12.3. General Characteristics of DAD Curves

• For a fixed duration, rainfall depth decreases as the area increases.

This characteristic indicates that as the area over which rain falls gets larger, the average depth of rainfall for a specific duration decreases. For example, if you have a small garden and it receives a rainfall of 10 mm in an hour, this same depth spread over a larger area, like a field, would result in a lower average depth of rainfall when considering the entire field.

Imagine you're pouring a cup of water over a small surface, like a plate. Almost all of the water forms a puddle of a certain depth. Now, if you pour that same cup of water over a larger surface like a table, the water will spread out and form a much shallower puddle. This is similar to how rainfall works over different areas.

• For a fixed area, rainfall depth increases with duration up to a certain limit.

This point conveys that if the area remains the same but the duration of rainfall increases, the average depth of rainfall will increase to a certain extent. However, after reaching a specific duration, the depth of rainfall will not continue to rise significantly. This reflects the reality that prolonged rain might lead to saturation and runoff rather than additional accumulation.

It's similar to watering a plant. If you water it for a few minutes, it absorbs quite a bit. But if you water it for too long, the soil becomes saturated, and the extra water just runs off and does not contribute to further absorption. Thus, there is a limit to how much 'depth' the soil can handle before it stops being effective.

• The curves tend to flatten out for large areas or durations due to spatial and temporal averaging.

This characteristic of DAD curves explains that as the area or duration increases, the differences in rainfall averages across the area tend to reduce. This is caused by averaging out the variations in rainfall that occur across different locations and times. For instance, a small storm may cause significant rainfall in one location but hardly any in another, but as we consider larger areas and longer durations, these local variations balance out.

Consider a classroom where each student has a different score in a math test. If you calculate the average score for just a few students, the average could be very different from the average score of a larger group. However, if you check the average score of the entire school, those individual extremes balance each other out, making the average less sensitive to the extremes.

DAD curves are usually plotted on log-log or semi-logarithmic scales, with area on the x-axis and rainfall depth on the y-axis.

DAD curves are often represented graphically on logarithmic scales to clarify relationships between the variables of area and rainfall depth. The log-log scale helps in visualizing data that vary over several orders of magnitude, making it easier to see trends and patterns that might not be obvious on a linear scale.

Think of a map where distance is represented logarithmically; scenarios involving very small areas can be appropriately scaled against vast regions. This way, it’s easier to identify how rainfall patterns change over different area sizes rather than being overwhelmed by large numbers on a linear graph.

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