While internal renewable water resources, represented by annual flow of rivers and recharge of aquifers generated from endogenous precipitation make-up the major part of a water balance, water generated outside the border of a country can also be important, such as natural inflows from upstream countries (groundwater and surface water), and part of the water of border lakes or rivers. Similarly, not all the water resources generated by endogenous precipitation in a particular country are available for that country. This is because, for example, a certain quantity of water must remain to maintain the natural flow of the river which ultimately leaves the nation's (nations') border. Thus, the water balance equation of a country also needs to include the external renewable water resources that naturally flow into that country and the amount of water generated by endogenous precipitation that naturally flows out of the country.