Abstract
Abstract. Rice paddy fields provide important ecosystem services (e.g., food production, water retention, carbon sequestration) to a large population globally. However, these benefits are diminishing as a result of rapid environmental and socioeconomic transformations, characterized by population growth, urbanization, and climate change in many Asian countries. This case study examined the responses of stream flow and watershed water balances to the decline of rice paddy fields due to urbanization in the Qinhuai River basin in southern China, where massive industrialization has occurred during the past 3 decades. We found that stream flow increased by 58 % and evapotranspiration (ET) decreased by 23 % during 1986–2013 as a result of a three-fold increase in urban areas and a reduction of rice paddy fields by 27 %. Both high flows and low flows increased significantly by about 28 % from 2002 to 2013. The increases in stream flow were consistent with the decreases in ET and leaf area index monitored by independent remote sensing MODIS (Moderate Resolution Imaging Spectroradiometer) data. Attribution analysis, based on two empirical models, indicated that land-use/land-cover change contributed about 82–108 % of the observed increase in stream flow from 353 ± 287 mm yr−1 during 1986–2002 to 556 ± 145 during 2003–2013. We concluded that the reduction in ET was largely attributed to the conversion of cropland to urban use. The effects of land-use change overwhelmed the effects of regional climate warming and climate variability. Converting traditional rice paddy fields to urban use dramatically altered land surface conditions from an artificial wetland-dominated landscape to an urban land-use- dominated one, and thus was considered an extreme type of contemporary hydrologic disturbance. The ongoing large-scale urbanization of the rice paddy-dominated regions, in humid southern China and East Asia, will likely elevate storm-flow volume, aggravate flood risks, and intensify urban heat island effects. Understanding the connection between land-use/land-cover change and changes in hydrological processes is essential for better management of urbanizing watersheds in the rice paddy-dominated landscape.
Highlights
Urbanization is a global phenomenon that poses profound threats to the local environment, society, and culture (Foley et al, 2005; McDonald et al, 2011)
The Qinhuai River basin (QRB) represents a typical landscape of the lower Yangtze River Delta region that is characterized by its flat topography with natural river networks which are severely modified, and land use which is dominated by paddy rice fields, dotted with small irrigation ponds that have been converted from natural wetlands over thousands of years (Fig. 2)
Based on the watershed balance theory and comprehensive analyses using different methods, including flow duration curve (FDC), climate elasticity model (CEM), and rainfall–runoff model (RRM), we attributed the dramatic increase in stream flow mainly to the changes in land-use and land-cover (LULC) and associated decrease in leaf area index (LAI), not climate (PET or P ), for the following three complementary reasons
Summary
Urbanization is a global phenomenon that poses profound threats to the local environment, society, and culture (Foley et al, 2005; McDonald et al, 2011). A simulation study in Taiwan suggested that rice paddy fields generated 55 % lower total runoff and 33 % lower peak flows than dry farms (Wu et al, 1997). Another simulation study that used the HEC-HMS model for a rice paddy-dominated watershed in southern China found that an increase in impervious surface areas from 3 to 30 % increased the peak-flow rate and storm volume (4–20 %), but had a very limited impact on total annual flow (< 6 %) (Wang et al, 2009; Du et al, 2011, 2012) and long-term water balances
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