Use of the SHE hydrological modelling system to investigate basin response to snowmelt at Reynolds Creek, Idaho

The first test of the snowmelt component of the Systeme Hydrologique Europeen (SHE) hydrological modelling system is reported, in which the SHE is used to simulate a 12 1/2 day period of snowmelt runoff for a 40 ha subbasin of the Reynolds Creek rangeland research basin in Idaho. The results show the SHE to be capable of successfully simulating hydrograph responses caused by snowmelt generated by heat inputs, especially net radiation and sensible heat transfer. The results also show how, in the absence of previous field studies of basin response, the SHE can be used to investigate different hypotheses of basin behaviour. In this case, four hypotheses of basin response are examined, based on different representations of the soil permeability and subsurface flow pathways: (1) impermeable (frozen) ground surface; (2) fully permeable soil profile throughout the basin; (3) reduced soil permeability away from the channel; (4) rapid, near-channel subsurface response. Comparison of the measured and simulated hydrographs supports Hypothesis 4, with a possible cause of the response being the rapid conversion. of a near-surface, tension-saturated capillary fringe into a pressure-saturated zone by the infiltrating meltwater. In the absence of direct measurements of net radiation and wind gradient, indirectly derived values are used as input to the snowmelt component. The implications which the uncertainty attached to these values holds for the simulation results is investigated in sensitivity tests which show that, with sensible heat transfer forming the major heat flux to the snowpack, greater uncertainty is introduced into the simulation results by the uncertainty in the wind gradient than by the uncertainty in the net radiation. The snowmelt simulations are based on an energy-budget option: a test of a temperature-based (degree-day) option shows comparable accuracy but only through empirical fitting. Overall the study shows that a greater hydrological expertise is required in operating SHE-type models compared with simpler models.