MULTIANNUAL CLIMATE CONTROL OF RESERVOIR STORAGE IN THE SEMIARID REGION OF PIAUÍ, BRAZIL

Reservoir storage is the central variable of water security in semiarid regions. Its relationship with climate has been little quantified in long records and with explicit contributing basins. This study evaluated the relationship between the storage of eight reservoirs in the semiarid region of Piauí, monitored by the Brazilian National Water and Sanitation Agency (ANA), and drought indices (SPI and SPEI, 3- to 48-month scales) from 1998 to 2025 (28 years), with each series starting after dam completion. The semiarid region was delimited by the Thornthwaite moisture index (Im < −20). The contributing basins were defined from ANADEM. Precipitation and evapotranspiration came from BR-DWGD and CHIRPS. The indices were compared with mixed-effects models and generalized additive models under 4-year blocked cross-validation. Scales of 24 to 48 months had lower mean error than shorter ones. With BR-DWGD, however, the differences among SPI24, SPI36 and SPI48, and relative to SPI12, were within the validation uncertainty. The 36-month SPI had the lowest mean error with CHIRPS, with a near-zero lag. The SPEI did not outperform the SPI. Each unit of SPI36 corresponded to about 12 to 13 percentage points of storage. The response varied among reservoirs, from 8 to 23 percentage points, and the fit under blocked validation was appreciable in only one of them. As a lagged estimate, the index did not improve on storage persistence at one month. The 4.4% error reduction at twelve months was not distinguishable from zero. We conclude that reservoir storage follows moisture accumulated over several years (24 to 48 months), not recent rainfall.