Browsing by Author "Jacobs, Suzanne R."

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Assessment of hydrological pathways in East African montane catchments under different land use
(2018) Jacobs, Suzanne R.; Rufino, Mariana; Butterbach Bahl, Klaus; Timbe Castro, Edison Patricio; Weeser, Bjorn; Breuer, Lutz
Conversion of natural forest (NF) to other land uses could lead to significant changes in catchment hydrology, but the nature of these changes has been insufficiently investigated in tropical montane catchments, especially in Africa. To address this knowledge gap, we aimed to identify stream water (RV) sources and flow paths in three tropical montane sub-catchments (27–36 km2 ) with different land use (natural forest, NF; smallholder agriculture, SHA; and commercial tea and tree plantations, TTP) within a 1021 km2 catchment in the Mau Forest complex, Kenya. Weekly samples were collected from stream water, precipitation (PC) and mobile soil water for 75 weeks and analysed for stable isotopes of water (δ2H and δ18O) for mean transit time (MTT) estimation with two lumped parameter models (gamma model, GM; and exponential piston flow model, EPM) and for the calculation of the young water fraction. Weekly samples from stream water and potential endmembers were collected over a period of 55 weeks and analysed for Li, Na, Mg, K, Rb, Sr and Ba for endmember mixing analysis (EMMA). Solute concentrations in precipitation were lower than in stream water in all catchments (p < 0.05), whereas concentrations in springs, shallow wells and wetlands were generally more similar to stream water. The stream water isotope signal was considerably damped compared to the isotope signal in precipitation. Mean transit time analysis suggested long transit times for stream water (up to 4 years) in the three sub-catchments, but model efficiencies were very low. The young water fraction ranged from 13 % in the smallholder agriculture sub-catchment to 15 % in the tea plantation sub-catchment. Mean transit times of mobile soil water ranged from 3.2–3.3 weeks in forest soils and 4.5–7.9 weeks in pasture soils at 15 cm depth to 10.4–10.8 weeks in pasture soils at 50 cm depth. The contribution of springs and wetlands to stream discharge increased from a median of 16.5 (95 % confidence interval: 11.3–22.9), 2.1 (−3.0–24.2) and 50.2 (30.5–65.5) % during low flow to 20.7 (15.2–34.7), 53.0 (23.0–91.3) and 69.4 (43.0–123.9) % during high flow in the natural forest, smallholder agriculture and tea plantation sub-catchments, respectively. Our results indicate that groundwater is an important component of stream water, irrespective of land use. The results further suggest that the selected transit time models and tracers might not be appropriate in tropical catchments with highly damped stream water isotope signatures. A more in-depth investigation of the discharge dependence of the young water fraction and transit time estimation using other tracers, such as tritium, could therefore shed more light on potential land use effects on the hydrological behaviour of tropical montane catchments.
Factors controlling the temporal variability of streamflow transit times in tropical alpine catchments
(2023) Mosquera Rojas, Giovanny Mauricio; Cárdenas Moreno, Irene Lucila; Jacobs, Suzanne R.; Crespo Sánchez, Patricio Javier; Larco Erazo, Karina Marlene
The mean transit time (MTT) of water is an essential descriptor of streamflow generation and catchment water storage. Research on how MTTs fluctuate over time and the variables influencing such variation is limited. In this study, bi-weekly stable isotopic data in precipitation and streamflow were used, together with daily records of hydrometeorological information, to investigate the temporal variability of streamflow MTTs. The data were collected over 8 years in a nested system of 8 tropical alpine catchments in the Zhurucay Ecohydrological Observatory in southern Ecuador (3,450 to 3,900 m a.s.l.). The temporal variability of streamflow MTTs was estimated using yearly periods and a 1-month moving window (i.e., 81 yearly calculated MTTs per catchment). The factors controlling the temporal variability of MTTs were identified using simple and multiple linear regression models with hydrometeorological parameters as explanatory variables. Results reveal that streamflow MTTs in all catchments were short (<1 year) and varied little among catchments (191.30 ± 47.10 days). A combination of hydrometeorological variables (i.e., precipitation, streamflow, and runoff coefficient) over antecedent periods up to 1 year was found to control MTT temporal variability. Overall, these findings point to the prevalence of low temporal variability of hydrological conditions in the investigated catchments. Our study is key to provide insights into the factors controlling the temporal variability of streamflow MTT in tropical catchments, overcoming data limitations of past investigations and with significant implications for improved water supply management.
Watershed services in the Humid tropics: opportunities from recent advances in ecohydrology
(FAPESP, 2018) Hamel, Perrine; Riveros Iregui, Diego; Ballari, Daniela Elisabet; Browning, Trevor N.; Célleri Alvear, Rolando Enrique; Chandler, David G.; Chun Sun, Kwak Pan; Destouni, Georgia; Jacobs, Suzanne R.; Scott, Jasechko; Johnson, Mark S.; Krishnaswamy, Jagdish; Poca, María; Pompeu, Patricia Vieira; Rocha, Humberto
In response to increasing pressures on water resources, watershed services management programs are implemented throughout the tropics. These programs aim to promote land management activities that enhance the quantity and quality of water available to local communities. The success of these programs hinges on our ability to (a) understand the impacts of watershed interventions on ecohydrology; (b) model these impacts and design efficient management programs; and (c) develop strategies to overcome barriers to practical policy development, including resource limitations or the absence of baseline data. In this paper, we review opportunities in ecohydrological science that will help address these three challenges. The opportunities are grouped into measurement techniques, modelling approaches, and access to resources in our hyperconnected world. We then assess management implications of both the knowledge gaps and the new research developments related to the effect of land management. Overall, we stress the importance of policy-relevant knowledge for implementing efficient and equitable watershed services programs in the tropics.