Résultat de la recherche
Salix psammophila afforestations can cause a decline of the water table, prevent groundwater recharge and reduce effective infiltration
Afforestation can reduce desertification and soil erosion. However, the hydrologic implications of afforestation are not well investigated, especially in arid and semi-arid regions. China has the largest area of afforestation in the world, with one-third of the world's total plantation forests. How the shrubs affect evapotranspiration, soil moisture dynamics, and groundwater recharge remains unclear. We designed two pairs of lysimeters, one being 1.2 m deep and the other one 4.2 m deep. Each pair consists of one lysimeter with bare soil, while on the other one a shrub is planted. The different water table depths were implemented to understand how depth to groundwater affects soil moisture and water table dynamics under different hydrological conditions. Soil moisture, water table depth, sap flow, and rainfall were measured concurrently. Our study confirms that for the current meteorological conditions in the Ordos plateau recharge is reduced or even prohibited through the large-scale plantation Salix psammophila. Shrubs also raise the threshold of precipitation required to increase soil moisture of the surface ground. For the conditions we analyzed, a minimum of 6 mm of precipitation was required for infiltration processes to commence. In addition to the hydrological analysis, the density of root distribution is assessed outside of the lysimeters for different water table depths. The results suggest that the root-density distribution is strongly affected by water table depth. Our results have important implications for the determination of the optimal shrub-density in future plantations, as well as for the conceptualization of plant roots in upcoming numerical models.
Estimating the Spatial Extent of Unsaturated Zones in Heterogeneous River Aquifer Systems
The presence of unsaturated zones at the river‐aquifer interface has large implications on numerous hydraulic and chemical processes. However, the hydrological and geological controls that influence the development of unsaturated zones have so far only been analyzed with simplified conceptualizations of flow processes, or homogeneous conceptualizations of the hydraulic conductivity in either the aquifer or the riverbed. We systematically investigated the influence of heterogeneous structures in both the riverbed and the aquifer on the development of unsaturated zones. A stochastic 1‐D criterion that takes both riverbed and aquifer heterogeneity into account was developed using a Monte Carlo sampling technique. The approach allows the reliable estimation of the upper bound of the spatial extent of unsaturated areas underneath a riverbed. Through systematic numerical modeling experiments, we furthermore show that horizontal capillary forces can reduce the spatial extent of unsaturated zones under clogged areas. This analysis shows how the spatial structure of clogging layers and aquifers influence the propensity for unsaturated zones to develop: In riverbeds where clogged areas are made up of many small, spatially disconnected patches with a diameter in the order of 1 m, unsaturated areas are less likely to develop compared to riverbeds where large clogged areas exist adjacent to unclogged areas. A combination of the stochastic 1‐D criterion with an analysis of the spatial structure of the clogging layers and the potential for resaturation can help develop an appropriate conceptual model and inform the choice of a suitable numerical simulator for river‐aquifer systems.
Heterogeneous or homogeneous? Implications of simplifying heterogeneous streambeds in models of losing streams
A common approach in modeling surface water–groundwater interaction is to represent the streambed as a homogeneous geological structure with hydraulic properties obtained by means of model calibration. In reality, streambeds are highly heterogeneous, and there are currently no methodical investigations to justify the simplification of this geologic complexity. Using a physically based numerical model, synthetic surface water–groundwater infiltration flux data were generated using heterogeneous streambeds for losing connected, losing transitional and losing disconnected streams. Homogeneous streambed hydraulic conductivities were calibrated to reproduce these fluxes. The homogeneous equivalents were used for predicting infiltration fluxes between streams and the aquifer under different hydrological conditions (i.e. for different states of connection). Homogeneous equivalents are shown to only accurately reproduce infiltration fluxes if both the calibration and prediction are made for a connected flow regime, or if both the calibration and prediction are made for a disconnected flow regime. The greatest errors in flux (±34%) using homogeneous equivalents occurred when there was a mismatch between the flow regime of the observation data and the prediction. These errors are comparatively small when compared with field measurement errors for hydraulic conductivity, however over long river reaches these errors can amount to significant volumes of water.
Inclusion of remote sensing information to improve groundwater flow modelling in the Chobe region (Botswana)
groundwater flow model has been built for the Chobe region in Botswana. Due to the scarcity of conventional data, alternative sources of information have been explored. METEOSAT and NOAA-AVHR images have been used to estimate the spatial distribution of the water balance. This spatial distribution has been correlated to local chloride measurements from which the recharge rate could be estimated. A digital elevation model has been used as a constraint for the maximum local piezometric head values. Finally, geomagnetic data were used as an indication for the presence of faults. These data have been used together with a limited number of traditional data (hydraulic head and transmissivity measurements) in an inverse calibration procedure. In the inverse conditioning, equally likely realizations, consistent with all the measurements (transmissivity, hydraulic head, digital elevation model, satellite images, chloride measurements and geomagnetic data) have been generated. The objective function contains an extra constraint on a statistical basis that guarantees that the calibrated recharge rate pattern does not deviate too much from the estimated water balance using the satellite image. The study demonstrates the importance of the digital elevation model and the satellite information in improving the groundwater model of the site.
Potential evaporation dynamics over saturated bare soil and an open waters surface
Actual evaporation (Ea) can be calculated as a fraction of potential evaporation (PE), which refers to the evaporation rate if supply water is unlimited. Potential evaporation depends on the available energy and the underlying material, and different approaches to estimate potential evaporation exist nowadays. This study provides a detailed analysis of the evaporation dynamics over fully saturated, sandy soil (PEs) and an open water surface (PEw). Moreover, the performance of commonly used methods to estimate PE is assessed. At the basis of these analyses is a lysimeter experiment in the Guanzhong Basin, China, which allowed a precise measurement of PE with a very high temporal resolution. Temperature profiles in lysimeters and meteorological data were also measured during the experiment. A comparison of PEs and PEw was carried out for seven consecutive days (August 11th to 17th, 2016). Results show that PEw is smaller than PEs on a daily scale, with PEw rates being bigger than PEs at night but smaller during daytime. Furthermore, the temporal dynamics of PEw lags 4–5 h behind PEs. In accordance with the energy balance equation, PE dynamics are mainly governed by “available energy”. The PE rates calculated by Penman-Monteith (PM) and Priestly-Taylor (PT) based on these measurements were also evaluated. The measured PE is relatively well reproduced by PM and PT equations. Finally, the effect of using different approaches to estimate PE on calculating Ea was tested by an integrated hydrological model that calculates water flow in the unsaturated zone by solving the Richards equation. The relative differences were up to 17.5%.
The influence of riverbed heterogeneity patterns on river-aquifer exchange fluxes under different connection regimes
Riverbed hydraulic conductivity (K) is a critical parameter for the prediction of exchange fluxes between a river and an aquifer. In this study, the role of heterogeneity patterns was explored using the fully integrated hydrological model HydroGeoSphere simulating complex, variably saturated subsurface flow. A synthetic 3-D river-aquifer reference model was constructed with a heterogeneous riverbed using nonmulti-Gaussian patterns in the form of meandering channels. Data assimilation was used to test the ability of different riverbed K patterns to reproduce hydraulic heads, riverbed K and river-aquifer exchange fluxes. Both fully saturated as well as variably saturated conditions underneath the riverbed were tested. The data assimilation experiments with the ensemble Kalman filter (EnKF) were carried out for four types of geostatistical models of riverbed K fields: (i) spatially homogeneous, (ii) heterogeneous with multiGaussian distribution, (iii) heterogeneous with non-multi-Gaussian distribution (channelized structures) and (iv) heterogeneous with non-multi-Gaussian distribution (elliptic structures). For all data assimilation experiments, state variables and riverbed K were updated by assimilating hydraulic heads. For saturated conditions, heterogeneous geostatistical models allowed a better characterization of net exchange fluxes than a homogeneous approximation. Among the three heterogeneous models, the performance of non-multi-Gaussian models was superior to the performance of the multi-Gaussian model, but the two tested non-multi-Gaussian models showed only small differences in performance from one another. For the variably saturated conditions both the multi-Gaussian model and the homogeneous model performed clearly worse than the two non-multi-Gaussian models. The two non-multi-Gaussian models did not show much difference in performance. This clearly shows that characterizing heterogeneity of riverbed K is important. Moreover, particularly under variably saturated flow conditions the mean and the variance of riverbed K do not provide enough information for exchange flux characterization and additional histogram information of riverbed K provides crucial information for the reproduction of exchange fluxes.
Uncertainty analysis of an integrated hydrological model using posterior covariance matrix from automatic calibration
Simulating Flood‐Induced Riverbed Transience Using Unmanned Aerial Vehicles, Physically Based Hydrological Modeling, and the Ensemble Kalman Filter
Abstract Flood events can change the riverbed topography as well as the riverbed texture and structure, which in turn can influence the riverbed hydraulic conductivity (Krb) and river-aquifer exchange fluxes. A major flood event occurred in the Emme River in Switzerland in 2014, with major implications for the riverbed structure. The event was simulated with the fully integrated hydrological model HydroGeoSphere. The aim was to investigate the effect of the spatial and temporal variability of riverbed topography and Krb on predictions of hydraulic states and fluxes and to test whether data assimilation (DA) based on the ensemble Kalman filter (EnKF) can better reproduce flood-induced changes to hydraulic states and parameters with the help of riverbed topography changes recorded with an unmanned aerial vehicle (UAV) and through-water photogrammetry. The performance of DA was assessed by evaluating the reproduction of the hydraulic states for the year 2015. While the prediction of surface water discharge was not affected much by the changes in riverbed topography and in Krb, using the UAV-derived postflood instead of the preflood riverbed topography reduced the root-mean-square error of predicted heads (RMSE [h]) by 24%. If, in addition to using the postflood riverbed topography, also Krb and aquifer hydraulic conductivity (Kaq) were updated through DA after the flood, the RMSE (h) was reduced by 55%. We demonstrate how updating of Krb and Kaq based on EnKF and UAV-based observations of riverbed topography transience after a major flood event strongly improve predictions of postflood hydraulic states.
Characterisation of river–aquifer exchange fluxes: The role of spatial patterns of riverbed hydraulic conductivities
Interactions between surface water and groundwater play an essential role in hydrology, hydrogeology, ecology, and water resources management. A proper characterisation of riverbed structures might be important for estimating river–aquifer exchange fluxes. The ensemble Kalman filter (EnKF) is commonly used in subsurface flow and transport modelling for estimating states and parameters. However, EnKF only performs optimally for MultiGaussian distributed parameter fields, but the spatial distribution of streambed hydraulic conductivities often shows non-MultiGaussian patterns, which are related to flow velocity dependent sedimentation and erosion processes. In this synthetic study, we assumed a riverbed with non-MultiGaussian channel-distributed hydraulic parameters as a virtual reference. The synthetic study was carried out for a 3-D river–aquifer model with a river in hydraulic connection to a homogeneous aquifer. Next, in a series of data assimilation experiments three different groups of scenarios were studied. In the first and second group of scenarios, stochastic realisations of non-MultiGaussian distributed riverbeds were inversely conditioned to state information, using EnKF and the normal score ensemble Kalman filter (NS-EnKF). The riverbed hydraulic conductivity was oriented in the form of channels (first group of scenarios) or, with the same bimodal histogram, without channelling (second group of scenarios). In the third group of scenarios, the stochastic realisations of riverbeds have MultiGaussian distributed hydraulic parameters and are conditioned to state information with EnKF. It was found that the best results were achieved for channel-distributed non-MultiGaussian stochastic realisations and with parameter updating. However, differences between the simulations were small and non-MultiGaussian riverbed properties seem to be of less importance for subsurface flow than non-MultiGaussian aquifer properties. In addition, it was concluded that both EnKF and NS-EnKF improve the characterisation of non-MultiGaussian riverbed properties, hydraulic heads and exchange fluxes by piezometric head assimilation, and only NS-EnKF could preserve the initial distribution of riverbed hydraulic conductivities.