Electrical resistivity tomography of irrigation event on potato in Belgium

The soil at the field site (N51 12:2420, E457:5220) is a podzol (WRB). Potatoes (Solanum tuberosum L., Zorba variety) were planted on 14/4/2018 and harvested on 10/8. The 2018 growing season was characterized by a wet, warm start in the spring, followed by a dry and warm end of spring and a very dry and very warm summer. The field was irrigated over a time span of two days per irrigation event with a hose reel sprinkler with a flow rate of 40 m³/hour. In total, the farmer applied 190 mm of water, distributed over seven irrigation events between 11/6 and 28/7. Between planting and harvest, the total reference evapotranspiration (ETo) was 439 mm and the total rainfall was 114 mm. We measured soil temperature (T) and volumetric water content (WC) with 5TE sensors at three depths beneath the second ridge (-0.20, -0.40 and -0.60 m) and at -0.10 m beneath the second furrow. These sensors were located at 0.3 m north from the ERT transect. The ERT transect is composed of 32 stainless steel electrodes covering a series of four ridges-furrows. Electrodes are 0.13 m long and have a 0.05m diameter. The minimum electrode spacing projected on the x-axis is 0.066m. We conducted an array of 1342 measurements (dipole-dipole and wenner-alpha) with the MPT-DAS-1 containing normal and reciprocal measurements. We invert the data using a smoothness constrained inversion. We used the QuadTree mesh of the SimPEG package Discretize.

The ERT measurements took place after a first dry period. During this period the reference evapotranspiration (ET0) varied around 10 mm.day-1 and a first irrigation event of 25 mm took place on 13/06. In the furrow, soil moisture reacts immediately after irrigation with a quick rise of more than 20% and then a strong decrease to the initial level over a timespan of about 8 days. The sensors in the ridge show little or no reaction to the irrigation event and the soil moisture is even showing a downward trend over the month. The start situation is characterized by a rather dry surface and also two soil horizons with different conductivities. The boundary between these two horizons is situated around 0.25cm. As the irrigation event occurs, conductivity increases, especially in the furrows and then decreases slowly again over time. As the sensitivity of the used electrode array is located mainly in the top 25 cm, the smoothness constrained-inversion smears structures downward.