Frequent water inputs to a semi-arid ecosystem at night - a lysimeter based study

Globally, ecosystems are water-limited on about one-third of the land area. In these ecosystems, it has been shown that even small water inputs often play a relevant role for a large number of species ensuring their survival. However, to date, such inputs from fog, dew, and adsorption of atmospheric water vapor, which are summarized as non-rainfall water input (NRWI), can rarely be studied because the necessary measurement infrastructure is scarce. Long-term measurements covering multiple seasons and years are especially rare. This limits our understanding of the role of NRWI in the surface water, energy, and carbon balance in ecosystems. 

In this contribution, we investigate surface water exchange processes in a semi-arid Savannah ecosystem over a period of a year. Five large high precision weighing lysimeters enable us to analyze water phase changes with a temporal resolution of five minutes. 

Our main finding is that across (almost) all seasons diel dynamics were characterized by evaporation at daytime and condensation at nighttime. Condensation processes varied between seasons. In winter, dew and fog regularly formed at night when soil moisture and atmospheric humidity were close to saturation. In summer, despite high mean night conditions of atmospheric vapor saturation deficit (15 hPa), water input via adsorption of atmospheric vapor formed due to dry topsoil moisture (< 10 %). In total NRWI occurred for at least 3 hours per day on 297 days (81 % of the year) with a mean duration of 6 hours per day. The relative contribution of NRWI to the total annual water input was 8 %. However, we found a large seasonal variability, with adsorption forming the major water input to the ecosystem during the summer drought period. In the year analyzed, it compensated for 19 % of the evaporation losses. 

Our results suggest a non-negligible contribution of NRWI to the water budget of a semi-arid ecosystem. Consequently, a better representation of the diel dynamics of evaporation and condensation could help us to increase our knowledge of eco-hydrological processes in semi-arid ecosystems. Especially during the dry season, data from daytime and nighttime hours should be taken into account in order not to bias the water balance towards evaporative losses.