4. Hydrograph model
The Hydrograph model is a distributed process-based hydrological modelling system (Vinogradov et al., 2011). It describes all components of the land hydrological cycle, including precipitation and its interception; snow accumulation and melting; evaporation from snow, soil, and vegetation cover; surface flow and infiltration; soil water dynamics and flow; heat dynamics and phase change in soil layers; underground flow formation, slope and channel flow transformation. The model requires the following atmospheric variables as its input: air temperature and humidity, the amount of precipitation. The output are flow hydrographs, water balance and state variables of basin elements. The model can be run at time steps from minutes and hours to daily.
The concept of runoff elements used in the Hydrograph model for spatial discretization of basins is a key concept. The catchment area consists of runoff elements of different levels – surface, soil and underground. The concept proposes the system of runoff elements characteristics, such as outflow time which include the time and intensity of outflow from elements, depending on the water storage (Vinogradov et al., 2011).
Within the discretization procedure, the basin territory is divided into several conditionally homogeneous parts called runoff formation complexes (RFC). It is assumed that the characteristics of soil, vegetation, topography, and other components of the landscape are constant within each RFC, while the runoff formation process is uniform. The main parameters of the model are the physical properties of the landscapes that may be observed in nature and are classified according to the types of soil (specific weight, specific heat capacity of soil particles, specific heat conductivity of dry soil particles, porosity, maximum water holding capacity, wilting point, ice impedance factor, infiltration coefficient, hydraulic parameter of subsurface system of runoff elements), vegetation (seasonal shadiness by vegetation cover, landscape albedo, interception storage capacity, coefficient of evaporation from the interception storage during the maximum development of vegetation cover, parameter of heat supply from atmosphere to soil surface, phenological dates) and other characteristics (Vinogradov et al., 2011; Semenov et al., 2013).
The method for simulating thermal dynamics in the upper layer of ground is incorporated in the Hydrograph model. It is based on several techniques that simplify the differential equation of thermal conductivity in the soil profile and allows bringing the system of differential equations to a system of linear algebraic equations without losing the quality of the simulation results (Lebedeva et al., 2015; Semenova et al., 2015).
The Hydrograph model has been successfully applied to simulate the river runoff formation in cold regions with a lack of ground-based observation data (Semenova et al., 2013; Semenova et al., 2015; Vinogradov et al., 2011). Here we present the results of parameterization and verification of the Hydrograph model for mountainous landscapes of the North-East of Russia based on the data of short-term special observations. For modeling, the Suntar River basin is presented as a hexagonal grid, with 32 representative points (RP) (Fig. 1). Each RP has its own set of point characteristics, which include the coordinates, latitude, elevation, slope aspect, slope inclination and lag time.