Soil porosity affects the movement and retention of water through the soil profile. As a result, water (and dissolved nutrients) can move faster through coarse-textured soils than fine-textured soils, conversely fine textured soils will tend to generate more surface runoff. The porosity is larger in coarse-textured soils (sands) than in fine-textured soils (clays). The rate of this movement is expressed as its hydraulic conductivity. The ease with which water can move through the soil profile depends on the porosity of the soils or in other words the amount of “free spaces” (voids) through which water can flow. Soil texture affects the movement and retention of water. Topography varies over several orders of magnitude, from centimetres (roughness) to hundreds of kilometres (mountain ranges). Topographic features such as slope and curvature can serve as indirect measures of many hydrological processes. The topography (shape of an area) has a direct impact on the hydrology. Permeable rocks, such as basalts and unconsolidated sediments such sands and loams, can contain one or more aquifers, where groundwater is stored and transmitted through intergranular pore space, fractures, vesicles and/or weathered zone of the rock. Some rocks (such as some fine-grained sedimentary rock, metamorphic rock and igneous rock) and some unconsolidated sediments (such as clay deposits) have low/limited permeability and water falling on these surfaces can generate surface water run-off. The underlying geology of catchments in a region strongly influences the hydrology. Physical characteristics Geology and topography

Solar radiation, temperature, wind and vegetation cover influence these processes. The processes of evaporation and transpiration (collectively ‘evapotranspiration’) cause the loss of water from the land to the atmosphere at a more local scale.

Rainfall, and other forms of precipitation, provide the source of surface water flows such as waterways and other, non-channelised overland flow, as well as groundwater. The climate at the regional and subregional scales determines the overall water in the system.

Hydrological processes runoff