Insights on the Rainfall-Runoff Response Controls in two High-Elevation Tropical Ecosystems

Abstract:

Although tropical mountainous ecosystems supply large quantities of high quality water for downstream populations, the factors influencing rainfall-runoff response in these ecosystems remain poorly understood. Here, we provide insights on the controls of such response in two tropical headwater catchments located in south Ecuador. The sites are the páramo basin of the Zhurucay River and the tropical montane cloud forest (TMCF) basin of the San Francisco River. Nested monitoring systems were used to record hydrometric data and to collect streamwater samples. Weekly water samples of rainfall, streamflow, and soils were collected for two years and analyzed for stable water isotopes. Hydrometric and meteorological data were used to conduct a hydrologic characterization of the catchments, while isotopic signals were used to estimate stream waters’ mean transit times (MTTs). Regression analysis was used to examine the influence of landscape features on MTT variability. The hydrologic characterization revealed rapid response of runoff to precipitation inputs at both biomes, though higher baseflows were identified at the TMCF. Runoff coefficient (runoff as a proportion of precipitation) is high at both sites. However, MTTs are different: shorter times were estimated at the páramo sites (0.15-0.73 yr) than at the TMCF catchments (2-5 yr). MTT variability is controlled by topography and the areal percentage of wetlands at the páramo, whereas geology controls it at the TMCF. These results reveal that although catchments at both biomes show flashy response to rainfall during events, different factors control their overall hydrologic response. Lateral shallow subsurface flow in the organic horizon of the soils and the combination of the high storage capacity of wetlands and the terrain slope controls runoff generation and regulation at the páramo; whereas deeper subsurface flow due to enhanced vertical infiltration within the fractured geology is the dominant control of runoff response at the TMCF.