Assessment of hydrological pathways in East African montane catchments under different land use

Abstract

Abstract. 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.

Global Isotope Hydrogeology―Review

“…Each bin in this lowermost histogram therefore represents all studies that report a groundwater (or pre‐event water) fraction that overlaps with the binned interval. The frequency describing annual variations in δ 18 O applied by Jacobs et al, , differs from the other studies.…”

“…The middle panel presents a histogram of young streamflow fractions reported for n =351 catchments. Data for catchments not shown in the uppermost panel but that are shown in the middle panel are from Clow et al (; n =11 catchments), Jacobs et al (; n =3 catchments), and Stockinger et al (; n =1). The bottom panel panel presents histogram of reported groundwater (or “pre‐event” water) from n =101 studies reported in Table ).…”

“…Relative to their areas, tropical and high‐latitude lands are underrepresented in the primary literature, whereas northern mid‐latitude catchments are overrepresented. Recent works are ameliorating this by studying tropical (Jacobs et al, ), high‐elevation, and high‐latitude systems (e.g., Ala‐aho et al, , ; Ren et al, ; Tetzlaff et al, ; Yi et al, ). Continuing streamflow, groundwater, and precipitation isotopic data set development in these regions may help to monitor consequences of atmospheric warming on hydrologic processes (Lyon et al, ; Wilusz et al, ).…”

Using isotopes to understand the evolution of water ages in disturbed mixed land‐use catchments

“…Previous studies have shown that controls on water ages are complex and can depend on the internal structure of the catchment and can be variously dominated by physiographical characteristics, including drainage density (Hrachowitz, Soulsby, Tetzlaff, Malcolm, & Schoups, ), topography (McGuire et al, ; Mosquera et al, ), soil cover (Hrachowitz et al, 2009), soil hydraulic properties (Heidbüchel, Troch, & Lyon, ; Muñoz‐Villers, Geissert, Holwerda, & McDonnell, ), land cover (Ma & Yamanaka, ), and their spatial variation. However, several of these characteristics can be affected by land management (e.g., Soulsby, Birkel, Geris, & Tetzlaff, ), which makes MTT a useful indicator to evaluate land use impacts on hydrological processes (Jacobs et al, ).…”

Rainfall‐Runoff Modeling Using Crowdsourced Water Level Data

“…This framework has been applied to a variety of catchments and is considered to describe the underlying hydrological processes sufficiently well (Jehn et al, ; Maier et al, ; Windhorst et al, ). The model structure represents the conceptual understanding of the rainfall‐runoff processes reported by Jacobs, Timbe, et al (). Daily precipitation and ET pot are the only model inputs required.…”