Hydrological Connectivity

Hydrological connectivity defines the spatial and temporal pattern of the links among surface or subsurface water networks flowing across a landscape, and hence, it determines the ease with which water may move across a landscape or through a river system. Hydrological connectivity is a dynamic property of the system, and its long-term integration can be expressed using probabilities whereby a point in the landscape connects with another point or with a receiving water body such as a river or a lake. The strength of the hydrological connectivity is a function of two factors: (1) the surface facilitating or impeding the movement of water and (2) the temporal structure of the rainfall or river flow time series driving the movement.

The degree to which the surface facilitates or impedes the movement of water is determined by the surface properties and their spatial structure. As water moves along a terrestrial flow path, at each point there are four options available for water movement: (1) evaporation, (2) entering depression storage, (3) infiltration, or (4) moving along the flow path down the slope. The water may move down the slope along the surface, through the subsurface within the soil, or through a groundwater aquifer. The characteristics of the landscape and storm determine the set of probabilities that the water will move in each of these directions. For example, in many semiarid environments, there is a two-phase pattern of infiltration rates controlled by vegetation, with areas under vegetation having far greater infiltration rates. This pattern leads to short overland flow travel distances and hence hydrological disconnection. Under temperate conditions, subtle topographic hollows can cause overland flow to infiltrate and cause disconnection. Flow concentrations, in the form of rills and channels, promote efficient transmission and hence facilitate connection. Points further from the river channel have longer travel times, and hence, there is a greater probability of disconnection along the flow path. Therefore, the catchment has to be in the runoff-generating and -transmitting state for longer for the flow to connect. This relationship with the temporal characteristics of storms led Bracken and Croke to define two types of disconnection: Type 1, where the points along a flow path that are more prone to wetness become dry before the material delivered from upstream has passed through them, and Type 2, where the event is of insufficient magnitude and duration to wet the driest point along a flow path.