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Water Budgets

Water Budgets

The water budget measures the amount between the inputs, outputs and stores of water in a system. There is a specific water budget equation to calculate the water budget of a drainage basin. The results can be presented in a water budget diagram. As well as drainage basins having their own water budget, there is a global water budget that monitors the flow of water between the Earth’s subsystems.

Water Budget Definition

The water budget aims to work out how much water there is in a system, such as a drainage basin. It does this by looking at the balance between the inputs and outputs of the system, usually over the course of a year. Essentially, this means that it looks at how much is going into a system compared with how much is leaving the same system.

In the case of drainage basins, the major inputs (ways that water enters the system) are precipitation and the water introduced into the systems because of human activity (e.g. via farming and waste), and the major outputs (ways the water leaves the system) are evapotranspiration, runoff and the flow of water via river channels (river discharge).

The hydrological cycle is what drives changes in the water budget.

If certain hydrological processes are altered or disrupted, the water balance will change. Where more water is entering the system than leaving, there is a water surplus. By contrast, where more water is leaving the system than entering, there is a water deficit. There can be problems with having a particularly strong water surplus or deficit. This can include flooding and/or drought.

Water Budget Equation

To calculate the water budget/water balance, there is an equation:

Precipitation (P) = discharge (Q) + evapotranspiration (E)

± changes in storage (S)

Let’s break this down further:

The equation has inputs on one side (left) and outputs on the other (right). Therefore, if precipitation is greater than the discharge, evapotranspiration and changes in storage, then there is a water surplus. Similarly, if the outputs exceed the inputs then there is a water deficit. The reference to water storage is important because it ensures that the water budget accounts for all the water within the system. For example, soil moisture counts as a mechanism of water storage. If the equation did not account for this, then it wouldn't accurately represent the amount of water in the system.

Water Budget Graph

One of the ways that the water budget is displayed is through a water budget graph:

Water budget, water budget graph, StudySmarterA graph showing the water budget, Hannah Leighton-Jones, StudySmarter Originals

This example illustrates how the relationship between evapotranspiration and precipitation affects the water budget. Where there is a water surplus, water stores can be recharged. Alternatively, where there is a water deficit, water from the stores is depleted. When thinking back to the water budget equation, it is clear that it is a very dynamic phenomenon.

What Affects The Water Budget?

We have briefly touched on the fact that the hydrological cycle is a huge contributor to changes in the water balance. However, there are also other, more indirect, factors at play. Let's explore the main influencers of the water budget further:

Naturally, if the inputs and outputs of water (through precipitation, evaporation, etc.) change, so does the water budget. This can happen for a number of reasons, including global warming/cooling and extreme weather events. However, there are also factors that affect the water budget more indirectly. For example, if the ground is more porous then it can absorb and store more water. This means that more water is kept in the system for longer. As such, the land surface can significantly alter the water balance. Humans often construct non-permeable land surfaces, which will increase surface runoff and allow water to pass to and through the river channels more quickly. In the same way, if groundwater stores are bigger, then less water leaves the system, as more is being conserved. Another key control of water within the system is the presence or absence of vegetation. Vegetation plays a role in the hydrological cycle by intercepting precipitation and absorbing water, for instance.

Human activity can also affect the outputs of the system. The demand for water is increasing as populations increase. This means that groundwater stores are often relied on to meet demands. This highlights how the water budget equation can potentially miss out on crucial ways that the water budget is influenced.

Examples Of The Water Budget

Now that we understand what the water budget is, how it is calculated and how it is presented, let’s look at some examples:

Drainage basin water budgets

The water budget of a drainage basin can tell us a lot about the system. It is also something that can severely impact the population and so it is important to calculate. Let’s think about the UK. Typically, higher temperatures in the summer coincide with higher levels of evapotranspiration. Similarly, lower temperatures in winter are associated with lower evapotranspiration. Therefore, the water budget diagram for a drainage basin in the UK usually resembles the earlier picture. Namely, during the summer there is often a water deficit and during the winter there is often a water surplus. This means that water stores have the chance to recharge in the winter months after being depleted. Although there is a typical pattern for UK water budgets, there can be events that disrupt this. For example, extreme weather events (e.g. storms or heatwaves) can alter the river discharge for a short time. As extreme weather events are projected to increase in frequency due to climate change, their influence over the water budget will also likely increase.

Global water budget

It isn’t just individual drainage basins that have a water budget. The global water budget applies the principles of the water budget equation to all of the water involved in the water cycle worldwide. Instead of moving through one system, the global water budget examines how water moves within the different subsystems of the Earth (atmosphere, biosphere, cryosphere, hydrosphere, and lithosphere). This is because the water cycle as a whole is a closed system, and so there are no inputs or outputs. Flows between the subsystems of the Earth happen in many different ways, including:

  • Evapotranspiration and condensation.
  • Cloud formation and precipitation.
  • Convection.
  • Glacial processes (e.g. accumulation and ablation).

A lot of these processes are very susceptible to the influence of climate change. Therefore, the global water budget is set to undergo considerable change as we continue contributing to climate change. For example, as global warming continues to warm the Earth and melt the ice, the cryosphere will lose significant amounts of water. Therefore, the water budget will shift.

Water Budgets - Key Takeaways

  • The water budget examines the difference between the inputs, outputs and changes in the storage of water within a system.
  • It can be calculated using the water budget equation and presented in a water budget diagram.
  • The water budget of a drainage basin is altered by changes in precipitation, evapotranspiration, discharge and water stores.
  • The global water budget is affected by changes in the flows of water between the Earth’s subsystems (atmosphere; biosphere; cryosphere; hydrosphere and lithosphere).

Frequently Asked Questions about Water Budgets

The global water budget is the total amount of inputs, outputs and stores of water in the global system.

The water budget is calculated by using the following equation: precipitation = evapotranspiration + discharge +/- changes in storage.

A water budget diagram is a way of presenting the differences between the inputs and outputs, and water stores in a system.

The water budget sources of moisture are precipitation and water introduced into the system by humans.

The balance between precipitation and evapotranspiration is a key component of determining the water budget. If precipitation exceeds evapotranspiration, there is a water surplus, whereas if evapotranspiration is higher than precipitation then there is a water deficit. 

Final Water Budgets Quiz

Question

What is the global water budget?

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Answer

The balance of inputs, outputs and stores of water in the global system.

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Question

What are the main inputs to the water budget?

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Answer

Precipitation and water introduced by humans

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Question

What are the main outputs in the water budget?

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Answer

Evapotranspiration, river discharge and water abstraction

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Question

What is it called when more water enters than leaves a system?

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Answer

Water surplus

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Question

What is it called when more water leaves a system than enters it?

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Answer

Water deficit

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Question

What is the equation to calculate the water budget?

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Answer

precipitation = evapotranspiration + discharge +/- changes in storage.

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Question

In the UK, in what months is there more likely to be a water deficit?

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Answer

Summer

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Question

In the UK, in what months is there more likely to be a water surplus?

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Answer

Winter

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Question

Why is there more likely to be a water deficit in the UK summer?

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Answer

Because higher temperatures results in more evapotranspiration (and so more water leaving the system than entering it).

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Question

Why is there more likely to be a water surplus in the UK winter?

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Answer

Because there tends to be more precipitation than evapotranspiration, so more water enters the system than leaves it.

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Question

What are the different subsystems of Earth?

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Answer

Atmosphere; biosphere; cryosphere; hydrosphere; and lithosphere

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Question

How does water flow between the different subsystems of Earth? (Name 6 examples)

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Answer

Evapotranspiration; condensation; cloud formation; precipitation; convection; glacial processes

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Question

Why is it important that water storage is included in the water budget equation?

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Answer

To make sure the equation accounts for all the water in the system.

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Question

Why do we bother to calculate the water budget?

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Answer

To work out how much water is in a system

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