Examining the Potentials and Limitations of Using Temperature Tracing to Infer Water Flux through Unsaturated Soils

Abstract

Irrigation agriculture is the primary consumer of water worldwide. Because groundwater resources are being depleted, there is an increasing need to measure subsurface flux to quantify consumptive use and return flow. This study used HYDRUS-1D to simulate one-dimensional vertical movement of water and heat. Temperature time series were analyzed for different materials, depths, and applied fluxes to understand the limitations of temperature methods for monitoring steady-state water flux under unsaturated conditions. The conditions used in this analysis were intended to explore the minimum standards needed for unsaturated, steady-state flux estimation. If these conditions cannot be met, then it is unnecessary to look at more challenging conditions (e.g., heterogeneity, transient flux, and unknown hydraulic and thermal properties). Previous work has suggested that there is a minimum flux that is measurable with temperature methods. The low saturated hydraulic conductivities of fine-grained materials limit the applicability of temperature methods to high fluxes that lead to nearsaturated conditions for these materials. Although coarser soils have higher hydraulic conductivities, thermal diffusivity increases significantly with decreasing water content and may still complicate flux measurements for unsaturated conditions. As a result, we conclude that temperature-based methods in unsaturated conditions will most likely fail even with the most simplifying assumptions for most soil types. Therefore temperature methods should be restricted to monitoring steady-state fluxes in coarse soils and/or conditions near saturation.Worldwide, irrigation agriculture is the primary consumer of water (?90%), accounting for approximately 70% of freshwater withdrawals, 49% of which is irrigation with either solely groundwater or a combination of surface and groundwater (Siebert et al., 2010). The high demand for groundwater use is dramatically altering the quality and quantity of available groundwater. Groundwater storage is being exploited at higher rates than groundwater recharge, although the exact rates of withdrawal and return flow are poorly understood (Siebert et al., 2010).Interest in reducing irrigation must be balanced by the need to provide irrigation excess to avoid soil salinization. This practice results in some water percolating through the root zone, with some of it recharging underlying aquifers as return flow. To better manage irrigated agriculture, measurement of downward flux at depth is required to quantify this flow. There are several methods used to understand aquifer recharge and directly measure percolation rates (Kalbus et al., 2006). However, the level of uncertainty in groundwater recharge estimates is difficult to determine (Healy, 2010;Siebert et al., 2010). As the global population rises, we will need practical methods to quantify agricultural consumption and return flow.The focus of this study was to explore the potential and limitations of using subsurface temperature signals to infer return flow. For b...

Theory, tools, and multidisciplinary applications for tracing groundwater fluxes from temperature profiles

“…Second, groundwater fluxes are also assumed to be at steady‐state, even for the transient thermal equation (Equation ). Groundwater fluxes vary seasonally in the vadose zone and in shallow aquifers and thereby limit heat tracing applications (Clutter & Ferré, ). Although these variations are damped with depth (Dickinson, Ferre, Bakker, & Crompton, ), long‐term changes in groundwater recharge due to climate change (e.g., Kurylyk & MacQuarrie, ; Scibek & Allen, ; Taylor et al, ) or land cover change (Ranjan et al, ; Scanlon et al, ) impact deeper groundwater flow regimes and likely invalidate steady‐state flow assumption in certain cases.…”

Dynamics of groundwater recharge near a semi-arid Mediterranean intermittent stream under wet and normal climate conditions

“…Therefore, in these basins, mountain front groundwater recharge linked to streamflow is often considered the primary source of recharge (de Vries and Simmers, 2002;Manning and Solomon, 2003;Simmers, 2003;Wilson and Guan, 2004;Dahan et al, 2008). The infiltration processes of streamflow have been widely analyzed (Ronan et al, 1998;Stonestrom and Constantz, 2003;Niswonger et al, 2005;Keery et al, 2007;Rau et al, 2010;Lautz, 2012;Clutter and Ferré, 2018;Kurylyk et al, 2019). However, there are few applied studies on the processes of groundwater recharge and its temporal variability in arid and semi-arid basins (Shanafield and Cook, 2014).…”