Summary
A snowmelt runoff model consisting of three submodels has been developed (Koike and others, Reference Koike, Takahasi and Yosino1986, Reference Koike, Takahasi and Yosino1987; Lu and others, Reference Lu, Koike and Hayakawa1989). The submodels, for estimating basin-wide snow water equivalent (SWE), basin-wide snowmelt rate, and runoff, require input of the following variables: meteorological data (e.g. air temperature, insolation, precipitation), basin characteristics (e.g. elevation, aspect, slope), and snow-covered area (SCA). Of these, SCA is usually obtained from remote sensing data. However, use of satellite data for real-time forecasting is not always practical, because it is restricted by weather conditions and its observation cycle.
The objective of this paper is to apply the distributed snowmelt runoff model to a basin with no continuous information on SCA. The study was conducted in Takaragawa basin of the upper Tone River, Japan. The basin is 19.6 km2 in area and elevation ranges from 800 to 1960 m a.s.l. Air temperature, humidity, insolation, net radiation, precipitation and wind velocity were collected hourly at the basin outlet. SCA was estimated by visual inspection four times during the snowmelt season.
Initial SWE should be equal to the sum of snowmelt during the snowmelt season. Therefore, if the initial distribution of SWE is obtained, the change of SCA can be simulated by using the snowmelt model, which can express the snowmelt distribution in the basin. On the basis of this concept, we estimated the change of SCA during the snowmelt season.
First, we have to estimate the spatial distribution of SWE in the basin on the starting day of snowmelt. This can be estimated from snow-line data collected within a basin. At any point on each snow line, the net heat input is calculated from the starting day in the basin to the day of the observation, to give the initial SWE.
The initial distribution of SWE is calculated by applying this method, if the snow line is observed frequently. This is usually not the case, and interpolation between calculated SWE at “points” is necessary. In this study, the interpolation model based on the topographical features of the basin, which are elevation and distance from the bisecting line of the basin, is used.
The runoff model consists of two components, one for direct runoff, and the other for base flow. Direct runoff is generated at meshed square elements and allowed to flow down. The flow in the channel network is routed through with the kinematic wave model. Manning’s law is used for the equation of motion. Channel width, roughness coefficient and separation ratio of snowmelt into two components, assumed to be constant over the basin, are determined by preliminary trial-arid-error analysis. The base flow is expressed by the lumped storage-drainage model, using a low-flow fractional recession equation. The main results obtained are as follows:
(1) In this study catchment, the distribution of SWE is approximated by a linear function of elevation and distance from the bisecting line of the basin.
(2) Using the initial distribution of snow water equivalent model and the snowmelt-distribution model, the change of SCA during snowmelt season is simulated successfully.
(3) The result of the application of the distributed snowmelt runoff model, including the simulated SCA, is in good agreement with the observed hydrograph except on rainy days.
Acknowledgements
Thanks are due to the Forestry and Forest Products Research Institute, Ministry of Agriculture, Forestry and Fishing, for providing us with the hydrological data.