The Surface Water Budget

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Water is an important determinant of the local climate. We know Nevada can have temperatures similar to Florida's, but the characteristics of the plants that grow in both regions are wildly different! Why do we have rainforests in South America but a relatively arid climate in Australia?

Previously, we delved into the concept of budgets when examining the energy dynamics of our planet. Budgets serve as valuable tools because they help us comprehend that certain variables, like energy, cannot be created or destroyed within a closed system. Instead, they shed light on how these elements are redistributed within the system—in our case, within the intricate framework of our climate system.

Water Budget: Key Components

To gain insights into how local climates are sustained, we can employ a local surface water budget for water. Imagine delineating a square area on a patch of ground in a park. We can assess the water budget of that square using four key components. Each component of the water budget represents a different way water can enter or leave our defined patch.

• Precipitation (P) – This includes any form of water (liquid or solid) that descends from the atmosphere onto our defined patch.
• Dewfall (or frost) (D) – This encompasses water vapor that condenses or is deposited directly onto our patch.
• Evaporation + transpiration (evapotranspiration) (E) – This refers to water leaving our patch, either through evaporation into the atmosphere or through release by plants.
• Horizontal “movement,” a.k.a. runoff ($\mathrm{\Delta }\mathrm{F}$) – This represents the lateral flow of water across the surface, exiting our square patch. A positive value indicates water moving away from our patch.

We can construct our water budget by simply adding these terms together:

Water Budget = $P+D-E-\mathrm{\Delta }F$

Water Budget Equation: Breakdown

Let's break down this equation. To increase the amount of water within our patch, we need to introduce more water into it. The primary sources in this equation for adding water are precipitation (P) and dewfall (D), which bring water from the atmosphere. On the contrary, we must account for water leaving our patch, represented by the negative terms. This includes both water returning to the atmosphere through evaporation and transpiration (E), as well as horizontal movement ($\mathrm{\Delta }\mathrm{F}$). Remember that when $\mathrm{\Delta }\mathrm{F}$ is positive, it indicates water is flowing away from our area, so subtracting a positive value results in less water remaining in our square.

Now that we understand the equation, let’s explore one crucial element: runoff, the horizontal movement of water across the surface. We’ve all seen runoff before—it’s the flow of water across surfaces, like rain streaming down a street during a heavy storm. Runoff occurs when the ground becomes saturated, frozen, or unable to absorb additional water (not unlike an asphalt road). Positive runoff typically signifies water flowing away from a particular location. For instance, if you stand at the top of a hill during a heavy rain, you are measuring positive runoff. Conversely, if you find yourself in a valley and water accumulates around you, you are experiencing negative runoff.

Water Budget's 5th Term: Storage

Now, it's important to recognize that the cumulative effect of all four of these terms doesn't instantaneously reach zero. In other words, at any given moment, the positive terms don't necessarily have to offset the negative terms precisely. For instance, we've all observed situations where there is more evaporation than precipitation during a hot, humid, sunny afternoon or significantly more runoff than dewfall during a heavy rainfall event. To account for these variations, we introduce a fifth and final term known as the “storage” term:

$$gw=P+D-E-\mathrm{\Delta }F$$

Think of the storage term as a mechanism for managing any surplus water from precipitation or dewfall and facilitating the release of water for processes like evaporation, transpiration, or runoff. If more precipitation occurs, it accumulates on our patch, much like your Venmo balance increases when you receive more money than you spend. The surface primarily stores water through three key mechanisms: soil moisture (reflecting the ground's level of saturation), groundwater storage (representing the water held beneath the surface we stand on), and snowpack (referring to water that remains locked into the surface as a solid and cannot flow like liquid water).

Schematic showing the five terms in the surface water budget with arrows representing precipitation, evaporation, groundwater, and runoff.

Credit: Colin Zarzycki © Penn State University is licensed under CC BY-NC-SA 4.0

Seasonal Cycle of Water Storage: Snowpack

I want to focus a bit more on this storage term. While it may seem quasi-insignificant, it can be significant for our lives. Water storage on both seasonal and shorter time scales is a critical component of freshwater availability. It acts as a buffer for local climates, particularly in regions that receive large amounts of snow in the winter, such as the northeastern United States or the Mountain West.

During periods of heavy snowfall or rainfall, not all the water runs off immediately. Instead, it gets absorbed into the ground, snowpack, or underground aquifers. This stored water is gradually released during drier months, helping stabilize local climate conditions by ensuring a continuous water supply. This stability is essential for ecosystems, agriculture, and human consumption, especially when precipitation is irregular.

For example, snowpack is a vital water source in many areas, supplementing rivers and streams during dry seasons. It acts as a natural reservoir, accumulating water (in the g_w term) during the winter months (via the P term) and gradually releasing it as snowmelt during the spring and summer (via the $\mathrm{\Delta }\mathrm{F}$ term).

In other words, as snow accumulates in the winter, it stores water that would otherwise be unavailable for immediate use. When temperatures rise in the spring and summer, this snowpack melts, providing a steady and reliable source of freshwater downstream. This slow release of water is invaluable for agricultural activities, ensuring a consistent water supply for crop irrigation exactly when it's needed most.

Agriculture heavily relies on this seasonal cycle of water storage provided by snowpack. In regions with pronounced snowmelt-driven water sources, such as the western United States, much of Europe, and parts of Asia, the timing and volume of snowmelt directly impact crop growth and yields. Farmers can efficiently manage their water resources, optimizing the use of available freshwater during the growing season.

This seasonally stored water also replenishes groundwater reserves, ensuring that agricultural activities have access to a sustainable and consistent supply of freshwater year after year. Thus, the seasonal storage and release of water from snowpack contribute significantly to the resilience and productivity of agricultural systems worldwide.

As we’ll discuss in this class, changes in this aspect of the water cycle can be very problematic for areas that rely on this annual water cycle!