DOI: 10.5880/fidgeo.2019.030
title: Rhizosphere soil moisture dynamics and sap flow – determining root water uptake in a case study in the Attert catchment in Luxembourg

description: The dataset consists of measured soil moisture, sap velocity, precipitation and solar radiation time series, additionally calculated time series of root water uptake (RWU) and sap flow, including a python script for calculating RWU. The measurements were collected at two sites in mixed beech stands (Fagus sylvatica L.) in contrasting geological settings (sandstone and slate). Both sites are situated within the Attert catchment in western Luxembourg and were part of the monitoring network of the CAOS project (DFG research unit “From Catchments as Organised Systems to Models Based on Functional Units”; Zehe et al. 2014). In the vegetation period of 2017, we measured soil moisture across two profiles in the trees’ rhizosphere. These time series are compared to sap flow measurements in nearby trees. Moreover, we include precipitation and solar radiation data for the study period. For conversion of soil moisture to soil matric potential, we provide van Genuchten parameters (van Genuchten, 1980) for soil water retention at both sites, based on a previous study (Jackisch, 2015). 1. Soil moistureSoil moisture was monitored using TDR tube probes (Pico Profile T3PN, Imko GmbH), which allow for installation with minimal disturbance using an acrylic glass access liner (diameter 48 mm). The liner tube was installed in the rhizosphere of the trees without any excavation using a percussion drill. For optimal contact of the liner with the surrounding soil, the drill diameter was 40 mm and the tube was installed more than one year prior to the recorded data set. Each TDR probe segment integrates the soil moisture measurement over its length of 0.2 m. The signal penetrates the soil about 0.05 m which results in an integral volume of approx. 0,001 m-3. The probes can be stacked directly on top of each other, permitting spatially continuous monitoring over the soil moisture profile. At the sandstone site, we were able to install a sequence of 12 probes reaching a depth of 2.4 m. At the slate site, percussion drilling was inhibited by the weathered bedrock. There, we installed a sequence of 9 probes reaching a depth of 1.8 m. Soil moisture is recorded in 15 min intervals and aggregated to 30 min means. 2. Sap flowSap velocities were monitored in four beech trees in the direct vicinity of the soil moisture profile (as part of the CAOS research unit). At the sandy site, the reference sap velocity time series could be obtained from the exact tree where the TDR sensors were installed. It had a diameter at breast height (DBH) of 64 cm. At the slate site, the sap velocity sensor of the intended tree failed 3 weeks after leaf out. There, we refer to a neighbouring beech tree with a DBH of 48 cm about 9 m from the TDR measurements The sap flow sensors (East30 Sensors) are based on the heat ratio method and measure simultaneously at 5, 18 and 30 mm depth within the sapwood. Installation and calculation of sap velocities followed the description in Hassler et al. (2018). The sensors were installed before leaf out of the vegetation period in 2017. The data is recorded in 30 min intervals. We provide both the measured sap velocities and the upscaled sap flows. We assume the two outer measurement points in the sapwood to be representative for the radial area between 0–11 mm and 11–24 mm. Both are the mid points between the sensor positions. The inner sensor is representing a flow field, which has been shown to follow a Weibull distribution (Gebauer et al., 2008) in the active sapwood. To estimate the sap velocity distribution at each time step, we fit the Weibull function with the beech-parameters of (Gebauer et al., 2008) to the observed measurements at the mid and inner point via a scaling factor. For a correct position reference, the bark thickness is removed after Rössler (2008). As an inner limit, the 95% percentile is used to mark the transition to the inactive sapwood (Gebauer et al., 2008) (“zero” sap velocity limit). The resulting time series is now reporting sap flow in L h-1 and is aggregated to daily values. 3. Meteorological dataAs further reference for the drivers of temporal dynamics in soil moisture and sap velocity we use 10 min solar radiation records (Apogee Pyranometer SP110) subsampled to the time stamps of the precipitation data. Corrected hourly radar stand precipitation at canopy level is obtained from combined data from DWD (Deutscher Wetterdienst, Germany), ASTA (Administration des Services techniques de l'agriculture, Luxembourg) and KNMI (Koninklijk Nederlands Meteorologisch Instituut, Netherlands) after Neuper and Ehret (2019). 4. Soil water retention propertiesSoil water retention properties of the sites are given for two layers. The data was assessed in a previous study using the free evaporation method of the HYPROP apparatus and the chilled mirror method in the WP4C (both Meter AG) with 250 mL undisturbed soil samples from the sites (Jackisch, 2015). Following this method, the matric potential is divided into bins (0.05 pF). All retention data of the reference soil samples is bin-wise averaged to form the basis for the fitting of a van Genuchten retention curve. We have aggregated the results of 44 and 41 soil samples in the subbasins of the sand and slate site.