1.1 Background

Small water bodies, which in this monograph refer to ponds, pools, ditches, springs, and small reservoirs largely located in headwater regions, are numerous and widely distributed across the world (Fig. 1.1). Our awareness of the importance of small water bodies has witnessed an increasing growth since the last decade (Biggs et al., 2017; Calhoun et al., 2017; Chen et al., 2019),due to their abundance, importance to freshwater systems and biodiversity,contributions to ecosystem services, and their vulnerability and sensitivity to a variety of human activities (Cohen et al., 2016; Creed et al., 2017; Golden et al., 2019; Sayer and Greaves, 2020; Lane et al., 2022). Meanwhile, our lack of knowledge on the functional services of these small water bodies is widely recognized, which affects practical and effectively legislative and regulative measures to protect and sustainable use of these waters (Biggs et al., 2005;Sullivan et al., 2019; Wade et al., 2022). This circumstance has led to many recent local and regional initiatives that highlight the importance of small water bodies and encourage collaborations between scientists, decision makers,local planners, travelers, and other interested parties who pursue their better management and utilization (Takeuchi et al., 2016; Xu et al., 2019; Wang et al., 2019), especially in the global trend of climate change, pandemic diseases and food insecurity (Duan et al., 2016; Thorslund et al., 2017; Chen et al., 2022). Focused on these novel important academic and management topics,our monograph aims to provide a series of hydro-environmental assessment of small water bodies from local to global scales. These assessments include nitrogen and non-point source pollution (water quality), hydrologic regulation effects (water resource), and inundation analysis and flood control management, as well as research perspectives at two scales of southern China and the world. As a pilot study of hydro-environmental assessment of small water bodies, this monograph can provide monitoring strategies and modeling tools for regions with abundant small water bodies and similar concerns on these waters. It can also provide reference to water resource management,water environment protection, regional planning and development, sustainable agriculture, and other areas related with small water bodies.

Deterioration of water quality, especially nitrogen and phosphorus, has attracted growing attention across the globe. The excessive normal nutrient and other dissolved substances including persistent organic pollutants and antibiotics have along accumulated in various landforms and flowed into the water environment along with rainfall runoff processes (Lane et al., 2018;Golden et al., 2019). Previous studies largely focused on large rivers, lakes and coasts, with fewer attention given to small water bodies in headwater regions (Biggs et al., 2017; Chen et al., 2018; Sayer et al., 2020). The neglect of these water bodies, which often serve as nutrient storage and retention in the watersheds, hinders our deeper understanding of nutrient accumulation processes and associated pollution control projects. This is because that small water bodies are usually interacted with surface, shallow subsurface and groundwater flows in the aquatic systems (Lane et al., 2018; Chen et al., 2020),and they are a mediator between upstream landforms and downstream waters.Early in the 1990s, scientists suggested that small water bodies had potential impact on watershed nutrient conditions using landscape-scale statistical methods (Whigham et al., 1988; Johnston et al., 1990). In the early 2000s,hydrological processes of small water bodies embedded in watershed were explored, providing glimpses of these waters in the timing and frequency of their water transport to downgradient rivers and lakes (Leibowitzet al., 2003;Richardson, 2003; Rains et al., 2006). Later in the 2010s, cumulative hydrological effects were further demonstrated in watershed-scales, but few studies directly targeted at small water bodies and their individual or grouped effects on downstream water quality (Lane et al., 2018; Golden et al., 2019; Chen et al., 2021). Although intrinsic characteristics of small water bodies’(e.g., water storage, connectivity, and bidirectional exchange with shallow groundwater;Xia et al., 2016; Chen et al., 2018) and extrinsic factors of subcatchment (e.g.,slope, land use types, and soil attributes; Li et al., 2019; Wang et al., 2018) and precipitation (e.g., intensity, duration, and concentration; Zhang et al., 2020) were considered as water quality influences (Fig. 1.2), fragmentary knowledge on small water bodies and their cumulative watershed-scale effects remains a challenge for both researchers and managers.

More essentially, small water bodies are landscape functional elements performing physical and hydrological effects on downstream systems (Marton et al., 2015; Rains et al., 2015; Schofield et al., 2018). Source, sink and refuge are the three main functions first identified for these waters (Leibowitz et al., 2008).Later in 2015, USEPA added another two functions, lag and transformation, into this conceptual framework. Such framework helps to study and understand the physical and hydrological connections between small water bodies and their downstream and adjacent water systems (Leibowitz et al., 2018; Schofield et al., 2018). Based on these insights, the interdisciplinary hydrological science community has started looking into several research subjects of small water bodies in various landscapes, such as spatial heterogeneity, physical interactions, and connectivity (Fig. 1.3). The area scale of studied watersheds ranges from a few hectares to several thousand square kilometers (Evenson et al., 2016; Evenson et al., 2018; Yeo et al., 2019; Chen et al., 2020; Zeng and Chu, 2021). However, due to the different intrinsic characteristics and extrinsic factors of small water bodies across the world, physical processes and related driving mechanisms are still not clear, especially involving various management and conservation measures (Calhoun et al., 2017; Lahoz-Monfort et al., 2019). On the other hand, more than 847,000 reservoirs have been constructed globally in the last 100 years, and approximately 95% of them are small reservoirs (Song et al. 2015), not mentioning those unmanaged small reservoirs. They provide water, irrigation, and sometimes hydroelectric energy,and more importantly, stabilize extreme inflows to mitigate floods or droughts(Chang, 2006; Rodrigues et al., 2012). Comparing with natural small water bodies, more attention is paid to water resource and flood control management for small reservoirs, including simultaneous considerations on the hydrological,geotechnical, and environmental aspects (Chen et al., 2017). Although a number of countries have employed flood forecasting systems, considering volume, peaks and duration of rainfall events, since the 1990s, their application and usage require a deeper understanding of flood processes and a more effective operational solution, especially for small reservoirs with a fast process of converging and rising of water levels (Werner et al., 2013; Cools et al., 2016).

Our mapping, measuring and modeling techniques for small water bodies have gradually developed since the last decade, but are still limited compared with those for rivers, coasts, and lakes (Golden et al., 2019; Swartz and Miller,2021). Scientists and decision-makers need to acquire where small water bodies locate before their hydro-environmental assessment. In addition to traditional satellite observation and field visit, including setting up ground instruments like cameras, water level meters, etc., Light Detection and Ranging (LiDAR) and novel radar-based remote sensing methods are started in detecting small water bodies, especially in forests and canopy environments (Lang and McCarty, 2009;Tiner et al., 2015; Wu et al., 2019). Many wetland inventories and lake database are available online, such as HydroLAKES and MERIT Hydro. These datasets,however, mostly don't have enough spatial resolution for capturing and assessing small water bodies, especially within changing landscapes (Tiner et al.,2015; DiBello et al., 2016). Accordingly, our measurements with high temporal resolution for the nutrient levels and hydrological regimes of small water bodies are lacking. This has been called on for more than 15 years, although it is resource intensive and challenge to measure those variables in watershed scales (Whigham and Jordan, 2003; Biggs et al., 2005; Chen et al., 2019). On the other hand, models, especially process-based models, are useful tools to assess how small water bodies affect hydrological processes and nutrient loads in watersheds. Recent model advances have enabled Soil and Water Assessment Tool (SWAT), Hydrological Simulation Program-Fortran (HSPF),and Annualized Agricultural Non-Point Source Pollution Model (AnnAGNPS),to incorporate small, scattered wetlands (Golden et al., 2017; Chen et al.,2018; Yasarer et al., 2018). The spatially-explicit representation with physical and functional attributes provides feasible means for detailed assessments on these waters (Evenson et al., 2015; Yasarer et al., 2018; Evenson et al., 2018;Yeo et al., 2019). However, model limitations still exist for consideration of MPSs, including bidirectional exchanges between ponded water and shallow groundwater (Rains et al., 2015), and cascading fill-spill relationships between intermittently connected ponds (Yu et al., 2015). These model deficiencies and lack in mapping and measuring together hinder the detailed assessment of small water bodies.

From a typological perspective, small water bodies can be ponds, pools,bogs, fens, marshes, playas, and others across different parts of the world(Mushet et al., 2015; Creed et al., 2017; Chen et al., 2022). Scientists and decision-makers hold various research interests among these small water body types, as they are closely associated with our life and daily living environment in different ways (Biggs et al., 2017; Hunter et al., 2017; Calhoun et al., 2017;Thorslund et al., 2017; Ghajarnia et al., 2020). In southern China, the farm pond, whose history can be dated back to 700s BCE, is a widely distributed type of small water body. Farm ponds were first constructed in Anhui Province,and were vigorously developed and flourished as a unique natural-economic-social complex across southern China since 1949 (Verhoeven et al., 2006;Liu et al., 2009; Yu et al., 2015). Some larger ponds were later enlarged as small reservoirs, due to appropriate topographic features of the surrounding landscape and hydrologic and vegetation conditions, so that they can hold and manage greater water resources (Yu, 2015). In conjunction with the surrounding woods, farmlands, cottages, and water courses, farm ponds and small reservoirs constitute a unique pastoral landscape in southern China(Fig. 1.4). These small water bodies with Chinese characteristics can alter the hydrological and nutrient cycling in the watershed, and lead to idiosyncratic riparian conditions to host flora and fauna. They also offer considerable social and cultural benefits, including improving physical and mental well-being and increasing awareness of environmental conservation at larger scales (Yu et al.,2015; Creed et al., 2017; Chen et al., 2022). For example, the 193 farm ponds can reduce annual irrigation water shortages from 306±26 to 89±48 mm in the Liuchahe catchment, Anhui Province (Yin et al., 2006), while reducing the flood peak from 2.5 to 0.3 m ³ s ^-1 during a heavy rainfall event (141 mm d ^-1 ; Liu et al.,2009). Although hydrologic regulation of these small water bodies has been gradually recognized in catchments (Wang et al., 2008; Longbucco, 2010; Feng et al., 2013), the comprehensive hydro-environmental assessment relating with regional planning, environmental protection, sustainable agriculture, etc., is still a multidisciplinary challenging issue, and can serve as a testbed for other types of small water bodies across the globe.

To sum up, global changes including all the future uncertainties due to human activities have significantly affected hydrological cycles and associated water resources, nutrient behaviors, and biological responses, especially during extreme rainfall and drought events. The widely distributed small water bodies,including various types around the world, play non-negligible roles in these watershed processes, but our understandings are lacking due to limited map resolution, coverage of measurement, and targeted modeling tools. Farm ponds and small reservoirs, as a a unique and pastoral landscape in southern China,can serve as a testbed for hydro-environmental assessment of other types of small water bodies. Hence, the objective of this book is to assess the hydro-environmental processes of farm ponds and small reservoirs in local regions of southern China, and propose research perspectives that inform both farm ponds and other small water body types around the world. lOr6QFu3ldu7F2tLTZkvsgVBadqDgG8afoWSwdJX61EvdpH96ykUp9znLGX6Kgs3