5. Discussion and Conclusions

5.1 Discussion

5.1.1 Mechanisms of Spatial Differentiation

The study reveals a distinct “high-in-center, low-in-west” rainfall pattern that is intimately linked to the region's complex interaction between topography and atmospheric dynamics. The exceptional rainfall in the Mazhu Midstream Area, averaging 5.68 mm/day, acts as a hydrological hotspot. This phenomenon is primarily driven by the "confluence zone enhancement" effect. As the geographic convergence point for multiple tributaries of the Yao River system, this area naturally channels airflow, forcing low-level convergence and subsequent uplift that triggers precipitation. This physical mechanism is further amplified by the region's land surface characteristics; the dense network of rivers, wetlands, and paddy fields significantly enhances local evapotranspiration. This creates a moisture recycling feedback loop, where locally generated water vapor supplements the moisture advected from the ocean, thereby sustaining high precipitation levels even during periods of weaker large-scale forcing.

In sharp contrast, the Nansha Plain Area in the west records the lowest daily rainfall of 3.47 mm/day. This deficit can be attributed to a combination of "lee-side subsidence" and "terrain blocking" effects. Located in the lee of the regional hill systems relative to the prevailing moisture-bearing southeasterlies, the area experiences descending air masses that suppress convective development. Furthermore, the transitional hills to the east act as a physical barrier, intercepting a portion of the low-level moisture transport from the East China Sea before it can reach the western plains. This distinct spatial gradient—where the central confluence zone receives nearly 64% more rainfall than the western rain shadow area—highlights the critical role of sub-regional geography in modulating the broader monsoon climate.

5.1.2 Climate Change Signatures and Comparison

The analysis confirms that the Eastern Zhejiang region is not immune to global climate shifts. The universal upward trends in rainfall, ranging from 4.00 to 7.99 mm/year across all 15 sub-regions, align strongly with the global “wet-gets-wetter” paradigm documented in the IPCC Sixth Assessment Report (AR6). This local intensification mirrors the thermodynamic Clausius-Clapeyron relationship, where a warmer atmosphere holds and precipitates more moisture. Specifically, our finding of a distinct “Coastal Enhancement” pattern—where coastal areas show an average trend of 7.45 mm/year—is consistent with recent studies in the Yangtze River Delta and broader Zhejiang Province. These studies link enhanced coastal precipitation to rising sea surface temperatures in the East China Sea, which intensify the land-sea thermal contrast and associated moisture flux convergence along the coast.

Significantly, the temporal evolution of these trends reveals a non-linear trajectory, characterized by a marked "post-2010 intensification" of rainfall variability. The occurrence of record-breaking high rainfall events, such as the ~3250 mm annual total observed in Mazhu in 2021, signals a shift towards a more volatile hydro-climatic regime. This observation aligns with findings by Su et al. regarding increased extremes in the Yangtze Basin, suggesting that the region is transitioning from a period of steady increase to one of high-amplitude oscillation. Crucially, the Hurst exponent analysis provides a predictive dimension to these observations. With values consistently exceeding 0.5 ($H>0.5$) across all sub-regions, the data provides novel and robust evidence that these upward trends are not merely temporary multi-decadal fluctuations (like the Atlantic Multidecadal Oscillation) but are persistent, long-term climate shifts driven by external forcing. This persistence implies that the "wetting" trend is locked in for the foreseeable future, necessitating a fundamental re-evaluation of long-term infrastructure planning assumptions.

5.1.3 Implications for Engineering Operations and Digital Twin Construction

The findings have direct and profound implications for the Eastern Zhejiang Water Diversion Project and the ongoing construction of its Digital Twin system. A notable source-receiver mismatch exists: the primary water source area, encompassing the Mazhu and Yao River Basin, has the highest rainfall but also the highest variability. Conversely, the key recipient areas, such as Nansha and Shaoyu, have the lowest baseline rainfall but are experiencing the fastest increasing trends. This dynamic fundamentally challenges the traditional operational criteria. For instance, the post-2010 intensification of rainfall extremes suggests that the static thresholds for "stop diversion" orders—currently based on a 24-hour forecast of 25-50mm of rain—may act as a rigid constraint that does not fully account for the increasing frequency of compound events, such as a local rainstorm coinciding with an upstream flood peak. Similarly, the high increasing trend observed in the coastal Cixi area implies a greater need for frequent pre-discharge operations at drainage nodes like the Sanxing Sluice to maintain safety margins.

In the context of Digital Twin integration, the scale-dependent characteristics of rainfall provide a blueprint for intelligent management. The observation that spatial correlation increases from 0.83 at the 3-month scale to 0.87 at the 12-month scale suggests that strategic planning should be coordinated regionally based on annual climate coherence. This scale relationship needs to be encoded into the "Knowledge Base" of the digital twin platform. Specifically, the "Business Rule Library" should adopt annual-scale trends to guide infrastructure capacity planning and updates to the L3 data bottom board. Meanwhile, the "Forecast Scheme Library" must prioritize 3-month scale variability for tactical flood scheduling and emergency response. Furthermore, the moderate-to-strong Hurst exponents ($H>0.5$) justify the inclusion of trend persistence as a dynamic weight factor in the "Intelligent Early Warning" models. This enhancement would allow the system to better predict water supply safety risks under changing climate conditions, transitioning from a reactive stance to a proactive, risk-based management approach.

5.2 Conclusions

Based on the systematic analysis of 62 years of daily rainfall data (1961-2022) from 15 stations in Eastern Zhejiang, this study draws five comprehensive conclusions that bridge climatic science and engineering application.

First, regarding the spatial pattern, the region exhibits a distinct “high-in-center, low-in-west” distribution structure. The Central zone, exemplified by the Mazhu Midstream Area, serves as a "rainfall chimney," receiving 64% more daily rainfall than the Western zone's Nansha Plain. This heterogeneity is not random but is structurally driven by the "confluence zone effect" of the Yao River system and local topographic lifting, challenging the assumption of uniform regional rainfall often used in broad-scale planning.

Second, in terms of long-term evolution, the region is undergoing a universal and significant wetting process. All 15 sub-regions show statistically significant upward trends ($p<0.05$), but the intensity of this change is spatially differentiated. A clear gradient of “Coastal (7.45 mm/yr) > River Basin (6.38 mm/yr) > Hilly (4.91 mm/yr)” has been identified, indicating that coastal zones are the primary "frontline" of climate change response, exhibiting greater sensitivity to marine warming than inland areas.

Third, concerning trend persistence, the Hurst exponent analysis confirms the robustness of these changes. With $H$ values consistently ranging between 0.56 and 0.63, the study verifies that the observed wetting trends possess "long-term memory." This finding is critical as it statistically negates the hypothesis of random fluctuation, confirming that the upward trajectory of rainfall is a persistent climate shift that will likely sustain or accelerate in the near future.

Fourth, with respect to scale dependence, the study uncovers a fundamental operational rule: rainfall characteristics are strictly scale-dependent. As the analysis window widens from 3 to 12 months, the amplitude of trend fluctuations collapses by approximately 90% (from ±16 mm to ±1.7 mm), while inter-regional spatial correlation strengthens by 5-11%. This mathematical relationship proves that while short-term weather is chaotic and local, long-term climate is organized and regional, providing a theoretical basis for multi-scale water management.

Fifth, regarding regional anomalies, the Mazhu Midstream Area is identified as a unique hydro-geographic entity. Its “increase-then-decrease” correlation pattern reveals a decoupling behavior: while it synchronizes with the region seasonally, its annual total rainfall often diverges due to its intense local hydrological cycling. This anomaly marks it as a critical risk point in the water network, requiring independent monitoring separate from the regional average.

5.3 Recommendations

Based on the research findings and the "Three Safeties" requirements—engineering safety, water supply safety, and water quality safety—of the water network, detailed recommendations are proposed to enhance the resilience and intelligence of the Eastern Zhejiang Water Diversion Project.

First, differentiated strategies should be implemented for source and recipient areas. For the source areas, particularly the Mazhu and Yao River Basin, the focus should be on prioritizing them as key water sources while simultaneously enhancing flood drainage capacity. The high rainfall in these areas should be utilized for local supply to reduce dependence on the Cao'e River diversion during wet periods, but this requires robust drainage infrastructure to handle the high variability. For recipient areas like Nansha and Shaoyu, the strategy should focus on ensuring water delivery and designing infrastructure that can accommodate rapidly increasing local rainfall trends. Additionally, the Xiaoshan Hub's capability to balance intake quality, specifically regarding chloride levels, and quantity under variable river flow conditions must be strengthened to ensure a stable supply.

Second, multi-scale operations should be adopted to align with the temporal characteristics of rainfall. For long-term planning, annual-scale statistics (12-month) should be used as the basis for infrastructure investment and capacity planning. These findings should be integrated into the Digital Twin L3 Data Bottom Board to support long-term scenario simulation and strategic decision-making. Conversely, for short-term operations, seasonal (3-month) and real-time monitoring should guide tactical flood scheduling. The "Stop Diversion" thresholds in the Business Rule Library should be refined based on updated probabilities of extreme rainfall events to prevent operational rigidity during critical weather windows.

Third, climate adaptation measures must be integrated into both infrastructure and early warning systems. Design standards for hydraulic structures should be upgraded to accommodate the post-2010 intensification of extreme events, ensuring that they can withstand the new normal of precipitation volatility. Safety monitoring for key nodes, such as the Cao'e River Grand Sluice, should be enhanced to address compound risks arising from the interaction of tides and floods. Furthermore, a multi-tier early warning system should be established that integrates trend persistence, indicated by the Hurst exponent, with real-time monitoring data. This should be coupled with the development of Intelligent Recognition Models capable of rapid response to sudden anomalies in water quality or quantity, thereby completing the "Four-Pre" defense line for the regional water network.

Table 5 provides a structured summary of these management recommendations, categorizing them by target area and operational scale.

Table 5. Recommendations for Water Diversion Project Management