3 Drainage Basins and Runoff

Defining a drainage basin

A drainage basin is the area of land that contributes water to a particular outlet. The outlet might be a point on a river, the outlet of a lake, or a river mouth. Catchment is another common term for a basin. In the United States, watershed often means the basin itself, although in some contexts it means the boundary between basins.

A basin’s size depends on where its outlet is defined. Large basins can contain smaller, nested basins: for example, the basin of a small creek may be part of the basin of the larger river it joins.

Finding divides and drainage areas

A drainage divide marks the boundary between neighboring basins. It commonly follows high ground, such as ridges and hills, so precipitation falling on opposite sides tends to flow toward different outlets. The drainage area is the land area enclosed by the divide.

To identify a basin on a topographic map, trace the high ground around the outlet. Keep the boundary across slopes and ridges rather than following stream valleys. Groundwater does not always follow surface topography, so the area contributing groundwater to a stream can differ from its surface-water basin.

How stream networks convey water

As water moves downslope, it gathers in small channels that join to form larger channels. Small upstream streams are headwaters; a smaller stream joining a larger one is a tributary; and the place where streams meet is a confluence. Together, the channels make a stream network that conveys water toward the basin outlet.

The network also connects places across the basin. Changes in runoff or water quality upstream can affect downstream reaches. Channel arrangement and density influence how quickly water is collected and conveyed.

Drainage density measures the total channel length relative to basin area:

Here, $D_d$ is drainage density, $L$ is the total length of channels, and $A$ is basin area. A dense network can route water to the outlet efficiently, although soils, vegetation, slope, and storage also affect basin response. Lakes, wetlands, reservoirs, and depressions can temporarily hold water and delay its movement downstream.

Runoff and streamflow response

Runoff is precipitation that moves across the land surface or through shallow pathways toward streams. Some precipitation instead evaporates, is taken up by plants, remains in soil, or infiltrates more deeply to recharge groundwater. Streamflow at a given point can include both relatively rapid runoff and slower contributions from groundwater. Not all precipitation falling in a basin leaves through its outlet as streamflow.

A basin’s response describes how much and how quickly streamflow changes after precipitation. On a streamflow hydrograph, peak flow is the highest flow during the event, while lag time is the delay between the rainfall peak and the streamflow peak.

Factors shaping basin response

Several interacting characteristics determine how much runoff reaches the outlet and how quickly it arrives:

• Precipitation: Its amount, intensity, duration, type (rain or snow), and distribution across the basin affect how much water is available and when it arrives. Rain falling faster than the ground can absorb it is more likely to produce surface runoff.

• Antecedent conditions: Wet or saturated soil has less capacity to absorb additional water than dry soil. Earlier precipitation can therefore make a later storm produce more runoff.

• Soil and underlying geology: Permeable soils and fractured rock can admit more water, while less-permeable materials tend to promote surface runoff. Infiltrated water may be stored, move slowly toward streams, or recharge groundwater.

• Vegetation and land use: Vegetation can intercept rainfall, slow surface flow, and support infiltration. Roads, roofs, and parking lots are impervious and route water rapidly toward drains and streams; urban development can therefore increase and speed up storm runoff.

• Relief and slope: Steep slopes tend to move water downslope faster, while gentler terrain can allow more time for infiltration or temporary storage.

• Basin size, shape, and network: These affect how runoff from different parts of the basin travels and combines at the outlet. Shape alone does not determine the flow pattern; channel layout, terrain, and storage matter too.

• Storage and water losses: Lakes, wetlands, reservoirs, soil storage, evaporation, and plant transpiration can reduce or delay the water reaching the outlet.

Comparing responses to a storm

During the same intense storm, a paved neighborhood with saturated soil and direct storm-drain connections may send water to a creek quickly, producing a sharp rise and high peak flow. A vegetated basin with more permeable soil and wetlands may absorb or temporarily store more water, producing a slower, less abrupt response.

These are general tendencies, not outcomes that apply in every case. Actual results depend on the storm and the basin’s combined characteristics.