Abstract
Sri Lanka’s village tank cascade systems are among the oldest community-managed irrigation systems in the world. They form the backbone of smallholder agriculture in the dry zone, supporting rural livelihoods through the collective management of tank-stored water and its distribution through shared canal networks (Ratnayake et al, 2021). The irrigation systems have endured for centuries, sustained not only by economic incentives but also by long-standing norms, cultural expectations, and informal institutions that govern cooperation among farmers (Ostrom, 1990; Ostrom and Gardner, 1993).
In these systems, Farmers’ Organisations (FOs) act as local social planners. Before each cultivation season, they assess the tank’s water level, determine the recommended extent of land to be cultivated, and coordinate collective canal cleaning to remove weeds, silt, and debris. These actions enable water to flow efficiently from the head end closest to the tank through the middle section to the tail end. In principle,
such coordination allows the community to achieve a social optimum: cultivating the maximum possible area under water constraints while maintaining the canals in good condition. However, individual incentives often diverge from this collective optimum. Both key decisions, which are how much land to cultivate and how much labour to contribute to canal cleaning, are vulnerable to free riding. Studies on gravitational irrigation systems have identified spatial asymmetry as a major factor contributing to this divergence (Dayton-Johnson, 2000b; Dayton-Johnson and Bardhan, 2002). Upstream, or head-end, farmers can over-cultivate and extract more water with little risk, whereas downstream farmers, located in the middle and tail ends, face shortages even when they comply with FO recommendations.
Despite these structural inequalities, empirical research shows that many irrigation communities have developed mechanisms to overcome asymmetric benefits and sustain cooperation (Weissing and Ostrom, 1991;Ostrom et al., 1994; Janssen et al., 2010). Through local rule enforcement, mutual monitoring, and shared norms, farmers have historically maintained collective canal cleaning and equitable water distribution. During years of good rainfall, this location asymmetry remains hidden by the abundance of water, but under drought conditions, unequal access becomes sharply visible. However, increasing climatic uncertainty now threatens these informal equilibria through frequent and unpredictable droughts. Climate variability amplifies the spatial dependence of irrigation outcomes, transforming irrigation management into a coupled-commons dilemma in which two interdependent public goods —canal condition and irrigation water —must be sustained through voluntary cooperation.
Recent empirical evidence from Sri Lankan tank systems illustrates this challenge. Analysis of secondary data and field surveys conducted between December 2024 and May 2025 reveals a clear behavioural pattern: in drought years, both middle- and tail-end plots were cultivated to a lesser extent than recommended by FOs. Farmers in these locations reported deliberately reducing cultivated area to manage water risk, but also decreasing canal-cleaning hours because they expected limited returns from a system in which upstream farmers overused water. These location-specific responses reveal a form of strategic adaptation: farmers in disadvantaged positions withdraw from both land use and collective labour in anticipation of others’ defection. This behaviour, where private adaptation to scarcity (reducing cultivated extent) crowds out contribution to the enabling public good (canal maintenance), is the central empirical puzzle motivating this study. The key question is whether such restraint by downstream farmers during drought also leads to reduced canal maintenance effort, thereby accelerating collective decline. Over repeated droughts, even small reductions in canal effort across many farmers could accumulate into long-term deterioration of water conveyance and institutional capacity.
To identify the mechanisms linking these behaviours, the study conceptualises farmers’ seasonal decisions as a coordination game. In good seasons, when water is abundant, the game simplifies to a single public goods problem, canal cleaning, where cooperation achieves the social optimum but remains vulnerable to free riding. In drought seasons, however, the game becomes a nested coordination problem: canal cleaning and cultivation extent interact. Water allocation depends on upstream over-cultivation, so downstream farmers’ payoffs are jointly determined by others’ cultivation decisions and total canal cleaning effort. Each farmer must decide whether to continue contributing labour to a canal that may not deliver sufficient water. This study, therefore, focuses on the behavioural responses of farmers at different spatial positions, head, middle, and tail ends, under varying environmental and institutional conditions. By experimentally replicating these irrigation environments, the research isolates the causal effects of drought and upstream defection on cooperative labour, providing new evidence on how spatial asymmetry and climate variability jointly shape collective action in self-managed irrigation systems.