Understanding Rain Gauges and Rainfall Mass Curves

This question asks which type of rain gauge among the given options cannot provide data to plot a mass curve of rainfall. To answer this, we first need to understand what a mass curve of rainfall is and how different rain gauges operate.

What is a Mass Curve of Rainfall?

A mass curve of rainfall is a cumulative plot showing the total rainfall depth accumulated over time during a storm or a specific period. The y-axis represents the cumulative rainfall depth (e.g., in mm or cm), and the x-axis represents time. The slope of the mass curve at any point gives the rainfall intensity at that moment. Plotting a mass curve requires a continuous or frequent recording of the accumulated rainfall over time.

Types of Rain Gauges

Rain gauges are instruments used to measure precipitation (rainfall). They can be broadly classified into two categories:

• Non-recording rain gauges: These collect rainfall over a period (usually 24 hours), and the total depth is measured manually at the end of the period. They provide the total rainfall for a duration but not its distribution over time.
• Recording rain gauges: These continuously or periodically record the amount of rainfall collected over time, producing a chart or digital data log. This detailed record allows for the determination of rainfall intensity and the plotting of mass curves.

Analyzing the Options

Let's examine each rain gauge type listed in the options:

1. Symons rain gauge: This is a standard non-recording rain gauge. It consists of a collector, a funnel, and a receiving bottle. The rainfall collected in the bottle is measured using a graduated measuring glass, typically once every 24 hours. Since it only provides the total rainfall over a long interval, it does not give a continuous record needed for a mass curve plot.
2. Weighting-bucket type: This is a recording rain gauge. It weighs the collected rainfall using a spring or lever mechanism connected to a pen that traces a cumulative rainfall curve on a revolving drum chart, or it sends electronic data. This provides a continuous record of accumulated rainfall over time.
3. Tipping-bucket type: This is a recording rain gauge, especially suitable for digital logging. It has a small, seesaw-like bucket divided into two compartments. When one compartment fills with a specific volume of water (corresponding to a small depth, e.g., 0.2 mm), it tips, emptying its contents and bringing the other compartment under the funnel. Each tip is recorded electronically. By counting the number of tips over time, both rainfall intensity and cumulative rainfall can be determined, allowing for mass curve plotting.
4. Natural-syphon type (Float type): This is also a recording rain gauge. Rainfall enters a float chamber, causing a float to rise. The movement of the float is transferred to a pen that records the accumulated rainfall on a chart. When the water level in the chamber reaches a certain height, a syphon tube automatically empties the chamber, and the pen drops back to zero, ready to record the next accumulation. This provides a continuous record.

Comparison of Rain Gauge Types

Based on the above analysis, the Symons rain gauge, being a non-recording type, does not provide the time-varying rainfall data necessary to construct a mass curve of rainfall. The other types are recording rain gauges and can provide the required data.

Conclusion

The rain gauge that is NOT able to give a plot of a mass curve of rainfall is the Symons rain gauge.

Revision Table: Rain Gauge Features

Additional Information on Rainfall Measurement

Understanding rainfall measurement is crucial in hydrology and meteorology. Recording rain gauges are vital for detailed analysis of storm events, including:

• Rainfall Intensity: The rate at which rainfall occurs (e.g., mm per hour). This is determined from the slope of the mass curve or the frequency of tips in a tipping-bucket gauge.
• Storm Duration: The length of time a rainfall event lasts.
• Time Distribution of Rainfall: How the rainfall is spread out over the duration of the storm. This information is critical for hydrological modeling, urban drainage design, and flood forecasting.

Different rain gauge types have pros and cons. Symons gauges are simple and reliable for basic total rainfall measurement. Recording gauges, while more complex and potentially requiring maintenance or power, provide the essential time-series data needed for advanced hydrological analysis and research.