Temporal and Spatial Partitioning of Water Resources in a Seasonal Dry Tropical Climate

Abstract:

In this study, we measured the δ²H and δ¹⁸O values of precipitation, lake water, groundwater and plant xylem and leaf water across different plant species, seasons and locations with varying distances to Lake Challa. Lake Challa is situated in equatorial East Africa, a tropical semi-arid area with a bimodal climate, characterized by two distinct monsoon rain seasons. The isotopic composition of precipitation was most depleted during the northeast monsoon, intermediate during southeast monsoon and enriched during the dry season. A seasonal trend in the isotopic composition of lake water was found with higher values around March and lower values around July-August. A clear spatial, but only a limited temporal, partitioning of water resource use by plants was observed. Plants at the crater rim relied on precipitation fallen during the early wet season throughout the year. This provides evidence for a “two water worlds hypothesis”, stating that plants rely more on bound than mobile water. Plants in the savannah depended on groundwater during the year, while plants at the lake shore used a water source that showed an admixture of lake water, deduced from enriched δ²H_xylem and δ²H_LMWL intersection values. Surprisingly, trees appear to preferentially exploit shallower water sources than shrubs at the lake shore. From all sampled species, only two trees showed a clear seasonal partitioning of water resources. Species from the family Capparaceae, Rutaceae and Tiliaceae, independent from location, relied on air moisture like dew and fog, as derived from positive xylem precipitation offsets (meaning δ²H_xylem plotted above the local meteoric water line). The xylem-leaf water deuterium enrichment averaged 28 ± 26‰. According to our results, the plant phenology is the primary factors influencing this enrichment factor, while growth form, location (proximity to the lake) and seasonality have negligible effects.