To calculate 75% of dependable rainfall from a probability distribution of annual rainfall, you'll need to follow these steps:

Let's say you have a probability distribution table of annual rainfall levels and their probabilities:

Rainfall Level (mm) Probability

500 0.10

600 0.20

700 0.30

800 0.25

900 0.10

So, in this example, 75% of dependable rainfall is approximately 562.5 mm.

Remember that the actual process may vary based on the specifics of the probability distribution table and whether interpolation is required. Always ensure that your calculations are accurate and appropriate for the context of the data you are working with.

1. Arrange the rainfall data (annual, monthly, decadal, or weekly) in descending order

2. Use one of the methods of computing probability, for instance, Weibull’s method:

P = m/n+1 X 100

Where P is the probability of occurrence, m is the rank of the rainfall value (after arranging the data in descending order) and n is the total number of observations

3. Plot the P values (x-axis) against rainfall values (y-axis) on the log-normal scale

4. Fit a trend line and use it to determine the rainfall values that correspond to 75% OR

5. Use the trend equation to compute the rainfall values by substituting 75 as the x value