7.5.2. Normal Ratio Method

Used when the normal annual rainfall at surrounding stations differs significantly (by more than 10%).

The Normal Ratio Method is applied specifically when there are substantial differences (greater than 10%) between the normal annual rainfall at a station with missing data and that of surrounding stations. This means if we have a gauge that cannot provide data, we look to other stations nearby to estimate what the missing value should be, but only if those other stations have a normal rainfall rate that differs significantly from ours.

Imagine you’re planning a garden and need to know how much water it typically needs. If your neighbor's garden (which is similar to yours) normally gets 30mm of rain and your garden usually gets 40mm, but you discover one week that your gauge is broken, you wouldn’t just look at your neighbor's gauge. You would look at others (your immediate area) that have very different rain amounts to make a better guess for your garden’s specific needs.

n (cid:18) (cid:19) P = 1 X N x ×P x n N i i=1

In the Normal Ratio Method, we use a formula to estimate the missing rainfall amount (P). The formula takes into account the normal rainfall of the station missing data (N) and the normal amounts from surrounding stations (N_i), while also incorporating the recorded rainfall amounts at these nearby stations (P_i). Essentially, this helps us create a weighted average that better reflects the missing station's rainfall based on known data.

Think of this method like using recipes when baking. If you’ve baked bread before and know it usually needs 500 grams of flour, yet your friend who bakes a similar recipe says they use 400 grams, but your older cousin says she uses 600 grams, you don't just choose any one of those amounts. Instead, you balance the ingredients based on how tightly packed or loose the flour might be (similar to the rainfall data adjustments), helping you land on a more accurate measurement for your baking.

Where: • N = normal rainfall at missing station • N = normal rainfall at station i • P = recorded rainfall at station i

In the formula, each symbol represents an important component. 'N' is the normal rainfall we expect for the station where data is missing, while 'N_i' represents the normal rainfall for nearby stations. Finally, 'P_i' indicates the actual recorded rainfall at these neighboring stations. This helps to ensure that our estimates are reflective and take existing data into account.

Consider you're determining how much sugar to add to tea. 'N' could be the amount you usually find perfect. Other friends ('N_i') might use less or more sugar in their similar tea recipes ('P_i'). You base your decision of how much sugar to add for today on what works well for you and a combination of feedback from those friends.

--