2.1.4 Nitrogen transport and human water use
For nitrogen processes, the schemes of human activities (including nitrogen discharge and water regulation) and the riverine DIN transport were incorporated into CAS-LSM via the coupling of the RTM. For nitrogen discharge, the nitrogen fertilizer data were updated using the scheme of Lu and Tian (2017), and the point source pollution was updated using the data merged by Morée et al. (2013). For water regulation, the surface water included the scheme of Hanasaki, Kanae and Oki (2006), and the groundwater scheme follows that of Zou et al. (2015) and Zeng et al. (2016). For the surface water and groundwater, the data were determined from several source data sets, including the Food and Agricultural Organization (FAO) water-use data set (http://www.fao.org/nr/water/aquastat/data/query/index.html), the Global Map of Irrigation Areas, version 5.0 (Siebert et al. 2013), the historical monthly soil moisture levels and saturated soil moisture levels simulated by CLM4.5 offline for the years 1965–2000 (Zeng et al. 2017), and the FAO water information system for 2010 (http://www.fao.org/nr/water/aquastat/main/index.stm). During DIN transport, a proportion of the DIN will be affected by the surface and groundwater regulation based on the mean DIN concentration, and the rate of withdrawal for surface and groundwater (Liu et al. 2019). For the nitrogen river transport, based on the water transport framework in CAS-LSM, the DIN inputs, including soil DIN runoff and leaching, nitrogen deposition, and point source nitrogen discharge, were used as tracers in the nitrogen transport module, along with a DIN retention module during the transport (Nevison et al. 2016). As nitrogen transport is associated with water temperature, this variable was also considered in CAS-LSM following the scheme of van Vliet et al. (2012). Anthropogenic heat discharge from thermoelectric power plants was also considered in CAS-LSM (Liu et al. 2019), where the input data for the emitted heat are from Raptis and Pfister (2016).