Abstract :
[en] Agricultural water scarcity is increasingly driven by climatic variability rather than by long-term declines in average water availability. In many regions, the dominant hydrological challenge is no longer a decline in annual rainfall totals per se, but the increasing mismatch between intra-annual rainfall distribution and critical crop water-demand periods. Short droughts, rainfall intermittency, and heat-induced evaporative demand peaks expose cropping systems to hydrological risks that cannot be mitigated solely through reductions in seasonal evapotranspiration. Crop diversification is widely promoted as an agroecological lever for improving water-use efficiency and resilience; however, its hydrological benefits remain fragmented across studies and are still insufficiently translated into mechanistic and design-oriented principles for climate-resilient agricultural systems. Importantly, diversification should not be interpreted as a universal solution, since its effectiveness depends strongly on climatic conditions, soil properties, operational feasibility, and crop functional complementarity. In this review, we synthesize current knowledge on crop diversification and water dynamics through the lens of the Hydrological Portfolio Effect (HPE). We argue that diversified cropping systems optimize water primarily by smoothing peak water demand, partitioning soil water in time and depth, and enhancing soil-moisture resilience following drought events, rather than by minimizing total water consumption. Nevertheless, reductions in total water consumption may emerge as a long-term collateral outcome of well-designed diversification systems through improved soil structure, reduced evaporation, and enhanced buffering capacity. Drawing on evidence from intercropping, crop rotations, service crops, and spatial crop mosaics, we reorganize the literature around three core mechanisms: phenological asynchrony, root-niche complementarity, and canopy–soil buffering. We further emphasize that hydrological optimization cannot be dissociated from other interacting agronomic processes, including nutrient cycling, radiation interception, soil biological activity, and management feasibility. We introduce a parsimonious set of operational indicators (Demand Peak Index, Phenological Asynchrony Score, Root-Niche Complementarity Proxy, and Soil-Moisture Resilience) and propose design rules for water-smart diversification across contrasting soil–climate contexts. By reframing diversification as a strategy for managing hydrological risk rather than average water use, this review provides a unifying framework and a forward-looking research agenda for agroecological water optimization under climate uncertainty. This framework generates testable hypotheses and operational indicators to evaluate how diversification reduces hydrological risk under climate variability.
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