How do you calculate change in water storage?

The change in water storage is fundamentally calculated by applying the principle of conservation of mass, which states that the total change in water volume within a defined system over a specific period is the difference between the total water entering and the total water leaving that system.

According to hydrological principles, the calculation can be precisely expressed as:

(Inflow volume in time increment dt) - (Outflow volume in time dt) = (change in volume of water stored)

This equation, often referred to as the water balance equation, is a core concept in hydrology and water resource management. It directly accounts for all water movements into and out of a specific area, such as a lake, reservoir, river reach, or even an entire basin.

Understanding the Components

To accurately calculate the change in water storage, it's crucial to understand the individual components of the water balance equation:

• Inflow Volume: This represents all the water entering the system during a specific time period (dt).
  • Sources of Inflow:
    • Precipitation: Rainfall, snowfall, or hail directly falling onto the water body or land surface that drains into it.
    • Surface Runoff: Water flowing over the land surface from surrounding areas into the system (e.g., streams, rivers).
    • Groundwater Inflow: Water seeping into the system from underground aquifers.
    • Direct Human Additions: Water diverted from other sources into the system (e.g., canal inflows, treated wastewater discharge).
• Outflow Volume: This represents all the water leaving the system during the same time period (dt).
  • Sources of Outflow:
    • Evaporation: Water turning into vapor and rising into the atmosphere from the surface of the water body.
    • Transpiration: Water released into the atmosphere by plants. (Often combined with evaporation as evapotranspiration).
    • Surface Outflow: Water flowing out of the system through rivers, streams, or engineered outlets.
    • Groundwater Outflow: Water seeping out of the system into underground aquifers.
    • Direct Human Withdrawals: Water removed from the system for human use (e.g., irrigation, municipal supply, industrial use).
• Change in Volume of Water Stored (ΔS): This is the resulting difference, indicating whether the total amount of water within the system has increased, decreased, or remained constant. A positive value means storage increased, a negative value means it decreased.

The Rate of Change

The reference also highlights the concept of dS/dt, which represents the rate of change in reach storage with respect to time. While the primary equation calculates the total change in volume over a discrete time increment (dt), dS/dt signifies the instantaneous rate at which the storage is changing. Think of it as the speed at which the water level is rising or falling at any given moment.

Practical Application and Examples

This calculation is vital for various water management activities:

• Reservoir Management: Water managers use this equation to predict how much water will be available in a reservoir for future use (e.g., drinking water, irrigation, hydropower) by monitoring inflows, outflows, and adjusting releases.
• Flood Forecasting: By understanding the rate of water storage change in river basins, hydrologists can predict potential flooding downstream.
• Aquifer Management: Assessing changes in groundwater storage helps determine the sustainability of water withdrawals from underground sources.
• Environmental Monitoring: Tracking water storage changes in wetlands or lakes can indicate the health of an ecosystem.

Here's a simplified example of how components contribute to the water balance:

By meticulously measuring or estimating these components, water professionals can maintain a balanced water budget, ensuring sustainable water resources for various needs. This hydrological tool is fundamental to understanding and managing the Earth's freshwater systems.