The drainage basin system

A drainage basin is the area of land drained by a single river and all its tributaries, bounded by the watershed.

It is described as an open system because both matter and energy cross its boundary. The matter is water, which enters as precipitation (an input) and leaves as channel flow, evaporation and evapotranspiration (outputs). The energy is solar energy (heat) and gravitational potential energy that drives the water downslope.

Because both water and energy can enter and leave across the basin boundary, it is open rather than closed — in a closed system only energy, not matter, would cross the boundary. Over the long term, when total inputs balance total outputs, the system is in dynamic equilibrium.

Infiltration is the transfer of water from the ground surface downwards into the soil. Its maximum rate is the infiltration capacity.

Percolation is the further, deeper transfer of water from the soil down into the underlying rock (bedrock), eventually reaching the groundwater store below the water table.

The key difference is the destination: infiltration moves water from the surface INTO the soil, whereas percolation moves it from the soil PAST it into the rock below.

One factor affecting the infiltration rate is soil type: a sandy soil has large pore spaces and a high infiltration capacity, so water soaks in quickly, whereas a clay soil has tiny pore spaces and a low infiltration capacity, so water soaks in slowly.

A short lag time and a high peak occur when rainwater reaches the channel quickly and in large amounts, and each of the three characteristics speeds water to the river.

Steep slopes mean water flows downhill rapidly under gravity. Overland flow and throughflow move quickly across and through the slope, so storm water arrives at the channel sooner, shortening the lag time.

Impermeable rock, such as granite or clay, does not allow water to percolate into the ground. With infiltration and percolation reduced, most rainfall cannot be stored and instead travels as overland flow — the fastest route to the channel. This raises the peak discharge and shortens the lag because little water is delayed in the soil or groundwater stores.

Urban land use reinforces this strongly. Tarmac and concrete are impermeable, reducing infiltration further, while drains, gutters and storm sewers deliberately route water quickly and directly into the river. This concentrates a large volume of runoff into a short time, producing a high, sharp peak and a very short lag time.

Together the three factors reinforce one another: the impermeable surface forces water to run off, the steep slope speeds that runoff, and the urban drainage delivers it rapidly to the channel, giving the flashy hydrograph described.

The annual (regime) hydrograph reflects the seasonal climate of the basin, so the variations follow the timing of the rains.

The very low discharge from January to May corresponds to the dry season. With little or no precipitation as input, the river is fed mainly by baseflow — the slow release of water from the groundwater store — which keeps it flowing at a low level. High temperatures in these months also increase evaporation and evapotranspiration (outputs), reducing discharge further.

The sharp rise to a peak in August is caused by the heavy monsoon rainfall. The large precipitation input rapidly fills the basin's stores; once the soil is saturated, saturation-excess overland flow delivers water quickly to the channel, raising discharge to its annual peak. High antecedent moisture from earlier monsoon rain makes each later storm produce more runoff, amplifying the peak.

The decline from September to December occurs as the monsoon ends and precipitation falls. Discharge recedes as stored storm water drains away and the river returns to being supported mainly by the slowly falling baseflow.

The shape of a storm hydrograph — its lag time, the steepness of the rising limb and the height of the peak — depends on how quickly and in what volume rainwater reaches the channel. This is controlled both by the physical characteristics of the basin (rock, soil, slope, drainage density, shape, vegetation and land use) and by the climate (the type, intensity, duration and timing of precipitation, temperature and antecedent moisture). This essay assesses their relative importance and argues that physical characteristics set the basin's underlying tendency to be flashy or subdued, but that climate, especially rainfall intensity and antecedent moisture, determines the response to any individual storm — so neither acts alone.

The physical characteristics are powerful controls. Impermeable rock such as granite or clay prevents percolation, forcing rain to run off as overland flow, the fastest route to the channel, which shortens the lag and raises the peak; permeable, porous rock such as chalk stores water as groundwater and releases it slowly as baseflow, producing a subdued hydrograph with a long lag. Soil texture works the same way: a clay soil has a low infiltration capacity and sheds water, whereas a deep sandy soil absorbs it. Steep slopes speed runoff, a high drainage density delivers water quickly to many channels, and a circular basin concentrates flow from all points at the outlet at the same time, producing a sharp peak. Land use is decisive here too: urbanisation replaces permeable soil with impermeable tarmac and adds drains that route water rapidly to the river, while forest intercepts and transpires water, reducing and delaying the peak. These characteristics are relatively permanent, so they give a basin a consistent character — the River Wye on impermeable uplands is naturally flashier than a chalk stream on the South Downs.

A worked example shows their force: when parts of a basin are urbanised, the storm hydrograph for the same rainfall develops a shorter lag and higher peak than before, because the physical surface has been changed. This strongly supports the view that physical characteristics determine hydrograph shape.

However, the same basin produces very different hydrographs from one storm to the next, which only climate can explain. A short, intense convective downpour exceeds the infiltration capacity and generates Hortonian overland flow, producing a flashy peak; the same total rainfall spread gently over two days largely infiltrates and produces a subdued response. Antecedent moisture is especially important: if the basin is already saturated from earlier rain, its stores cannot absorb more, so saturation-excess overland flow begins almost at once, giving a short lag and high peak — whereas the identical storm on a dry basin would be partly soaked up. Snowmelt adds a further climatic control, releasing a large input when temperatures rise in spring. Because the physical characteristics are fixed, it must be these climatic variables that explain why one storm floods and another does not.

The most accurate assessment is that the two operate at different scales and interact. The physical characteristics set the basin's baseline sensitivity — they decide how flashy it CAN be — while climate provides the trigger and determines the response to each specific event. Crucially, they reinforce one another: intense rain on an already-saturated, impermeable, urbanised, steep basin produces the most extreme peak, because every control is pushing the same way; the same rain on a permeable, forested, dry basin is buffered.

In conclusion, physical characteristics do not, on their own, determine the shape of a storm hydrograph. They establish the basin's underlying tendency, but it is climate — above all rainfall intensity and antecedent moisture — that determines the response to any particular storm. The shape of a real hydrograph is therefore the product of the two interacting, and assigning primacy to either in isolation oversimplifies a system in which the controls combine.

A drainage basin is an open system in which inputs of precipitation are converted into the output of channel flow through a sequence of stores (interception, surface, soil moisture and groundwater) and transfers (infiltration, throughflow, percolation, overland flow and baseflow). River discharge — the volume of water passing a point per second, in cumecs — depends on how much water is held back in stores and how quickly the transfers deliver it to the channel. This essay weighs the importance of stores against transfers and argues that, while stores set how much water is delayed and buffered, it is the transfers that actually deliver water to the river and therefore most directly control the timing and peak of discharge — so the statement is only partly correct.

Stores are clearly important in controlling discharge because they hold water back and release it slowly. The interception store on vegetation catches rain before it reaches the ground, some of which evaporates and never reaches the channel; the soil-moisture store absorbs infiltrating water; and the groundwater store below the water table holds the largest, slowest-moving volume. A basin with large, effective stores — a forested basin on porous, permeable chalk — buffers a storm: water enters the groundwater store and is released gradually, giving a long lag, a low peak and a reliable baseflow that keeps the river flowing through droughts. In this sense the size and capacity of the stores determine how subdued the discharge response is, which strongly supports the statement.

The transfers, however, are arguably more decisive because they determine the SPEED at which water reaches the channel, and therefore the lag time and the height of the peak — the features that matter most for discharge and flooding. Overland flow is the fastest transfer: when impermeable surfaces, intense rain or saturation prevent infiltration, water runs straight to the channel, producing a short lag and a high peak. Throughflow and baseflow are far slower, delivering water gradually. The very distinction between a flashy and a subdued hydrograph is, in practice, a distinction between which TRANSFER dominates: rapid overland flow versus slow baseflow. The two types of overland flow show this clearly — Hortonian flow when rainfall intensity exceeds infiltration capacity, and saturation-excess flow when the soil is already full — both bypassing the stores and spiking discharge.

In truth, stores and transfers cannot be separated, because they govern each other. A store only buffers discharge if a transfer (infiltration, percolation) can move water into it fast enough; once a store is full — high antecedent moisture saturating the soil — infiltration stops and the dominant transfer switches to rapid overland flow, sending discharge sharply up. The 2007 summer floods in parts of England illustrate this: prolonged rain saturated the soil-moisture store, so further rain could not infiltrate and ran off as overland flow, producing extreme discharge. Here it was the FILLING of the store that switched the controlling transfer — neither factor alone explains the flood.

In conclusion, I agree only partly with the statement. Stores are important because their capacity sets how far a basin can buffer and delay discharge, and they sustain baseflow between storms. But it is the transfers — above all whether water moves as slow baseflow or fast overland flow — that most directly determine the timing and peak of discharge. Most accurately, stores and transfers are interdependent: the state of the stores decides which transfer dominates, and that transfer then controls the discharge. The stores are therefore not 'more important' than the transfers; the two function as one system.