Quantifying the impact of landscape on hydrological variables is essential for the sustainable development of water resources. Understanding how landscape changes influence hydrological variables will greatly enhance the understanding of hydrological processes. Important vegetation parameters are considered in this study by using remote sensing data and VIC-CAS model to analyse the impact of landscape changes on hydrology in upper reaches of the Shule River Basin (URSLB). The results show there are differences in the runoff generation of landscape both in space and time. With increasing altitude, the runoff yields increased, with approximately 79.9% of the total runoff generated in the high mountains (4200-5900 m), and mainly consumed in the mid-low mountain region. Glacier landscape produced the largest runoff yields (24.9% of the total runoff), followed by low-coverage grassland (LG; 22.5%), alpine cold desert (AL; 19.6%), mid-coverage grassland (MG; 15.6%), bare land (12.5%), high-coverage grassland (HG; 4.5%) and shrubbery (0.4%). The relative capacity of runoff generation by landscapes, from high to low, was the glaciers, AL, LG, HG, MG, shrubbery and bare land. Furthermore, changes in landscapes cause hydrological variables changes, including evapotranspiration, runoff and baseflow. The study revealed that HG, MG, and bare land have a positive impact on evapotranspiration and a negative impact on runoff and baseflow, whereas AL and LG have a positive impact on runoff and baseflow and a negative impact on evapotranspiration. In contrast, glaciers have a positive impact on runoff. After the simulation in four vegetation scenarios, we concluded that the runoff regulation ability of grassland is greater than that of bare land. The grassland landscape is essential since it reduced the flood peak and conserved the soil and water.

Runoff processes in glacier and paramo catchments in the Andean region are of interest as they are vitally important to serve the water needs of surrounding communities. Particularly in Northern Ecuador, the runoff processes are less well-known due to the high variability of precipitation, young volcanic ash soil properties, soil moisture dynamics and other local factors. Previous studies have shown that the melting of glaciers contributes to runoff generation and that the paramo ecosystem plays an important role in regulating runoff during periods of low precipitation. Data collection and experimental investigations were carried out in a catchment of 15.2 km(2) and altitude ranging between 4000 and 5700 m above sea level. Environmental tracers and hydrochemical catchment characterization were used for identifying runoff sources and their respective contributions during dry and wet conditions. Dry conditions are defined as periods where precipitation was absent for at least three consecutive days and wet conditions imply rainfall events. This study highlights the importance of the paramo on contributing to total runoff during baseflow (70% of total runoff) and the capacity of the paramo to dissipate the stream energy and buffer the peak flow during rainfall conditions. Electrical conductivity together with stable isotopes were identified as conservative tracers that characterize the end-member concentrations.

In permafrost regions, temperature and precipitation play decisive roles in hydrological processes. Soil freeze-thaw cycles in the active layer cause the runoff generation process to show multiple mechanisms and seasonal alternating patterns. In this study, based on the precipitation-runoff modeling system implemented in the Java modeling framework Object Modeling System (PRMS-OMS), we developed a runoff generation model with temperature-induced variable source area (TVSA) for permafrost regions by introducing an active layer parameterization scheme, an active layer freeze-thaw module, a glacier module, and the sub-permafrost groundwater module. The TVSA model was calibrated and validated using field observations in Fenghuoshan (FHS), a typical permafrost watershed, and Tuotuohe (TTH), a typical permafrost-glacier catchment, on the Qinghai-Tibet Plateau (QTP). During the calibration period, parameters related to the thermal conditions of the active layer and runoff processes were calibrated using the observed freeze and thaw depth data and the discharge data. For the validation period, the model successfully reproduced the freeze-thaw processes (average root mean squared error = 0.205 m) and discharges (FHS, Nash-Sutcliffe efficiency coefficient (NSE) = 0.99; TTH, NSE = 0.85). The TVSA model is a powerful tool for the system identification of variable source runoff generation processes and the associated physical mechanism under temperature control in permafrost basins; additionally, the model accurately reproduces hydrological processes and the associated response to climate change.

This study proposes a new process-based framework to characterize and classify runoff events of various magnitudes occurring in a wide range of catchments. The framework uses dimensionless indicators that characterize space-time dynamics of precipitation events and their spatial interaction with antecedent catchment states, described as snow cover, distribution of frozen soils, and soil moisture content. A rigorous uncertainty analysis showed that the developed indicators are robust and regionally consistent. Relying on covariance- and ratio-based indicators leads to reduced classification uncertainty compared to commonly used (event-based) indicators based on absolute values of metrics such as duration, volume, and intensity of precipitation events. The event typology derived from the proposed framework is able to stratify events that exhibit distinct hydrograph dynamics even if streamflow is not directly used for classification. The derived typology is therefore able to capture first-order controls of event runoff response in a wide variety of catchments. Application of this typology to about 180,000 runoff events observed in 392 German catchments revealed six distinct regions with homogeneous event type frequency that match well regions with similar behavior in terms of runoff response identified in Germany. The detected seasonal pattern of event type occurrence is regionally consistent and agrees well with the seasonality of hydroclimatic conditions. The proposed framework can be a useful tool for comparative analyses of regional differences and similarities of runoff generation processes at catchment scale and their possible spatial and temporal evolution.

Rain on snow (ROS) is a complex phenomenon leading to repeated flooding in many regions with a seasonal snow cover. The potential to generate floods during ROS depends not only on the magnitude of rainfall but also on the areal extent of the antecedent snow cover and the spatio-temporal interaction between meteorologic and snowpack properties. The complex interaction of these factors makes it difficult to accurately predict the effect of snow cover on runoff formation for an upcoming ROS event. In this study, the detailed physics-based snow cover model SNOWPACK was used to assess the influence of snow cover properties on converting rain input to available snowpack runoff during 191 ROS events for 58 catchments in the Swiss Alps. Conditions identified by the simulations that led to excessive snowpack runoff were a large snow-covered fraction, spatially homogeneous snowpack properties, prolonged rainfall events, and a strong rise in air temperature over the course of the event. These factors entail a higher probability of snowpack runoff occurring synchronously within the catchment, which in turn favours higher overall runoff rates. The findings suggest that during autumn and late spring, flooding due to ROS is more likely to happen, whereas during winter a coincidence of the above conditions in the study area is quite rare. For example, an autumn event which occurred in October 2011 resulted from a combination of spatially homogeneous snowpack conditions following a recent snowfall and high, but not exceptional rainfall, and led to major flooding. The results of this study provide key factors to assess in advance of an incoming ROS event and emphasize the importance of detailed snow monitoring for flood forecasting in snow-affected watersheds.

The hydrological regimes of the major rivers in cold regions of the world have changed remarkably since the 1960s, but the mechanisms underlying these changes have not yet been fully understood. Specifically, changes in freeze-thaw processes can affect the thicknesses of the permafrost layer and active layer, storage capacity for liquid water and subsequent surface runoff. In this study, we investigated the hydrological processes and runoff generation in the Yellow River source (YRS) region, which is located in a permafrost region, over the past four decades using ground observations. The impacts of runoff processes are assessed in terms of the spatiotemporal distributions of precipitation, air temperatures, and soil water dynamics in the active layer. The Mann-Kendall test and correlation analysis were employed to detect the variations in runoff and annual changes in influencing factors. Runoff generation in different periods was influenced by different main factors. In the spring flood and recession flow period (SP_FRP, which ranges from late March to mid-June), soil water storage played an important role in runoff generation. With the increased air temperature and enhanced soil ground thawing process, liquid water storage capacity of the soil increased, and constrained runoff. After the soil was saturated in the summer flood and recession flow period (SU_FRP, which ranges from late June to late October), runoff increased with precipitation. In the dry period (DP, which ranges from early November to Mid-March), little precipitation, high potential evaporation (308 mm) and increased air temperature (0.43 degrees C/decade) caused decreases in snow accumulation and water content. Without a sufficient water supply, river runoff decreased in DP. These results show that precipitation played an important role in runoff generation in SU_FRP. In permafrost regions, the freeze-thaw processes of the active layer of soils, heavy evaporation and the air temperature play primary roles in runoff generation during SP_FRP and DP.