Connecting a lake and its upstream watersheds: a joint water balance approach

In estimating lake water balance (WB), simultaneous determination of the watershed WB is beneficial. Nevertheless, such a joint water balance (JWB) approach has not been implemented, likely due to practical difficulties. We applied this novel approach to Lake Kasumigaura, the second-largest lake in Japan, and its upstream watersheds to demonstrate its relevance in understanding the lake–watershed system. To estimate WB components, various methods were applied to available data, statistics, and previous studies. The JWB approach indicated that the lake–watershed system is largely connected through river discharge and that anthropogenic water transportation also plays a role. The results also demonstrated large WB errors likely due to river discharge errors and smaller errors in other WB components. Also, the present hydrological health of the area was demonstrated; however, a need for periodic updates of JWB was also identified. Difficulties of JWB applications in other areas and possible solutions are discussed.