Soil Moisture In Unsaturated Zone Research Articles

Investigations of the unsaturated zone at two radioactive waste disposal sites in Lithuania

ABSTRACTThe unsaturated zone is an important part of the water cycle, governed by many hydrological and hydrogeological factors and processes and provide water and nutrients to the terrestrial ecosystem. Besides, the soils of the unsaturated zone are regarded as the first natural barrier to a large extent and are able to limit the spread of contaminants depending on their properties. The unsaturated zone provides a linkage between atmospheric moisture, groundwater, and seepage of groundwater to streams, lakes, or other surface water bodies. The major difference between water flow in saturated and unsaturated soils is that the hydraulic conductivity, which is conventionally assumed to be a constant in saturated soils, is a function of the degree of saturation or matrix suction in the unsaturated soils. In Lithuania, low and intermediate level radioactive wastes generated from medicine, industry and research were accumulated at the Maisiagala radioactive waste repository. Short-lived low and intermediate levels radioactive waste, generated during the operation of the Ignalina Nuclear Power Plant (INPP) and arising after the INPP decommissioning will be disposed of in the near surface repository close to the INPP (Stabatiske site). Extensive data sets of the hydraulic properties and water content attributed to unsaturated zone soil profiles of the two radioactive waste disposal sites have been collected and summarized. Globally widespread radionuclide tritium (3H) and stable isotope ratio (18O/16O and 2H/1H) distribution features were determined in precipitation, unsaturated zone soil moisture profiles and groundwater.

Improving soil moisture profile reconstruction from ground-penetrating radar data: a maximum likelihood ensemble filter approach

Abstract. The vertical profile of shallow unsaturated zone soil moisture plays a key role in many hydro-meteorological and agricultural applications. We propose a closed-loop data assimilation procedure based on the maximum likelihood ensemble filter algorithm to update the vertical soil moisture profile from time-lapse ground-penetrating radar (GPR) data. A hydrodynamic model is used to propagate the system state in time and a radar electromagnetic model and petrophysical relationships to link the state variable with the observation data, which enables us to directly assimilate the GPR data. Instead of using the surface soil moisture only, the approach allows to use the information of the whole soil moisture profile for the assimilation. We validated our approach through a synthetic study. We constructed a synthetic soil column with a depth of 80 cm and analyzed the effects of the soil type on the data assimilation by considering 3 soil types, namely, loamy sand, silt and clay. The assimilation of GPR data was performed to solve the problem of unknown initial conditions. The numerical soil moisture profiles generated by the Hydrus-1D model were used by the GPR model to produce the "observed" GPR data. The results show that the soil moisture profile obtained by assimilating the GPR data is much better than that of an open-loop forecast. Compared to the loamy sand and silt, the updated soil moisture profile of the clay soil converges to the true state much more slowly. Decreasing the update interval from 60 down to 10 h only slightly improves the effectiveness of the GPR data assimilation for the loamy sand but significantly for the clay soil. The proposed approach appears to be promising to improve real-time prediction of the soil moisture profiles as well as to provide effective estimates of the unsaturated hydraulic properties at the field scale from time-lapse GPR measurements.

Experimental Study and Numerical Modeling of Surface/Subsurface Flow at Field Scale

Infiltration which occurred in the process of runoff on farmland surface causes the increasing of soil moisture in unsaturated zone. It’s necessary to comprehend the dynamic process during farmland runoff process, analyze the coupling mechanism of water movement and solute transport among surface/subsurface zone. In this paper, a physically-based, spatially-distributed model was built for simulation of surface/subsurface flow and the interactions between two domains. The system is represented by the three-dimensional saturated–unsaturated flow equation for the subsurface, coupled with the diffusion wave equation for overland flow, Ground surface unevenness at the grid scale is incorporated via the concept of detention storage. The system of equations is discretized using a fully implicit procedure, with the Newton-Raphson method to handle non-linearity of the equations. In order to assess this modeling approach, the simulations compared with the experiment results. It shows that the model can simulate the process of farmland runoff production and soil water movement accurately. Finally, Statistics and mass balance analysis was also performed to examine the model accuracy and response to changes of specific input data.

Impacts of Unsaturated Zone Soil Moisture and Groundwater Table on Slope Instability

The combined effect of soil moisture in unsaturated soil layers and pore-water pressure in saturated soil layers is critical to predict landslides. An improved infinite slope stability model, that directly includes unsaturated zone soil moisture and groundwater, is derived and used to analyze the factor of safety’s sensitivity to unsaturated zone soil moisture. This sensitivity, the change in the factor of safety with respect to variable unsaturated zone soil moisture, was studied at local and regional scales using an active landslide region as a case study. Factors of safety have the greatest sensitivity to unsaturated zone soil moisture dynamics for shallow soil layers (<2 m) and comparatively deep groundwater tables (1 m). For an identical groundwater table, the factor of safety for a 1 m thick soil mantle was four times more sensitive to soil moisture changes than a 3-m thick soil. At a regional scale, the number of unstable areas increases nonlinearly with increasing unsaturated zone soil moisture and with moderately wet slopes exhibiting the greatest sensitivity.

Soakage testing: modelling the effect of antecedent hydrology

Current design methods for sustainable drainage system infiltration devices such as soakaways, permeable pavements and infiltration basins require knowledge of the empirical soil infiltration coefficient, which is determined using a soakage test procedure. However, the control that seasonal antecedent hydrological conditions, specifically unsaturated zone soil moisture and water table position, have on the infiltration coefficient is uncertain. This paper uses exploratory numerical modelling to investigate this uncertainty. The results show that antecedent water table position has a significant effect on the infiltration coefficient, while for a properly conducted test the seasonal antecedent soil moisture is less important. To avoid overestimating the infiltration coefficient, soakage testing should ideally be performed under the most onerous hydrological conditions and three successive drain downs must be used. Model outputs such as those presented can be used to reduce the design infiltration coefficient to account for tests made during periods of lower groundwater level and higher design water table positions. Additionally, the current infiltration coefficient calculation is shown to be biased towards the initial, more rapid, infiltration rate. Therefore, an alternative full-depth infiltration coefficient calculation is proposed. Finally, a potential detrimental feedback is hypothesised between infiltration device performance and groundwater mounding. Significant design implications are discussed.

Effects of initialization on response of a fully-distributed hydrologic model

Summary Knowledge of initial conditions is very important to correctly model the basin response at the storm event scale. Of particular interest is the influence of topography and soil type on the principal hydrologic variables and runoff generation mechanisms as a function of antecedent wetness conditions. This study addresses the influence of initial states on the short-term hydrologic response and characterizes the effects of topography and soils on the dissipation of the influence of the initialization conditions. Two case studies are considered: a synthetic two-dimensional planar hillslope with various assumed slope magnitudes and soil types; and a real basin (∼800 km2) with actual land-surface characteristics. These case studies have been chosen in order to investigate the hydrological behavior at two different scales. Various precipitation rates and durations are used to force both domains, each initialized assuming different positions of spatially variable water table. The results illustrate the existence of complex mechanisms that modulate the watershed response depending on the catchment initial state and rainfall forcing. Factors contributing to the unsaturated zone soil moisture movement are discussed in relation to different soils and topographic properties. The role of rainfall intensity in determining the degree of control exerted by initial conditions on the total runoff as well as its partition into various mechanisms is examined for both case studies. Differences in the sensitivity of certain terrain locations to the initialization and the rainfall rates are revealed. In particular, the real case study points out that the effects of initial conditions strongly depend on rainfall intensity and that the spatial hydrologic variations is influenced by the initial groundwater position.

Estimating profile soil moisture and groundwater variations using GRACE and Oklahoma Mesonet soil moisture data

In this study we estimate a time series of regional groundwater anomalies by combining terrestrial water storage estimates from the Gravity Recovery and Climate Experiment (GRACE) satellite mission with in situ soil moisture observations from the Oklahoma Mesonet. Using supplementary data from the Department of Energy's Atmospheric Radiation Measurement (DOE ARM) network, we develop an empirical scaling factor with which to relate the soil moisture variability in the top 75 cm sampled by the Mesonet sites to the total variability in the upper 4 m of the unsaturated zone. By subtracting this estimate of the full unsaturated zone soil moisture anomalies, we arrive at a time series of groundwater anomalies, spatially averaged over a region approximately 280,000 km2 in area. Results are compared to observed well level data from a larger surrounding region, and show consistent phase and relative inter‐annual variability.

Comparing Performance and Parameterization of a One‐Dimensional Unsaturated Zone Model across Scales

The utility of an unsaturated zone soil moisture model is not only its ability to describe the soil moisture dynamics at a given point but also the possibility to generalize the results to larger areas. In this study we investigated the predictive performance of a one‐dimensional unsaturated zone soil moisture model when applied at point, field, response unit, and catchment scales, using detailed field observations from a 0.42‐km2 catchment in the Netherlands. Our main question was how model parameterization and model performance could be compared across these scales. We considered two different calibration–validation schemes and three performance statistics. In all cases we applied the same Levenberg–Marquardt optimization scheme. Differences between calibration–validation schemes (interpolation vs. extrapolation) were surprisingly small. Using one particular model parameterization across the various aggregation levels, the optimal Mualem–van Genuchten parameters for a coarser aggregation level can be derived from an underlying level by simple arithmetic averaging. The different performance indices (RMSE, index of agreement, and Nash–Sutcliffe coefficient) were highly variable between observation locations and for different aggregation levels. Overall, the indices were more favorable at higher aggregation levels, and in correspondence with errors reported in comparable studies. The unsaturated‐zone model did not, however, provide satisfactory predictions of independent flux observations, in this case daily catchment discharge. Moreover we did not succeed in deriving a meta‐model to scale model performance indices with aggregation level. Our case study therefore supports the view that multiscale calibration studies that use both state and flux observations are required to compare results from unsaturated zone models at different aggregation levels.

Modelling the energy balance of a natural jarrah ( Eucalyptus marginata) forest

This paper presents the development of the energy balance components of a catchment scale model (COUPLE) that couples soil moisture storage, surface energy balance and the atmospheric boundary layer. The model determines the energy balance at a canopy surface and the soil surface by coupling bulk stomatal resistance and soil surface vapour pressure to the soil moisture content. Heat storage in the trunks and branches is modelled using the “force-restore” method widely used for soil heat flux. We test the parameterisations in the model against field data, and compare values obtained with those from the literature. Literature parameterisations for soil surface vapour pressure and stomatal conductance are assessed. While some existing parameterisations were found to be adequate, others required modification for this environment. In particular, the relationship between canopy conductance and vapour pressure deficit was found to be exponential, rather than the commonly used linear one. Evaporation at the soil surface was well modelled by combining the simple surface vapour pressure representation suggested by Lee and Pielke [J. Appl. Meteorol. 31 (1992) 480], and a catchment scale unsaturated zone soil moisture storage. Energy flux data were collected during two field exercises in a natural eucalypt forest with a canopy height of 25 m during spring and summer, giving data for periods of minimum and maximum water stress. We show that while the inclusion of more locally appropriate parameterisations only slightly improved the overall energy and water balance of the model, the partitioning of moisture flux between soil evaporation and transpiration was substantially improved. The data indicated that heat storage in the deep canopy was a substantial proportion of the energy budget at the beginning and end of the day, with heat stored in the trunks the major component of it. The “force-restore” method proved adequate to simulate this, and is relatively simple to implement.

Unsaturated zone soil moisture behavior under temperate humid climatic conditions — Tensiometric observations and numerical simulations

Soil moisture behavior in the unsaturated zone in a temperate humid climate is discussed based on tensiometric observations in an experimental basin near Tokyo, Japan, and on related numerical simulations. Initially, through an examination of observed data, some salient features of annual soil moisture fluctuations are pointed out. Characteristics of soil water movement during rainfall and continuously dry periods are closely investigated Next, the hydraulic properties required for simulation purposes are examined and determined, based on the results of laboratory tests on soil samples taken from the tensiometer site. Finally, changes of potential profile in the unsaturated zone both during rainless periods and during rainfall are simulated by a numerical model using Richards' flow equation, and the results are discussed in comparison with the observed data.