Water resources: total external renewable

That part of the country's long-term average annual renewable water resources which are not generated in the country. It includes inflows from upstream countries (groundwater and surface water), and part of the water of border lakes and/or rivers. The ERWR take into account the quantity of flow reserved by upstream (incoming flow) and/or downstream (outflow) countries through formal or informal agreements or treaties. Therefore, it may vary with time. In extreme cases, it may be negative when the flow reserved to downstream countries is more than the incoming flow. The rules applied for computing the long-term annual renewable water resources by country are given in the following template as an example: http://www.fao.org/nr/water/aquastat/data/wrs/readPdf.html?f=SEN-WRS_eng.pdf.