Runoff Analysis in a Loam Upland (II)

Abstract

This report is the result of a runoff analysis by the infiltration method in a loam upland covered with volcanic ashes. In this basin the infiltration capacity is large, but the subsoil is not so permeable as the surface soil. Therefore, at the time of heavy rainfall, the infiltration capacity is much influenced by the maximum infiltration rate of subsoil. Moreover, when the ground water level is raised up to the ground surface by heavy rainfall, the area without any infiltration capacity increases to occupy a large percentage of the basin.
On these conditions, the authors calculated the runoff at the time of the typhoon No.22 in 1958 when the rainfall amounted to 280mm. The values calulated by the following methods coincided with the measured hydrograph.
(1) In regard to the movement of surface soil water, the authors assumed such formulas as follows:
dM/dt=-cM, M=M₀e^-ct, f=fc+qi, qi=K(M-M)
where, M: excess moisture content of surface soil over the field capacity;dM/dt: the rate of M moving into subsoil; M_o: maximum value of M; t: time from M_o to M; f: infiltration capacity; f_c: maximum infiltration rate of subsoil; q_i: seepage rate from surface soil that provides the source of interflow; M: the minimum value of M when q_i first appears; c and K: constants.
source of interflow; M: the minimum value of M when q_i first appears; c and K: constants. The infiltration capacity before reaching M_o is very large. In the intermisson between rainfalls themoisture in the surface soil moves down into subsoil, and the infiltration capacity is recovered. The condition corresponding to M_o usually takes place before the surface soil becomes saturated. This is be-cause the ground surface is undulatory, and the excess water in surface soil is easily discharged by seepage.
(2) In case of heavy rainfall the channel runoff in the basin floods. Therefore, the infiltration method is to be modified as follows:
i=f+re re=ΔS+
qs=ΔSf+ΔSe+qe ΣΔSc=Sc qe=K. Scm
where, i: rainfall intensity; re: rainfall excess;ΔS: the rate of surface detention; S: surface detention; q_s: surface runoff; ΔS_f: the rate of flooding storage; ΔSc: the rate of channel storage; S_c: channel storage; q_c: channel runoff; K and M: constants.
(3) The runoff from each part of the basin has a time lag in concentrating at the end of the basin. The time lag is influenced by many factors, and the authors assumed here as follows:
T=K.Q^-e
where, T: time lag; Q: discharge; K and c: constants.