Reservoir Live Storage Explained

Live storage in a reservoir is the volume of water that can be withdrawn and utilized, excluding the dead storage which is unusable. Calculating the live storage requirement is fundamental for designing reservoirs that can consistently meet water demands (like irrigation, domestic supply, or industrial use) despite variations in natural water availability (inflow).

Mass Curve Analysis for Storage Determination

The primary method used to determine the live storage requirement for a reservoir is mass curve analysis. This technique involves plotting the cumulative inflow into the reservoir against time. It's a graphical method that helps visualize the balance between water supply and demand over extended periods.

The steps typically involved are:

• Plotting Cumulative Inflow: Construct a curve where the Y-axis represents the total accumulated inflow (e.g., river discharge) over time (X-axis).
• Plotting Cumulative Demand: On the same graph, plot the cumulative water demand over the same time period. If the demand is constant, this will be a straight line; if it varies, it will be a curve.
• Identifying Maximum Deficit: The critical step is to find the maximum vertical distance between the cumulative inflow curve and the cumulative demand line. This gap represents the maximum shortfall (deficit) between the water available and the water needed during a specific period.

Why Mass Curve Analysis is Crucial for Live Storage

The maximum deficit identified through mass curve analysis directly corresponds to the minimum reservoir capacity needed to ensure that demand can always be met. This required capacity is the live storage requirement. This method accounts for the sequential nature of inflows and demands, making it superior to methods that only consider total amounts.

Evaluating Alternatives for Reservoir Sizing

Let's consider why the other options are less suitable for determining the requirement:

• Topographical Survey: A topographical survey maps the physical terrain. It's essential for calculating the volume of water a reservoir *can* hold at different levels once a site is selected, but it doesn't indicate how much storage is *needed* based on inflow and demand patterns.
• Annual Demand: While the total annual demand is a crucial input for the analysis (used to draw the demand line on the mass curve), simply knowing the total doesn't suffice. The timing and distribution of demand throughout the year, relative to the timing of inflow, are critical factors that only the mass curve can effectively illustrate.
• Double Mass Curve Analysis: This technique is primarily used to check the consistency of hydrological data (like rainfall or streamflow) over time or to compare the consistency between two different stations. It is not the standard method for determining the required storage capacity of a reservoir.

In summary, mass curve analysis is the appropriate technique for calculating the necessary live storage requirement because it directly addresses the temporal variations between water supply and demand.