*5.1. Regional Water Reuse*

Water "lost" to irrigation return and groundwater through connectivity can be beneficial on the regional level for other users [40–43]. Results (Figure 3) show positive irrigation return (except G1 during 2017–2050) and negative connectivity. This suggests that, from one perspective, IE policy can yield water for redistribution as increasing unit water supply in the field. From another perspective, IE policy sacrifices regional connectivity. In general, groundwater at the regional scale is not sufficiently recharged. Since the 1960s, IE policy impacts of upstream areas upon downstream resources have been reported in the literature [44,45]. However, real savings at a regional scale require encompassing water managemen<sup>t</sup> for downstream re-allocation. Simons et al. [45] showed that the recoverable flow from a water user could be reused multiple times by downstream water users. The "saved" amount in water conveyance impacts downstream uses that may rely on this portion of water.

Connectivity reductions indicate inadequacies of recharge from reduced irrigated agriculture on a regional scale. Similar to the irrigation return flow, the connectivity exhibits the time lag of mass movement in the water cycle. It buffers the disturbance of cumulative consequences such as water quality deterioration, soil contamination, edaphon alternation, and groundwater table depression. Declining connectivity and irrigation return reduce water availability for other users, as these non-consumptive flows play a vital role in instream water supplies [46,47].

In a broad context, connectivity contributes to groundwater resilience. In simulations, a loss of connectivity between surface water and groundwater is one of the unintended consequences of IE policy. Pringle [48] argued that human alterations of hydrologic processes that eliminate hydrologic connectivity have already influenced ecological patterns regionally and globally. As Gleick et al. [49] demonstrated, traditional IE improvement failed to depict the co-benefits, including water quality, reductions in water-related energy costs, ecosystem health, and improved crop quality. Although valuable, these benefits are not always as tangible as direct and immediate benefits because they do not yield "new water." As Lexartza-Artza and Wainwright [50] suggested, practical studies should consider the opposite structural and functional components of connectivity and system boundaries. In water balance closure, any water use changes in one part of the region or basin will impact another water use. The consideration that water is "lost" when not applied directly to anthropogenic uses is a misleading perception. Applying these labels presents a lack of quantification of non-consumptive values. The definitions of natural and anthropogenic flows should be embedded in water managemen<sup>t</sup> to represent multiple water users' interdependency. IE may maximize water supply locally ye<sup>t</sup> concurrently need synergies in a broader context. Synergistic practices such as aquifer storage and recovery should be included in future strategic water managemen<sup>t</sup> alongside the efficiency practice to guarantee ecological benefits.
