I think the expression "memoria hiperanual" refers to the "big picture", i.e., the base flow (caudal base) AND the changes in the flow over a period of several years.

First of all, the "hiperanual" part: EuroDicAutom gives these two expressions that contain it, both relating to your context. This is one of them:


TERM media hiperanual del caudal medio mensual

Reference ICID-Comité Español de Riegos y Drenajes, 1977

Definition mean monthly discharge for the same month averaged ***over a specific number of successive years ***

Reference ICID
(1)
TERM averaged mean monthly discharge
http://ec.europa.eu/eurodicautom/Controller

I found a number of references in English to "multi-year" in this context:

Design and operation of large water-supply systems with high volumes of storage or many linked sources (e.g. surface water supplies in different climatic zones, or surface and groundwater supplies) require definition of ***multi-year average flows.***
http://tinyurl.com/ytgfkn

***multi-year average*** stream base flow volumes and groundwater levels;
http://www.thewaterhole.ca/publications/SoE Summary FINAL.ht...

Multi year data
(April-Oct data from 1980-1984) used at USGS station to better represent mean conditions.
http://www.deq.state.mt.us/wqinfo/TMDL/SunRiverFinalTMDL/Fin...

(Note that the previous example shows "a specific number of successive years" as in the EuroDic definition).