River basin hydrology with respect to dominating hydrological processes

Moving from headwaters to tailwater along with a river in arid endoreic catchment, the following dominant hydrological processes and geomorphologic characteristics are encountered: mountainous area with dominant runoff generating processes, vicinal area of mountain gap with interactive runoff generating and absorbing processes, and plain area with dominant runoff absorbing processes. This pattern with a runoff spreading zone widely occurs in Central Asia endoreic catchments as a typical example in the Kuche River of the Tarim Basin is shown in Fig.1. Similar runoff spreading zone are also reported by other researchers in semiarid endoreic catchment of Africa (Peugeot et al., 2003).

Mountainous area, which is characterized by steeper slopes and larger precipitation, acts as a catchment collecting rainfall, melting snow or glacial ice etc. It can be considered as a runoff generating area (RGA). Plain areas with minute slopes, little precipitation and high evaporation act as a dissipative area distributing and consuming water. The dominating hydrological processes can also be grouped into lateral and vertical processes. Lateral fluxes are related to water dispersive processes roughly parallel to the terrain surface such as river forks, river overruns, river water diversions into irrigated areas, and groundwater flow. Vertical fluxes occur by redistribution processes of surface water such as evapotranspiration, infiltration and percolation during all the dispersive processes. Apart from the hydrological cycle characteristics that runoff is recharged and concentrates into riverway in RGA, river water is dispersed and consumed into atmosphere in plain area after mountain gap of arid endoreic area. It can therefore be considered as runoff absorbing area (RAA). With interactive runoff generating and absorbing processes, vicinal area of mountain gap acts as a catchment collecting water while consuming much water. Both rainfall-runoff and runoff-evaporation processes are obvious in this transition zone, as the situation in semi-arid area.

Although the dominating hydrological processes are rainfall-runoff processes in RGA and runoff-evaporation processes in RAA respectively, it's not easy to specify them strictly without a facile and determinate index. The relationship between long-term evaporation and precipitation can be impacted by climate change and human activities and changes in different spatial and temporal scale. For the case in the Yellow River Basin, runoff generating processes are obvious and runoff abstractions, which are impacted by human activities and considered as the reason of river flow cutoff in the lower reaches besides climate change, are also influential (Ren et al., 2002), both runoff generating and absorbing processes should be considered and represented in significant hydrological modeling of this region. It is reasonable for hydrological modeling to focus on runoff generating processes in RGA but runoff absorbing processes in RAA. This paper will introduce a model focusing on runoff absorbing processes in an extreme arid RAA.