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Drainage Basins as open systems

The drainage basin is the area of land drained by a river system (a river and its tributaries). It includes the surface run-off in the water cycle, as well as the water found in the ground. Drainage basins are separated by watersheds. This is the area that separates on drainage basin from another, indeed, the water shed is the upper limit around the drainage basin.

A drainage basin is an example of an open system because it is open to inputs from outside, such as precipitation, and is responsible for outputs out of the system, such as output of water into the sea and evaporation of water into the atmosphere. It can also be seen as a cascading system, as the outputs from drainage basins become the inputs to other systems such as the coastal system.
You can see a diagram of the drainage basin below.  The North East of England has 3 major drainage basins, the Tyne, Tees and Wear, and several other smaller drainage basins.  These are tiny compared to the world’s largest drainage basins, such as the Nile, Amazon and Mississippi 1, which covers 3,225,000km2.

The drainage basin – major flows and stores


The drainage basin as an open system
The drainage basin starts with an input from the water cycle – precipitation, after the processes of condensation and cloud formation. This precipitation can be stored on the surface as snow or ice (surface storage), or can be intercepted by trees and vegetation.  On the trees or vegetation the water can be dripped off leaves, flow down the vegetation as stem flow or be taken up and lost as transpiration.  Some water is stored in plants as vegetation storage (aka biospheric water). The combined losses of water through transpiration and evaporation are known as EVAPOTRANSPIRATION.

Drainage Basin Flow Chart
It could also fall straight into the ground where it can infiltrate (into the soil then percolate (the downward movement of water within the rock under the soil surface) into the rock underneath if the soil and rock are permeable.  If the rock is not permeable or the soil stores are full then surface runoff will occur.  This water will then work its way through the soil (soil flow/throughflow) or rock (groundwater flow - the slow movement of water through underlying rocks) or over the land as overland flow (the tendency of water to flow horizontally across land surfaces when rainfall has exceeded the infiltration capacity of the soil and all surface stores are full) and into streams and rivers.  This is also known as run-off - all the water that enters a river channel and eventually flows out of the drainage basin.  These small tributary streams join at confluences and the river will grow in size and strength. 
These processes are true for hillslopes and larger scale drainage basins.

Factors affecting infiltration rate 3
The infiltration capacity is the maximum rate at which water can be absorbed by a given soil per unit area under given conditions.  There are several factors which affect how much water can be infiltrated by a soil;
1. Soil type (texture, structure, hydrodynamic characteristics) - the soil characteristics influence capillary forces (the ability of a liquid to flow in narrow spaces without the assistance of, or even in opposition to, external forces like gravity) and adsorption. This can be seen in the table below for uncompacted soils;
Infiltration rates for various soils

Source: Minnesota Pollution Control Agency 4
2. Soil coverage. Vegetation has positive influence on infiltration by increasing the time of water penetration in soil, this is because plant roots can create channels for the water to soak through.
3. The topography and morphology of slopes – it has been suggested that steeper slopes have lower infiltration rates and more surface runoff.
4. The flow supply (how intense is the rain? is there drainage?).
5. How humid are the soils already - this is an important factor of infiltration regime. The infiltration regime changes for dry or wet soils.
6. Soil compaction due to rain drop impact and other effects. The use of hard agricultural equipment can have consequences on the surface layer of soil.
In reality, infiltration rates are affected by a combination of these factors.  These could be a potential NEA study. If the rainfall intensity is greater that the infiltration rate then water will remain at the surface of the soil as surface storage and overland flow can occur.  Similarly, if soil pores are full no water can infiltrate and the soil is said to be saturated.  In both cases there is the potential for localised flooding.

Factors affecting soil storage
Rain water can also be stored in the ground as soil storage. Soils consist of particles and pores. Those pores can be filled with air but also with water. The amount of pores is a soil is different for different types of soil. The pores in a clay soil account for 40% to 60% of the volume. In fine sand this can be 20%–45%
The soil particles have small pores in them where water can enter (soil water) and between the particles are larger pores that can be filled. The soil is filled with water up a certain level. This level goes up and down with changing weather conditions.6 This water level is the ground water level also known as the water table.  This can best be studied at local scale such as a valley or hill slope, as shown below.

Drainage Basin Hill slope flows

Factors affecting interception rates
Interception is another important factor controlling flows and stores within the drainage basin.  It is basically when plants trap water or stop it from reaching the ground during a rainfall event.  The rates of interception are controlled by;
• The plant type and shape – for example coniferous trees intercept 25-35% of annual precipitation whilst       deciduous trees intercept 15-25% of annual precipitation, but just as much as coniferous trees during the growing season. Grasses have high interception capacity during the growing but then either die (annual plants) or loose mass (perennial plants); also they are grazed and harvested (spring wheat intercepts 11-19% of precipitation before harvest).
• Plant density
• Plant structure in terms of size, flexibility, strength and pattern of branches. Leaves- their texture, surface area and orientation
• Plant community structure - secondary interception occurs in stratified forest communities where water drips from the canopy and is intercepted by lower plants
• Precipitation intensity - water can be delivered too quickly for the plants to intercept
• Precipitation duration - absolute interception storage increases with increasing storm duration
• Wind speed - promotes interception loss by evaporation
• Type of precipitation: rain versus snow 5

All of this can be considered in the diagram above.  Note that most ocean water evaporated is returned back the sea as ocean precipitation.

The water balance
The Water balance is the balance between inputs and outputs within a drainage basin. When the INPUTS and OUTPUTS of the drainage basin system are balanced over longer periods of time it is said to be in dynamic equilibrium.  However, there will be periods were there is more rainfall than others leading to more erosion, or drier periods where there will be deposition occurring.  All of this can be interpreted as a systems diagram.  This can also be shown in a water budget, which is;

Precipitation (P)Runoff (Q) - Evapotranspiration (E) +or- Change in storage (ΔS)

A positive balance indicates that inputs are greater than outputs and water will be stored in the system.
A negative balance indicates that outputs are greater than inputs and stores will deplete and be used up.
This can be shown on a soil moisture budget graph, examples for Harrogate and Malaga are shown below. 

Soil moisture budgets

This graph comprises 3 basic sections.  Where potential evapotranspiration (is a measure of the ability of the atmosphere to remove water from the surface through the processes of evaporation and transpiration assuming no control on water supply2) is above precipitation soil moisture utilisation will take place, that is, soil stores will be used and not replaced.  This could continue until soil moisture depletion, where there is no water left in the soil and plants will begin to die.  Once precipitation rises above evapotranspiration soil moisture recharge will occur, and soil stores will be replaced slowly.  Once the field capacity of the soil is reached (the normal amount of water a soil can hold) then we reach a situation of soil moisture surplus.

NEXT TOPIC - Hydrographs & River Regimes

1 - Wikipedia (2018). Mississippi River. Accessed 13th October 2018 from
2 - Michael Pidwirny (2014). Introduction to the Hydrosphere. FUNDAMENTALS eBOOK. Accessed 13th October 2018
3 - Infiltration - Accessed 13th October 2018
4 - Accessed 13th October 2018
5 - University ofRegina (2018). Hydrology. Accessed 13th October 2018 
6 - Floodsite Project (2008). Water Storage in soil. Accessed 13th October 2018

Written 13/10/2018 - by Robert Gamesby



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