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The history of farm ponds in China dates back to the Sinic civilization over 2,600 years ago. Quebei, the earliest pond on record, was constructed during the Spring and Autumn Period (770BC to 476BC) under the leadership of Sun Shu-Ao, who was the Prime Minister of the Chu State (Zhou et al., 2016a). As described in The Commentary on the Waterways Classic ,Quebei received water from three streams and had five outlets for irrigation and flood discharge. Large-scale water conservancies became prevalent during the Han Dynasty (202 BC to 220 AD) as a result of the national policy of valuing agriculture above other industries. The total ponded area at that time was surprisingly large. For example, it was reported that the ponded area in Runan county, Henan Province, totaled 300 km 2 , and its water storage capacity was far greater than that in the county in the 1970s (Hang, 2001). Owing to political infighting and warfare, the number of large ponds declined in both Tang and Song Dynasties (618 AD to 1279 AD). Even so, there was an increase in the number of small ponds, especially in the low mountainous and hilly areas of the Yangtze and Huai River basins. In the Qing Dynasty (1616 AD to 1911 AD),although pond irrigation was rarely recorded in historical files, small-scale water conservancies were widely implemented and voluntarily maintained by farmers (Lü and Chen, 2014). Along with the rapid population growth and an increasing food demand after the foundation of the PRC, small ponds flourished everywhere in southern China. However, their numbers and water storage capacity have declined sharply in recent decades due to flood damage, sludge blockage, and intentional filling or other destructive activities. As shown in Fig.2.2, farm ponds played an indispensable role in China's traditional agriculture,and their number and function have been closely related to the policies of the central and local governments as well as the maintenance by farmers.
Fig. 2.2 Variations in the total number of farm ponds throughout Chinese history. A star indicates the amount estimated from references for a historical period highlighted in the same color. Additional comments are provided to explain some major features related to the farm ponds for each period. Citations: a Zhang (2009), b Li and Liu(1986), c Lü and Chen (2014), d Tan (2005), and e Hang(2001) and Yu et al. (2015).
Farm ponds refer specifically to the abovementioned small-scale water conservancies to distinguish them from artificial lakes and reservoirs (Chou et al., 2013; Gao et al., 2016). Although farm ponds are rarely mapped at large scales (Tiner et al., 2015; Wu, 2018), the latest Google imagery (source:the 2018 Digital Globe) offers a feasible means to view their shapes and distributions in southern China. As demonstrated by the eleven examples presented in Fig. 2.3, farm ponds are distributed across (1) almost every province and municipality, (2) four rainfall isohyets with average annual rainfall of 800 mm, 1,200 mm, 1,600 mm, and 2,000 mm, (3) landscapes with elevation ranging from 30 m to > 1,500 m, and (4) all four large river basins (the Yangtze River basin, the Pearl River basin, the southeastern river basins, and the southwestern river basins) and both island provinces (Taiwan and Hainan).
Fig. 2.3 Boundaries of southern China and examples of farm ponds. From left to right and top to bottom, the images show (1) Taoyuan county, Taiwan, (2) Youbu town, Zhejiang Province, (3) Dingan county, Hainan Province, (4) Hongshan town, Hunan Province, (5)Luchuan county, Guangxi Province, (6) Chenji town,Jiangsu Province, (7) Huanglu town, Anhui Province,(8) Huwei town, Chongqin Municipality, (9) Luliang county, Yunnan Province, (10) Yongping county,Yunnan Province, and (11) Hangwang town, Shanxi Province. An average elevation of the area is provided on the bottom right corner of each image.
Farm ponds have a variety of common names in Chinese, such as 堰 (yàn in Mandarin, literally meaning weir), 坝 (accordingly, bà and dyke), 陂 (bēi and pond), 堨 (ài and blockage), 捺 (nà and restraint), 塘 (tánɡ and pond),and 荡 (dànɡ and marsh). These names were derived from local customs and traditions, and they are also used to reflect various topographic features of the surrounding landscape (e.g., plains, valleys, and hills), hydrologic and vegetation conditions, and embankment structures of a farm pond (Yu, 2015). Despite these differences, advances in measuring systems have enabled the quantification of pond characteristics in various dimensions. Specifically, a single pond can be described by its shape, area, perimeter, depth, hydroperiod (i.e., annual water level variation and flood frequency), and other parameters (Tan, 2005). From a watershed or landscape perspective, groups of ponds can be described by their density (i.e., area proportion of water surface to farmland), connectivity(i.e., whether there is water transport between ponds via ditches or streams),fragmentation (i.e., a measure of dispersion and aggregation), contagion (i.e., a measure of spatial distribution), and other indicators (Huang et al., 2012; Fang et al., 2014).