The Solar Paradox: Pure Social Diffusion and Competitive Resource Capture in Semi-Arid Irrigated Land Expansion

Solar-powered irrigation is expanding rapidly across semi-arid regions, but the mechanisms through which this technology diffuses in informal groundwater economies-where the majority of wells operate without permits-remain poorly understood. We address this gap through a spatiotemporal analysis of 3,201 solar wells identified via satellite census in Sidi Bouzid, Tunisia, combined with 10,000 control points. Cox proportional hazards models reveal that terrain accessibility is the dominant spatial determinant: slope reduces the adoption hazard by 66% (HR = 0.339, p < 0.001), with proximity to urban centers (HR = 0.700) and lower elevation (HR = 0.718) also significant. Bass diffusion model estimation yields a near-zero innovation coefficient (p = 0.001) alongside a high imitation coefficient (q = 0.679)-the first documented case of p ≈ 0 in agricultural technology diffusion, indicating a complete absence of institutional influence over the adoption process. We attribute this to a multi-layer governance failure in which legal barriers, financial exclusion, and extension collapse block all formal technology transfer channels. A neighbor effects model reveals that higher local adopter density slows subsequent adoption (HR = 0.606, p < 0.001), creating an apparent paradox with the high aggregate imitation. We reconcile this through a Competitive Diffusion Framework distinguishing an aggregate-level Information Effect from a local-level Congestion Effect linked to aquifer stress. The Finance Law 2025's pivot to retroactive regularization of unauthorized wells confirms that informal diffusion has outpaced regulatory capacity, marking a transition from administrative exclusion to fiscal capture in Tunisia's groundwater governance.