Untenable nonstationarity: An assessment of the fitness for purpose of trend tests in hydrology

The detection and attribution of long-term patterns in hydrological time series have been important research topics for decades. A significant portion of the literature regards such patterns as ‘deterministic components’ or ‘trends’ even though the complexity of hydrological systems does not allow easy deterministic explanations and attributions. Consequently, trend estimation techniques have been developed to make and justify statements about tendencies in the historical data, which are often used to predict future events. Testing trend hypothesis on observed time series is widespread in the hydro-meteorological literature mainly due to the interest in detecting consequences of human activities on the hydrological cycle. This analysis usually relies on the application of some null hypothesis significance tests (NHSTs) for slowly-varying and/or abrupt changes, such as Mann-Kendall, Pettitt, or similar, to summary statistics of hydrological time series (e.g., annual averages, maxima, minima, etc.). However, the reliability of this application has seldom been explored in detail. This paper discusses misuse, misinterpretation, and logical flaws of NHST for trends in the analysis of hydrological data from three different points of view: historic-logical, semantic-epistemological, and practical. Based on a review of NHST rationale, and basic statistical definitions of stationarity, nonstationarity, and ergodicity, we show that even if the empirical estimation of trends in hydrological time series is always feasible from a numerical point of view, it is uninformative and does not allow the inference of nonstationarity without assuming a priori additional information on the underlying stochastic process, according to deductive reasoning. This prevents the use of trend NHST outcomes to support nonstationary frequency analysis and modeling. We also show that the correlation structures characterizing hydrological time series might easily be underestimated, further compromising the attempt to draw conclusions about trends spanning the period of records. Moreover, even though adjusting procedures accounting for correlation have been developed, some of them are insufficient or are applied only to some tests, while some others are theoretically flawed but still widely applied. In particular, using 250 unimpacted stream flow time series across the conterminous United States (CONUS), we show that the test results can dramatically change if the sequences of annual values are reproduced starting from daily stream flow records, whose larger sizes enable a more reliable assessment of the correlation structures

CONFIGURE: An Optimisation Framework for the Cost-Effective Spatial Configuration of Blue-Green Infrastructure

This paper develops a Blue-Green Infrastructure (BGI) performance evaluation approach by integrating a Non-dominated Sorting Genetic Algorithm II (NSGA-II) with a detailed hydrodynamic model. The proposed Cost OptimisatioN Framework for Implementing blue-Green infrastructURE (CONFIGURE), with a simplified problem-framing process and efficient genetic operations, can be connected to any flood simulation model. In this study, CONFIGURE is integrated with the CityCAT hydrodynamic model to optimise the locations and combinations of permeable surfaces. Permeable zones with four different levels of spatial discretisation are designed to evaluate their efficiency for 100-year and 30-year return period rainstorms. Overall, the framework performs effectively for the given scenarios. The application of the detailed hydrodynamic model explicitly captures the functioning of permeable features to provide the optimal locations for their deployment. Moreover, the size and the location of the permeable surfaces and the intensity of the rainstorm events are the critical performance parameters for economical BGI deployment.Comment: Paper submitted for publication in Environmental Modelling and Software. 26 pages, 11 figure

Evaluation of future climate change impacts on semi-arid Cobres basin in southern Portugal

This study evaluated future climate change impacts on hydrological and sediment transport processes for the medium-sized (705 km2) agriculture dominated Cobres basin, Portugal, in the context of anti-desertification strategies. We used the Spatial-Temporal Neyman-Scott Rectangular Pulses (STNSRP) model—RainSim V3, a rainfall conditioned weather generator—ICAAM-WG, developed in this study but based on the modified Climate Research Unit daily weather generator (CRU-WG), and a PBSD hydrological model—SHETRAN, to downscale projections of change. Climate projections were derived from the RCM HadRM3Q0 output, provided by the ENSEMBLES project, for the SRES A1B scenario for the period 2041–2070. The RainSim V3 and ICAAM-WG models are demonstrated to be able to reproduce observed climatology for the period 1981–2010. The SHETRAN model reproduces hourly runoff with Nash-Sutcliffe Efficiency (NSE) of 0.86 for calibration (2004–2006) and 0.74 for validation (2006–2008) for basin outlet; it reproduces hourly sediment discharge with NSE of 0.48 for the storm from October 23rd 2006 to October 27th 2006. We found that future mean climate is drier, with mean annual rainfall decreased by 88 mm (19%), mean annual PET increased 196 mm (16%) and consequent mean annual runoff and sediment yield decreased respectively 48 mm (50%) and 1.06 t/ha/year (45%). The future mean annual AET decreases 41 mm (11%), which occurs mainly in spring indicating a more water-limited future climate for vegetation and crop growth. Under current conditions, November to February is the period in which runoff and sediment yield occur frequently; however, it is reduced to December to January in future, with changes in the occurrence rate of 50%. On the other hand, future wet extremes are more right-skewed. Future annual maximum discharge and sediment discharge decrease for extremes with return periods (T) less than 20 years and the decreases are especially greater for those with T less than 2 years; besides, both quantities present the same or slightly lower magnitudes as those with T larger than 20 years. The annual maximum discharge (sediment discharge) series, under control climate, follows the GEV distribution with location parameter of 64.6 m3/s (164.4 kg/s), scale parameter of 46.5 m3/s (120.3 kg/s) and shape parameter of 0.09 (-0.24); under future climate, the annual maximum discharge series follows the gamma distribution with scale parameter of 75.2 m3/s and shape parameter of 0.97 and the annual maximum sediment discharge series follows the three-parameter lognormal distribution with location parameter of -46.2 kg/s, mean of 5.3 kg/s and standard deviation of 0.78. This study has confirmed the increasing concerns of water scarcity and drought problems in southern Portugal; but it also indicated the mitigation of sediment transport for most of time in the future except heavy events. However, the results should be interpreted carefully since we did not consider possible changes of land-use in the future, as well as the climate and hydrological modelling uncertainties

Flood Risk Management In Small Urban River Using A Sustainable Urban Drainage System: Wortley Beck, Leeds, Uk

In the UK, urban river basins are particularly vulnerable to flash floods due to short and intense rainfall. This paper presents potential flood resilience approaches for the highly urbanised Wortley Beck river basin, south west of the Leeds city centre. The reach of Wortley Beck is approximately 6km long with contributing catchment area of 30km2 that drain into the River Aire. Lower Wortley has experienced regular flooding over the last few years from a range of sources, including Wortley Beck and surface and ground water, that affects properties both upstream and downstream of Farnley Lake as well as Wortley Ring Road. This has serious implications for society, the environment and economy activity in the City of Leeds. The first stage of the study involves systematically incorporating Wortley Beck’s land scape features on an Arc-GIS platform to identify existing green features in the region. This process also enables the exploration of potential blue green features: green spaces, green roofs, water retention ponds and swales at appropriate locations and connect them with existing green corridors to maximize their productivity. The next stage is involved in developing a detailed 2D urban flood inundation model for the Wortley Beck region using the CityCat model. CityCat is capable to model the effects of permeable/impermeable ground surfaces and buildings/roofs to generate flood depth and velocity maps at 1m caused by design storm events. The final stage of the study is involved in simulation of range of rainfall and flood event scenarios through CityCat model with different blue green features. Installation of other hard engineering individual property protection measures through water butts and flood walls are also incorporated in the CityCat model. This enables an integrated sustainable flood resilience strategy for this region

Model Validation Using Crowd-Sourced Data From A Large Pluvial Flood

On 28 June 2012, Newcastle upon Tyne, UK, was hit by a large convective storm delivering 45 mm of rain in less than two hours. Although not large on a world scale, the event had a big local impact. Widespread areas of the city were inundated and traffic was blocked in and around the city for about 6 hours. The following morning there was very small amount of visible evidence that the event happened. To gather data about the event through crowd sourcing Newcastle University organised and publicised a web page inviting public to upload their flood photographs, pin them on the map and optionally write a comment. In a more classical manner Newcastle City Council sent questionnaires to all the residents in the streets from which they received any reports about the flood and asked them to describe the flood in and around their properties. Data gathered from these sources were used to validate and calibrate the model of this event simulated using the hydrodynamic modelling software CityCAT developed at Newcastle University. CityCAT combines very accurate numerical methods with advanced software architecture providing both ease of use and accuracy in performance. Combination of those two main properties enables modelling of complex flow situations such as propagation of shocks and flows over initially dry areas, commonly in urban flash floods. Agreement between the gathered data and modelling results was satisfying at a majority of places when reduced rainfall input accounting for the drainage network was used. Crowd-sourced data, photographs and questionnaires, have proven to be effective tools in model calibration/validation