More than 50% of the groundwater recharge in Norway takes place during snowmelt. Given the possible threat to groundwater quality caused by potentially rapid transport through the unsaturated zone, ...it is important to understand the infiltration processes that take place during snowmelt, and the factors that control the temporal and spatial variability of such processes. Here, we report on the results of an experimental study of infiltration during the snowmelt period of 2001. The study was carried out at a well-characterised field plot, close to Oslo Airport. In order to examine the spatial and temporal variability of snowmelt infiltration, a series of electrical resistivity surveys were carried out using electrodes installed below the ground surface and in shallow boreholes. The results from this time-lapse survey reveal significant changes over time, and suggest that localised infiltration takes place. The patterns of inferred increases in saturation are consistent with observed reductions in snow cover and appear to be principally controlled by variations in microtopography. Resistivity changes observed at depth, using the borehole-based electrodes, show rapid percolation through the unsaturated profile. Such behaviour is consistent with observed rapid changes in local groundwater levels. The results confirm the potential threat to groundwater quality during snowmelt and illustrate the spatial scale of processes that require adequate attention in groundwater management in vulnerable areas.
There is growing interest in the use of geophysical methods for hydrological model parameterization. Empirical induced polarization (IP)–hydraulic conductivity (K) relationships have been developed, ...but these are only applicable to sediments in which the IP response shows limited variation with electrical current frequency. Here we examine the spectral IP response of samples taken from a UK sandstone aquifer and compare measured parameters with physical and hydraulic properties. We demonstrate the limited value of existing IP‐K models due to the inherent IP frequency dependence of these samples. Our results show how the mean relaxation time, τ, is a more appropriate measure of IP response for these sediments. A significant inverse correlation between the surface area to pore volume ratio and τ is observed, suggesting that τ is a measure of a characteristic hydraulic length scale. This is supported by a measured strong positive correlation between log τ and log K. Our measurements also reveal evidence of a relationship between τ and a dominant pore throat size, which leads to postulations about the parallelism between the spectral IP behavior and unsaturated hydraulic characteristics. Additional experiments show how the relaxation time is affected by degree of fluid saturation, indicating that saturation levels must be accounted for if our empirical relationships are applied to vadose zone studies. Our results show clear evidence of the potential value of frequency‐based IP measurements for parameterization of groundwater flow models.
A saline tracer test has been performed at an industrial site, located in the Po River delta region (North-Eastern Italy). This tracer test was aimed at identifying possible hydraulic connections ...between a shallow unconfined aquifer and a deeper confined aquifer. Hydraulic head difference between the two aquifers is maintained via an active hydraulic barrier (pumping wells) screened in the deeper formation. The movement of the tracer in the subsurface has been monitored via electrical resistivity tomography (ERT) from the surface, using a network of twelve 48-electrode lines, for a total time period of about 45 days. This non-invasive geophysical method offers several advantages over direct water sampling from monitoring boreholes: (a) a wide area is investigated in a relatively short time; (b) a 2D picture of the subsurface is produced, in this case to a depth of about 15–20 m; (c) because of the above, it is possible to follow the tracer motion even if the tracer does not reach the few boreholes where water can be sampled. The latter is a major advantage over traditional methods, where it is not uncommon to miss the tracer plume altogether, or sample it only partially. The site is characterized by fairly saline groundwater, and the presence of diffuse silt and clay, that make the system rather conductive. Under such unfavourable conditions, careful inversion of resistance ratios was needed to identify changes in resistivity of as much as 50% with respect to background. The migration of the tracer was successfully monitored and confirmed by direct sampling of water from few boreholes. The results of the time lapse survey confirm that the system is very heterogeneous, and that some downward migration of the tracer through the confining layer can take place over relatively short time periods, maybe through some manmade connections.
Geophysical surveys are now commonly used in agriculture for mapping applications. High‐throughput collection of geophysical properties such as electrical conductivity (inverse of resistivity) can be ...used as a proxy for soil properties of interest (e.g., moisture, texture, salinity). Most applications only rely on a single geophysical survey at a given time. However, time‐lapse geophysical surveys have greater capabilities to characterize the dynamics of the system, which is the focus of this work. Assessing the impact of agricultural practices through the growth season can reveal important information for the crop production. In this work, we demonstrate the use of time‐lapse electrical resistivity tomography (ERT) and electromagnetic induction (EMI) surveys through a series of three case studies illustrating common agricultural practices (cover crops, compaction with irrigation, and tillage with N fertilization). In the first case study, time‐lapse EMI reveals the initial effect of cover crops on soil drying and the absence of effect on the subsequent main crop. In the second case study, compaction leading to a shallower drying depth for potatoes (Solanum tuberosum L.) was imaged by time‐lapse ERT. In the third case study, larger changes in electrical conductivity over time were observed in conventional tillage compared with direct drill using time‐lapse EMI. In addition, different N application rates had a significant effect on the yield and leaf area index but only ephemeral effects on the dynamics of electrical conductivity, mainly after the first application. Overall, time‐lapse geophysical surveys show great potential for monitoring the impact of different agricultural practices that can influence crop yield.
Geophysical methods, such as electromagnetic induction (EMI), can be effective for monitoring changes in soil moisture at the field scale, particularly in agricultural applications. The electrical ...conductivity (σ) inferred from EMI needs to be converted to soil moisture content (θ) using an appropriate relationship. Typically, a single global relationship is applied to an entire agricultural field; however, soil heterogeneity at the field scale may limit the effectiveness of such an approach. One application area that may suffer from such an effect is crop phenotyping. Selecting crop varieties based on their root traits is important for crop breeding and maximizing yield. Hence, high‐throughput tools for phenotyping the root system architecture and activity at the field scale are needed. Water uptake is a major root activity and, under appropriate conditions, can be approximated by measuring changes in soil moisture from time‐lapse geophysical surveys. We examine here the effect of heterogeneity in the θ–σ relationship using a crop phenotyping study for illustration. In this study, the θ–σ relationship was found to vary substantially across a field site. To account for this, we propose a range of local (plot specific) θ–σ models. We show that the large number of parameters required for these models can be estimated from baseline σ and θ measurements. Finally, we compare the use of global (field scale) and local (plot scale) models with respect to ranking varieties based on the estimated soil moisture content change.
Core Ideas
Early‐time amplitude analysis requires understanding of site, system, and methodology.
The AEA and CFA both have benefits and disadvantages.
Time window selection is important when shallow ...reflectors may be present.
Early time GPR results are highly frequency dependent.
Improving irrigation efficiency requires accurate assessment of the soil moisture distribution in time and space, but obtaining accurate observational data is challenging. Early‐time signal (ETS) amplitude analysis of ground‐penetrating radar (GPR) data may permit such rapid noninvasive characterization. In this study we performed controlled irrigation experiments using a multifrequency GPR system to compare two statistics used to quantify the ETS: average envelope amplitude (AEA) and carrier frequency amplitude (CFA). Supporting data were provided by direct measurements, electrical resistivity imaging (ERI), and synthetic modeling. In our first experiment, both statistics successfully related the ETS for 250 and 400 MHz GPR data to increasing water content. However, the 400 MHz AEA lost sensitivity at later stages of the irrigation process, whereas the 400 MHz CFA remained sensitive to changes in water content. The 1000 MHz data did not show the expected relationships, possibly due to shallow reflectors, such as the wetting front, which the higher frequency antennae would have a greater chance of detecting, as supported by synthetic modeling. In our second experiment, we focused on the effect of the time window on calculating ETS statistics. We demonstrate that, when there is interference in the ETS, using a shorter time window instead of the more common first positive half cycle improves correlation with soil moisture content. Our work shows that the GPR ETS data respond to changes in soil water content in similar fashion to ERI data.
Freshwater resources are becoming increasingly scarce in coastal areas, limiting crop productivity in coastal farmlands. Although the characteristic of crop water use is an important factor for water ...conservation in coastal farmlands, it has not been studied extensively. This study aimed to depict the water use process of soil-plant systems under saline stress in coastal ecosystems and optimize water management. An intensive observation experiment was performed within China's Yellow River Delta to identify the water use processes and crop coefficients (K
) and also quantify the impacts of salt stress on crop water use. The results show that shallow groundwater did not contribute to soil water in the whole rotation; K
values for wheat-maize, wheat-sorghum, and wheat-soybean rotation systems were 45.0, 58.4, and 57% less, respectively, than the FAO values. The water use efficiency of the maize (8.70) and sorghum (9.00) in coastal farmlands was higher than that of the soybean (4.37). By identifying the critical periods of water and salt stress, this paper provides suggestions for water-saving and salinity control in coastal farmlands. Our findings can inform the sustainable development of coastal farmlands and provide new insights to cope with aspects of the global food crisis.
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