A.1 Drainage basin hydrology and geomorphology

In the drainage basin open system, every component is classified as an input (water entering), flow (water in motion between locations), store (water held temporarily), or output (water leaving the system entirely).

Throughflow is the lateral movement of water sideways through the soil, downslope, after infiltration has occurred. It moves water between the soil store and the river channel — that is, it connects locations, which is exactly what a flow does.

Because throughflow moves water from one location to another (through soil towards the channel) rather than holding it in place, it is unambiguously a flow, not a store, input, or output.

The correct answer is that throughflow is a flow, specifically the lateral movement of water through the soil towards a river channel.

The question asks which option correctly classifies throughflow within the drainage basin open system.

Calling throughflow 'a store' is incorrect. Stores hold water in place (e.g. soil moisture, aquifers). Throughflow is water actively moving — it cannot be stored and moving simultaneously in this classification.

Inputs bring water into the basin from outside (e.g. precipitation); outputs remove water from the basin entirely (e.g. evaporation, discharge to the sea). Throughflow stays within the basin and moves water between internal locations, so it is neither.

By elimination, throughflow must be a flow — the lateral subsurface movement of water through soil towards the river channel — which matches the open system definition of a flow exactly.

Both events deliver the same total volume of rainfall (80 mm), so total volume is not the differentiating factor. The critical difference is intensity: the rate at which rain falls per unit time.

Infiltration capacity is the maximum rate at which soil can absorb water. A gentle storm delivering rain slowly allows most water to infiltrate; an intense storm delivers rain faster than the soil can absorb it, so the surplus becomes overland flow.

Overland flow reaches the river channel far faster than throughflow or base flow. More rapid, direct delivery of water to the channel produces a higher and earlier discharge peak — a sharper flood hydrograph.

The correct explanation is that high rainfall intensity exceeds infiltration capacity, forcing excess water to travel as overland flow, which reaches the channel quickly and raises peak discharge.

The question asks why an intense storm produces a larger flood peak than a gentle storm of equal total rainfall.

The scenario explicitly states both events deliver 80 mm. Total volume is identical, so this option cannot explain a difference in flood peak and is eliminated.

While prolonged rain can eventually saturate soil, the question asks specifically why the intense storm produces a larger peak. If saturation from the gentle event were the key factor, it would tend to increase the gentle storm's flood peak — not explain why the intense one is larger.

Evaporation is an output that removes water from the basin. More evaporation would reduce, not increase, the flood peak. This option is scientifically incorrect in this context.

The remaining option — intensity exceeding infiltration capacity and producing overland flow — is correct and directly explains the larger flood peak.