Interactions of Soil Moisture and Soil Temperature in an Irrigated Field.

Two-minute interval soil temperature and soil moisture were measured at six depths in a wastewater spray irrigation field from 2008 to 2012. The relationship and dynamics of soil moisture and soil temperature at two irrigated sites – one cropped and another forested - were compared with two corresponding non-irrigated sites. Determination of long-term trends for the entire period revealed negative correlations between temperature and moisture at most, but not all, depths (slopes from 0.02 to -0.35%VWC/°C). Seasonal and annual variation was studied, with summers displaying the narrowest temperature ranges (~40% of average Spring/Fall range) but the widest moisture ranges (~120% of average Spring/Fall range) at all depths. Event-driven temperature-moisture dynamics were studied in deep soil (90 – 100 cm), revealing rapid transfer of thermal conditions at the surface, mediated by shallow soil conditions (± 2.5°C change at 90 cm depth possible from an individual event). Finally, diurnal temperature variation was found to be influenced by soil moisture at high mean temperatures, interpreted as a result of increased thermal diffusivity.