Performance of Priestley–Taylor model for estimating evaporation with and without snow coverage over a temperate meadow in Inner Mongolia,China

To promote the awareness of Priestley–Taylor (PT) model and extend the knowledge of hydrological model under snow, performance of PT model was illustrated by comparing with eddy covariance (EC) measurements over a temperate meadow. The results showed that the mean half‐hourly averages and directly calculated daily value of PT parameter α is 0.47 ± 0.27 and 0.51 ± 0.32, respectively. Vapour pressure deficit V_PD is the mainly factor influences α in the snow‐covered period, while soil moisture in the snow‐free period. PT method underestimated evaporation in the whole study period and showed remarkable difference in different phases. It performed best in the snow‐melting phase, better in the presnow phase, worse in the fresh snow phase and worst in the snow‐melted phase. These studies indicated that PT model could be used to estimate evaporation over temperate meadow in dormant season, but utility of a variable PT parameter α during different phases should be considered.     

PROJECTED FLOW ALTERATION AND ECOLOGICAL RISK FOR PAN‐EUROPEAN RIVERS

Projection of future changes in river flow regimes and their impact on river ecosystem health is a major research challenge. This paper assesses the implications of projected future shifts in river flows on in‐stream and riparian ecosystems at the pan‐European scale by developing a new methodology to quantify ecological risk due to flow alteration (ERFA). The river network was modelled as 33 668 cells (5′ longitude × 5′ latitude). For each cell, modelled monthly flows were generated for an ensemble of 10 scenarios for the 2050s and for the study baseline (naturalized flows for 1961–1990). These future scenarios consist of combinations of two climate scenarios and four socio‐economic water‐use scenarios (with a main driver of economy, policy, security or sustainability). Environmental flow implications are assessed using the new ERFA methodology, based on a set of monthly flow regime indicators (MFRIs). Differences in MFRIs between scenarios and baseline are calculated to derive ERFA classes (no, low, medium and high risk), which are based on the number of indicators significantly different from the baseline. ERFA classes are presented as colour‐coded pan‐European maps. Results are consistent between scenarios and show that European river ecosystems are under significant threat with about two‐thirds at medium or high risk of change. Four main zones were identified (from highest to lowest risk severity): (i) Mediterranean rim, southwest part of Eastern Europe and Western Asia; (ii) Northern Europe and northeast part of Eastern Europe; (iii) Western and Eastern Europe; and (iv) inland North Africa. Patterns of flow alteration risk are driven by climate‐induced change, with socio‐economics as a secondary factor. These flow alterations could be manifested as changes to species and communities, and loss of current ecosystem functions and services. Copyright © 2013 John Wiley & Sons, Ltd.     

A Quantitative Framework to Derive Robust Characterization of Hydrological Gradients

If ecological management of river ecosystems is to keep pace with increasing pressure to abstract, divert and dam, we must develop general flow–ecology relationships to predict the impacts of these hydrologic alterations. Regional flow gradient analyses are a promising tool to quickly reveal these functional relationships, but there are considerable uncertainties in this method because of variability in the historical extent of flow data across different rivers, combined with multiple indices characterizing the ecological attributes of flow regimes. In response, we outline an objective framework for analysing spatial hydrologic gradients that addresses three major sources of uncertainty: robust estimation of flow indices, the potential for temporal trends to confound spatial variation in flow regimes and the statistical robustness to detect underlying hydrological gradients. The utility of our framework was examined in relation to flow regimes across multiple braided river catchments in Canterbury, New Zealand. We found that a subset of flow indices could be robustly estimated using only 10 years of flow data, although indices that captured flow ‘timing’ required longer time series. Temporal trends were unlikely to confound conclusions from a spatial hydrologic gradient analysis, and there were three statistically supported hydrologic gradients related to flow magnitude, flow variability and low flow events. The widespread application of robust spatial flow gradient analyses has the potential to further our understanding of how altered flow regimes affect the ecology of freshwater and riparian ecosystems, thereby providing the evidence base to inform river management. Copyright © 2016 John Wiley & Sons, Ltd.     

The impact of Japanese knotweed on river discharge at the watershed scale in New Jersey,USA

Recently, New Jersey has experienced several droughts and declared several critical water shortage areas, spurring interest in reducing freshwater resource depletion. Japanese knotweed (Reynoutria japonica Houtt.) is an invasive plant species that may have a significant effect on stream water loss. In order to assess the impact of this species on river-level water flow, we estimated total knotweed distribution along two tributaries of the Passaic River in New Jersey, USA using a combination of field measurements and GIS to calculate total daily water loss to the atmosphere from three stands of knotweed along these rivers. We measured total leaf area of each stand and transpiration rates across each stand from sun-up to sun-down. The average water loss was 8.5 L wate/day/m² of ground area covered in knotweed. Knowing the total distribution of knotweed along each river, the amount of river length covered by knotweed stands and the total water lost to the atmosphere per amount of knotweed along each river we were able to estimate the total amount of water transpired to the atmosphere by knotweed per river on a daily basis during its growing season. These results were compared to summer low flow rates to assess the impact on river flow during the growing season for Japanese knotweed. Our results suggest that knotweed along these rivers is reducing total flow by an average of 8% (approximately 800,000–1,400,000 L/day) during the summer months. This is important as the impact of invasive species on water resources in temperate areas is currently under-studied in ecohydrology.     

Vegetation recruitment on the ‘white’ sandbars on the Nakdong River at the historical village of Hahoe,Korea

Hahoe, a historical village, is famous for two large nonvegetated sandbars. These sandbars have become covered with vegetation after construction of the two upstream dams. To identify the contributing factors to vegetation recruitment, flow regime, soil moisture, flood intensity and climate conditions before and after two dams were investigated. The occurrence of drought caused flow change and significant encroachment of riparian vegetation in just 3 years after the second dam was in place. Numerical results show that the dimensionless shear stress was a useful factor for predicting vegetation recruitment. Decrease in peak flows and aggradation of the sandbars resulted in more of the sandbar area experiencing low shear stress, hence facilitating vegetation colonization. In conclusion, the development of optimal dam operation rule to increase the difference of peak release flows between germination and flood season may be required to manage the riparian vegetation sustainably and economically in the regulated river.     

生态水文双向耦合模型的研发与应用:Ⅰ模型原理与方法

陆地生态水文耦合的关键过程是以植被气孔行为调控的植物蒸腾-光合作用的双向耦合,并共同影响着水、能量通量、碳的收支平衡,构建陆地生态水文双向紧密耦合模型是研究其相互作用与反馈的重要基础。本研究基于分布式时变增益模型(DTVGM),改进并发展了其中水、能量通量、光合作用及植物生长等关键模块,耦合以CASACNP为基础的生物地球化学模型,提出了耦合模型的架构、模拟方法及模型主要原理和计算方法,构建了一个耦合陆面水文与生物地球化学循环的生态水文双向紧密耦合模型,为陆面水文-生物地球化学过程的模拟及应用提供基础。     

Quantifying uncertainty in estimation of hydrologic metrics for ecohydrological studies

Hydrologic metrics have been used extensively in ecology and hydrology to summarize the characteristics of riverine flow regimes at various temporal scales but there has been limited evaluation of the sources and magnitude of uncertainty involved in their computation. Variation in bias, precision and overall accuracy of these metrics influences the ability to correctly describe flow regimes, detect meaningful differences in hydrologic characteristics through time and space, and define flow‐ecological response relationships. Here, we examine the effects of two primary factors—discharge record length and time period of record—on uncertainty in the estimation of 120 separate hydrologic metrics commonly used by researchers to describe ecologically relevant components of the hydrologic regime. Metric bias rapidly decreased and precision and overall accuracy markedly increased with increasing record length, but tended to stabilize >15 years and did not change substantially >30 years. We found a strong positive relationship between the degree of overlap of discharge record and similarity in hydrologic metrics when based on 15‐ and 30‐year discharge periods calculated within a 36‐year temporal window (1965–2000), although hydrologic metrics calculated for a given stream gauge tended to vary only within a restricted range through time. Our study provides critical guidance for selecting an appropriate record length and temporal period of record given a degree of metric bias and precision deemed acceptable by a researcher. We conclude that: (1) estimation of hydrologic metrics based on at least 15 years of discharge record is suitable for use in hydrologic analyses that aim to detect important spatial variation in hydrologic characteristics; (2) metric estimation should be based on overlapping discharge records contained within a discrete temporal window (ideally >50% overlap among records); and (3) metric uncertainty varies greatly and should be accounted for in future analyses. Copyright © 2009 John Wiley & Sons, Ltd.     

白洋淀流域生态水文过程演变及其生态系统退化驱动机制研究

白洋淀是华北地区最大的天然淡水湖泊湿地和重要的生态功能区,在自然和人为驱动影响下,生态系统呈明显的退化趋势。通过对白洋淀流域生态水文过程演变分析,剖析了其生态系统退化的特征及驱动机制。气候干旱是白洋淀生态系统退化的环境背景条件,而上游水库的截流、水利工程的建设及水资源的开发利用等自然、人为因素的耦合作用,加速了白洋淀生态系统退化的过程。基于白洋淀生态系统退化的特征和驱动机制,从流域生态水文过程演变出发,为湿地的保护提出了措施和建议。     

On the predictive ability of mechanistic models for the Haitian cholera epidemic

Predictive models of epidemic cholera need to resolve at suitable aggregation levels spatial data pertaining to local communities, epidemiological records, hydrologic drivers, waterways, patterns of human mobility and proxies of exposure rates. We address the above issue in a formal model comparison framework and provide a quantitative assessment of the explanatory and predictive abilities of various model settings with different spatial aggregation levels and coupling mechanisms. Reference is made to records of the recent Haiti cholera epidemics. Our intensive computations and objective model comparisons show that spatially explicit models accounting for spatial connections have better explanatory power than spatially disconnected ones for short-to-intermediate calibration windows, while parsimonious, spatially disconnected models perform better with long training sets. On average, spatially connected models show better predictive ability than disconnected ones. We suggest limits and validity of the various approaches and discuss the pathway towards the development of case-specific predictive tools in the context of emergency management.     

Application of the ‘natural flow paradigm’ in a New Zealand context

The natural flow paradigm (NFP) emphasizes the need to partially or fully maintain or restore the range of natural intra‐ and interannual variation of hydrologic regimes to protect native biodiversity and the evolutionary potential of aquatic, riparian and wetland ecosystems. Based on our studies of natural and managed flow regimes in New Zealand, we do not believe that all components of the natural flow regime are necessary to achieve the objectives of the NFP, either partially or fully, because many aquatic species have very flexible niches and life‐history requirements (i.e. there is ‘ecological redundancy’). Obviously, maintaining the natural flow regime will maintain the hydrologic and hydraulic conditions necessary for sustaining natural ecosystems. However, if there is adequate knowledge of what ‘values’ need to be maintained in a waterway, and the aspects of the flow regime that are required to maintain those values are also known, then regimes can be designed that target these requirements and thus optimize conditions for the ‘values’. We believe that an assessment of ecosystem requirements using information on river processes together with habitat requirements and life‐history strategies of biota can achieve the best balance between resource use and sustaining ecosystem function and value, and show examples where changes to natural flow regimes have maintained, or even improved, instream values in some New Zealand rivers. We caution that simple flow‐based rules, such as those that might be developed under the NFP, could be unnecessarily restrictive on multiple use of water in New Zealand while, at the same time, preclude the opportunity for enhancement of key ecosystem values in many waterways. Copyright © 2008 John Wiley & Sons, Ltd.