We investigated year-round water level fluctuations in Myoginohana floodplain, which is the largest reed wetland in Lake Kasumigaura and where there has been considerable focus on their functions such as biodiversity and water purification. As a result of having observed spatial and temporal changes of water levels in the floodplain and the lake, a series of three following mechanisms of water cycle was clarified. 1) In flood peak, the whole floodplain had same water level with the lake and the river. 2) In discharge period, when the water level in the floodplain got lower than the height of the floodplain edge, the water levels in the floodplain decreased more slowly whereas the water level of the lake fell immediately by the artificial operation of the water gate. And different reductions of water levels were observed within the floodplain caused by their topographic conditions. 3) Then if there was no longer inundation, only evapotranspiration let the water level decrease. Furthermore, water balance in each flood event was calculated to evaluate the volume of water exchange between the floodplain and the lake or the river. As the result, about 58-78% of water level rise in a flood was caused by inflow from the river. Total inflow of four floods in 2006 was estimated about 1300mm, which was same amount of average annual precipitation. Hence, it's inferred that water exchange in floods contribute to material exchange and make influence for water quality in the floodplain.
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