Does spatial variation of grassland canopy affect soil water fluxes?

Gökben Demir1, Beate Michalzik2, Janett Filipzik1, Johanna Metzger1, Anke Hildebrandt1
1 Chair of Terrestrial Ecohydrology, Friedrich Schiller University Jena, Institute of Geoscience
2 Chair of Soil Science,Friedrich Schiller University Jena, Institute of Geography

V 15.1 in Grundwasserqualitätsentwicklung – Erkenntnisse aus Langzeitstudien in der Kritischen Zone

24.03.2022, 10:30-10:45, HS 2

Vegetation canopies create heterogeneous patterns in water input, net precipitation, because of precipitation portioning by leaves and stems. This spatial heterogeneity is often thought to be crucial for soil hydrology, due to impacting on soil moisture patterns, soil water flow, and preferential flow, bypassing the rooting zone and locally enhancing deep drainage. Net precipitation can create concentrated water input to the soil, which potentially creates saturation not only in topsoil but also in the soil-bed rock interface. Although several studies about canopy influence on soil water fluxes have been conducted in forest ecosystems, grasslands are often paid less attention to, which is due to a presumption that grassland canopies do not induce spatial heterogeneity. Yet some observations in short vegetation showed existence of a relation between canopy and soil moisture patterns, which have so far been related to transpiration. Also, recently it has been shown that also grassland canopies affect spatial net precipitation patterns. However, still, how grassland canopy-induced heterogeneity interacts with soil fluxes is little known because of scarcity of experimental evidence. Therefore, in this study, we investigated the effect of net precipitation patterns on soil wetting response in grassland by including antecedent soil moisture status. We conducted weekly precipitation sampling in Hainich CZE (Thuringia, Germany) on the research site, which was equipped with soil moisture sensors, along with grass height measurements. We employed linear mixed effect models to understand the role of canopy height and dynamics for impacting spatial patterns of soil moisture response to rainfall. Also, we calculated weekly spatial average increase in soil water content to investigate soil storage recharge in the growing season. The water balance showed that early in the growing season, rainfall was stored in topsoil rather than subsoil. Yet later in the season, drier topsoil conditions enhanced bypass flow. Also, through the growing season, overall storage in the soil column decreased, which indicates preferential flow. Based on the mixed-effects models, we found that spatial variation in grass height is a significant driver along with precipitation and pre-event soil moisture status, determining soil wetting patterns. In taller canopy locations, soil wetting was suppressed, indicating bypass flow. Our results suggest that seasonally drier conditions and grassland stemflow kick off the preferential flow. The spatial variation of the canopy affects soil moisture wetting patterns and thereby likely water flow into the deeper subsurface not only on woody ecosystems but also in herbaceous vegetation. 



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