The sensors were installed at vertical infiltration profiles inside the upper soil layer of each of the four partitions. In addition, a meteorological station was fixed at the top of the embankment. All the sensors are connected by wires to a datalogger (CR1000, Campbell Scientific, Logan, UT, USA) that was used in combination with two multiplexers due to the large amount of sensors. The data of all sensors are recorded at a constant sampling rate of 5 min. Every 24 h, the data files are sent via FTP to the university sever. The power supply of the entire monitoring system is provided by solar panels and batteries.
The experiment includes four different zones: (i) South slope with vegetation (SV), (ii) South slope without vegetation (SnV), (iii) North slope with vegetation (NV), (iv) North slope without vegetation (NnV). Each of the four zones is equipped by a vertical profile of different sensors (Figure 2
) and the devices that measure the surface runoff and seepage. Thus, a complete analysis of the soil–vegetation–atmosphere interaction is possible by incorporating observations gathered by the meteorological station. The installation of the sensors was performed in two main phases. First, the setup of the non-vegetated profiles (SnV and NnV) was performed in spring 2017. Second, the vegetated profiles (SV and NV) were installed in autumn 2017. Finally, some complementary sensors were mounted at the beginning of 2018. Figure 3
shows a photograph of the embankment after the installation of the sensors looking towards the North-faced slope.
2.3.1. Vertical Sensor Profiles
Each vertical profile measures air and soil temperature, relative humidity, barometric pressure, heat flux, pore water pressure (PWP), and volumetric water content (VWC) at different positions. A complete list of all the sensors and the recorded parameters is given in Table 1
. Net solar radiation devices were installed at each of the orientations (North and South). The general distribution of the different devices is shown by the example of vertical profile NnV (Figure 4
a). In addition, photographs of the soil texture format profiles NnV and NV during the sensors installation are shown in Figure 4
b,c respectively. They show the sandy loamy soil with isolated gravel particles at both trenches, while the presence of organic material in the form of plant roots is visible in the vegetated North (NV) profile.
Not each vertical profile has the distribution of sensors as shown in Figure 4
a and the final number of devices installed in each profile ranges from 13 to 14 (Table 2
). The parameter that is measured at most positions is temperature, which is monitored at least at 7 positions along each vertical profile, while PWP and VWC are registered at a minimum of 3 and 4 positions, respectively. The total number of records measured at all sensors for each of the non-vegetated slopes (SnV and NnV) and for each of the vegetated slopes (SV and NV) is 27 and 29, respectively. Some sensors had technical problems during the first year or were installed in the second phase. That’s why the time series of specific sensors are not complete when presented in the results section.
In the following, the different types of devices will be described and some characteristics discussed. In this study, two types of sensors are considered: (1) devices measuring temperature changes and heat flux along the atmosphere–soil interface, and (2) devices focusing on the infiltration of rainfall into the soil layer.
Air and soil temperature is measured at different positions close to the surface. In the most superficial part of the soil layer, three thermistors encapsulated in an aluminum housing (107, Campbell Scientific, Logan, UT, USA) were buried. The rest of the soil temperature is monitored by the thermistors incorporated in other sensors (MPS-6, Decagon Devices, Pullman, WA, USA, and 5TE, Decagon Devices, Pullman, WA, USA). Another sensor (VP-4, Decagon Devices, Pullman, WA, USA) measures the air temperature at 9.5 cm above the terrain surface.
The heat flux across a soil section is measured close and parallel to the surface, where most of the heat transfer is expected. For this reason, a thermopile (HFP01, Hukseflux, Delft, The Netherlands) is installed at 8 cm depth and transforms the measured voltage into heat flux. Finally, wind speed and direction is also registered at each vertical profile by a cup and vane anemometer installed 15 cm above the terrain surface.
Water content and pore water pressure are fundamental parameters to understand the atmosphere–vegetation–soil interactions related to rainfall infiltration into the soil. In the experiment, multiple devices register these processes in the vertical profiles, principally tensiometers and soil moisture sensors.
Two different types of tensiometers were installed for measuring pore water pressure: (i) porous ceramic disc tensiometers (MPS-6, Decagon Devices, Pullman, WA, USA), and (ii) porous ceramic cup tensiometer (T4, UMS, München, Germany). The MPS-6 dielectric water potential sensors are designed to measure suction values up to 100 MPa and are located close to the surface (Table 2
), where high suction values are expected. In contrast, the UMS T4 is a tensiometer that measures PWP in a negative and positive range. Thus, its installation is at the lower part of the monitored soil layer, where low suction or positive PWP values are expected. The UMS T4 device is a rather delicate sensor in comparison with the robust MPS-6. The UMS T4 tensiometers were refilled and calibrated in the laboratory before their installation. The refilling was made with de-aired water, ensuring no air bubbles remained inside the ceramic cup, which would lead to an incorrect pressure reading. In addition, the pressure measured by the transducer has to be corrected twice: (i) due to the elevation difference between the pressure transducer and the ceramic cup, and (ii) because of the different power excitation (our system supplies 12 V, while the manufacturer calibration is performed at 10.6 V). The installation of the UMS T4 tensiometers has to be carried out very carefully. To prevent surface runoff running down into the borehole along the tensiometer, a rubber water-retaining disk was slipped around the sensor at the soil surface and a perfect fit between the previously drilled hole and the device was performed.
The volumetric water content (VWC) is measured by Decagon 5TE sensors (Decagon Devices, Pullman, WA, USA), which are installed at different depths in the soil layer (Table 2
). The 5TE uses an electromagnetic field to measure the dielectric permittivity of the surrounding medium. Prior to field installation, a calibration of the sensor was performed in the laboratory using soil samples with a similar bulk density as the embankment slope. The VWC that was recorded by the sensor was compared with the one determined by the standard procedure incorporating the void ratio, the gravimetric water content, and the specific gravity of the soil samples. The calibration results showed that there was no significance difference with the equation given by the manufacturer, therefore this equation was finally used for the transformation of the sensor reading into VWC.