A taxonomy of hydrological processes and watershed function

This paper presents a taxonomy (hierarchical organization) of hydrological processes; specifically, runoff generation processes in natural watersheds. Over 120 process names were extracted from a literature review of papers describing experimental watersheds, perceptual models, and runoff processes in a range of hydro-climatic environments. Processes were arranged into a hierarchical structure, and presented as a spreadsheet and interactive diagram. For each process, additional information was provided: a list of alternative names for the same process, a classification into hydrological function (e.g. partitioning, flux, storage, release) and a unique identifier similar to a hashtag. The taxonomy provides a method to label and search hydrological knowledge, thereby facilitating synthesis and comparison of processes across watersheds.

Climate dominates changes in flood magnitude and timing across China during 1960-2017

<p>Riverine floods are exhibiting temporal shifts in both magnitude and timing under the context of climate change as well as human alternations of the river systems (i.e., construction of reservoirs and land management practices). A nation-wide assessment of changes in riverine floods is still lacking over China, despite the societal perception that recent Chinese flood trends are dictated by drastic environmental changes associated with rapid economic development. Here we examine changes in flood magnitude and timing based on the most comprehensive database of annual maximum flood peak discharge (AMF) over China during the period 1960-2017. We find both increasing and decreasing trends in AMF magnitude and timing. Trends in AMF magnitudes range from -4.29% to 2.86% (per year relative to long-term mean flood peak discharge). Decreased AMF magnitudes are observed in central and northern China, while increased AMF magnitudes mainly in northwestern and southern China. The shifts of AMF timing range from -16 days to +18 days per decade. Changes in AMF timing show less spatial consistency than that in AMF magnitude. We categorize the gauged watersheds into human-modified and natural categories. Flood changes in natural watersheds can only be attributed to climate variability. The spatial pattern of changes in AMF magnitude and timing in human-modified watersheds resembles those in natural watersheds, pointing towards the dominant role of climate in dictating recent flood changes over China. Impacts of reservoirs and land management practices are only isolated cases. We further provide a predictive understanding of climatic controls on flood hazards over China (and East Asian countries) by establishing connections between changes in AMF magnitude/timing and climate indices. Our analyses, together with similar efforts in other continents, contribute to a general understanding of space-time dynamics of riverine floods around the globe.</p>

Sustainability of soil organic carbon in consolidated gully land in China’s Loess Plateau

Massive gully land consolidation projects, launched in China’s Loess Plateau, aim to restore 2667 $$\mathrm{km}^2$$ km 2 agricultural lands in total by consolidating 2026 highly eroded gullies. This effort represents a social engineering project where the economic development and livelihood of the farming families are closely tied to the ability of these emergent landscapes to provide agricultural services. Whether these ‘time zero’ landscapes have the resilience to provide a sustainable soil condition such as soil organic carbon (SOC) content remains unknown. By studying two watersheds, one of which is a control site, we show that the consolidated gully serves as an enhanced carbon sink, where the magnitude of SOC increase rate (1.0 $$\mathrm{g\,C}/\mathrm{m}^2/\mathrm{year}$$ g C / m 2 / year ) is about twice that of the SOC decrease rate (− 0.5 $$\mathrm{g\,C}/\mathrm{m}^2/\mathrm{year}$$ g C / m 2 / year ) in the surrounding natural watershed. Over a 50-year co-evolution of landscape and SOC turnover, we find that the dominant mechanisms that determine the carbon cycling are different between the consolidated gully and natural watersheds. In natural watersheds, the flux of SOC transformation is mainly driven by the flux of SOC transport; but in the consolidated gully, the transport has little impact on the transformation. Furthermore, we find that extending the surface carbon residence time has the potential to efficiently enhance carbon sequestration from the atmosphere with a rate as high as 8 $$\mathrm{g\,C}/\mathrm{m}^2/\mathrm{year}$$ g C / m 2 / year compared to the current 0.4 $$\mathrm{g\,C}/\mathrm{m}^2/\mathrm{year}$$ g C / m 2 / year . The success for the completion of all gully consolidation would lead to as high as 26.67 $$\mathrm{Gg\,C}/\mathrm{year}$$ Gg C / year sequestrated into soils. This work, therefore, not only provides an assessment and guidance of the long-term sustainability of the ‘time zero’ landscapes but also a solution for sequestration $$\hbox {CO}_2$$ CO 2 into soils.

Scaling Effects of Elevation Data on Urban Nonpoint Source Pollution Simulations

The scale effects of digital elevation models (DEM) on hydrology and nonpoint source (NPS) pollution simulations have been widely reported for natural watersheds but seldom studied for urban catchments. In this study, the scale effect of DEM data on the rainfall-runoff and NPS pollution was studied in a typical urban catchment in China. Models were constructed based on the DEM data of nine different resolutions. The conventional model performance indicators and the information entropy method were applied together to evaluate the scale effects. Based on the results, scaling effects and a resolution threshold of DEM data exist for urban NPS pollution simulations. Compared with natural watersheds, the urban NPS pollution simulations were primarily affected by the local terrain due to the overall flat terrain and dense sewer inlet distribution. The overland process simulation responded more sensitively than the catchment outlet, showing prolonged times of concentration for impervious areas with decreasing DEM resolution. The diverse spatial distributions and accumulation magnitudes of pollutants could lead to different simulation responses to scaling effects. This paper provides information about the specific characteristics of the scale effects of DEM data in a typical urban catchment, and these results can be extrapolated to other similar catchments as a reference for data collection.

Flood Routing Efficiency Assessment: an Approach Using Bivariate Copulas

Flood control reservoirs are widely recognized as effective structural practices in order to mitigate the flood risk in natural watersheds. Nevertheless, the flood frequency distribution in the downstream reach is strongly affected by a certain number of characteristics of the upstream flood hydrographs. When a direct statistical method is utilized, a multivariate approach should therefore be utilized to accurately assess reservoir performances. In this paper, a flood frequency distribution of the routed flow discharge is derived from a bivariate joint distribution function of peak flow discharges and flood volumes of hydrographs entering the reservoir. Such a joint distribution is constructed by using the copula approach. Reservoir performances are also exploited to categorize event severity and to estimate their bivariate return periods. The method is applied to a real-world case study (Sant’Anna reservoir, Panaro River, northern Italy), and its reliability is verified through continuous simulations. Bearing in mind the popularity that design event methods still have in practical engineering, a final evaluation of the performance assessment achievable by simulations of synthetic hydrographs derived from a flood reduction curve is finally proposed.

Natural and managed watersheds show similar responses to recent climate change

Changes in climate are driving an intensification of the hydrologic cycle and leading to alterations of natural streamflow regimes. Human disturbances such as dams, land-cover change, and water diversions are thought to obscure climate signals in hydrologic systems. As a result, most studies of changing hydroclimatic conditions are limited to areas with natural streamflow. Here, we compare trends in observed streamflow from natural and human-modified watersheds in the United States and Canada for the 1981–2015 water years to evaluate whether comparable responses to climate change are present in both systems. We find that patterns and magnitudes of trends in median daily streamflow, daily streamflow variability, and daily extremes in human-modified watersheds are similar to those from nearby natural watersheds. Streamflow in both systems show negative trends throughout the southern and western United States and positive trends throughout the northeastern United States, the northern Great Plains, and southern prairies of Canada. The trends in both natural and human-modified watersheds are linked to local trends in precipitation and reference evapotranspiration, demonstrating that water management and land-cover change have not substantially altered the effects of climate change on human-modified watersheds compared with nearby natural watersheds.

Characterising hydrological response in urban watersheds based on inter-amount time distributions

Urban watersheds are typically characterised by a more flashy nature of the hydrological response compared to natural watersheds. Predicting the degree of flashiness associated with urbanisation is not straightforward, as it is influenced by interactions between impervious cover, basin size, drainage connectivity and stormwater management infrastructure. In this study, we present an alternative approach of analysing hydrological response variability and basin flashiness, based on the distribution 5 of inter-amount times.We analyse inter-amount time distributions of streamflow time series for 17 (semi)urbanised basins in North Carolina, US, ranging from 13 km2 to 238 km2 in size. We show that in the inter-amount times analysis, sampling frequency is tuned to the local variability in the flow pattern, resulting in more balanced representation of high and low flow periods in the time series. This helps to stabilize the variance of the distribution across scales and leads to more robust scaling behaviour. We show that inter-amount times distributions can be used to detect regulation effects on flow patterns, 10 identify critical sampling resolutions and characterise flashiness of hydrological response. The possibility to use both the classical approach and the inter-amount time framework to identify minimum observable scales and analyse flow data opens up interesting areas for future investigation.

Misalignment of watershed and jurisdictional boundaries: the importance of scale

The misalignment of the boundaries of watersheds and jurisdictions is cited often as a barrier to effective water governance, but the validity of the assertion depends on watershed scale and the decisions or processes involved. The paper probes these decision processes and their alignment with scales of natural watersheds and with governance processes. Two examples from the USA provide context and data to inform the discussion, one from the humid eastern part and one from the drier western part. Ultimately, the spatial and governance scales determine the complexity of decisions. The major issue is the level and nature of negotiations and how stakeholders communicate and work with each other to resolve issues in a form of pragmatic federalism, where the concept merges into decentralization to the subwatershed level. At smaller scales, negotiations can be worked out in person-to-person venues but at larger scales institutions have their own trajectories and inertia. Ultimately, watershed boundaries can be effective for joint planning and assessment, but decisions follow governance patterns. Basin boundaries do provide venues for coordination mechanisms to mediate conflicts.

A coupled FE model for the joint resolution of the shallow water and the groundwater flow equations

Purpose – A new coupled finite element model has been developed for the joint resolution of both the shallow water equations, that governs the free surface flow, and the groundwater flow equation that governs the motion of water through a porous media. The paper aims to discuss these issues. Design/methodology/approach – The model is based upon two different modules (surface and ground water) previously developed by the authors, that have been validated separately. Findings – The newly developed software allows for the assessment of the fluid flow in natural watersheds taking into account both the surface and the underground flow in the way it really takes place in nature. Originality/value – The main achievement of this work has dealt with the coupling of both models, allowing for a proper moving interface treatment that simulates the actual interaction that takes place between surface and groundwater in natural watersheds.