HYDRO-CLIMATOLOGICAL STUDY ON THE MID-WINTER RUNOFF FROM THE SNOWY REGIONS IN JAPAN

Abstract

1. It is a well known fact that the snow cover greatly affects on the hydrological conditions. In Japan, Japanese Sea side of the country is covered with heavy snow during winter, but Pacific side receives only a little precipitation in the same season. Differences in hydrological condition in these regions are analyzed in this report. 2. For the comparison of the amount of discharge in different regions, it is usual method to calculate “specific discharge”. It is said that the amount of specific discharge increases in narrow drainage basin which locates in the mountaneous region and receives more precipitation than in the flat basin (Hanazawa, 1960). Such tendency is not obtained for winter runoff, because precipitation occurs as solid form in the Japanese Sea side and other region does not receive remarkable precipitation. The relations between amounts of specific discharge and catchment area of river basins are represented in Fig. 2, in which the following characteristics are pointed out. (1) The amount of specific discharge does not vary in relation to the area of drainage basin in winter months. (2) The difference between snowy region and region without snow cover is evident (Fig. 1 and Fig. 2). (3) Effect of geological and vegetal condition on winter runoff is considered to be very small. 3. In mid-winter, surface ablation of the snow is not observed except for several warm days. For the verification of this tendency, heat gain at the snow surface is calculated at several stations by use of radiational and bulk-aerodynamical calculations basing on monthly mean climatic data. The values are negative except for one station (Table 2), meaning that the surface cooling is dominant instead of melting. The calculation does not indicate the amount of net-exchange at the snow surface, because it cannot separate the daily and nocturnal heat exchanges. Then, the values in Trable 2 are rough estimates of heat gain. Besides the above calculation, examination based on empirical relationship between daily mean air temperature and ablation amount was made. Daily and inter-diurnal variation of runoff is condiserably small in mid-winter, by which the existence of continuous water supply is assumed. Then the effect of soil heat flux on the ablation of snow is considered as the main factor mid-winter runoff from the snowy region. 4. The amount of total ablation is given by the sum of surface ablation (Qs) and that by soil heat flux at the bottom of snow cover (Qb). M=Qs+QbThe first item in right side of the equation is neglected by the preceding consideration, then the amount of total ablation for mid-winter is represented by the following relation. M=c•ρ•λ_??_Calculation of the soil heat flux is substituted by the change in soil heat storage by use of vertical profile of soil temperature obtained at weather stations (Table 3). A part of the amount of heat which is lost from the soil may be drainaged into river by ground water temperature, but it is assumed here that the all heat is consumed in bottom melting of snow. On the other hand, river runoff (R) is assumed to be separated into two items, R=MW'+RGwhere, MW' is water supply by ablation caused by soil heat flux and RG is basic ground water supply. Assuming that the ground water supply (RG) would be represented by the amount of discharge of the river basins in the regions whithout snow cover, the amount of MW' for the unit surface area is calculated (Table 4). This value would coincide with the amount of change in soil heat storage, i.e. -c•ρ_??_dZ=80(R-RG/S)where S denotes surface area of drainage basin.