Topographic delineation is critical to watershed hydrologic modeling, which may significantly influence the accuracy of model simulations. In most traditional delineation methods, however, surface depressions are fully filled and hence, watershed-scale hydrologic modeling is based on depression-less topography. In reality, dynamic filling and spilling of depressions affect hydrologic connectivity and surface runoff processes, especially in depression-dominated areas. Thus, accounting for the internal hydrologic connectivity within a watershed is crucial to such hydrologic simulations. The objective of this study was to improve watershed delineation to further reveal such complex hydrologic connectivity. To achieve this objective, a new algorithm, HUD-DC, was developed for delineation of hydrologic units (HUs) associated with depressions and channels. Unlike the traditional delineation methods, HUD-DC considers both filled and unfilled conditions to identify depressions and their overflow thresholds, as well as all channels. Furthermore, HUs, which include puddle-based units and channel-based units, were identified based on depressions and channels and the detailed connectivity between the HUs was determined. A watershed in North Dakota was selected for testing HUD-DC, and Arc Hydro was also utilized to compare with HUD-DC in depression-oriented delineation. The results highlight the significance of depressions and the complexity of hydrologic connectivity. In addition, HUD-DC was utilized to evaluate the variations in topographic characteristics under different filling conditions, which provided helpful guidance for the identification of filling thresholds to effectively remove artifacts in digital elevation models.

Correction of digital elevation models (DEMs) for flat areas is a critical process for hydrological analyses and modeling, such as the determination of flow directions and accumulations, and the delineation of drainage networks and sub-basins. In this study, a new algorithm is proposed for flat correction/removal. It uses the puddle delineation (PD) program to identify depressions (including their centers and overflow/spilling thresholds), compute topographic characteristics, and further fill the depressions. Three different levels of elevation increments are used for flat correction. The first and second level of increments create flows toward the thresholds and centers of the filled depressions or flats, while the third level of small random increments is introduced to cope with multiple threshold conditions. A set of artificial surfaces and two real-world landscapes were selected to test the new algorithm. The results showed that the proposed method was not limited by the shapes, the number of thresholds, and the surrounding topographic conditions of flat areas. Compared with the traditional methods, the new algorithm simplified the flat correction procedure and reduced the final elevation increments by 5.71–33.33%. This can be used to effectively remove/correct topographic flats and create flat-free DEMs.

The SCS curve number (SCS-CN) method has gained widespread popularity for simulating rainfall excess in various rainfall events due to its simplicity and practicality. However, it possesses inherent structural issues that limit its performance in accurately simulating rainfall excess and infiltration over time. The objective of this study was to develop a modified CN method with temporally varying rainfall intensity (MCN-TVR) by combining a soil moisture accounting (SMA) based SCS-CN method with the SMA method in the Hydrologic Engineering Center’s Hydrologic Modeling System (HEC-HMS). In the MCN-TVR, the SMA-based SCS-CN method is utilized to simulate the cumulative rainfall excess and infiltration, while the SMA method in the HEC-HMS serves as an infiltration control function. A key advantage of the MCN-TVR is that it eliminates the need for additional input parameters by inherently linking the parameters in the two SMA-based methods. Sixteen hypothetical 24 h SCS Type II rainfall events with different soil types and five real rainfall events for the Rush River Watershed in North Dakota were used to assess the performances of the MCN-TVR method and the SMA-based SCS-CN method. In the hypothetical simulations, the rainfall excess simulated by the SMA-based SCS-CN and MCN-TVR models was compared to that simulated by a Green–Ampt model. Discrepancies were observed between the rainfall excess simulated by the SMA-based SCS-CN and Green–Ampt models, especially for coarse soils under relatively light rainfall. However, the MCN-TVR model, incorporating an infiltration control function, demonstrated its improved performance closer to the Green–Ampt model. For all the hypothetical events, the Nash–Sutcliffe efficiency (NSE) coefficient of the rainfall excess simulated by the MCN-TVR method compared to the Green–Ampt model was greater than 0.99, while the root mean standard deviation ratio (RSR) was less than 0.03. In the real applications, the SMA-based SCS-CN model failed to provide acceptable simulation of the direct runoff for rainfall events with durations of less than the time of concentration. In contrast, the MCN-TVR model successfully simulated the direct runoff for all the events with NSE values ranging from 0.65 to 0.91 and RSR values from 0.31 to 0.56.

Lakes are key factors in maintaining ecosystems in semi-arid regions. However, due to data shortage, most studies used remote-sensing data and water-balance models to analyze lake variations in semi-arid ungauged closed watersheds, resulting in the oversimplified assessment of lake variations and their associated hydrologic processes. This study aimed to enhance the understanding of the mechanisms behind the water supplement and consumption of lakes and reveal the influences of hydrological processes on lake variations in such watersheds. Physically based and lake-oriented hydrologic modeling, remote-sensing technology, and results from previous studies were comprehensively integrated to achieve the research objective. The Hongjiannao (HJN) watershed in Northwest China was selected as the study area of this research. The calibration and validation results demonstrated that remote-sensing data and results from previous studies indeed guaranteed the accuracy of the lake-oriented model. Further hydrologic and statistical analyses revealed the linkage between lake variations and their associated hydrologic processes, and the mechanisms behind the linkage. Specifically, rainfall and snowmelt were found to be the most stable sources of HJN Lake, particularly in dry years. Due to the differences in recession rates, groundwater inflow was more stable than upstream inflow and inflow from the contributing area of HJN Lake. The correlations between hydrologic processes and the storage variation of HJN Lake varied significantly at daily and monthly time scales, which can be explained by the generation mechanisms of these processes. This study provided valuable guidance for water resources management and ecosystem protection in the HJN watershed and can be further applied for hydrologic simulations in other similar watersheds.

Groundwater resource is crucial for the development of agriculture and urban communities in valley basins of arid and semiarid regions. This research investigated the groundwater chemistry of a typical urbanized valley basin on the Tibetan Plateau to understand the hydrochemical status, quality, and controlling mechanisms of groundwater in arid urbanized valley basins. The results show groundwater is predominantly fresh and slightly alkaline across the basin, with approximately 54.17% of HCO₃-Ca type. About 12.5% and 33.33% of sampled groundwaters are with the hydrochemical facies of Cl-Mg·Ca type and Cl-Na type, respectively. Groundwater is found with the maximum TDS, NO₃⁻, NO₂⁻, and F⁻ content of 3066 mg/L, 69.33 mg/L, 0.04 mg/L, and 3.12 mg/L, respectively. Groundwater quality is suitable for domestic usage at all sampling sites based on EWQI assessment but should avoid direct drinking at some sporadic sites in the urban area. The exceeding nitrogen and fluoride contaminants would pose potential health hazards to local residents, but high risks only existed for infants. Both minors and adults are at medium risk of these exceedingly toxic contaminants. Groundwater quality of predominant sites in the basin is suitable for long-term irrigation according to the single indicator of EC, SAR, %Na, RSC, KR, PI, and PS and integrated irrigation quality assessment of USSL, Wilcox, and Doneen diagram assessment. But sodium hazard, alkalinity hazard, and permeability problem should be a concern in the middle-lower stream areas. Groundwater chemistry in the basin is predominantly governed by water-rock interaction (silicate dissolution) across the basin in natural and sporadically by evaporation. Human activities have posed disturbances to groundwater chemistry and inputted nitrogen, fluoride, and salinity into groundwater. The elevated nitrogen contaminants in groundwater are from both agricultural activities and municipal sewage. While the elevated fluoride and salinity in groundwater are only associated with municipal sewage. It is imperative to address the potential anthropogenic contaminants to safeguard groundwater resources from the adverse external impacts of human settlements within these urbanized valley basins.