Irrigation-induced land water depletion aggravated by climate change – Nature

Executive Summary

A comprehensive analysis using seven Earth system models reveals that the historical expansion of agricultural irrigation has significantly exacerbated water scarcity, posing a direct challenge to the achievement of Sustainable Development Goals (SDGs), particularly SDG 6 (Clean Water and Sanitation) and SDG 2 (Zero Hunger). The study demonstrates that irrigation expansion substantially decreases the net water influx from the atmosphere to land, compounding drying trends induced by climate change (SDG 13, Climate Action). In regions like South Asia, this has amplified the negative trend of net water influx from -0.664 to -1.461 mm yr⁻² post-1960. Consequently, the rate of local terrestrial water storage depletion has been drastically enlarged by irrigation, for instance, from -2.559 to -16.008 mm yr⁻¹. These findings attribute significant land water loss to the combined pressures of irrigation expansion and climate change, underscoring the urgent need for sustainable water management solutions to reverse these detrimental trends and ensure the viability of water resources for future generations.

Introduction: Irrigation’s Role in Sustainable Development

Balancing Food Security (SDG 2) and Water Scarcity (SDG 6)

Agricultural irrigation is a critical component in achieving global food security, a cornerstone of SDG 2 (Zero Hunger). It supports approximately 40% of global food production on just 17% of the world’s cropland. However, this vital practice is the largest consumer of freshwater, accounting for over 70% of global withdrawals and 90% of consumption. This intensive water use creates a direct conflict with SDG 6 (Clean Water and Sanitation), which aims to ensure the availability and sustainable management of water for all. The rapid expansion of irrigated agriculture, driven by population growth, has led to a continuous rise in irrigation water withdrawal, placing immense pressure on local and global water availability.

Historical Context and Unsustainable Practices

The expansion of irrigation has accelerated since the mid-20th century, spurred by initiatives such as the ‘Green Revolution’ in India and the ‘Reclamation Act of 1902’ in the United States. While these efforts boosted agricultural output, they also initiated patterns of unsustainable water use. This has led to significant environmental consequences, including:

• Streamflow reductions
• Lake shrinkage
• Widespread declines in groundwater levels

These impacts threaten the long-term viability of freshwater resources and aquatic ecosystems, undermining SDG 15 (Life on Land). The concept of ‘sustainable irrigation’ has emerged to address these challenges, aiming to maintain crop yields without depleting water resources. However, recent studies indicate that half of the irrigation expansion in the 21st century has occurred in water-stressed regions, highlighting a continued trend of unsustainable development.

Analysis of Historical Irrigation Expansion (1901-2014)

Global and Regional Expansion Trends

Data from the Land-Use Harmonization 2 (LUH2) dataset, utilized in the IRRigation Model Intercomparison Project (IRRMIP), shows a 5.4-fold increase in global irrigated area from approximately 0.5 million km² in 1901 to 2.7 million km² in 2014. The analysis focuses on four key regions that have experienced significant expansion:

1. South Asia: Experienced rapid growth after 1960, driven by government-supported private pumping wells, leading to a heavy reliance on groundwater.
2. Mediterranean: Saw a surge in irrigated farmland after the 1960s due to post-war development projects.
3. Central North America: Expansion began in the early 20th century, linked to the ‘Reclamation Act of 1902’, with growth rates varying over time.
4. West Central Asia: The most rapid expansion occurred between 1970 and 1990, associated with the construction of dams and reservoirs.

Consequent Increase in Water Withdrawal

The expansion of irrigated land directly corresponds to an increase in irrigation water withdrawal (IWW). Globally, simulated IWW rose from a range of 200–500 km³ yr⁻¹ in 1901 to 900–4,000 km³ yr⁻¹ in 2014. All seven Earth System Models (ESMs) used in the study concur that IWW has increased steadily across the four focus regions, further straining water resources and complicating efforts to achieve SDG 6.

Impacts on Water Cycles and Terrestrial Resources

Alterations in Water Fluxes: Precipitation, Evapotranspiration, and Runoff

Irrigation acts as a substantial forcing in the Earth system, altering the land’s energy balance and water cycle, which has direct implications for SDG 13 (Climate Action). The study found that:

• Evapotranspiration: Irrigation expansion substantially enhances land evapotranspiration. In the period 1996–2014, this increase reached 58.2 mm yr⁻¹ in South Asia and 36.9 mm yr⁻¹ in West Central Asia.
• Precipitation: Changes in precipitation are less pronounced and vary significantly by region. Irrigation expansion led to a multi-model mean precipitation reduction in South Asia but an increase in Central North America and West Central Asia.
• Runoff: The impact on runoff is inconsistent across models, largely due to different implementations of water withdrawal sources (e.g., from local runoff versus external sources).

Net Water Loss and Depletion of Terrestrial Water Storage (TWS)

A critical finding is that the additional evapotranspiration from irrigation is not sufficiently offset by changes in precipitation. This results in a reduction of the net water input from the atmosphere to land (Precipitation minus Evapotranspiration, or P-ET), leading to the depletion of local water resources.

• In South Asia, irrigation expansion exacerbated the negative trend in P-ET after 1960, accelerating it to -1.461 mm yr⁻².
• In the Mediterranean, irrigation contributed to an overall decreasing trend of -0.425 mm yr⁻² after 1960.
• In West Central Asia, irrigation reversed a slightly positive trend into a negative one.

This net water loss directly translates to a decline in Terrestrial Water Storage (TWS). Satellite observations and model simulations both confirm a significant decreasing trend in TWS over irrigation hotspots. Models that explicitly account for groundwater withdrawal show that irrigation expansion is the dominant driver of this depletion, with the rate of loss accelerating in the post-1960 period. This unsustainable abstraction of water resources is a major impediment to achieving water security under SDG 6.

Implications for Sustainable Development Goals

Aggravating Climate Change Impacts (SDG 13)

The study demonstrates that irrigation-induced water depletion is not an isolated issue but is aggravated by climate change. In many regions, irrigation expansion exacerbates existing drying trends, making ecosystems and communities more vulnerable to climate-related hazards. This feedback loop between land use and climate systems complicates efforts to build resilience and adapt to climate change, a key target of SDG 13 (Climate Action).

Threats to Water Security and Ecosystems (SDG 6 & SDG 15)

The widespread depletion of groundwater and surface water resources directly threatens the long-term water security for millions of people. This unsustainable consumption compromises the core objectives of SDG 6 (Clean Water and Sanitation). Furthermore, the degradation of freshwater resources and reduced streamflow negatively impact aquatic and terrestrial ecosystems, hindering progress toward SDG 15 (Life on Land).

Challenges to Food Security (SDG 2)

While irrigation is essential for food production, its current unsustainable trajectory jeopardizes the very food systems it is meant to support. Depleting water resources threatens the long-term viability of irrigated agriculture in hotspot regions, potentially leading to future food shortages and undermining the goal of SDG 2 (Zero Hunger).

Pathways to Sustainable Irrigation and Water Management

Policy and Technological Solutions

Addressing the challenge of irrigation-induced water depletion requires a multi-faceted approach that aligns with SDG 12 (Responsible Consumption and Production). Immediate solutions are needed, particularly in arid and semi-arid regions. Key strategies include:

• Adoption of Water-Saving Technologies: Widespread implementation of drip and sprinkler irrigation can significantly reduce water consumption without compromising crop yields.
• Sub-optimal Irrigation: In severely water-scarce regions, sacrificing a portion of crop yield to conserve water may be a necessary strategy.
• Strong Policies: Government policies are required to prevent the ‘paradox of irrigation efficiency,’ where water savings from technology lead to further expansion of irrigated areas rather than overall water conservation.

Broader Strategies: Global Trade and Consumption Patterns (SDG 12 & SDG 17)

Beyond local management, global strategies are essential for achieving a sustainable balance between food and water systems, reflecting the interconnectedness highlighted in SDG 17 (Partnerships for the Goals).

1. Optimizing Global Food Trade: Promoting the trade of water-intensive crops from water-rich to water-scarce regions (virtual water trade) can reduce the overall global water footprint of agriculture.
2. Dietary Changes: Shifting consumption patterns towards less water-intensive foods can significantly lower the demand for irrigation water, contributing to both environmental sustainability and public health.
3. International Collaboration: Financial and technical support is needed to help developing nations, particularly in regions like sub-Saharan Africa, implement sustainable irrigation infrastructure and management practices.

1. SDGs Addressed in the Article

The article on agricultural irrigation and its impact on water resources addresses several interconnected Sustainable Development Goals (SDGs). The analysis of the text reveals strong connections to the following goals:

• SDG 6: Clean Water and Sanitation: This is the most central SDG discussed. The article’s primary focus is on how agricultural irrigation, the largest consumer of freshwater, leads to “water scarcity issues,” “terrestrial water storage depletion,” and “groundwater depletion.” It explicitly discusses the need for “sustainable irrigation” to avoid “freshwater resources depletion.”
• SDG 2: Zero Hunger: The article directly links irrigation to food production and security. It states that irrigation “considerably enhances crop yields and plays a crucial role in ensuring food security, supporting ~40% of global food and fibre production.” The challenge presented is balancing this need for food production with the sustainable use of water resources.
• SDG 13: Climate Action: The article establishes a clear link between irrigation, water depletion, and climate change. It notes that irrigation’s impact “further [aggravates] the existing drying trends caused by climate change” and that irrigation itself acts as a “historical climate forcing” by altering the land energy balance and water cycle.
• SDG 15: Life on Land: The impact of unsustainable water use on ecosystems is highlighted. The article mentions that sustainable irrigation aims to avoid “aquatic ecosystems degradation.” It also links irrigation expansion to “streamflow reductions or lake shrinkage,” which directly impacts freshwater ecosystems.

2. Specific SDG Targets Identified

Based on the issues discussed, several specific SDG targets can be identified as being directly relevant to the article’s content.

1. SDG 6: Clean Water and Sanitation

   • Target 6.4: By 2030, substantially increase water-use efficiency across all sectors and ensure sustainable withdrawals and supply of freshwater to address water scarcity.
     
     Explanation: The entire article revolves around this target. It details how the “rapid expansion of irrigated cropland has driven a continuous rise in… irrigation water withdrawal,” leading to unsustainable conditions. The text calls for “water-saving irrigation techniques” and discusses how “half of the irrigation expansion in the twenty-first century has been deemed unsustainable,” directly addressing the need for efficient and sustainable water use.
   • Target 6.5: By 2030, implement integrated water resources management at all levels, including through transboundary cooperation as appropriate.
     
     Explanation: The article implicitly calls for better management by highlighting the complex “land–atmosphere interactions” that are often neglected in assessing irrigation’s impact. It shows how irrigation in one area can alter precipitation patterns elsewhere, demonstrating the need for an integrated approach that considers the entire water cycle, not just local withdrawals.
   • Target 6.6: By 2020, protect and restore water-related ecosystems, including mountains, forests, wetlands, rivers, aquifers and lakes.
     
     Explanation: The article provides evidence of damage to these ecosystems, stating that studies have “linked irrigation expansion to streamflow reductions or lake shrinkage in recent decades” and that unsustainable practices lead to “aquatic ecosystems degradation.” The widespread decline in groundwater levels is a direct impact on aquifers.

2. SDG 2: Zero Hunger

   • Target 2.4: By 2030, ensure sustainable food production systems and implement resilient agricultural practices that increase productivity and production, that help maintain ecosystems, that strengthen capacity for adaptation to climate change… and that progressively improve land and soil quality.
     
     Explanation: The article frames the core problem as a conflict between food production and sustainability. It acknowledges that irrigation is crucial for “ensuring food security” but warns that current expansion is often unsustainable. The concept of “‘sustainable irrigation’—a practice aimed at ensuring crop yields while avoiding freshwater resources depletion” is a direct reference to the principles of this target.

3. SDG 13: Climate Action

   • Target 13.1: Strengthen resilience and adaptive capacity to climate-related hazards and natural disasters in all countries.
     
     Explanation: The article highlights that the negative impacts of irrigation are “aggravated by climate change.” This indicates that as climate change worsens drying trends, agricultural systems become less resilient. The call for “immediate solutions to tackle the negative trends” is a call for building resilience in the face of combined pressures from water use and climate change.

3. Indicators Mentioned or Implied

The article provides several quantitative measures and discusses trends that can serve as direct or proxy indicators for measuring progress toward the identified targets.

• Indicator 6.4.2 (Level of water stress: freshwater withdrawal as a proportion of available freshwater resources): This is the most relevant indicator. The article provides data points that directly inform it:
  • It states that irrigation is the “largest contributor to freshwater withdrawal and consumption, accounting for over 70% of global freshwater withdrawals and ~90% of total consumption.”
  • It quantifies the depletion, noting that in South Asia, irrigation expansion enlarged the “terrestrial water storage depletion rate… from −2.559 ( ± 0.094) to −16.008 ( ± 0.557) mm yr−1.” This is a direct measurement of water stress exceeding replenishment rates.
• Indicator 6.6.1 (Change in the extent of water-related ecosystems over time): The article provides qualitative and quantitative evidence for this indicator.
  • It mentions “widespread declines in groundwater levels.” Specific examples are given, such as in the US High Plains where “groundwater levels are estimated to have dropped by an average of 4 metres between the 1950s and 2007,” and in North India where levels “declined at a rate of ~4.0 cm yr−1 during the early 2000s.” These are direct measurements of changes in the extent of aquifers.
  • The mention of “streamflow reductions or lake shrinkage” also directly relates to this indicator.
• Indicator 2.4.1 (Proportion of agricultural area under productive and sustainable agriculture): The article implies this indicator by defining and assessing the sustainability of irrigation.
  • It cites a study concluding that “half of the irrigation expansion in the twenty-first century has been deemed unsustainable.” This suggests that the sustainability of agricultural areas can be measured by their impact on water resources, using the water stress and depletion metrics mentioned above.

4. Summary Table of Findings

Source: nature.com