Characteristics of discharge and suspended-sediment load in a subarctic river, Tanana River, Alaska, were examined by observations in the glacier-melt and permafrost-melt seasons (June to September) of 2002 and 2003. The drainage basin (area: 6.48 × 104km2) of the river is occupied by the ca. 96 % permafrost region of the diluvium to alluvium and the ca. 4 % glacier-covered region in the Alaska Range and the Wrangell Mountains. The hydrographs of the Tanana River are explained by the superimposition of rainfall runoffs and baseflow from the permafrost region and glacier-melt runoffs from the glacial region. The suspended-sediment load, meanwhile, varied synchronously with the discharge, but its magnitude nonlinearly responded to the discharge. The nonlinearity of sediment load occurred only under local rainfalls, when the sediment supply from river channels was possibly insufficient. The runoff analyses by a tank model were carried out for the hydrographs of 2002 and 2003. The rainfall onto the whole drainage basin and the meltwater input into the glaciers were estimated by the Thiesen method and the positive degree-day approach (PDDA), respectively. The analytical results, reasonably reproducing the hydrographs, indicated that the glacier-melt runoffs occupy 60 % and 66 % of the total discharge in 2002 and 2003, respectively.
The purpose of this study is to develop an easier method than Thornthwaite’s to estimate the potential evapotranspiration. The data used are monthly climate (temperature) grid data of 1km resolution (Japan Meteorological Agency). We calculated potential evapotranspiration by the Thornthwaite’s method at first.
We got an approximate expression as follows, verifying by Thornthwaite’s value;
where PE is potential evapotranspiration in our method, AT is a yearly accumulated temperature, a and b are coefficients defined as follows;
b=–0.0069 ln (Tm)+0.0245
where Tm is each monthly mean temperature.
This method is much more simplified than Thornthwaite’s method, and has practical advantages with enough accuracy.