This paper proposes a method by which to estimate the Water level hydrograph of Lake Shinji corresponding to rainfall in summer (Jun. -Sept.). By transforming the hydrograph into a simple triangle model as in Figure 1, the relations between X (rainfall depth in mm at Matsue) and H_1 (increment in water level in cm from the initial water level to the peak), T_1 (time in hr. from the beginning to the end of hydrograph) , T_2 (time in hr. of the rising limb) can be expressed by
h_1 = 0.8490x - 0.2642 + 0.1988ε_1
t_2 = 0.2703x + 1.1099 + 0.1814ε_2
t_1 = 0.2644h_1 + 0.0619x + 1.1346 + 0.0850ε_3
where x is log X, h_1 log H_1, t_1 log T_1, t_2 log T_2, and ε_1～ε_3 stochastic variables.
Based on the observed data, the initial water level H_2 (T. P, cm) and the time lag from the rainfall beginning to the rising of hydrograph T_3 (hr.) can be estimated by Monte Calro Simulation Method. When X is given, a triangle model of the hydrograph is obtained with the combination of H_1, H_2, T_1, T_2, and T_3 above mentioned.