi-arid regions
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Intensive irrigated agriculture substantially modifies the hydrological cycle and often has major environmental impacts. The article focuses upon a specific concern—the tendency for progressive long-term increases in the salinity of groundwater recharge derived from irrigated permeable soils and replenishment of unconfined aquifers in more arid regions. This process has received only scant attention in the water-resource literature and has not been considered by agricultural science. This work makes an original contribution by analysing, from scientific principles, how the salinisation of groundwater recharge arises and identifies the factors affecting its severity. If not proactively managed, the process eventually will impact irrigation waterwell salinity, the productivity of agriculture itself, and can even lead to land abandonment. The types of management measure required for mitigation are discussed through three detailed case histories of areas with high-value groundwater-irrigated agriculture (in Spain, Argentina and Pakistan), which provide a long-term perspective on the evolution of the problem over various decades.
KeywordsSoil science Groundwater recharge Salinization Irrigation engineering Groundwater management
Impact de l’irrigation agricole sur la salinité de la recharge des eaux souterraines: une préoccupation majeure en matière de pérennité dans les régions semi-arides
L’irrigation agricole intensive modifie considérablement le cycle hydrologique et a souvent des impacts environnementaux majeurs. L’article porte sur une préoccupation spécifique—la tendance à l’augmentation progressive à long terme de la salinité de la recharge des eaux souterraines provenant des sols perméables irrigués et de la reconstitution des niveaux piézométriques des aquifères libres dans les régions plus arides. Ce processus n’a reçu que peu d’attention dans la littérature sur les ressources en eau et n’a pas été pris en compte par les sciences agricoles. Ce travail apporte une contribution originale en analysant, à partir de principes scientifiques, comment se produit la salinisation de la recharge des eaux souterraines et identifie les facteurs affectant sa sévérité. S’il n’est. pas géré de manière proactive, le processus aura éventuellement une incidence sur la salinité des puits utilisés pour l’irrigation, sur la productivité de l’agriculture elle-même et peut même mener à l’abandon de terres. Les types de mesures de gestion requises pour l’atténuation sont discutés à travers trois cas détaillés d’agriculture irriguée (en Espagne, en Argentine et au Pakistan), qui offrent une perspective à long terme de l’évolution du problème sur plusieurs décennies.
Impacto de la agricultura de regadio en la salinidad de recarga de agua subterranea: una gran preocupacion para la sostenibilidad en las regiones semiaridas
La agricultura intensiva con regadío modifica sustancialmente el ciclo hidrológico y a menudo tiene importantes impactos ambientales. El trabajo se centra en una preocupación específica—la tendencia a incrementos progresivos a largo plazo en la salinidad del agua subterránea en la recarga derivada de la irrigación de suelos permeables y de la recarga de acuíferos no confinados en regiones más áridas. Este proceso ha recibido escasa atención en la literatura de los recursos hídricos y no ha sido considerada por la ciencia agrícola. Este trabajo realiza una contribución original al analizar, desde principios científicos, cómo surge la salinización de la del agua subterránea en la recarga e identifica los factores que afectan su severidad. Si no se gestiona de manera proactiva, el proceso eventualmente impactará en la salinidad de los pozos de agua para riego, la productividad de la agricultura en sí misma, e incluso puede llevar al abandono de la tierra. Los tipos de medidas de gestión requeridas para la mitigación se discuten a través de tres casos detallados de áreas con alto valor de la agricultura con riego de agua subterránea (en España, Argentina y Pakistán), que proporcionan una perspectiva a largo plazo sobre la evolución del problema durante varias décadas.
Impacto da agricultura irrigada na salinidade da recarga de águas subterrâneas: uma grande preocupação quanto à sustentabilidade em regiões semiáridas
Agricultura intensiva irrigada modifica substancialmente o ciclo hidrologico e frequentemente tem grandes consequencias ambientais. O artigo centra-se em um conceito específico—a tendência para o aumento progressivo de longo prazo na salinidade da recarga da água subterrânea derivada de solos permeáveis irrigados e reabastecimento de aquíferos confinados em regiões áridas. Este processo recebeu pouca atenção na literatura sobre recursos hídricos e não foi considerado pela ciência agrícola. Este trabalho faz uma contribuição original para analisar, a partir de principios cientificos, como a salinização da reacarga de água subterrâneas ocorre e identificar os fatores que afetam sua severidade. Se não for gerenciado de forma proativa, o processo eventualmente terá impacto na salinidade do poço de irrigação, na produtividade agrícola e pode até levar ao abandono da terra. Os tipos de medidas de manejo necessárias para a mitigação são discutidos através de três históricos detalhados de áreas com agricultura irrigada de alto valor por água subterrânea (na Espanha, Argentina e Paquistão), que fornecem uma perspectiva de longo prazo sobre a evolução do problema ao longo de várias décadas.
Irrigated agriculture and groundwater
Understanding the impact of irrigated agriculture on the hydrological cycle is critical for effective management of groundwater resources and for promoting more sustainable irrigated agriculture (Seibert et al. 2010; Foster and Shah 2012). Irrigated crop-cultivation practices change the soil-water regime and modify groundwater recharge rates (Foster et al. 2000; Leduc et al. 2001). Where flood irrigation techniques with surface water are practiced on permeable soils, they are a major source of groundwater recharge and often the predominant one in arid terrains (Foster and Perry 2009; Jimenez-Martinez et al. 2009).
The impacts of irrigated agriculture on groundwater quality are also profound (Scanlon et al. 2007), with dissolved salts in irrigation water being concentrated by soil evapotranspiration and subsequently leached from permeable soils to groundwater—as so-called ‘irrigation return-flow’ (the term used by agricultural engineers and scientists for ‘irrigation in excess of plant needs’ that will drain from the soil and ‘return’ to groundwater (or surface water) bodies according to the physical properties of the sub-soil profile). The effect is compounded if the irrigation water itself has significant salinity, which may be the case where groundwater or wasterwater is used. On the other hand, irrigation-canal seepage can provide helpful dilution if they carry low-salinity surface water.
In recent years water security concerns have centred on groundwater depletion by withdrawls for irrigated agriculture, and only limited attention has been paid to the more insidious (and more chronic) problem of progressive aquifer salinisation of groundwater recharge by irrigation return-flows, which is occurring in many semi-arid regions (Llop 2002; Foster et al. 2002; Garduno and Foster 2010; Ó Dochartaigh et al. 2010). Analysis of the phenomenon of groundwater recharge salinisation, and its serious long-term implications, is the primary objective of the present report.
Mobilisation of geogenic salinity from connate water at depth in aquitards or aquifers, due to over-deepening and/or excessive waterwell pumping
Intrusion of saline water into aquifers due to inadequately-controlled construction and/or operation of irrigation waterwells in coastal areas
Salinisation as a result of infiltration of reject brines from desalination plants or hydrocarbon exploitation, if discharged to ephemeral (losing) watercourses or seep from retention ponds
Salinisation from land waterlogging where the water table is very shallow, and accompanied by direct phreatic evaporation with rapid salinisation of shallow groundwater.
Historically, salinisation from land waterlogging has been the focus of major programmes of applied research and is well documented in the agricultural literature, but should not be confused with the mechanism of salinisation of groundwater recharge described here.
Analysis of the recharge salinisation process
Rain water has very low total dissolved solids (TDS) and slightly acidic pH—with Cl and Na generally in the range 10–20 mg/L (although higher in coastal zones due to aerosol effects). On coming into contact with the land surface, rainfall acquires Ca and HCO3, and commonly reaches 200–500 mgTDS/L in natural aquifer recharge. On permeable uncultivated land, the vadose-zone Cl profile can be used to estimate historic rates of groundwater recharge (Edmunds and Tyler 2002), and seasonal differences in soil-water Cl content allows calculation of actual evaporation (Bouhlassa et al. 2016).
Groundwater supply salinity is augmented by mobilisation of connate water from depth in aquifers or underlying aquitards.
Major natural salt accumulation in the vadose zone by native vegetation existed prior to the development of irrigated agriculture.
A high proportion of the aquifer recharge area is under irrigated agriculture.
The number of days annually that soil moisture is maintained by groundwater irrigation is elevated.
Consequences of groundwater salinisation
Groundwater with electrical conductivity (EC) >2,000 μS/cm has been classified as moderately saline for use in crop irrigation (Rhoades et al. 1992)—there is no ‘standard conversion’ from the readily measured parameter EC to TDS (total dissolved solids or salinity in mg/L), since it varies somewhat with the predominant salts (Na-Cl, Ca-SO4, Ca-HCO3, Mg-HCO3) in solution; however, an average conversion factor of ×0.65 is used here. Whilst it is feasible to irrigate with water of EC up to 5,000 μS/cm (3,250 mgTDS/L) for less sensitive crops (e.g. onions), use on more sensitive crops (e.g. some cereals) will impact their growth and reduce productivity, and seriously damage the most sensitive crops (including many vegetables, fruit trees and grape vines (Shani and Dudley 2001)). Increasing salinity will also reduce the utility of groundwater for public and industrial water supply.
Moreover, soil sodicity often increases with increasing irrigation-water salinity and this can lead to decreasing soil permeability and breakdown of soil structure (Quirk and Schofield 1955). The sodium adsorption ration (SAR) of water, calculated by SAR = Na/[(Ca + Mg)/2) ½] is an indicator of potential detrimental impact on the permeability of finer-grained soils (Hillel et al. 2008)—with a SAR of >10 being definitely detrimental to texture and fertility, and values of 3–10 affecting some more sensitive soils and crops. The SAR concept has subsequently been extended by the CROSS (cation ratio of soil structural stability) concept (Rengasamy and Marchuk 2011), which provides a further indication of the hazard of irrigation water with elevated sodic salinity.
Shallow groundwater salinisation: outline of case profiles
Context of selected cases
Groundwater salinisation by irrigation return flows is a serious long-term concern for the sustainability of both water resources and irrigated agriculture (Foster and Cherlet 2014). The three case histories included here provide a long-term (20–40 year) perspective on the evolution of the problem, are widely drawn geographically and cover a wide range of intensive groundwater-fed irrigated agriculture. The groundwater data for each area, which have been compiled in this report, have been obtained from field surveys, monitoring archives and (in the case of the Campo de Dalias, Spain) from published research also. The cases selected are ones in which significant data from field research are available (compared to many other otherwise comparable locations), but even here the lack of systematic long-term monitoring data introduces some constraints on interpretations. It is believed that the process of groundwater recharge salinisation from irrigation water-returns is occurring very widely in the more arid regions with significant irrigated cultivation, but given its slow insidious impact and the generally poor level of field water-quality monitoring, it is not yet widely identified or reported.
Campo de Dalias, Almeria, Spain
The aquifer system has a complex geometry (Fig. 3), which has been researched in considerable detail in recent years (Pulido-Bosch 2005). In the present context, the main interest is the upper aquifer, which is present over much of the plain and comprises Pliocene calcarenites/conglomerates overlain by Quaternary alluvial outwash fans. This unit is underlain by a thick series of impermeable Pliocene marls containing brackish connate water. The Triassic dolomitic limestone aquifer, which forms most of the neighbouring Sierra de Gador recharge area, extends highly confined at depth beneath most of the Campo de Dalias.
Well-drained ‘artificial’ greenhouse soils allowing excess irrigation to leach accumulated salts, with about 75% of farmers applying 30–60 mm after transplanting crops and a similar lamina in the non-cropping period
Rainfall on greenhouses being directed to soakaways from where most infiltrates directly to shallow groundwater
Large manure applications (2,300–4,600 kgN/ha) applied on greenhouse construction, with a subsequent application (600–1,700 kgN/ha) widely every 2–5 years and considerable additional ferti-irrigation resulting in high-nitrate irrigation returns to shallow groundwater.
Marked water-table rebound in the upper aquifer (Fig. 4)
Creation of an aquatic wetland in La Cañada de Las Norias (a neotectonic land-surface depression) known as La Balsa del Sapo, whose water currently has EC > 5,000 μS/cm, coupled with persistent inundation of some glasshouse infrastructure and rural dwellings (Molina et al. 2015)
Some initial signs of the ‘freshening-up’ of the upper aquifer to 1,500–3,000 mgTDS/L (Fig. 4), due to a probable reduction in irrigation-return salinity consequent upon the use of a low salinity groundwater for irrigation (<1,000 mgTDS/L) and some artificial recharge from greenhouse drains.
All of the aforementioned interpretations have been corroborated by isotopic data (Vallejos et al. 1997; Diaz-Puga et al. 2016), which reveal that only a few shallow waterwells in piedmont locations record 3H at measurable (post-1965) levels. This suggests that rainfall recharge has mixed with a large volume of older groundwater in the main aquifers, whose C-isotopes suggest an age of more than 1,000 years.
Taking every opportunity of managed recharge of surface run-off to the shallow aquifer during high-intensity rainfall episodes in the Sierra Gador
Closing or partly grouting-up those waterwells considered to be most responsible for mobilising salinity from the underlying Pliocene aquitard
Evaluating trade-offs between reducing the annual salt load in irrigation returns (and thus shallow groundwater salinity) and reducing cropping intensity and/or the proportion of intensively cropped land
Optimising total N applications from organic manures, inorganic fertilisers and groundwater irrigation so as to reduce excessive NH4/NO3 leaching to shallow groundwater, which is not in the best interest of farmers themselves.
Carrizal Aquifer, Mendoza, Argentina
Upstream construction of the Potrerillos Dam and hydropower plant—radically changing the riverflow regime and reducing by 60% the riverbed length over which recharge occurs continuously, although increased unit seepage rates will occur from ‘clear water infiltration’
Progressive reduction in irrigation returns to groundwater—related to growth in the irrigated area with pressurised water application.
After the first waterwells were drilled in the 1950s, the Carrizal Valley became a horticultural area, but during the 1990s it was discovered to have exceptional soil and a microclimate for export-quality viticulture and fruit production. This created a consumptive water demand on all cultivated land of 3–4 mm/day during October–March (totaling 700–800 mm/a), which was met from a major expansion of groundwater use (with more than 1,000 waterwells having been drilled). Modern irrigated agriculture systems were installed that utilized pressurised ferti-irrigation, anti-hail nets and minimal tillage with glyphosate herbicides and copper fungicides. Today agricultural land prices on the western flanks of this broad valley are high (from US$ 30–50,000/ha for land with productive vines and water use rights, to US$ 4,000/ha for uncultivated dryland). This is putting tremendous pressure on the groundwater resource administration, since there are now 600–700 active production waterwells.
Intensive groundwater development, with competition for available resources amongst the many waterwell users, has given rise to concern about falling groundwater levels and increasing groundwater salinity in some areas. For this reason, a ‘groundwater use restriction zone’ was declared in 1997, with the aim of constraining the expansion of the irrigated area, stabilising groundwater levels and maintaining outflow to the Arroyo Carrizal. The elaboration of a MODFLOW numerical aquifer model suggested a long-term equilibrium of the groundwater system in 2000, but with significant withdrawals from aquifer storage (25–60 Mm3/a) to support abstraction in periods of below average riverflow. The model provided a physical basis for assessing the aquifer salt balance and groundwater salinity problem.
Overall salinity stratification with elevated EC of 2,600–4,400 μS/cm (1,690–2,860 TDS mg/L) down to 70 m below ground level (bgl), but with deeper screen intakes recording 1,000–2,400 μS/cm (650–1,560 TDS mg/L; Figs. 5 and 6)
A major increase in salinity since the 1960s, when shallow and deep groundwater was found to have EC = 1,800 and 1,000 μS/cm (1,170 and 650 TDS mg/L) respectively during an early reconnaissance by the United Nations Development Proramme
A similar distribution of major elements (such as Na, Cl, Ca, SO4, NO3 and certain minor elements such as Li, Sr), with for example NO3 and Cl at levels of 20–60 mg/L and 150–750 mg/L in shallow groundwater compared to levels of NO3 < 10 mg/Land Cl < 100 mg/L at depth.
Increased groundwater salinity in the southeastern section of the valley has resulted in the substitution of onion and garlic cultivation for more profitable viticulture, with a corresponding fall in land values. The observed 2004–2005 distribution of groundwater salinity, accompanied by elevated anthropogenic nitrate, suggests that it can be primarily attributed to increasing salinisation of irrigation returns, consequent upon significant recirculation of salts through groundwater irrigation and the initial leaching of barren desert soils when first brought into irrigated cultivation. The process has been more marked given that natural groundwater residence times in this aquifer system are likely to be in excess of 100 years. Hydrocarbon exploitation in the general area could have led to some seepage of oilwell formation water (containing Na = 24,000 mg/L and Cl = 39,000 mg/L), but overall it does not appear that this has been a major factor in groundwater salinisation.
Diverting more water from the Mendoza River into the Carrizal Valley (during periods of peak flow) for managed aquifer recharge and continuing to constrain consumptive groundwater use.
Licensing of replacement waterwells in areas where shallow groundwater is already salinized should be accompanied by a reduction of annual abstraction and appropriate waterwell construction.
Some existing shallow saline groundwater can be used for cultivation of less-sensitive cash crops (like onions), where it is possible to mix it with lower-salinity groundwater, but this should still be subject to reducing annual abstraction.
Rechna Doab, Punjab, Pakistan
To understand the groundwater salinisation processes, it is essential to appreciate the hydrologic evolution of this arid alluvial-outwash plain, which was originally underlain by saline water (Fig. 7) and into which fresh (low salinity) groundwater was introduced below riparian zones as a result of riverbed seepage. Progressively during 1850–1930, the construction of weir-controlled irrigation canals to reach land distant from the main rivers, led to widespread freshwater infiltration and formation of a continuous layer of groundwater across the low interfluves, with the main rivers changing from potentially ‘losing’ to ‘gaining’ groundwater. Critically also, major seepage from the irrigation canal network resulted in a rising water table that led to soil waterlogging and secondary soil salinisation in some zones. Currently, freshwater occurs to depths of >100 m in a 20–30-km-wide zone beneath the river flood plains, but in the centre of the Rechna Doab the freshwater lens is only 10–20 m thick, and widely compromised by somewhat saline irrigation-return waters (Fig. 7).
In the 1950s, waterwell pumping was introduced to improve soil drainage and to mitigate soil sodic salinisation—and it was also soon found useful to supplement irrigation water-supply during times of reduced surface-water availability. Subsequently there was a boom in privately drilled irrigation boreholes. The intensive utilisation of groundwater was critical to sustaining agricultural production during the 1998–2001 drought, and an associated lowering of the water table also further reduced the land area experiencing soil salinisation. However, severe water-table depletion in some areas, as a result of intensive irrigation waterwell pumping, caused up-coning of connate water and salt mobilisation to the land surface (van Steenbergen and Gohar 2005).
It is not straightforward to establish salinity trends because of the mixed nature of most samples taken from production tubewells, considerable lateral and vertical salinity variations, and the high cost of representative sampling (MacDonald et al. 2016). However, it is instructive to consider the issue from first principles. In low-lying flat alluvial terrain, underlain by thin fresh groundwater bodies, vertical flux (both downward and upward) tends to predominate over horizontal flow. It is thus valid to consider the water and salt balance. In such areas, irrigation canals deliver reasonable quality water for irrigation, with salinity usually in the range 180–250 mgTDS/L. If typical canal water is taken as having 200 mgTDS/L, when this water was first used for gravity-furrow irrigation, fractionation of its salt content will have occurred, and if 40% of the applied irrigation lamina infiltrated to shallow groundwater, this recharge would have contained 500 TDSmg/L. Decades later, when supplementary irrigation with shallow groundwater became established, and provided say 50% of the total annual irrigation lamina, then soil fractionation would have generated an irrigation water return to groundwater containing 875 mgTDS/L. Subsequently, year-on-year, this process would inevitably lead to continuously rising groundwater salinity, eventually to exceed the FAO-UN guideline level of 1,500 mgTDS/L (above which negative impacts on crop yields must be anticipated).
Drilling deeper irrigation tubewells. While initially this may provide a groundwater supply of lower TDS, it runs a high risk of up-coning connate water.
Improving field irrigation efficiency. As regards groundwater, this amounts simply to delaying impacts by reducing irrigation returns whilst increasing their salinity.
Aeolian salt deposition. In arid climates there exists the possibility of occasional dust storms with winds capable of transporting particles of salty soil (Shiga et al. 2011).
The groundwater resources of the Punjab have already proven to be of critical importance to the food security of Pakistan; however, sustaining this role and developing their full potential will require a more proactive and adaptive approach to water-resource management (Qureshi et al. 2008). Central to this strategy will be that decisions on the lining of irrigation canals, and procedures for canal-water allocation and diversion entitlements, must take account of local groundwater conditions. There is also likely to be a periodic need to remove some salt load completely from various parts of the land–groundwater system (with appropriate safe disposal), as an additional element of integrated management. This will need to be compensated by increased use of surface water for gravity irrigation in the corresponding areas at times of excess riverflow, such that some dilution of existing groundwater salinity can be achieved. It will also be necessary to address the problem of mobilisation of connate groundwater, especially on the interfluves, requiring allocation and delivery of increased canal flows and reduction of waterwell depths and pumping rates.
There is also a pressing need to undertake applied hydrogeological research (using geophysical survey, isotopic techniques, other tracers, hydrochemical analyses and numerical modeling) to determine the three-dimensional groundwater flow dynamics and quality genesis with more confidence. This would help elucidate temporal trends in groundwater salinity from the large quantity of historic monitoring data of mediocre quality. In parallel, it will be essential to strengthen the groundwater-monitoring infrastructure to enable the benefits of water-resource-management measures to be critically assessed,
Land and water management options
The persistence and complexity of the problems arising in all cases of groundwater recharge salinisation from irrigated agriculture is such that they can only be properly addressed through carefully planned integrated management of land, surface and groundwater resources. The implementation of essential management measures will require awareness raising, institutional change and capacity building—and it will only be after improved surface-water and groundwater management is in place, that measures to improve soil salinity and fertility will accrue and the long-term benefits of more sustainable, higher-yielding, agricultural cropping will be assured.
Reducing the overall consumptive use of groundwater by substituting more surface-water irrigation or down-sizing the groundwater-irrigated area
Increasing the rate of freshwater recharge, by making excess flows in any main irrigation canals available for this purpose and/or capturing local storm-water run-off to recharge lagoons.
Reducing annual cropping intensity (by eliminating crops of lower market value) so as to reduce the annual salt load generated by soil evapotranspiration processes
Reducing the proportion of cropped land that is irrigated, whilst ensuring the highest possible economic return on those crops still cultivated
Constraining groundwater abstraction to prevent aquifer water levels from falling below the level required for ‘natural discharge’, such that throughflow and drainage are maintained to avoid progressive salt accumulation in the groundwater systems.
A high priority for water resource agencies should be to provide incentives for farmers to maximize opportunities to supplement groundwater recharge from streambeds and soakaway-drains on their land. Such additional recharge will provide valuable dilution of saline (and nutrient rich) recharge from irrigated agricultural land.
The case profiles presented in this report reveal that the salinisation of groundwater recharge from intensive agricultural cropping using waterwell irrigation is a gradual but insidious phenomenon under more arid climatic conditions, whose management poses complex and costly challenges.
The phenomenon of groundwater recharge salinisation is not related to soil waterlogging and phreatic salinisation, since it has been demonstrated to be occurring where the water table is 5–50 m deep (although the process will also be occurring on alluvial plains of low relief).
Water resource agencies (in collaboration with their agricultural counterparts) need to evaluate salt balances (in addition to those for water and nutrients) periodically at aquifer sub-catchment level, to assess the risk of serious salinisation of groundwater recharge and to guide management interventions.
Improved integration of land and water management by agricultural and water resource agencies is a pressing need to specify, implement and refine mitigation measures for the control of the salinisation of groundwater recharge.
The first author wishes to acknowledge past discussion of the processes of groundwater salinisation and options for their management in the context of the field areas presented in the report with the following: Amilcar Alvarez and Hector Garduño with respect to Mendoza, Argentina, and Mohammed Basharat and Frank van Steenbergen with respect to Punjab, Pakistan. He also thanks Chris Perry for a long-term sustained dialogue on the impacts of irrigated agriculture on groundwater, Antonio Pulido-Bosch (a co-author of this report) for many valuable interchanges down-the-years on groundwater irrigation issues, John Barker for valued advice on approaches to analytical modelling of the groundwater recharge salinisation process, and Gill Tyson for her excellent work improving the illustrations in this report.
Part of the survey work described in the field case-history areas was supported by funding from the Junta de Andalucia (Almeria, Spain), the World Bank GW-MATE Programme (Mendoza, Argentina) and the UK Department for International Development (Rechna Doab, Pakistan).
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