Formulation of ranfall interception and transpiration processes and evaluation of the alleviation functions of forest for flood and drought

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Based on the rainfall inception model by the authors, the interception rate is given by the product of rainfall intensity and potential evaporation rate, and then the relation between the interception loss and gross rainfall with a constant intensity for a rainfall event results in : I = a ・ h_I ・ R + c, where I is the interception loss , R the rainfall in a rainfall event, h_I the potential evaporation rate, and a and c are the coefficient and constant Furthermore, considering the nonlinear property of the interception-rainfall relation, a statistical form was proposed.After the daily transpiration has been confirmed to be almost proportional to net radiation or potential evaporation by using many observation data not only in Japan but also over the world, the proportional coefficient of the relation between transpiration and potential evaporation is formulated as a function of vapor pressure deficit and runoff depth that is related to soil moisture.The model parameters of interception evaporation and transpiration could be identified using several year data of daily rainfall, runoff discharge, air temperature and relative humidity for several watersheds of the Naka River , Hiji River, Sarukawa and Tatsunokuchi.The alleviation functions of forest soil layers for flood and drought can be evaluated by means of using a rainfall-runoff model capable for separating overland flow component from observed discharge and using the interception evaporation rate estimated on a rainy day and the daily transpiration rate estimated on a non-rainy day, and these property of interception, transpiration and forest soil layers are very important for evaluating the alleviation functions for flood and drought.