Showing papers by "Lu Zhang published in 2016"

Simulating runoff under changing climatic conditions: Revisiting an apparent deficiency of conceptual rainfall-runoff models

Abstract: Hydrologic models have potential to be useful tools in planning for future climate variability. However, recent literature suggests that the current generation of conceptual rainfall runoff models tend to underestimate the sensitivity of runoff to a given change in rainfall, leading to poor performance when evaluated over multiyear droughts. This research revisited this conclusion, investigating whether the observed poor performance could be due to insufficient model calibration and evaluation techniques. We applied an approach based on Pareto optimality to explore trade-offs between model performance in different climatic conditions. Five conceptual rainfall runoff model structures were tested in 86 catchments in Australia, for a total of 430 Pareto analyses. The Pareto results were then compared with results from a commonly used model calibration and evaluation method, the Differential Split Sample Test. We found that the latter often missed potentially promising parameter sets within a given model structure, giving a false negative impression of the capabilities of the model. This suggests that models may be more capable under changing climatic conditions than previously thought. Of the 282[347] cases of apparent model failure under the split sample test using the lower [higher] of two model performance criteria trialed, 155[120] were false negatives. We discuss potential causes of remaining model failures, including the role of data errors. Although the Pareto approach proved useful, our aim was not to suggest an alternative calibration strategy, but to critically assess existing methods of model calibration and evaluation. We recommend caution when interpreting split sample results.

Advances in hydrological modelling with the Budyko framework: A review

Abstract: Substantial climate change and intensive anthropogenic activities hamper efforts to explain and predict the variability of the terrestrial water balance. The Budyko framework has recently seen a renaissance in hydrological research due to the framework’s comprehensiveness and effectiveness for studying the effects of global change on water resources. In this paper, the development of the Budyko framework is analysed first. The temporal and spatial variability for the Budyko hypothesis are subsequently elaborated. On finer temporal scales, more processes need to be considered, and the degree of control exerted by individual factors on the water balance varies with the spatial scale considered. Finally, perspectives regarding better understanding and application of the Budyko framework at the catchment scale are provided. A representative and diverse catchment data set is required to estimate the parameter in the model. The co-evolution of landscape characteristics and climate properties would be beneficial...

Predicting shifts in rainfall‐runoff partitioning during multiyear drought: Roles of dry period and catchment characteristics

Abstract: While the majority of hydrological prediction methods assume that observed interannual variability explores the full range of catchment response dynamics, recent cases of prolonged climate drying suggest otherwise. During the ∼decade-long Millennium drought in south-eastern Australia significant shifts in hydrologic behavior were reported. Catchment rainfall-runoff partitioning changed from what was previously encountered during shorter droughts, with significantly less runoff than expected occurring in many catchments. In this article, we investigate the variability in the magnitude of shift in rainfall-runoff partitioning observed during the Millennium drought. We re-evaluate a large range of factors suggested to be responsible for the additional runoff reductions. Our results suggest that the shifts were mostly influenced by catchment characteristics related to predrought climate (aridity index and rainfall seasonality) and soil and groundwater storage dynamics (predrought interannual variability of groundwater storage and mean solum thickness). The shifts were amplified by seasonal rainfall changes during the drought (spring rainfall deficits). We discuss the physical mechanisms that are likely to be associated with these factors. Our results confirm that shifts in the annual rainfall-runoff relationship represent changes in internal catchment functioning, and emphasize the importance of cumulative multiyear changes in the catchment storage for runoff generation. Prolonged drying in some regions can be expected in the future, and our results provide an indication of which catchments characteristics are associated with catchments more susceptible to a shift in their runoff response behavior.

Bias in streamflow projections due to climate-induced shifts in catchment response

Abstract: Demand for quantitative assessments of likely climate change impact on runoff is increasing and conceptual rainfall-runoff models are essential tools for this task. However, the capacity of these models to extrapolate under changing climatic conditions is questionable. A number of studies have found that model predictive skill decreases with changed climatic conditions, especially when predicting drier climates. We found that model skill only declines under certain circumstances, in particular, when a catchment's rainfall-runoff processes change due to changed climatic drivers. In catchments where the rainfall-runoff relationship changed significantly in response to prolonged dry conditions, runoff was consistently overestimated. In contrast, modeled runoff was unbiased in catchments where the rainfall-runoff relationship remained unchanged during the dry period. These conclusions were not model dependent. Our results suggest that current projections of runoff under climate change may provide overly optimistic assessments of future water availability in some regions expecting rainfall reductions.

Long‐term streamflow trends in the middle reaches of the Yellow River Basin: detecting drivers of change

Abstract: x; "> The catchments in the Loess Plateau, in China ’ s middle reaches of the Yellow River Basin, experienced unprecedented land use changes in the last 50 years as a result of large-scale soil conservation measure to control soil erosion. The climate of the region also exhibited some levels of change with decreased precipitation and increased temperature. This study combined the time-trend analysis method with a sensitivity-based approach and found that annual stream fl ow in the Loess Plateau decreased signi fi cantly since the 1950s and surface runoff trends appear to dominate the stream fl ow trends in most of the catchments. Annual base fl ow exhibited mostly downward trends, but signi fi cant upward trends were also observed in 3 out of 38 gauging stations. Mean annual stream fl ow during 1979 2010 decreased by up to 65% across the catchments compared with the period of 1957 1978, indicating signi fi cant changes in the hydrological regime of the Loess Plateau. It is estimated that 70% of the stream fl ow reduction can be attributed to land use change, while the remaining 30% is associated with climate variability. Land use change because of the soil conservation measures and reduction in precipitation are the key drivers for the observed stream fl ow trends. These fi ndings are consistent with results of previous studies for the region and appear to be reasonable given the accelerated level of the soil conservation measures implemented since the late 1970s. Changes in sea surface temperature in the Paci fi c Ocean, as indicated by variations in El Nino – Southern Oscillation and phase shifts of the Paci fi c Decadal Oscillation, appear to have also affected the annual stream fl ow trends. The framework described in this study shows promising results for quantifying the effects of land use change and climate variability on mean annual stream fl ow of catchments within the Loess Plateau.

A new method to partition climate and catchment effect on the mean annual runoff based on the Budyko complementary relationship

Abstract: Effect of climate change and catchment change on the long-term water balance is of considerable interest at a range of spatial scales. The total differential of runoff within the Budyko framework, which has been widely used to attribute the change in runoff to the effect of climate and catchment changes, is not precise in that there is always some residual between the observed and estimated change in runoff. The objective of this study is to propose and evaluate a new partition method based on the Budyko complementary relationship for runoff. Algebraic identities have ensured that the change in runoff can be decomposed into two components precisely without any residuals using this complementary method. In addition, the complementary method allows estimation of the upper and lower bounds of the climate effect and catchment effect. The new method was compared with the total differential method and an extrapolation method for 15 catchments in Australia. Results show that the average range of the catchment effect using the complementary method was 6.7 mm for 14 of the 15 catchments, which is much smaller than that estimated with the total differential method (51.5mm). The average of the upper and lower bounds was shown to be in good agreement with the effect of climate and catchment changes estimated using the extrapolation method (R2 = 0.98 for both). Correlation analysis indicates that the average of these bounds is the best estimate of the magnitude of the climate and catchment effect for the 15 catchments examined. This article is protected by copyright. All rights reserved.

Automated Selection of Pure Base Flows from Regular Daily Streamflow Data: Objective Algorithm

Abstract: The identification of base flow from an available record of streamflow measurements for recession rate analysis is often a difficult and time-consuming process affected by unavoidable errors and subjective aspects. Here, several stringent criteria are proposed for automated selection of base flows for this method and are applied to select pure base flows in 26 catchments from the United States, Australia, and China. The characteristic drainage timescale parameter (K) of the linear response function was chosen as the effectiveness measure of the automated method; the K values estimated using this method are 44.5±13.2 days compared with 45.7±10.5 days estimated manually for the catchments with long records. The effects of different imposed criteria on the base flow selection are also quantified using this parameter. The proposed algorithm provides a faster and more objective methodology for automated selection of base flows for recession rate analysis and will thus greatly facilitate its application.

Future Changes in Floods and Water Availability across China: Linkage with Changing Climate and Uncertainties

Abstract: Future changes in floods and water availability across China under representative concentration pathway 2.6 (RCP2.6) and RCP8.5 are studied by analyzing discharge simulations from the Inter-Sectoral Impact Model Intercomparison Project (ISI-MIP) with the consideration of uncertainties among global climate models (GCMs) and hydrologic models. Floods and water availability derived from ISI-MIP simulations are compared against observations. The uncertainties among models are quantified by model agreement. Only model agreement >50% is considered to generate reliable projections of floods and water availability and their relationships with climate change. The results show five major points. First, ISI-MIP simulations have acceptable ability in modeling floods and water availability. The spatial patterns of changes in floods and water availability highly depend on the outputs of GCMs. Uncertainties from GCMs/hydrologic models predominate the uncertainties in the wet/dry areas in eastern/northwestern Chi...

Effects of water and salinity on plant species composition and community succession in Ejina Desert Oasis, northwest China

Abstract: Ecologic patterns and community succession are generally controlled by hydrologic mechanisms, especially for plant distributions which are sensitive to habitat conditions. The hydrology characteristics of ecosystems mainly influenced plant ecological processes in water and salinity changes. In this paper, we analyzed the composition and characteristic of natural plant community, divided the plant community classes and discussed the effect of water and salinity gradients on plant species and community classes in Ejina Desert Oasis. The results demonstrated that Populus euphratica, Tamarix chinensis and Phragmites communis were the most important plant species that had the highest important values among forest, shrubs and herbaceous. Six plant community patterns were classified by cluster analysis in Ejina Desert Oasis. Species richness and species diversity were the highest near West River and East River channels of the core oasis area. The distributions of plant community were mainly influenced by the following factors: distance from river channel, groundwater level, soil water content, soil salinity and groundwater salinity. The water and salinity factors, which controlled the distributions of plant, were the main driving forces for ecosystem succession. The plant community succession is becoming toward the type of shrub + herb or low shrub with very drought-tolerant from the type of forest + shrub + herb with tall and high water consumption, when habitat conditions change from good to poor. The water gradients had more significant and more directed effect than salinity gradients on plant species and communities.

Modelling Seasonal and Inter-annual Variations in Carbon and Water Fluxes in an Arid-Zone Acacia Savanna Woodland, 1981–2012

Abstract: Changes in climatic characteristics such as seasonal and inter-annual variability may affect ecosystem structure and function, hence alter carbon and water budgets of ecosystems. Studies of modelling combined with field experiments can provide essential information to investigate interactions between carbon and water cycles and climate. Here we present a first attempt to investigate the long-term climate controls on seasonal patterns and inter-annual variations in water and carbon exchanges in an arid-zone savanna-woodland ecosystem using a detailed mechanistic soil–plant–atmosphere model (SPA), driven by leaf area index (LAI) simulated by an ecohydrological model (WAVES) and observed climate data during 1981–2012. The SPA was tested against almost 3 years of eddy covariance flux measurements in terms of gross primary productivity (GPP) and evapotranspiration (ET). The model was able to explain 80 and 71% of the variability of observed daily GPP and ET, respectively. Long-term simulations showed that carbon accumulation rates and ET ranged from 20.6 g C m−2 mon−1 in the late dry season to 45.8 g C m−2 mon−1 in the late wet season, respectively, primarily driven by seasonal variations in LAI and soil moisture. Large climate variations resulted in large seasonal variation in ecosystem water-use efficiency (eWUE). Simulated annual GPP varied between 146.4 and 604.7 g C m−2 y−1. Variations in annual ET coincided with that of GPP, ranging from 110.2 to 625.8 mm y−1. Annual variations in GPP and ET were driven by the annual variations in precipitation and vapour pressure deficit (VPD) but not temperature. The linear coupling of simulated annual GPP and ET resulted in eWUE having relatively small year-to-year variation.