Executive Summary:

Irrigation development has made up a considerable share of the Bank’s infrastructure investment portfolio. However, after a surge from the 1960s to 1980s in the level of public investment in the irrigation sector, enthusiasm for continued investment (particularly in new irrigation development) has softened, except in Sub-Saharan Africa. There may be multiple reasons for this change. Irrigation’s success—with other investments in agriculture—removed the imperative for continued rapid expansion of the global food supply. Despite success in increasing food output, large-scale irrigation systems are often viewed as failing to live up to their potential, and they require consistent injections of outside capital for operations and maintenance (O&M). Irrigation is associated with negative externalities, particularly related to the environment. Finally, irrigation’s potential role in reducing poverty, especially large-scale irrigation, may have changed as areas suitable for new irrigation development have declined and populations urbanize. Nonetheless, the pressure to meet growing food demand and provide low-cost food to an increasingly urban world persists. Continued irrigation investments, even if not following past models, must play a key role in relieving that pressure.

To assess empirical evidence of the irrigation impacts and guide future investment, we review a wide range of published literature (over 500 articles with over 250 cited in the bibliography) on irrigation impact.

The reviewed studies show broad variation in scope and focus, evaluation design and methods used to infer impact, and the physical and institutional nature of the irrigation systems being evaluated. The impacts measured also ranged widely, from direct and indirect production effects to impacts on poverty, nutrition, and groundwater depletion. While this variation made systematic comparison across studies difficult, these key conclusions emerged:

• Irrigation is linked to increased agricultural output through direct production effects and its role in increasing the productivity of complementary inputs.

• Irrigation is strongly associated with decreases in poverty, particularly among direct beneficiaries and urban consumers.

• Irrigation is linked to a wide range of other impacts, including ones associated with the nutrition, health, and environment sectors.

• Studies that measured irrigation impacts beyond the farm or system scales have often found indirect secondary effects of similar or greater magnitude than direct primary effects. Secondary effects could be positive (e.g., multiplier effects to the overall economy) or negative (e.g., off-site environmental effects that offset the gains in production or poverty reduction).

• The range and magnitude of irrigation impacts, particularly those beyond production effects, were highly dependent on the location and circumstances in which irrigation occurred. In addition, impacts changed over time in magnitude and sometimes sign.

• Irrigation impacts—positive and negative—were unequally distributed socially, spatially, and temporally. Virtually every study that measured differential impacts has found them to be significant.

These conclusions suggest considerations to ensure the highest returns for future irrigation investment. Investment planning should expand attention to irrigation benefits and costs and explicitly consider direct and indirect impacts. During planning, interventions to reduce negative environmental impacts of irrigation and increase public awareness of indirect environmental benefits, such as through decreased pressure on marginal land, would improve investment outcomes and public support. Given the socially differential impacts of irrigation, continued, explicit focus on distribution effects are needed if poverty reduction and equity goals of investments are to have the greatest likelihood of success.