Instrumental set-up

2.2.2.1 Standard meteorological measurements

Continuous measurements of micrometeorological and standard meteorological variables have been performed since March 2016. At each agroforestry plot, one eddy covariance mast with a height of 10 m (Fig. 2.1b) was installed, and at each monocultural plot one eddy covariance mast with a height of 3.5 m (Fig. 2.1c) was installed. Each mast in the agroforestry and the monocultural plots was equipped with an identical instrumental set-up. An overview of all installed instruments is given in Table 2.1. The data were logged and stored on a CR6 data logger (Campbell Scientific, Inc., Logan, UT, USA). The meteorological data were regularly sent to a database via mobile phone network.

2.2.2.2 Conventional eddy covariance installation

Fluxes of sensible heat and momentum were continuously measured with a uSONIC-3 Omni (METEK GmbH, Elmshorn, Germany) ultrasonic anemometer. CO2 and water vapour fluxes were measured in campaigns during the vegetation periods of 2016 and 2017. During the 2016 campaign, fluxes were measured separately during two consecutive periods of 4 weeks at the agroforestry and monocultural plots, whilst in 2017 both plots were sampled simultaneously over a time period of approximately 4 weeks (see Table2.5 for exact dates).

During the campaigns, the instrumentation specified in Table 2.1was complemented by a LI-7200 (LI-COR Inc., Lincoln, NE, USA) enclosed-path infrared gas analyser (Burba et al., 2012). The data were measured together with the three-dimensional wind velocity and the sonic temperature and stored on the same data logger (CR6, Campbell Scientific,

Figure 2.1: (a) SIGNAL sites (map source: Bundesamt für Kartographie und Geodäsie, 2011), (b) the agroforestry plot in Dornburg with eddy covariance mast and (c)the monocultural agriculture plot in Forst (Lower Lusatia) with eddy covariance mast.

Inc., Logan, UT, USA) as used for the meteorological variables. The water vapour and CO2 mole fractions were sampled with a sampling frequency of 20 Hz. The intake tube was of 1 m length and had an inner tube diameter of 5.3 mm (2016) and 8.2 mm (2017). The separation of the gas analysers intake tube relative to the centre of the sonic anemometer was different for each plot and is summarized in Table 2.6. The flow rate was kept constant at 15 slpm.

2.2.2.3 Low-cost eddy covariance (EC-LC) installation

The low-cost eddy covariance set-up shared the same ultrasonic anemometer (uSONIC-3 Omni) as used for the conventional EC set-up. The water vapour mole fraction was derived from the combined digital pressure, relative humidity and air temperature sensor BME280 manufactured by Robert Bosch GmbH, Stuttgart, Germany (hereafter named thermohygrometer). Figure 2.2 depicts the low-cost set-up. The measuring principles of the air pressure sensor and the relative humidity sensor are resistive and capacitive, respectively. The temperature sensor readings are based on diode voltage measurements.

Table 2.1: Instrumentation for flux and meteorological measurements used at all five agro-forestry and five monocultural agriculture plots.

Variable Height [m] Instrument Company

Standard meteorological measurements

3-D wind components,𝑢,𝑣,𝑤, 3.5, 10 uSONIC-3 Omni METEK GmbH, Elmshorn, Germany sonic temperature,𝑇_s,

wind speed and -direction

Net radiation,𝑅_N 3, 9.5 NR-Lite2 net radiometer Kipp & Zonen, Delft, The Netherlands Global radiation,𝑅_G 3, 9.5 CMP3 pyranometer Kipp & Zonen, Delft, The Netherlands Relative humidity, RH, 2 Hygro-thermo transmitter-compact Thies Clima, Göttingen, Germany

air temperature,𝑇 (Model 1.1005.54.160)

Precipitation 1 Precipitation transmitter Thies Clima, Göttingen, Germany

(Model 5.4032.35.007)

Atmospheric pressure, ppp 0.5 Baro transmitter Thies Clima, Göttingen, Germany (Model 3.1157.10.000)

Ground heat flux,𝐺 −0.05 Hukseflux HFP01 Hukseflux, Delft, The Netherlands Soil temperature,𝑇Soil −0.02,−0.05, DS18B20

−0.10,−0.25,−0.5 Conventional eddy covariance measurements

𝑢,𝑣,𝑤,𝑇s 3.5, 10 uSONIC-3 Omni METEK GmbH, Elmshorn, Germany

Water vapour mole fraction,𝐶𝐻2𝑂𝑣 3.5, 10 LI-7200 LI-COR Inc., Lincoln, NE, USA Carbon dioxide mole fraction,𝐶_𝐶𝑂2 3.5, 10 LI-7200 LI-COR Inc., Lincoln, NE, USA Low-cost eddy covariance measurements

𝑢,𝑣,𝑤,𝑇s 3.5, 10 uSONIC-3 Omni METEK GmbH, Elmshorn, Germany

RH,𝑇, ppp 3, 9.5 BME280 Robert Bosch GmbH, Stuttgart, Germany

The ultrasonic anemometer measured the three-dimensional wind speed and the ultrasonic temperature at a frequency of 20 Hz, whereas the thermohygrometer measured the air temperature, relative humidity and air pressure at a sampling frequency of 8 Hz. The specified response time of the thermohygrometer for relative humidity measurements is 1 s, in order to overcome 63 % of the step change from 90 % to 0 % or 0 % to 90 % relative humidity at a 25^∘C air temperature.

The response time of the temperature sensor of the thermohygrometer was not explicitly stated. Therefore, we estimated the response time in a lab experiment. We exposed the temperature sensor to a rapid temperature change about 10^∘C warmer than ambient air temperature. The time constant𝜏 was then directly proportional to the slope of the linear regression fit:

with the measurement time, 𝑡, the air temperature at the first time step,𝜗(𝑡= 1), the ambient air temperature, 𝜗_Ambient, and air temperature at variable time step, 𝜗(𝑡=𝑡_var).

The time constant achieved for the temperature sensor was 23.3±0.9 s as a mean of four replications. During the lab experiment the thermohygrometer was placed inside the same housing as deployed in the field.

The thermohygrometer was placed 0.5 m below the centre of the sonic anemometer in a polyvinyl chloride (PVC) housing to protect the thermohygrometer from precipitation.

The PVC housing consisted of an outer and an inner cylinder. The inner cylinder was perforated on the top to provide a continuous air flow of 15 L min⁻¹, which was generated by a ventilator (HA30101V3-0000-A99, Sunonwealth Electric Machine Industry Co. Ltd.,

Fresnes Cedex, France). The ventilator was placed below the thermohygrometer inside the inner cylinder. The volume of the inner cylinder was 98.1 cm³.

The absolute accuracy tolerance of the relative humidity sensor was specified as ±3 % for 20 % to 80 % relative humidity at 25^∘C air temperature. For the temperature sensor an absolute accuracy tolerance of ±0.5^∘C at 25^∘C air temperature was given and for a temperature range of 0 to 65^∘C an absolute accuracy tolerance of±1^∘C was specified. The pressure sensor has an absolute accuracy tolerance of±1 hPa for a pressure range from 300 to 1100 hPa at air temperature between 0 and 65^∘C (Bosch Sensortec GmbH,2016).

Digital data from the thermohygrometer were recorded via the i2c protocol and stored on a Raspberry Pi model B+ (Raspberry Pi Foundation, Cambridge, UK). The thermohy-grometer has very low power consumption of approximately 3.6𝜇A at a sampling frequency of 1 Hz. The power draw of the thermohygrometer is 9.4e-5 W at a measuring frequency of 8 Hz, when powered with 3.3 V and if all three variables are measured at the same time.

The Raspberry Pi has a maximum power consumption of about 1.1 W.

The key potential of the low-cost EC set-up is for replicated measurements of evapotrans-piration across different ecosystems. The relative cost of the low-cost set-up (featuring a sonic anemometer, a Raspberry Pi and the thermohygrometer of low cost) is often less then 10 % of a typical conventional EC set-up. Besides a precipitation protection and a stable power supply, the thermohygrometer is also low maintenance. The mean time before failure of the sensor in our study was approximately 2 years.

Figure 2.2: Low-cost eddy covariance instrumentation, featuring a uSONIC-3 Omni sonic anemometer and a BME280 thermohygrometer. The thermohygrometer is placed in a ventilated PVC housing below the sonic anemometer.