For a large catchment, contours joining points of equal monthly rainfall values were plotted. The plot is known as:
Isohyetal map
Understanding Rainfall Maps: Isohyetal Maps Explained
The question asks about a specific type of map used for large catchments, where lines connect points that have recorded the same amount of monthly rainfall. These connecting lines are essentially contours representing equal rainfall values.
What are Isohyets?
In meteorology and hydrology, lines that connect points of equal rainfall amount are called isohyets. The word "isohyet" comes from the Greek words "isos" meaning equal, and "hyetos" meaning rain.
Identifying the Correct Map Type
A map that displays these isohyets is known as an isohyetal map. These maps are particularly useful for visualizing the spatial distribution of rainfall over an area, especially for large areas like river catchments.
Let's look at why the other options are not correct:
- Isobar map: An isobar map shows lines (isobars) connecting points of equal atmospheric pressure, not rainfall.
- Thiessen polygon: The Thiessen polygon method is a technique used to calculate the average rainfall over an area using data from scattered rain gauges. It divides the area into polygons, but the map itself is not called a Thiessen polygon map in the context of showing rainfall contours.
- Mass curve: A mass curve, in the context of rainfall, is a cumulative plot of rainfall depth against time for a specific storm or period. It shows the total rainfall accumulated up to a certain time, not spatial contours of equal rainfall.
Therefore, a map with contours joining points of equal monthly rainfall values is specifically called an isohyetal map.
Using Isohyetal Maps for Rainfall Analysis
Isohyetal maps are commonly used to estimate the average rainfall over a catchment area. By calculating the area between consecutive isohyets and multiplying by the average rainfall value between those isohyets, hydrologists can determine the total rainfall volume or average depth over the entire catchment.
| Map Type | Lines Represent | Quantity Measured |
|---|---|---|
| Isohyetal map | Isohyets | Equal Rainfall |
| Isobar map | Isobars | Equal Atmospheric Pressure |
| Isotherm map | Isotherms | Equal Temperature |
Conclusion on Rainfall Contours
The plot known for having contours joining points of equal monthly rainfall values for a large catchment is the isohyetal map.
Revision Table: Key Hydrology Terms
| Term | Definition |
|---|---|
| Isohyet | A line connecting points of equal rainfall. |
| Isohyetal map | A map showing the spatial distribution of rainfall using isohyets. |
| Catchment area | The area from which rainfall drains into a particular river or reservoir. |
| Thiessen Polygon Method | A method to estimate average rainfall over an area from point measurements. |
| Mass Curve (Rainfall) | A plot of cumulative rainfall depth versus time. |
Additional Information: Rainfall Data Presentation
Representing rainfall data effectively is crucial in hydrology and water resource management. Various methods are used depending on the purpose:
- Point Rainfall: Rainfall measured at a single location using a rain gauge.
- Areal Rainfall: The average rainfall over a specific area, like a catchment. This can be estimated using methods like the arithmetic mean, Thiessen polygons, or isohyetal method.
- Rainfall Hyetograph: A plot showing the intensity of rainfall over time during a storm.
Isohyetal maps provide a visual representation of rainfall distribution patterns, highlighting areas that receive more or less rainfall, which is vital for water resource planning, flood forecasting, and drought monitoring, especially for large catchment areas.
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