Hydrology and Earth System Sciences Discussions

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EISSN : 1812-2116
Current Publisher: Copernicus GmbH (10.5194)
Total articles ≅ 3,796
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Marc Schleiss
Hydrology and Earth System Sciences Discussions, Volume 2019, pp 1-35; doi:10.5194/hess-2019-449

Spatial downscaling of rainfall fields is a challenging mathematical problem for which many different types of methods have been proposed. One popular solution consists in redistributing rainfall amounts over smaller and smaller scales by means of a discrete multiplicative random cascade (DMRC). This works well for slowly varying, homogeneous rainfall fields but often fails in the presence of intermittency (i.e., large amounts of zero rainfall values). The most common workaround in this case is to use two separate cascade models, one for the occurrence and another for the intensity. In this paper, a new and simpler approach based on the notion of equal-volume areas (EVAs) is proposed. Unlike classical cascades where rainfall amounts are redistributed over grid cells of equal size, the EVA cascade splits grid cells into areas of different sizes, each of them containing exactly half of the original amount of water. The relative areas of the sub-grid cells are determined by drawing random values from a logit-normal cascade generator model with scale and intensity dependent standard deviation. The process ends when the amount of water in each sub-grid cell is smaller than a fixed bucket capacity, at which point the output of the cascade can be re-sampled over a regular Cartesian mesh. The present paper describes the implementation of the EVA cascade model and gives some first results for 100 selected events in the Netherlands. Performance is assessed by comparing the outputs of the EVA model to bilinear interpolation and to a classical DMRC model based on fixed grid cell sizes. Results show that on average, the EVA cascade outperforms the classical method, producing fields with more realistic distributions, small-scale extremes and spatial structures. Improvements are mostly credited to the higher robustness of the EVA model to the presence of intermittency and to the lower variance of its generator. However, improvements are not systematic and both approaches have their advantages and weaknesses. For example, while the classical cascade tends to overestimate small-scale extremes and variability, the EVA model tends to produce fields that are slightly too smooth and blocky compared with observations.
Hydrology and Earth System Sciences Discussions; doi:10.5194/hess-2019-447

Recent virtual and experimental investigations have shown that the young water fraction Fyw (i.e. the proportion of catchment outflow younger than circa 2–3 months) increases with discharge in most catchments. The discharge sensitivity of Fyw has been defined as the rate of increase in Fyw with increasing discharge (Q), and has been estimated by the linear regression slope between Fyw and Q, hereafter called DS(Q). The combined use of both metrics, Fyw and DS(Q), provides a promising method for catchment inter-comparison studies that seek to understand streamflow generation processes. Here we explore the discharge sensitivity of Fyw in the intensively sampled small Mediterranean research catchment Can Vila. Intensive sampling of high flows at Can Vila allows young water fractions to be estimated for the far upper tail of the flow frequency distribution. These young water fractions converge toward 1 at the highest flows, illustrating a conceptual limitation in the linear regression method for estimating DS(Q) as a metric of discharge sensitivity: Fyw cannot grow with discharge indefinitely, since the fraction of young water in discharge can never be larger than 1. Here we propose to quantify discharge sensitivity by the parameter of an exponential-type equation expressing how Fyw varies with discharge. The exponential parameter (Sd) approximates DS(Q) at moderate discharges where Fyw is well below 1; however, the exponential equation and its discharge sensitivity metric better capture the non-linear relationship between Fyw and Q and are robust with respect to changes in the range of sampled discharges, allowing comparisons between catchments with strongly contrasting flow regimes.
, , Quan J. Wang, Chun-Hsu Su, Nathan Eizenberg
Hydrology and Earth System Sciences Discussions, Volume 2019, pp 1-20; doi:10.5194/hess-2019-432

The high spatio-temporal variability of precipitation is often difficult to characterise due to limited measurements. The available low-resolution global reanalysis datasets inadequately represent the spatio-temporal variability of precipitation relevant to catchment hydrology. The Bureau of Meteorology Atmospheric high-resolution Regional Reanalysis for Australia (BARRA) provides a high-resolution atmospheric reanalysis dataset across the Australasian region. For hydrometeorological applications, however, it is essential to properly evaluate the sub-daily precipitation from this reanalysis. In this regard, this paper evaluates the sub-daily precipitation from BARRA for a period of 6 years (2010–2015) over Australia against point observations and blended radar products. We utilise a range of existing and bespoke metrics for evaluation at point and spatial scales. We examine bias in quantile estimates and spatial displacement of sub-daily rainfall at a point scale. At a spatial scale, we use the Fractions Skill Score as a spatial evaluation metric. The results show that the performance of BARRA precipitation depends on spatial location with poorer performance in tropical relative to temperate regions. A possible spatial displacement during large rainfall is also found at point locations. This displacement, evaluated by comparing the distribution of rainfall within a day, could be quantified by considering the neighbourhood grids. On spatial evaluation, hourly precipitation from BARRA are found to be skilful at a spatial scale of less than 100 km (150 km) for a threshold of 75 % quantile (90 % quantile) at most of the locations. The performance across all the metrics improves significantly at time resolutions higher than 3 h. Our evaluations illustrate that the BARRA precipitation, despite discernible spatial displacements, serves as a useful dataset for Australia, especially at sub-daily resolutions. Users of BARRA are recommended to properly account for possible spatio-temporal displacement errors, especially for applications where the spatial and temporal characteristics of rainfall are deemed very important.
Yohei Sawada
Hydrology and Earth System Sciences Discussions, Volume 2019, pp 1-72; doi:10.5194/hess-2019-324

It is expected that hyperresolution land modeling substantially innovates the simulation of terrestrial water, energy, and carbon cycles. The major advantage of hyperresolution land models against conventional one-dimensional land surface models is that hyperresolution land models can explicitly simulatelateral water flows. Despite many efforts on data assimilation of hydrological observations into those hyperresolution land models, how and when surface water flows driven by local topography matter for data assimilation of soil moisture observations has not been fully clarified. Here I perform two minimalist synthetic experiments where soil moisture observations are assimilated into an integrated surface-groundwater land model by an ensemble Kalman filter. A horizontal background error covariance provided by overland flows is important to adjust the unobserved state and parameter variables. However, the non-Gaussianity of the background error provided by the nonlinearity of a topography-driven surface flow harms the performance of data assimilation. It is difficult to efficiently constrain model states at the edge of the area where the topography-driven surface flow reaches by linear-Gaussian filters, which brings the new challenge in land data assimilation for hyperresolution land models. This study highlights the importance of surface lateral flows in hydrological data assimilation.
, Andrew J. Pitman, , Beilei Zan, Congbin Fu
Hydrology and Earth System Sciences Discussions; doi:10.5194/hess-2019-397

To resolve a series of ecological and environmental problems over the Loess Plateau, the Grain for Green Program (GFGP) was initiated at the end of 1990s. Following the conversion of croplands and bare land on hillslopes to forests, the Loess Plateau has displayed a significant greening trend with soil erosion being reduced. However, the GFGP has also affected the hydrology of the Loess Plateau which has raised questions whether the GFGP should be continued in the future. We investigated the impact of revegetation on the hydrology of the Loess Plateau using high resolution simulations and multiple realisations with the Weather Research and Forecasting (WRF) model. Results suggests that land cover change since the launch of the GFGP has reduced runoff and soil moisture due to enhanced evapotranspiration. Further revegetation associated with the GFGP policy is likely to increase evapotranspiration further, and thereby reduce runoff and soil moisture. The increase in evapotranspiration is associated with biophysical changes, including deeper roots that deplete deep soil moisture stores. However, despite the increase in evapotranspiration our results show no impact on rainfall. Our study cautions against further revegetation over the Loess Plateau given the reduction in water available for agriculture and human settlements, without any significant compensation from rainfall.
, Francis H. S. Chiew, , , , Lu Zhang
Hydrology and Earth System Sciences Discussions; doi:10.5194/hess-2019-375

Realistic projections of changes to daily rainfall frequency and magnitude, at catchment scales, are required to assess the potential impacts of climate change on regional water supply. We show that quantile-quantile matched (QQM) bias-corrected daily rainfall from dynamically downscaled WRF simulations of current climate produce biased hydrological simulations, in a case study for the State of Victoria, Australia (237 629 km2). While the QQM bias correction can remove bias in daily rainfall distributions at each 10 km2 grid point across Victoria, the GR4J rainfall-runoff model underestimates runoff when driven with QQM bias-corrected daily rainfall. We compare simulated runoff differences using bias-corrected and empirically scaled rainfall for several key water supply catchments across Victoria and discuss the implications for confidence in the magnitude of projected changes for mid-century. Our results highlight the imperative for methods that can correct for temporal and spatial biases in dynamically downscaled daily rainfall if they are to be suitable for hydrological projection.
, Suchul Kang, Elfatih A. B. Eltahir
Hydrology and Earth System Sciences Discussions pp 1-26; doi:10.5194/hess-2019-238

The hydrology of Morocco is characterized by a significant spatial variability. Precipitation follows a sharp gradient decreasing from the North to the South. In order to redistribute water, a project is proposed to transfer 860 million m3 per year from the wet north to the arid southern regions, {Water Highway}. The present study aims to address the viability of the project including the effects of climate change in the watersheds located in the North. We perform Regional Climate Model (RCMs) simulations over the study region using boundary conditions from five different global circulation models (GCMs) and following two emissions scenarios RCP4.5 (with mitigation) and RCP8.5 (business as usual). The impact on precipitation is assessed and the decrease of available water quantity is estimated. Under RCP8.5 the project is likely unfeasible. However, under the RCP4.5 a rescaled version of this project may be feasible depending on how much water is allocated to satisfy the local water demand.
, , Daniel Princz, Howard Wheater, , Alain Pietroniro,
Hydrology and Earth System Sciences Discussions; doi:10.5194/hess-2019-249

This analysis documents projected changes in daily precipitation and temperature characteristics over Canada based on a 15-member ensemble which had been downscaled using the Canadian Regional Climate Model–CanRCM4 at 50 km resolution by the Canadian Centre for Climate Modelling and Analysis (CCCma) under Representative Concentration Pathway (RCP) 8.5. In this study, the historical CanRCM4 simulations are first compared against observations for validation purposes. Then, a multivariate bias correction algorithm is applied to the CanRCM4 outputs to adjust the data against the EU WATCH Forcing Data ERA-Interim reanalysis (WFDEI). We analyze changes in mean and extremes for two 30-year non-overlapping future periods: 2021–2050 and 2071–2100 relative to 1979–2008. The results indicate that daily mean precipitation is projected to increase over Canada, with larger increases expected in the 2080s. However, decreases are projected in summer precipitation over the Canadian Prairies by the year 2100. Mean air temperature is projected to intensify towards the northern high latitude regions, particularly in the winter season. Precipitation and temperature extreme events may increase more than the mean. By examining the behavior of precipitation distribution tails, the mean of the probability distributions of wet extremes over the Saskatchewan (SRB) and Mackenzie River basins (MRB) is projected to shift to the right with global warming. For temperature extremes, minimum temperature may warm faster compared to daily maximum temperatures, particularly in the winter and towards the Arctic region.
José Manuel Tunqui Neira, , Gaëlle Tallec, Jean-Marie Mouchel
Hydrology and Earth System Sciences Discussions; doi:10.5194/hess-2019-325

This Technical Note deals with the mathematical representation of concentration-discharge relationships and with the identification of its parameters. We propose a two-sided power transformation alternative to the classical log-log transformation, and a multicriterion identification procedure allowing determining parameters that are efficient, both from the concentration and the load points of view.
, Lachun Wang, Xiang Li, Jie Gao
Hydrology and Earth System Sciences Discussions; doi:10.5194/hess-2019-322

Time series of annual maximum instantaneous peak discharge from 1120 stations with record lengths of at least 50 years are used to examine flood peak distributions across China. Abrupt change rather than slowly varying trend is the dominant mode of the violation of stationary assumption for annual flood peaks over China. The dominance of decreasing trends in annual flood peak series indicates a weakening tendency of flood hazard over China in recent decades. Delayed (advanced) occurrence of annual flood peaks in southern (northern) China point to a tendency for seasonal clustering of floods across the entire country. We model the upper tails of flood peaks based on the Generalized Extreme Value (GEV) distributions for the stationary series, and evaluate the scale-dependent properties of flood peaks. The relations of GEV parameters and drainage area show spatial contrasts between northern and southern China. Weak dependence of the GEV shape parameter on drainage area highlights the critical role of space-time rainfall organizations in dictating the upper tails of flood peaks. Landfalling tropical cyclones play an important role in characterizing the upper-tail properties of flood peak distributions especially in northern China and southeastern coast, while the upper tails of flood peaks are dominated by extreme monsoon rainfall in southern China. Severe flood hazards associated with landfalling tropical cyclones are characterized with tropical cyclones experiencing extratropical transition, and persistent moisture transport/interactions with regional topography as demonstrated by Typhoon Nina (1975).
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