Improving runoff generation knowledge through baseflow campaigns

Predictions from Global Climate Models point to increasing water scarcity across the Global South. However we currently lack understanding of how these predicted changes propagate through the hydrological cycle to smaller-scale systems. This lack of understanding is due to limited knowledge of hydrological processes, which persists in tropical, subtropical, and arid environments of the Global South. Clear blueprints exist for experimental catchment setups that allow us to understand spatiotemporal catchment processes (e.g., the Long Term Ecological Research LTER, the Critical Zone Observatory network CZO, and the Terrestrial Environmental Observatories TERENO); yet, it is not feasible to maintain such dense experimental networks in the understudied Global South. Reconciling snapshot baseflow campaigns as sources of data can be an alternative to expanding hydrological observation, particularly in developing countries where long-term records are scarce and pressure on water resources is growing. Here, we present results from baseflow campaigns in small nested catchments across different landscape elements to improve a rainfall-runoff model (geomorphologic instantaneous unit hydrograph) and provide insights into spatial patterns of flow, catchment water storage, and estimates of streamflow sources. The Peri Lake Experimental Catchment (19 km²) is characterized by granite and diabase dike and covered by the Atlantic rainforest. We measured baseflow discharge and sampled isotopes (δ18O and δ2H) at 25 catchments (areas ranging from 0.02 to 5.33 km²). Through combining flow velocity and discharge, we incorporated spatial variations of velocity in the channels during runoff, using a constant relationship between velocity and celerity. The Nash values were above 0.80, and we eliminated the need for concentration time formulas, where uncertainty reaches 500%. Combining isotopes and discharge enhanced our knowledge of the role of geology, with the Spearman coefficient between the percentage of granite and specific discharge being -0.68 (p-value < 0.05). We conceptualize that the diabase dikes are shallower with greater permeability, functioning as a conductor and supplier of water during baseflow. Simulations with a 3D surface-subsurface hydrological model  verify the capacity of the observed baseflow patterns in this catchment with heterogeneous geology. The results suggest that measurements in nested catchment during baseflow conditions reflect the heterogeneity of the different sources that contribute to streamflow. Snapshot measurement and sampling  campaigns are a powerful tool to understand runoff generation patterns in subtropical and tropical catchments of the Global South where continuous monitoring is hard to implement.