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Title: Monitoring scale-specific and temporal variation in electromagnetic conductivity images

Author
item HUANG, J - University Of New South Wales
item SCUDIERO, ELIA - University Of California
item BAGTANG, MICHAEL - Scotts Brother Dairy
item Corwin, Dennis
item TRIANTAFILIS, JOHN - University Of New South Wales

Submitted to: Irrigation Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/17/2016
Publication Date: 3/4/2016
Citation: Huang, J., Scudiero, E., Bagtang, M., Corwin, D.L., Triantafilis, J. 2016. Monitoring scale-specific and temporal variation in electromagnetic conductivity images. Irrigation Science. doi: 10.1007/s00271-016-0496-6.

Interpretive Summary: Stream tubes are spatial domains of soil that behave similarly with respect to the flow of water and transport of solutes through the vadose zone (i.e., soil comprising the zone from the soil surface to the groundwater table). Stream tubes are of particular interest because they simplify the parameterization of field-scale solute transport models by providing a spatial means of addressing the complex spatial heterogeneity of field soils. Fundamental to the delineation of stream tubes is a 2-D and 3-D understanding of the moisture regime within an entire field. One way to do this is to use electromagnetic (EM) induction instruments because the apparent soil electrical conductivity (ECa – mS/m) has been used to map average volumetric soil moisture across landscapes. The use of EM induction is particularly advantageous because it can be mobilized to provide detailed geo-referenced maps of salinity, water content, and/or clay content with minimal cost and effort. In this paper we show how the electrical conductivity can be correlated with measurements of clay and water content at various depths. In addition, we show how the spatio-temporal variation in soil water content is scale specific. We demonstrate this by using 2-dimensional multi-resolution analysis (MRA) to characterize the spatio-temporal variation in water content beneath a pivot irrigated alfalfa (Medicago sativa) along a 350-m transect at different depths. The dominant scales of the variation are 37 m, 18 m and 9 m. We conclude that the variations in water content at these scales are controlled by both the location of irrigation sprinklers (~ 6 m) and the compacted soils due to the tire tracks (~ 40 m). The distribution of the different MRA components at different scales indicates important management implications and important concerns for delineating stream tubes. At the shorter scale of 4.5 m the MRA showed that there was little variation horizontally, which indicates irrigation is fairly uniform across the field and in particular in the topsoil. At a larger spatial scale of 18 m the MRA indicated that there was variation between subsurface and subsoil. We attribute this to the wetting front. At the spatial scales of 37 and 75 m the large variation in the horizontal direction is attributable to the change in soil texture across the field and the decreasing trend in salinity. This information and the methodlogy is of value to agricultural and environmental modellers at universities and government agencies worldwide.

Technical Abstract: In the semi-arid and arid landscapes of southwest USA, irrigation sustains agricultural activity; however, there are increasing demands on water resources. As such spatial temporal variation of soil moisture needs to be monitored. One way to do this is to use electromagnetic (EM) induction instruments because the apparent soil electrical conductivity (ECa – mS/m) has been used to map average volumetric soil moisture (cm3/cm3) across catenary sequences. However, depth-specific variation'which is important for irrigation management, has not been explored. In this paper we show how the calculated true electrical conductivity (mS/m) can be correlated with measurements of clay and volumetric soil moisture at various depths. In addition, we aim to show how other soil properties influence true electrical conductivity and how the spatio-temporal variation in volumetric soil moisture is scale-specific. We demonstrate this by using 2-dimensional multi-resolution analysis (MRA) to characterize the spatio-temporal variation in true electrical conductivity beneath a pivot irrigated alfalfa (Medicago sativa) along a 350-m transect and at different depths. The dominant scales of the variation in the ' are 37 m, 18 m and 9 m. We conclude that the variations in soil true electrical conductivity at these scales are controlled by both the location of irrigation sprinklers (~ 6 m) and the compacted soils due to the tire tracks (~ 40 m). The distribution of the different MRA components at different scales indicates important management implications. Firstly, and at the shorter scales of 4.5 m the MRA showed that there was little variation in the horizontal, which indicates irrigation is fairly uniform across the field and in particular in the topsoil. Secondly, and at spatial larger scale of 18 m the MRA indicated that there was variation between subsurface and subsoil true electrical conductivity. We attribute this to the wetting front and on days 1 and 4 in particular. Lastly, and at the spatial scale of 37 and 75 m the large variation in the horizontal direction is attributable to the change in soil texture across the field and the decreasing trend in ECe.