Direct Rapid Recharge Controls the Hydrological Response in a Peri‐Urban Mountainous Tropical Catchment

ABSTRACT In the wet tropics, the seasonal changes in rainfall amounts are altering catchment storage and, consequently, groundwater‐to‐surface water connectivity. However, estimations of water source contributions to streamflow remain poorly constrained across mountainous headwater catchments. Here, we use stable isotopes (δ 18 O and δ 2 H), 222 Rn activity and electrical conductivity (EC) to characterise the hydrological responses in the northern mountainous region of the Central Valley, Costa Rica. Temporal trends revealed a clear response in groundwater–surface water connectivity to rainfall inputs. Streamflow transitioned from being dominated mainly by groundwater during the dry season to a greater contribution of subsurface flow during the wet season. We found that direct rapid recharge regulates this hydrological connectivity during the rainiest periods, as demonstrated by increased 222 Rn activity in both springs and streams. Using a mass balance approach, we estimated monthly groundwater contributions to streamflow using 222 Rn activity (5.9%–63.1%) and EC (37.2%–84.0%). A Bayesian endmember mixing model yielded similar mean groundwater contributions compared to the mass balance approach depending on tracer selection (EC– 222 Rn: 56.2% ± 12.7%, δ 18 O– 222 Rn: 51.1% ± 11.6% and δ 18 O–EC: 89.2% ± 5.2%). Our findings are useful for future assessments of the impact of climate change in rainfall amount and seasonality on groundwater recharge and streamflow generation in tropical mountainous catchments.