What specific question can you answer about a reservoir's water supply using a 'mass curve analysis'?

The specific question a mass curve analysis can answer about a reservoir's water supply is: "What is the minimum reservoir storage capacity required to reliably meet a specified water demand throughout a historical period of streamflow, particularly identifying the storage necessary to sustain supply during critical dry periods or droughts?"

To understand this, a 'mass curve analysis' is a graphical method used in hydrology and water resource engineering. It involves plotting the cumulative inflow of water into a proposed or existing reservoir over a period of time against the cumulative demand for water over the same period.

The 'mass curve' itself is a continuous line graph where the x-axis represents time, and the y-axis represents the 'cumulative volume of water' that has flowed into the reservoir up to that point. 'Cumulative inflow' means that at any given time, the volume shown on the y-axis is the total sum of all water that has entered the reservoir from the beginning of the recorded period up to that specific moment. This curve generally rises, but its slope varies, directly reflecting changes in the actual rate of inflow; a steeper slope indicates higher inflow rates, while a flatter slope indicates lower inflow rates.

A 'demand line' is then superimposed on the same graph. This line represents the 'cumulative volume of water demanded' from the reservoir over time. If the 'demand' (the amount of water needed for uses like municipal supply, irrigation, or industry) is constant per unit of time, this line will be straight with a constant upward slope. If the demand varies, the slope of the demand line will change accordingly.

To determine the required 'reservoir capacity' (the volume of water the reservoir must be able to hold), one draws lines parallel to the cumulative demand line, positioned as 'tangents' to the peaks (highest points) of the mass curve. A 'tangent' line touches the mass curve at a single point without crossing it locally. The largest vertical distance measured between such a tangent line originating from a peak on the mass curve and any subsequent low point (trough) on the mass curve below that tangent directly indicates the largest 'deficit' that would occur if the reservoir started full at the tangent point and attempted to meet the specified demand. This largest vertical distance represents the 'minimum active storage capacity' required in the reservoir to ensure continuous supply without failure during that historical period.

The 'critical period' for reservoir design is identified as the time span between the peak of the mass curve (where a tangent demand line originates) and the subsequent lowest trough on the mass curve below that tangent. During this period, the natural inflow is less than the demand, meaning the reservoir must draw upon its stored water to meet the supply requirements. The calculated storage ensures that the reservoir does not run dry during this specific critical deficit period. For instance, if the mass curve shows a prolonged period of very low natural inflow, the analysis reveals precisely how much water needs to be stored *beforethat dry period begins to bridge the gap until inflows recover, thus preventing a water supply shortage. This analysis provides a factual, data-driven determination of the specific volume of water storage needed based on historical hydrological conditions and a defined water demand.