Water Quality in Drainage and Irrigation Channels of Agricultural Landscapes: A Narrative Review with Bibliometric Analysis

Drainage ditches, irrigation canals and multifunctional channels are pervasive features of agricultural landscapes and often represent the only remaining aquatic habitats where natural wetlands have disappeared. This narrative review synthesises evidence on surface-water quality in open channels that are purpose-built or strongly modified by humans, excluding closed subsurface drainage and large navigational canals. The evidence base comprises 36 peer-reviewed studies, predominantly from temperate Europe and North America, complemented by a keyword co-occurrence analysis of 851 Scopus-indexed publications, in which channel-specific terms formed peripheral nodes within five clusters defined by broader themes. The reviewed studies show that hydrological regime, residence time, sediment characteristics and vegetation regulate nitrogen, phosphorus, sediment and pesticide dynamics, so that the same channel may act as a pollution source, transport pathway or biogeochemical buffer. Permanent removal is largely confined to nitrogen via denitrification, whereas phosphorus and sediment retention is predominantly reversible. Evidence for irrigation-dominated, semi-arid and tropical systems, greenhouse gas trade-offs and network-scale functioning remains limited. On this basis, the review proposes a channel typology and candidate functional indicators of water-quality performance as conceptual proposals requiring empirical validation, and discusses them in relation to the Water Framework Directive and the EU Nature Restoration Law.