Supply and demand dynamics of hydrologic ecosystem services in the rapidly urbanizing Taihu Lake Basin of China

Although it is challenging to integrate supply and demand to comprehensively understand urbanization effects on ecosystem services, this knowledge is essential, especially in rapidly urbanizing areas. A spatially explicit approach was developed here to synthesize supply and demand dynamics of the two most important hydrologic services in the rapidly urbanizing Taihu Lake Basin (TLB) of eastern China. The supply of water purification (WPS) and flood mitigation (FMS) were measured as nitrogen removal and runoff retention, respectively. Whereas the demand for water purification (WPD) was quantified as the difference between total and permitted nitrogen loading based on relevant water quality standards, the demand for flood mitigation (FMD) was estimated as the vulnerability to potential flood damage, including economic losses and causalities. We found a spatial mismatch where high WPS and FMS occurred in mountainous areas while high WPD and FMD concentrated in urban and agricultural areas across the basin. WPS and WPD decreased by 10% and 20%, respectively during 2000-2015, due mainly to loss of croplands to urban expansion. This was also the main cause of decreased FMS by 7% but increased FMD by 67%, which underscored the importance of conserving croplands in rapidly ur-banizing regions of the TLB. Overall, land use composition had strong associations with WPD (r2 >= 0.57) and FMS (r2 >= 0.38) at the sub-basin scale, while the configuration of multiple land uses, such as urban sprawl, cropland fragmentation, and riparian buffers were crucial in influencing WPS. In comparison, FMD was most sensitive to urban expansion (r2 = 0.74), economic development (r2 = 0.81), and population growth (r2 = 0.93). These findings provide new insights into sustainable land management for coordinating supply of and demand for hydrologic services in the TLB and other urbanizing watersheds of the world.