Concept of drainage basins and catchments

Vegetation is an important natural control on the hydrological response of a drainage basin. Leaves and branches intercept part of the rainfall, while roots, leaf litter, and soil structure increase infiltration and temporary storage. Consequently, less water reaches the channel immediately as surface runoff, and the flood peak may be lower and delayed. Some water is also returned to the atmosphere through evapotranspiration. Therefore, option A is correct. Option B is wrong because vegetation does not always cause flooding; it generally moderates rapid runoff. Options C and D describe processes that do not occur in a drainage basin.

A watershed divide is a relatively high ridge, hill, or elevated surface that separates neighbouring drainage basins. Rain falling on one side generally follows the slope into one stream network, while rain on the other side flows toward a different network. In this way, the divide controls the boundary and direction of surface runoff and helps define the area draining to a particular river outlet. It does not stop rainfall, alter water vapour into salt, or build mountains from oceans. Therefore option A is correct. The divide is mainly a topographic boundary; it may also influence subsurface flow, although underground water does not always follow the exact surface boundary.

A small, steep drainage basin usually has a short travel distance and strong gravitational assistance. After heavy or intense rainfall, water can move rapidly as surface runoff down steep slopes and reach the channel in a short time. The result may be a sharp rise in discharge, a short lag time, and a relatively high flood peak, especially where soils are thin, vegetation is sparse, or the ground is already saturated. Thus option A is correct. Option B ignores the effect of slope and rapid concentration. Option C is wrong because not all water becomes groundwater, and option D is wrong because rainfall does not instantly evaporate. Actual response also depends on rainfall intensity, soil permeability, land use, and basin shape.

Flood response depends not only on how much rain falls but also on how quickly water reaches the channel and how much is temporarily stored. Soil with high infiltration can absorb more rainfall, whereas compact or saturated soil produces greater surface runoff. Steep slopes usually speed up runoff, while gentle slopes, wetlands, lakes, floodplains, and permeable ground can delay and store water. Vegetation intercepts rain, increases infiltration, and slows overland flow. Therefore option A correctly identifies the combined basin controls. The other options are unsuitable: rainfall is not necessarily identical in timing or intensity, rivers do not all have the same length, and ocean colour has no meaningful role in catchment flood response. Basin characteristics help explain different hydrographs from similar storms.