1. Introduction

1.1 Regional Hydrological and Socio-Economic Context

The Eastern Zhejiang region, situated on the highly developed eastern coast of Zhejiang Province, China, serves as a critical economic engine for the Yangtze River Delta. Characterized by a typical subtropical monsoon climate, the region operates under the dual control of the Meiyu front and typhoons. In late spring and early summer, the Pacific Subtropical High strengthens and encounters cold air masses from the north, forming a quasi-stationary front that results in continuous overcast and rainy weather known as the Meiyu flood season. Subsequently, in summer and autumn, the region is dominated by the Pacific Subtropical High and frequently impacted by tropical storms, leading to heavy rainstorms during the Typhoon flood season. While annual precipitation is abundant, exceeding 1400 mm, the spatiotemporal distribution is severely uneven. Over 60% of rainfall concentrates in these two flood seasons, creating a sharp oscillation between summer floods and winter-spring droughts. Spatially, water resources diminish from the mountainous southwest to the coastal northeast, contrasting sharply with the region's economic and demographic gradients.

This region stands as one of the most densely populated and economically vibrant areas in Zhejiang Province. The water receiving area spans approximately 15,614 square kilometers, accounting for 14.8% of the province's total area, yet it supports a beneficiary population of about 17.5 million people, which is 26.7% of the provincial total. Economically, the region contributes a Gross Domestic Product (GDP) of approximately 2.14 trillion CNY based on 2021 data, representing 36.6% of the provincial economy. As an integral component of the Yangtze River Delta Economic Zone, the area possesses a superior geographical location and a comprehensive transportation network. The Hangzhou-Ningbo Canal traverses the entire plain from west to east, linking the water transport systems of the Qiantang, Cao'e, and Yong rivers. Maritime transport is anchored by the major Ningbo Port, comprising the Ningbo, Zhenhai, and Beilun port areas, which facilitates global trade. The industrial foundation is robust, featuring traditional sectors like machinery, textiles, and building materials, alongside five key emerging industries being cultivated: electronic information, modern medicine, petrochemicals, textiles, and clothing. The agricultural landscape has also evolved, shifting from traditional double-cropping rice to single-cropping rice and a diverse array of economic crops such as vegetables and fruits, increasing the demand for precise and timely water regulation.

The Eastern Zhejiang Water Diversion Project, which commenced full operation in June 2014, represents the largest cross-basin water transfer infrastructure in Zhejiang Province. It serves 19 counties (districts) across the cities of Hangzhou, Shaoxing, Ningbo, and Zhoushan, delivering an average annual diversion volume of 890 million m³. Since its inception, the project has transported a cumulative total of over 5 billion cubic meters of water—equivalent to 35 West Lakes annually—effectively alleviating water pressure in these four major cities. This project acts as a lifeline for regional water security and stands as a vivid practice of the "Sixteen Character" water management philosophy of "Water saving priority, spatial balance, systematic management, and two-handed exertion." Furthermore, the region is at the forefront of digital water management, piloting initiatives like the "Zheli Nine Dragon Joint Water Control" application and building a provincial water conservancy data warehouse. However, in the context of accelerating global climate change, recent hydro-meteorological observations indicate that precipitation patterns in this region are undergoing significant and non-stationary shifts. The frequency and intensity of extreme precipitation events—varying from prolonged droughts to localized torrential rains—have increased markedly. These shifts pose unprecedented challenges to the project's operational logic: water source stability is threatened by shifting seasonal baselines, while conveyance channel safety is jeopardized by flash floods. Therefore, a systematic, multi-scale analysis of rainfall trends and variability specifically within the project's receiving areas is not merely an academic exercise but an urgent engineering necessity for adaptive management.

1.2 Research Progress on Precipitation Variability

1.2.1 Global Precipitation Change and the "Wet-Gets-Wetter" Paradigm

The Sixth Assessment Report (AR6) of the Intergovernmental Panel on Climate Change (IPCC) confirms unequivocally that human influence has intensified the global water cycle [1]. Since the 1950s, precipitation variability over mid-latitude land areas has increased, a phenomenon consistent with the thermodynamic Clausius-Clapeyron relationship, which predicts a ~7% increase in atmospheric water-holding capacity per degree Celsius of warming [2]. Recent high-impact studies have further solidified the "wet-gets-wetter" paradigm. Madakumbura et al. (2021) provided robust evidence that anthropogenic climate change is driving an intensification of extreme precipitation events globally, particularly in monsoon regions [16]. Furthermore, Kotz et al. (2022) demonstrated that the variability of precipitation is increasing at a rate faster than mean precipitation itself, leading to more volatile hydro-climatic regimes that challenge traditional water resource management assumptions [17].

1.2.2 Regional Responses in Eastern China

In China, the response to global warming exhibits significant regional heterogeneity. While northern China has experienced complex shifts between drying and wetting, eastern and southern China generally show a trend of increasing precipitation intensity [11,12]. For the Yangtze River Basin and the southeast coastal zone, recent studies have identified a "dual-intensification" pattern: both the frequency of extreme heavy rainfall and the duration of dry spells are increasing. Zhou et al. (2023) analyzed the changing characteristics of the East Asian Summer Monsoon, revealing that the northward shift of the rain belt has led to more concentrated and intense rainfall events in the Yangtze River Delta [18]. Li et al. (2022) focused on the spatiotemporal evolution of precipitation extremes in Zhejiang, identifying a significant correlation between rising sea surface temperatures (SST) in the East China Sea and the amplification of coastal typhoon rainfall [19]. Zhang et al. (2024) further highlighted that urbanization in this megalopolis region may be exacerbating these trends through the urban heat island effect, creating localized "rain islands" [20].

1.2.3 Methodological Advances in Trend Detection

The detection of trends in non-stationary hydrological series relies on robust statistical methods. The Mann-Kendall (MK) test [4,5] and Sen's slope estimator [7] remain the gold standards for assessing trend significance and magnitude due to their resilience against outliers [6]. However, recent methodological advancements emphasize the need for integrated frameworks. The Hurst exponent [8] has seen renewed application in evaluating the "long-term memory" or persistence of detected trends, offering a predictive dimension often missing in traditional trend analysis [21]. Moreover, the recognition of scale dependence has driven the adoption of multi-scale approaches. Wavelet analysis and sliding window techniques are increasingly used to decompose hydrological time series, revealing how statistical characteristics (such as variance and correlation) evolve from short-term weather noise (days to months) to long-term climate signals (years to decades) [9,10]. Wu et al. (2023) demonstrated the utility of multi-scale entropy in characterizing the changing complexity of rainfall regimes in changing environments [22].

1.2.4 Engineering Implications for Water Diversion

The intersection of climate change and large-scale water diversion projects is a growing field of inquiry. Research on the South-to-North Water Diversion Project has highlighted the vulnerability of inter-basin transfer systems to simultaneous droughts in source and receiving areas [23]. For the Eastern Zhejiang Water Diversion Project, the challenge is acute: the system must balance flood control during the typhoon season with water supply during dry spells. However, most existing studies focus on the physical infrastructure or hydraulic optimization [24], with fewer studies systematically linking long-term climatic trends to operational resilience. Recent work by Liu et al. (2023) calls for "climate-adaptive scheduling" that dynamically integrates trend detection into real-time decision-making frameworks [25].

1.3 Research Gaps and Objectives

1.3.1 Research Gaps

Despite substantial progress, critical gaps remain regarding the specific context of the Eastern Zhejiang Water Diversion Project: Lack of Fine-Scale Analysis: Most existing studies operate at provincial or basin scales (e.g., Qiantang River Basin). There is a notable lack of high-resolution analysis targeting the 15 specific sub-regional receiving areas. This "granularity gap" means that local variations—such as the difference between coastal plains and inland hills—are often smoothed out, leading to one-size-fits-all management strategies that are inefficient. Limited Integrated Assessment: While individual methods (MK, Sen's) are widely used, few studies systematically combine trend significance, magnitude, and persistence (Hurst exponent) into a unified framework. Understanding whether a trend is a temporary fluctuation or a persistent shift is crucial for long-term infrastructure planning. Insufficient Multi-scale Analysis: The scale dependence of rainfall variability and inter-regional correlations in this coastal transition zone remains under-investigated. It is unclear how the spatial synchronization of rainfall events changes from seasonal to annual scales, which is vital for coordinating multi-reservoir operations. Engineering Gap: There is a disconnect between climatic research and engineering application. Most studies stop at trend detection without translating findings into operational recommendations for source-receiver coordination and risk management under the specific constraints of the water diversion project.

1.3.2 Research Objectives

Addressing these gaps, this study focuses on the 15 sub-regions of the Eastern Zhejiang Water Diversion Project. Based on a continuous, quality-controlled 62-year dataset (1961-2022), it aims to achieve four objectives: Spatial Analysis: To quantify the multi-year average daily rainfall and identify spatial clustering patterns (high/medium/low-value areas) to provide a precise baseline for regional water balance assessments. Trend Assessment: To detect trend significance, quantify magnitudes, and evaluate persistence using an integrated MK-Sen-Hurst framework, specifically identifying "hotspots" of rapid climate response that require prioritized adaptation measures. Multi-scale Analysis: To characterize rainfall variability and the evolution of inter-regional correlations at 3-, 6-, and 12-month scales, distinguishing between local weather noise and regional climate signals to inform multi-time-scale scheduling. Engineering Application: To synthesize these findings into scientific recommendations for differentiated water supply strategies, source-receiver coordinated optimization, and adaptive management, directly enhancing the project's resilience to future climate uncertainty.