Yingying Yao

Systematic assessment of the uncertainty in integrated surface water‐groundwater modeling based on the probabilistic collocation method

Systematic uncertainty analysis (UA) has rarely been conducted for integrated modeling of surface water-groundwater (SW-GW) systems, which is subject to significant uncertainty, especially at a large basin scale. The main objective of this study was to explore an innovative framework in which a systematic UA can be effectively and efficiently performed for integrated SW-GW models of large river basins and to illuminate how process understanding, model calibration, data collection, and management can benefit from such a systematic UA. The framework is based on the computationally efficient Probabilistic Collocation Method (PCM) linked with a complex simulation model. The applicability and advantages of the framework were evaluated and validated through an integrated SW-GW model for the Zhangye Basin in the middle Heihe River Basin, northwest China. The framework for systematic UA allows for a holistic assessment of the modeling uncertainty, yielding valuable insights into the hydrological processes, model structure, data deficit, and potential effectiveness of management. The study shows that, under the complex SW-GW interactions, the modeling uncertainty has great spatial and temporal variabilities and is highly output-dependent. Overall, this study confirms that a systematic UA should play a critical role in integrated SW-GW modeling of large river basins, and the PCM-based approach is a promising option to fulfill this role.

Quantitative assessment of groundwater vulnerability using index system and transport simulation, Huangshuihe catchment, China

Groundwater vulnerability assessment has been an increasingly important environment management tool. The existing vulnerability assessment approaches are mostly index systems which have significant disadvantages. There need to be some quantitative studies on vulnerability indicators based on objective physical process study. In this study, we tried to do vulnerability assessment in Huangshuihe catchment in Shandong province of China using both contaminant transport simulations and index system approach. Transit time of 75% of hypothetical injected contaminant concentration was considered as the vulnerability indicator. First, we collected the field data of the Huangshuihe catchment and the catchment was divided into 34 sub areas that can each be treated as a transport sub model. Next, we constructed a Hydrus1D transport model of Huangshuihe catchment. Different sub areas had different input values. Thirdly, we used Monte-Carlo simulation to improve the collected data and did vulnerability assessment using the statistics of the contaminant transit time as a vulnerability indicator. Finally, to compare with the assessment result by transport simulation, we applied two index systems to Huangshuihe catchment. The first was DRASTIC system, and the other was a system we tentatively constructed examining the relationships between the transit time and the input parameters by simply changing the input values. The result of comparisons between the two index systems and transport simulation approach suggested partial validation to DRASTIC, and the construction of the new tentative index system was an attempt of building up index approaches based on physical process simulation.

Exploring scale‐dependent ecohydrological responses in a large endorheic river basin through integrated surface water‐groundwater modeling

Ecohydrological processes in a water-limited environment are sensitive to both climate conditions and human activities, but the response mechanisms have rarely been explored for large endorheic river basins via an integrated modeling approach. This study established an integrated surface water-groundwater model for the Heihe River Basin (HRB), Chinas second largest endorheic river basin, using GSFLOW as the modeling platform. Evapotranspiration (ET) and Leaf Area Index (LAI) data independently derived from remote sensing products were compared and correlated, respectively, with the modeling results. Scale-dependent interrelationships among ecological, hydrological, and human-impact (i.e., diversion and pumping) variables were revealed through multiple regression analyses. Major study findings include: (1) the independent ET and LAI data enabled the modeler to crosscheck the modeling results from a unique angle not possible with conventional groundwater and streamflow observations; (2) controlling factors for the temporal variability of ET and LAI exhibit notable scale-dependence, reflecting distinctive climate, and human impacts on different land covers; and (3) there exists an intricate linkage between the hydrological regimes in the lower HRB and the middle HRB, essentially equivalent to a tradeoff between the ecosystem health of the lower HRB and the sustainable development of the middle HRB. Overall, the integrated modeling assisted by the independent ET and LAI data has provided a coherent understanding on the regional water cycle, and led to new insights on tackling the existing water conflicts in HRB.

Numerical modeling of regional groundwater flow in the Heihe River Basin, China: Advances and new insights

Numerical groundwater modeling is an effective tool to guide water resources management and explore complex groundwater-dependent ecosystems in arid regions. In the Heihe River Basin (HRB), China’s second largest inland river basin located in arid northwest China, a series of groundwater flow models have been developed for those purposes over the past 20 years. These models have elucidated the characteristics of groundwater flow systems and provided the scientific basis for a more sustainable management of groundwater resources and ecosystem services. The first part of this paper presents an overview of previous groundwater modeling studies and key lessons learned based on seven different groundwater models in the middle and lower HRB at sub-basin scales. The second part reviews the rationale for development of a regional basin-scale groundwater flow model that unifies previous sub-basin models. In addition, this paper discusses the opportunities and challenges in developing a regional groundwater flow model in an arid river basin such as the HRB.

A field demonstration of the entropy-weighted fuzzy DRASTIC method for groundwater vulnerability assessment

Abstract Groundwater vulnerability assessment based on the DRASTIC index has been widely used since the 1980s to map potential risks of groundwater contamination. However, its applicability and usefulness are affected by two uncertain and subjective factors. One is the discretization of continuous input variables and the other is the assignment of different weights to the index variables. In this study, an entropy-weighted fuzzy-optimization approach was developed to augment and improve the classic DRASTIC method by reducing the uncertainties associated with variable discretization and weight assignment. The modified DRASTIC method was applied to a study site in Shandong, north China. The entropy-weighted fuzzy-optimization approach is shown to provide a more rigorous delineation of the relative vulnerability distribution. Meanwhile, the new approach does not require the use of more parameters. The results suggest that this approach significantly improves and enhances the ability of the classic DRASTIC method in a more systematic and rigorous way. Editor D. Koutsoyiannis Citation Yu, C., Zhang, B.X., Yao, Y.Y., Meng, F.H., and Zheng, C.M., 2012. A field demonstration of the entropy-weighted fuzzy DRASTIC method for groundwater vulnerability assessment. Hydrological Sciences Journal, 57 (7), 1420–1432.

Assimilation of temperature and hydraulic gradients for quantifying the spatial variability of streambed hydraulics

Understanding the spatial and temporal characteristics of water flux into or out of shallow aquifers is imperative for water resources management and eco-environmental conservation. In this study, the spatial variability in the vertical specific fluxes and hydraulic conductivities in a streambed were evaluated by integrating distributed temperature sensing (DTS) data and vertical hydraulic gradients into an ensemble Kalman filter (EnKF) and smoother (EnKS) and an empirical thermal-mixing model. The formulation of the EnKF/EnKS assimilation scheme is based on a discretized 1D advection-conduction equation of heat transfer in the streambed. We first systematically tested a synthetic case and performed quantitative and statistical analyses to evaluate the performance of the assimilation schemes. Then a real-world case was evaluated to calculate assimilated specific flux. An initial estimate of the spatial distributions of the vertical hydraulic gradients was obtained from an empirical thermal-mixing model under steady-state conditions using a constant vertical hydraulic conductivity. Then, this initial estimate was updated by repeatedly dividing the assimilated specific flux by estimates of the vertical hydraulic gradients to obtain a refined spatial distribution of vertical hydraulic gradients and vertical hydraulic conductivities. Our results indicate that optimal parameters can be derived with fewer iterations but greater simulation effort using the EnKS compared with the EnKF. For the field application in a stream segment of the Heihe River Basin in northwest China, the average vertical hydraulic conductivities in the streambed varied over three orders of magnitude (5 × 10−1 to 5 × 102 m/d). The specific fluxes ranged from near zero (qz < ±0.05 m/d) to ±1.0 m/d, while the vertical hydraulic gradients were within the range of -0.2 to 0.15 m/m. The highest and most variable fluxes occurred adjacent to a debris-dam and bridge pier. This phenomenon is very likely the result of heterogeneous streambed hydraulic characteristics in these areas. Our results have significant implications for hyporheic micro-habitats, fish spawning and other wildlife incubation, regional flow and hyporheic solute transport models in the Heihe River Basin, as well as in other similar hydrologic settings. This article is protected by copyright. All rights reserved.

A field demonstration of groundwater vulnerability assessment using transport modeling and groundwater age modeling, Beijing Plain, China

Groundwater vulnerability is often regarded as the resistance of aquifers to pollutions, which is in fact the groundwater environment vulnerability. Similarly the renewable capacity of groundwater system can be regarded as the indicator for groundwater resources vulnerability. In this paper, the concept of groundwater vulnerability is extended to include both the groundwater environment vulnerability and groundwater resources vulnerability, and the extended overall groundwater vulnerability is assessed by examining the solute transport process from ground surface to the water table (for groundwater environment vulnerability), and evaluating the renewable capacity of groundwater system (for groundwater resources vulnerability). For this case study in Beijing Plain, a 1D-modeling, 2D-mapping model is constructed in HYDRUS to simulate the solute transport process through vadose zone. The transit times and solute concentrations at the water table are selected as the indicators for groundwater environment vulnerability. Meanwhile, a 3D groundwater age model in MODFLOW/MT3DMS is employed to model the age distribution in saturated zone, and the modeled ages are taken as the indicators for groundwater resources vulnerability. These two models are coupled together by exchanging their vertical infiltrations and water table elevations. Overall vulnerability is finally assessed by taking all the indicators into account. The result suggests the overall vulnerability can finely represent the synthetic impact of solute transport characteristics (groundwater environment vulnerability in vadose zone) and groundwater system renewability (groundwater resources vulnerability in saturated zone). The most vulnerable regions of Beijing Plain are mostly in agricultural areas, the medium vulnerable areas mostly appear at pumping sites, and the least vulnerable areas are mostly distributed in recharge regions.

Eco‐hydrological effects associated with environmental flow management: A case study from the arid desert region of China

In arid regions, stream-flow regulation impacts the hydrological processes and riparian ecosystems in both direct and indirect ways. Thus, understanding the degree of eco-hydrological effects over different spatiotemporal scales is critical for effective stream-flow regulation. This study investigates the multiple hydro-ecological effects caused by a mandatory implementation of stream-flow regulation in a typical endorheic river basin, the Heihe River Basin (HRB), in northwest China. The changes in the stream-flow regime, spatiotemporal patterns of vegetation and groundwater flow system were quantified using stream-flow data, Leaf Area Index (LAI) products of 1 km resolution and a 3D numerical groundwater flow model. The results show the stream leakage to groundwater is strongly correlated with the disturbance by stream-flow regulation, moderately with the evaporation and inflow to terminal lake, but only weakly with the total vegetation state and cropland change. Even though the average groundwater level for the entire lower HRB has an upward trend in response to increasing stream-flow, it has no significant correlation with flow regulation. The groundwater system acts as a “temporary bank” to buffer the regulated stream-flow for the entire lower HRB. The spatiotemporal patterns of the LAI distribution point to the stream-flow regulation as triggers for the increase in cropland and vegetation patches in the flat terrain along the river branches. This case study in the lower HRB has provided valuable database and insights for environmental flow management of other arid watersheds.

What controls the partitioning between baseflow and mountain block recharge in the Qinghai‐Tibet Plateau?

Mountainous areas are referred to as “water towers” since they are the source of water for many low-lying communities. The hydrologic budgets of these areas, which are particularly susceptible to climate change, are typically poorly constrained. To address this, we analyzed the partitioning between baseflow and mountain block recharge (MBR) using a regional groundwater model of the northern Qinghai-Tibet Plateau run with multiple scenarios. We found that ~19% of precipitation is recharged, approximately 35% of which becomes MBR, while 65% discharges as baseflow. This partitioning is relatively independent of the recharge rate but is sensitive to exponential depth decrease of hydraulic conductivity (K). The MBR is more sensitive to this exponential decrease in K than baseflow. The proportion of MBR varies from twice to half of baseflow as the decay exponent increases by more than fivefold. Thus, the depth dependence of K is critical for quantifying hydrologic partitioning in these sensitive areas.

HPC Environment on Azure Cloud for Hydrological Parameter Estimation

High performance of data-intensive computation is required to deal with the complexity of analysis and simulation for hydrological modeling jobs like parameter estimation. The vigorously developing cloud computing has emerged as a promising platform for HPC (High Performance Computing) of science community. This paper presents our work in developing and implementing HPC environment on Azure cloud for applications of hydrological parameter estimation. According to the requirements of hydrological modeling, we design and construct a HPC environment on Azure cloud. After deploying parameter estimation applications on the HPC environment, a case study on groundwater uncertainty analysis in Heihe River Basin using the HPC environment is presented. Our work demonstrates that Azure cloud can advantageously complement traditional high performance computing infrastructure and help hydrological researchers improve model computing efficiency by handy process steps.