## Materials and Methods  
### The Temperature Profiler Data-Logger  
We designed a microcontrolled data-logger that is attached to a flat panel installed on the top of a wooden bar with several spaced thermistor, which will be buried to measure the soil temperature profile. A thermistor is also placed on the flat panel, so we can measure its temperature. A sketch and a photograph of the fabricated system used to measure the temperature of the flat panel and the soil temperature profiles is shown in Figure 2.  
A block diagram of the data-logger is presented in Figure 3. An ultra low-power microcontroller MSP430F2122 is used to manage the whole system. Seven thermistors UPS7982 (a 10 k\(\Omega\)\(\pm\) 1% vinyl encapsulated NTC manufactured by US Sensors) were connected, through a linearising resistor divider and an eight channel multiplexer (ADG708 BRU, from Analogue Devices), to the internal 10 bit analogue-to-digital (A/D) converter in the MSP430F2122, that digitizes the voltage at thermistors. The microcontroller wakes-up every twelve minutes and reads all the sensors. The digitized values of temperature (from the flat panel and the buried sensors) are stored in the internal memory of the microcontroller, and after five measurements (every hour), the data stored in the microcontroller is transmitted to an external USB flash memory, via an FT232 RL interface (from FTDI). Since the microcontroller runs with a Real Time Clock \(f=32.768\) KHz crystal, every packet of data includes the time stamp of the measurement. At the zero hour of each day, the microcontroller opens a new file in the USB flash drive (the file name is the actual date, in the format \"Pxxx.txt\" (where xxx is the number of the day, starting with \"P000.txt\"). The data acquired during the whole day is written to this file. This simplifies searching for the data of any specific day. To remove the flash drive, a push-button must be pressed, and if a green LED in the PCB turns on, it is safe to remove the USB flash drive. After this push-button is pressed, the system checks if the microcontroller is sending data to the USB port. If the microcontroller is communicating with the USB, the system waits for the end of the transmission, interrupts any future communication between the microcontroller and the USB, and lights up the LED. If the system detects that no transmission is occurring, it interrupts any future communication between the microcontroller and the USB and turns on the LED. Figure 2: Sketch and photograph of the flat panel with temperature sensor and data-logger. In both cases the microcontroller holds the next packet of data until the USB flash drive is reinserted in the circuit and a second push button is pressed. When this push-button is pressed, the microcontroller checks if the USB flash driver is present. If present, it blinks the LED three times, informing that the communication was re-established and the last packets of data (that were on hold) are immediately transmitted. With this procedure, the USB flash drive can be left out of the circuit for up to 72 h before being reinserted. The circuit is most of the time in sleeping mode (with the microcontroller running in low-power-mode and the other ICs in shut-down mode), so that the current consumption of the circuit in this state is only \(\approx 2.5\,\upmu\)A. Since the circuit wakes up only for about one second every twelve minutes (to read the sensors and write the data to the microcontroller) and for about five seconds every one hour (to write the data to the USB flash drive), we powered the system with 4 \(\times\) 1.5 V D-type battery cells in series, which is enough to operate the system for more than one year. Concerning the temperature measurements with thermistors, since the UPS7982 is a 10 \(\,\upmu\Omega\,\pm 1\%\), to compensate for these deviations in the value of the thermistors, a calibration (implemented in the software that converts the voltage to temperature) was necessary.