Water balance and aridity index
The water balance compares how much rain falls against how much water evaporates. The aridity index uses this comparison to measure how dry a place truly is.
Real World
Saudi Arabia's Rub' al Khali has an aridity index below 0.05, meaning potential evapotranspiration exceeds precipitation by more than twenty times, classifying it as hyper-arid under the UNEP system.
Exam Focus
Define aridity index as P÷PET and state the threshold values (e.g. <0.2 = arid) — precise figures earn application marks.
Essay Framework
Use PEEL to structure every paragraph. Tap each step for guidance and an example.
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