Water Budgets

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

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

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.

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

A 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.

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.

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.

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:

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.