2. Study Area and Data

2.1 Overview of the Study Area

2.1.1 Geographic Location and Physical Water Network

The study area is located in the eastern part of Zhejiang Province, China, covering 15 typical sub-regions distributed across five administrative zones: Xiaoshan, Shaoyu, Yuyao, Cixi, and Ningbo (Figure 1). This region serves as the primary water receiving area for the Eastern Zhejiang Water Diversion Project, which spans a research scope of 5,941 km². The topography features a complex transition from coastal plains in the east to inland hills in the west, which significantly influences regional hydrological processes. The characteristics of the three major topographic zones are summarized in Table 1a.

The regional hydrology is structurally defined by three major river basins: the Qiantang River, the Cao'e River, and the Yong River. The Qiantang River, the largest in Zhejiang, spans a vast basin of 55,491 km² across four provinces. It originates from the border regions of Anhui and Jiangxi, with its two main sources, the Lan River and Xin'an River, merging at Meicheng. The main stream flows through the Fuchun River section before entering the tidal estuary at Hangzhou Bay, where the river width expands and tidal forces dominate. The Cao'e River basin, covering 6,080 km², features terrain that slopes from south to north, with mountains and hills accounting for two-thirds of the area. Its main stream, receiving major tributaries like the Changle and Huangze rivers, flows through a fan-shaped convergence zone near Shengzhou before entering the coastal plain. The downstream section below the Cao'e River Estuary Grand Sluice has been transformed from a tidal channel into a stable inland river since the sluice's completion in 2008. The Yong River, located in eastern Zhejiang, is formed by the confluence of the Fenghua River and the Yao River at Sanjiangkou in Ningbo City. The Yao River, originating from Siming Mountain at an elevation of 715 meters, flows through the Yuyao and Ningbo plains and serves as a critical conduit for the regional water diversion network, connecting the inland hills to the East China Sea.

The physical water network of the study area is anchored by the Eastern Zhejiang Water Diversion Project, which connects these natural river systems through a comprehensive 294 km main diversion line. The system operates on a "diversion/lifting combined with river network conveyance" model, effectively linking the Qiantang, Cao'e, and Yong rivers into a unified hydraulic entity. The network begins at the Xiaoshan Hub, located at the confluence of the Qiantang, Fuchun, and Puyang rivers. Known as the "dragon head," it features dual functions of gravity diversion and pumping, with a design capacity of 50 m³/s. Water is conveyed through the Xiaoshao Plain river network via backbone rivers like the Hang-Yong Canal and Sanjiang River, eventually reaching the channel upstream of the Cao'e River Grand Sluice. This channel acts as a massive intermediate regulating reservoir with a capacity of 146 million m³, capable of balancing inflows from the Qiantang estuary and the Cao'e River's own upstream runoff. The Grand Sluice itself is a monumental structure with 28 holes and a net width of 560 meters, designed for tide blocking and flood discharge.

From the Cao'e River, the water diversion splits into two primary routes to serve the eastern regions. The North Line, or the Cao'e River to Cixi Diversion, originates at the Sanxing Sluice in Shangyu. It utilizes gravity flow through the Yao River plain network, passing through a series of control structures including the Puqian, Moushan, Sitang, and Qitang sluices, to deliver water to the districts of Shangyu, Yuyao, and Cixi. The total length of this diversion line is 85 km, with a design flow of 60 m³/s. The South Line, or the Cao'e River to Ningbo Diversion, starts at the Shangyu Hub. It routes water through the Yao River main stream (Hang-Yong Canal), traversing the Fenghui Plain and the Upper and Lower Yao River areas, ultimately supplying the Ningbo urban area. Key structures along this 92 km route include the Tongming Sluice and the Shushan Grand Sluice. Furthermore, the Zhoushan Mainland Diversion extends this network offshore; it lifts water from the Lixidu Pump Station in the Lower Yao River area and transports it via pressurized pipelines across the seabed to the Huangjinwan Regulating Reservoir on Zhoushan Island, ensuring water security for the archipelago with a design flow of up to 5 m³/s.

Table 1a. Topographic Zones and Characteristics

Table 1b. The 15 Sub-regions of the Study Area

2.1.2 Hydro-climate Characteristics

The region is characterized by a subtropical monsoon climate with substantial precipitation variability. Annual precipitation multi-year averages range between 1273.5 mm and 1454.8 mm, with high inter-annual variability as historical maximums reach 2050.1 mm and minimums drop to 682.2 mm. Regarding seasonal distribution, rainfall is highly concentrated in the Meiyu season (June-July) and Typhoon season (August-September), which together account for over 60% of the annual total. This concentration often leads to alternating flood and drought risks.

Figure 1. Location of the study area and distribution of 15 sub-regions

2.2 Data Sources

This study utilizes daily precipitation observation records from 1961 to 2022 (62 years) for 15 representative meteorological/hydrological stations within the study area. Data were obtained from the Zhejiang Provincial Meteorological Bureau and local hydrological monitoring networks. The dataset includes 22,630 daily values per station, capturing multiple decadal climate cycles and providing a robust foundation for long-term trend analysis.

2.3 Data Quality Control

To ensure reliability, a rigorous four-stage data quality control system was applied. The first stage involved continuity assessment, which systematically identified time series gaps; missing data accounted for less than 2% for any station. The second stage focused on outlier detection, applying the 3σ criterion (Pauta criterion) to flag statistical anomalies; flagged values were cross-checked against adjacent stations and synoptic records to distinguish measurement errors from genuine extreme events. The third stage addressed interpolation, where small gaps were filled using linear regression with the most highly correlated neighboring stations, validated against seasonal climatology. The fourth stage comprised homogeneity testing, employing the Standard Normal Homogeneity Test (SNHT) and sliding t-test methods to detect and correct breakpoints caused by station relocations or instrument changes.

The final quality-controlled dataset exceeds 98% completeness and has been verified for homogeneity.