A new study of Spain’s Guadalquivir River Basin is challenging one of the most influential assumptions in modern water policy: that making individual farms more efficient will automatically save water for the entire river system. Published in Water Resources Management, the research warns that basin-wide water-efficiency targets could become a policy trap when they ignore how water moves between users, landscapes, reservoirs, aquifers, and ecosystems. The central problem is hydrological rather than simply technological. Water that appears to be “wasted” at one location may return to a river or groundwater system and become an essential supply for users downstream. When irrigation systems are upgraded, those return flows can shrink, meaning that an improvement at farm level may produce little net saving across the basin—or even intensify pressure on already limited resources.
The study responds to European Union policy initiatives calling for basin-specific water-efficiency targets. The European Commission’s 2012 Blueprint for Water and its 2025 European Water Resilience Strategy both emphasize the need to use water more efficiently and to apply an “efficiency first” principle. Similar ambitions appear in the United Nations Sustainable Development Goal 6.4, which seeks a substantial increase in water-use efficiency and more sustainable freshwater withdrawals. Yet the researchers argue that efficiency is not a single, universally transferable measurement. It may refer to the proportion of water consumed by crops, the amount abstracted from a river, the economic value generated per cubic metre, or the quantity of water that remains available for other users and ecosystems. These definitions can produce radically different conclusions, especially in basins where water is reused several times before leaving the system.
The distinction between withdrawal, consumption, and return flow is crucial. If a farmer withdraws 100 units of water and crops consume 50 through evapotranspiration, the remaining 50 may flow back into a river, recharge an aquifer, or become available to another user. At the farm scale, the operation may appear only 50 percent efficient. At the basin scale, however, the water may be used repeatedly, producing a much higher overall efficiency. The authors illustrate this with a simplified cascade of users. When each user consumes half of the water withdrawn, repeated downstream reuse raises total basin efficiency to about 87.5 percent. Increasing local efficiency from 50 to 80 percent raises basin efficiency to approximately 96.6 percent, but the gain at basin level is only 9.1 percentage points. A large technological improvement at each farm therefore delivers a much smaller improvement in the performance of the whole river system.
The same example reveals an uncomfortable consequence of modernization. Suppose an upstream irrigation user consumes 60 units but withdraws 120. At 50 percent efficiency, 60 units return to the hydrological system. If the user adopts technology that raises efficiency to 80 percent while maintaining the same consumption, the required withdrawal falls to 75 units and the return flow declines to only 15. On paper, 45 units have been saved. But those units may previously have supplied downstream farms, wetlands, aquifers, or environmental flows. If the original downstream conditions must be maintained, the water authorities—not the individual user—must decide where those apparent savings go. Otherwise, the modernization project can reduce river outflow and leave other water users unable to meet their legal entitlements.
The Guadalquivir River Basin provides a real-world test of these dynamics. Covering approximately 57,527 square kilometres and supporting around 4.2 million people, the basin is one of Spain’s most important agricultural regions. Agriculture accounts for roughly 86 percent of total water use, while irrigated land covers about 856,429 hectares. The basin has experienced major irrigation modernization over the past two decades, with drip systems now widespread and sprinkler irrigation increasingly important in lower-basin areas. Olive cultivation dominates much of the upper basin, while the middle and lower regions contain vegetables, winter crops, citrus, almonds, rice, wheat, sunflower, and cotton. Despite technological progress, the basin remains effectively closed: demand is close to or above reliably available supplies, and drought restrictions are imposed in roughly 20 to 30 percent of years.
To examine what efficiency improvements actually do, the researchers used a hydro-economic model calibrated with real data from the Guadalquivir system. The model represents the basin as a network of connected nodes, including agricultural and urban demand points, reservoirs, gauging stations, diversions, consumption points, and return flows. Its economic component uses Positive Mathematical Programming to simulate farmer responses to changes in water availability, crop choices, irrigation efficiency, and prices. In the baseline scenario, agricultural consumption was estimated at about 2,468 to 2,458 cubic hectometres, while total applied water was approximately 3,458 to 3,468 cubic hectometres. Average local efficiency, including conveyance and distribution losses, was 71.2 percent. Yet once recoverable return flows were included, basin efficiency reached 80.7 percent because roughly 409 cubic hectometres of return water from upper and middle areas could be reused downstream.
The model found that raising local efficiency by a further five percentage points increased average local efficiency to 76.2 percent, but basin efficiency rose by only 3.5 percentage points, reaching 84.2 percent. Larger local improvements generated progressively smaller basin-level gains. A 15 percent increase in local efficiency, for example, lifted basin efficiency from 80.7 to 90.7 percent—an improvement of about ten percentage points rather than the full local increase. Geography also mattered. A 10 percent efficiency improvement in the upper or middle basin had almost no effect on overall basin efficiency when return flows were assumed to remain fully reusable. By contrast, a similar improvement in the lower basin increased overall efficiency by about 6.8 percent because water lost there was more likely to leave the basin and discharge into the sea. When the researchers assumed that only 80 percent of return flows could be reused, efficiency improvements in the upper and middle sectors began to produce modest basin-wide gains, while the lower basin remained the most influential location.
The findings also expose the danger of the rebound effect. Water that is technically saved may not remain in the river. Farmers may use it to expand irrigated acreage, switch to more water-intensive crops, increase irrigation frequency, or raise production. In a closed basin, improved technology can therefore increase total consumptive use rather than reduce it. Evidence cited by the authors includes a European Court of Auditors assessment concluding that public funds intended to improve irrigation efficiency have often encouraged greater water use instead of genuine savings. Research from the Guadalquivir has similarly linked the expansion of efficient irrigation systems with increased pressure on water resources. The problem is not that drip irrigation, sprinklers, or water-saving devices are ineffective. They can reduce losses, energy use, and local withdrawals. The problem is that without strict allocation rules, their benefits may be absorbed by new demand.
For policymakers, the study’s message is not to abandon efficiency, but to stop treating it as a universal scorecard. Basin targets should be built on detailed water-balance assessments that distinguish consumed water from recoverable and non-recoverable flows, account for seasonal and multi-year storage, include groundwater and non-conventional supplies, and identify the environmental functions of return flows. Indicators such as the EU’s Water Exploitation Index Plus can be useful, but they may exaggerate scarcity when they ignore reservoirs, aquifers, desalination, reclaimed wastewater, transfers, or drought-adaptation rules. Effective policy will require volumetric caps, transparent monitoring, consumption-based rights, enforceable environmental flows, and rules ensuring that public investment savings are retained for ecosystems or wider public use. The Guadalquivir case suggests that local efficiency can be valuable, but only governance can determine whether it becomes a real basin-scale water saving. Without that governance, a greener-looking irrigation system may quietly leave rivers, wetlands, and downstream communities with less water.
Subject of Research: Basin-scale water-use efficiency, irrigation modernization, return flows, water reuse, and river-basin policy
Article Title: Water Efficiency Targets at the Basin Scale: Useful Guide or Policy Trap?
Article References: Expósito, A., Gutiérrez-Martín, C., Delgado-Ramos, F. et al. “Water Efficiency Targets at the Basin Scale: Useful Guide or Policy Trap?” Water Resources Management 40, Article 512 (2026).
Image Credits: AI Generated
DOI: https://doi.org/10.1007/s11269-026-04836-4
Keywords: Water-use efficiency; water savings; environmental objectives; river basin; water policy
