Dynamics of Rainfall‐NDVI Relationships: Testing the Pulse‐Reserve and Two‐Layer Hypotheses

Abstract

Water availability significantly affects vegetation in dry regions. We studied how Normalized Difference Vegetation Index (NDVI) relates to rainfall pulses, or soil water reservoirs, across three continents: Australia, North America, and the Middle East, from 2011 to 2022. High‐resolution satellite images, at daily and tens‐of‐meters scales, were used to compute NDVI for four vegetation formations: herbaceous vegetation, shrubs, forest, and wheat fields, alongside corresponding daily rainfall data. First, we determined the minimal representative area for these vegetation formations and study sites. We found that this area ranged 0.02–0.14 , with variation depending more on vegetation formation than on geographic location. Second, our results, across soil texture, continents, climate conditions, and vegetation formations, challenge the Pulse‐Reserve hypothesis while supporting Walter's Two‐Layer hypothesis. Our results show that shallow‐rooted vegetation responds to ∼3‐month and deep‐rooted vegetation to ∼1‐year rainfall accumulation. These findings emphasize the importance of long‐term water storage rather than immediate rainfall pulses in semiarid ecosystems.