USING ELECTRICAL RESISTIVITY IMAGING TO EXAMINE SOIL MOISTURE AND TREE TRANSPIRATION INTERACTIONS

Determining the relationships between tree transpiration, soil moisture content, and stream discharge is important in understanding how natural processes affect the amount of water available for human use. Electrical Resistivity (ER) imaging presents a minimally invasive method that could be used to explore subsurface flow paths of water and how these are affected by tree transpiration. We used ER to examine interactions between ponderosa pine transpiration and soil moisture throughout an 8 by 8 grid of stainless steel electrodes inserted 20cm into the soil. We collected 30-48 continuous hours of soil ER data per week for seven weeks and measured sapflow on eight nearby trees at 30 min intervals. We found diel fluctuations in soil ER with 25 ohm-m differences between peak and minimum values. These diel shifts in soil ER are strongly correlated with diurnal patterns of tree transpiration observed in sap-flow data, with correlation coefficients ranging from 0.86-0.95. In contrast, correlation coefficients of approximately 0.20 for data obtained from soil moisture sensors against sap flow data demonstrate that, unlike the more spatially detailed ER measurements, single point measurements are less likely to pick up subtle or deep moisture changes adjacent to the root zone of a tree. Diel fluctuations in soil ER lagged behind sapflow by an average of 4 hrs. We hypothesize these lags are due to trees initially using stored water from the xylem. Thus, even though the tree begins transpiring it does not immediately draw water from the soil. Future work will include correcting ER data to ensure data are purely representative of soil moisture trends. Furthermore, the data will be transformed into subsurface images of the soil and, ideally, will show the flow patterns of water in the soil throughout the day.