Objective Characterization of Rain Microphysics: Validating a Scheme Suitable for Weather and Climate Models

Improving the atmospheric component of hydrological models is beneficial for applications such as water resources assessment and hydropower operations. Within this goal, precise characterization of rain microphysics is key for climate and weather modeling, and thus for hydrometeorological applications. Such characterization can be achieved by analyzing the evolution in time of the particle size distribution (PSD) of hydrometeors, which can be measured at ground using disdrometers for validation. The estimation, however, depends on the choice of the PSD form (the shape) and on the parameters to define the exact shape. In the case of modeling rain microphysics, two approaches compete: the use of the number concentration of drops decoupled from the shape of the distribution (the [N-T, E(D), E(D-2)] and the {N-T, E(D), E[log(D)]} models), and the (N-0, , ) model that embeds in N-0 both the shape of the distribution and the number concentration of drops. Here we use a comprehensive dataset of disdrometer measurements to show that the N-T-based approaches allow a more precise characterization of the drop size distribution (DSD) and also a physically based modeling of the microphysical processes of rain since N-T is analytically independent of the shape of the DSD {parameterized by E(D), and E(D-2) or E[log(D)]}. The implication is that numerical models would benefit from decoupling the number of drops from the shape of distribution in their modules of precipitation microphysics in order to improve outputs that eventually feed hydrological models.

Influence of dam operation on water resources management under different scenarios in the Zambezi River Basin considering environmental objectives and hydropower

Théodora Cohen Liechti

Worldwide, almost 50,000 dams over 15 m height have been built during the last six decades with an aggregated storage capacity of 6,000 km3. The fact that large dams, by increasing irrigation and hydroelectricity production, can sustain development and reduce poverty has led developing countries to undertake major investment in dam construction. However, scientific progress is required in order to design and manage the future complex hydrologic/hydraulic systems in sustainable ways. The African Dams project (ADAPT) aimed at enhancing the scientific basis of integrated water resource management. New models for the real-time control and multi- objective optimization of large hydraulic structures were created and data resources enhanced through field survey. The case study considered to apply this new knowledge is the Zambezi River basin located in southern Africa. It contains many of southern Africa’s largest and most intact freshwater and estuarine wetlands, e. g. the Kafue flats, the Mana Pools and the Zambezi delta as well as several free-flowing yet unprotected river reaches. Three of Africa’s largest dams (Kariba, Cahora Bassa and Kafue) inundate hundreds of square kilometers of river habitat and modify the natural flow patterns that sustain floodplains. Increasing human activity by cities and industry is causing a regional energy shortage, leading governments and investors to plan yet more dams in the basin. In the framework of the ADAPT project, the major contribution of the present research is to set-up a hydrologic-hydraulic model of the whole catchment area which includes all relevant elements as hydraulic structures and schemes as well as floodplains. The multi- objective modelling and simulation define how dam operation can be adapted to get the highest environmental results under highest energy production. Three main steps structure the project: (1) the evaluation of the quality of available input data, (2) the definition of the specific hydrological processes needed for hydraulic-hydrological modeling of the Zambezi Basin along with the establishment of a calibration outline, (3) the assessment of the impacts of the planned new hydraulic structures and the refurbishment of the existing hydropower plants on the flow regime at critical points of the basin. At first, three operational and acknowledged satellite derived precipitation products (the Tropical Rainfall Measuring Mission product 3B42 -TRMM 3B42-, the Famine Early Warning System product 2.0 -FEWS RFE2.0- and the National Oceanic and Atmospheric Administration/Climate Prediction Centre (NOAA/CPC) morphing technique -CMORPH-) are analyzed in terms of spatial and temporal distribution of the precipitation. They are compared to ground data at daily, 10-daily and monthly time steps. Based on the results, TRMM 3B42 was chosen as input data for the hydrological modeling. Secondly, the Soil and Water Assessment Tool (SWAT 2009), a semi-distributed physically based continuous time model, was selected to simulate the hydrology of the basin. Due to the specificities of the Zambezi River basin, two main additional functions were developed. (1) A floodplain sub-model based on a reservoir approach was implemented. The model separates the outflow of the reservoir simulating the floodplain into main channel flow and flow over the floodplain area. (2) The hydropower plant operations are simulated based on the rule curve and the technical characteristics of the dams. The pertinence of the implemented approach was verified by modeling the existing hydropower plants. Given the complexity and the size of the basin, an automated calibration procedure based on A Multi- ALgorithm Genetically Adaptive Multi-objective method (AMALGAM) was applied to optimize the relative error and the volume ratio at multiple discharge stations. The observed volume at the artificial reservoir derived from the measured water level was included in the calibration. Thirdly, scenarios combining different levels of environmental requirements as well as multiple hydropower development schemes were simulated at a daily time step with the hydraulic-hydrological model. The hydropower operation rules are simulated in detail. The mean annual energy produced, the firm power and the spilled volume during flood season are computed for each scenario. The impact on flow regime is characterized by Pardé coefficients, a set of indicators based on the Range of Variability Approach (RVA) and duration curves. In a global perspective, the analysis shows that it is possible to reach a compromise between energy production and environmental sustainability. Finally, the data of two Global Circulation Models (GFDL-CM2.0 and CCCma- CGCM3) for the emission scenario SRES A2 of the IPCC report were used to simulate the hydrological input during 2045-2065 and 2080-2100 periods. The prediction of the climate models diverge in terms of precipitation as GFDL-CM2.0 forecasts an increase while CCCma-CGCM3 a diminution but agree on the increase of temperature. The impacts of climate change were assessed both on energy production and flow pattern changes. The development and application of a hydraulic-hydrological model to a large river basin in a context of data scarcity and particular hydrologic units is particularly important for water resource management. The use of open source data and adapted calibration tools allows operators, authorities and researchers to assess together the impacts of hydropower development.

EPFL2013

A Generic Framework to Produce Probabilistic Rainfall Scenarios Incorporating Climate Change

Xavier Beuchat

The present work addresses the problematic of forecasting impacts of climate change on future rainfall regimes and their consequences on urban stormwater infrastructures. Researches carried out allowed to develop an integrated framework for producing high resolution probabilistic rainfall projections suitable for studying hydrological processes at the scale of urban drainage. Downscaling is at the core of the methodology as the predictions of the numerical General Circulation Models (GCMs) employed by the climate scientific community to model climate evolution are too coarse for hydrological impact studies. The proposed downscaling approach respects the scales of the physical processes characterizing precipitations and consists in three steps: i) Daily rainfall series at the location of interest are downscaled from coarse-gridded monthly GCMs projections (scale of weather events); ii) The generated daily series are further downscaled to the hourly time-step (scale of storms dynamics); iii) Finally, hourly series are disaggregated to sub-hourly level (scale of raincells). Daily downscaling is achieved by a statistical procedure, based on Generalized Linear Models (GLMs), seeking to relate large-scale atmospheric variables, corresponding to the scale of GCMs, to local daily rainfall series. The proposed methodology is assessed using three contrasted situations in Switzerland (Geneva, Sion and Säntis) and is shown to perform well in reproducing historical rainfall statistics (including extremes and inter-annual variability) in the present-day climate; furthermore, projections were shown to be consistent with the simulations of physically-based dynamical models (i.e. Regional Climate Models). Projections for the second part of the 21th century indicate considerably drier summers, but no significant tendency toward more extreme events was detected except for Säntis. Finally, extensions of the methodology were presented allowing to downscale other atmospheric variables than rainfall. Sub-daily rainfall downscaling is achieved using a stochastic hourly rainfall generator based on Poisson clusters model which aims at conceptualizing storm dynamics in a simple way. To provide sensible results such generators have to be fitted on historical rainfall statistics computed at different levels of temporal aggregation. In the present context, this raises a fundamental problem as the required fitting statistics at the sub-daily time-scale are not available for the future. Shortcomings of existing methods led us to develop a novel approach based on Multivariate Adaptive Regression Splines (MARS) which were so far seldom used in hydrology. The proposed MARS models are conditioned on climate and fit thus particularly well in the general downscaling framework. In addition, atmospheric predictors allow to account naturally for seasonal variations meaning that a single MARS model holds for the whole year, whereas existing models are specific to each month of the year and are therefore not robust against the seasonal changes that might induce global warming. The methodology was applied to generate hourly rainfall series from daily data simulated by the GLMs at Geneva for the end of the 21th century. Climate change was found to impact significantly summer storm dynamics: raincells are predicted to be shorter but more intense, and storms are projected to be less frequent. A frequency analysis made on the simulated hourly rainfall series revealed a significant increase of hourly rainfall return levels. Hourly rainfall series are further disaggregated to the 10-minute level using a cascade-based model. Using case studies in the Geneva area, the performances of this sub-hourly rainfall disagreggator (in particular the reproduction of extreme values) were shown to be equivalent when fitted on statistics derived from the temporal levels 10-minute to 1-hour (sub-hourly fitting set), or from the 1-hour to 3-hour levels (supra-hourly fitting set). In consequence, the supra-hourly statistics of the hourly rainfall series generated by the stochastic Poisson clusters model can be used to fit the disagreggator model in order to simulate 10-minute rainfall series. Projections of 10-minute rainfall at Geneva for the end of the 21th century indicate an increase of extreme events intensities. Uncertainties in the proposed downscaling procedure are dealt with using, whenever feasible, probabilistic models (i.e. GLMs, hourly rainfall generator, MARS model and sub-hourly disagreggator), and relying on a large number of General Circulation Models projections conditioned on various greenhouse gases emissions scenarios. The present work concludes with a case study illustrating how the developed downscaling methodology may be used to evaluate different strategies of sustainable stormwater management. The Industrial Zone of Plan-les-Ouates (ZIPLO) taken as example is a small urbanized area of Geneva. Urban drainage was characterized using a semi-distributed rainfall-runoff model, and climate change (under the higher greenhouse gas emissions scenario) was shown to increase significantly the peak discharge flows at the ZIPLO outlet. Different sustainable stormwater options were then evaluated in order to limit the peak discharge flows under the joint scenario of climate change and projected urbanization increase.

EPFL2011

Operation of Complex Hydropower Schemes and its Impact on the Flow Regime in the Downstream River System under Changing Scenarios

Martin Peter Bieri

Hydropower is the world’s most important renewable electricity source. More than 40% of European hydroelectric energy is produced in Alpine countries. High-head storage hydropower plants (HPP) contribute significantly to peak energy production as well as electricity grid regulation. Future plant management is faced with several challenges concerning modified availability of water resources due to climate change as well as new economic constraints associated with legal, political and electricity market issues. HPP operation results in unsteady water release to the downstream river system. Hydropeaking is the primary factor of flow regime alteration, impacting the river ecosystem. Even when the biological response to hydropeaking is not fully understood, the recently adapted law on water protection prescribes its mitigation in Switzerland. In this research project, a novel integrative approach to model and assess the impact of the operation of a complex hydropower scheme on the downstream river system is developed. It contains (1) a precipitation-runoff model extended for long-term simulations of glacierized Alpine catchment areas, (2) an operation tool for high-head storage HPP, (3) flow regime generation with cost estimation of hydropeaking mitigation measures and (4) a habitat model of reference river morphologies for a target species. The upper Aare River (Hasliaare) in Switzerland is an Alpine stream, affected by hydropeaking from a complex hydropower scheme with several storage volumes and power houses. Since the 1930s, seasonal water transfer from summer to winter and the amplitude and frequency of daily peak discharge have been continuously increased. Furthermore, the dynamic braided river network with various mesohabitats gave way to a mainly monotonous channel. Although diversity of species and biomass of aquatic biota have drastically decreased, the potential of redevelopment remains. Investigations to improve the river morphology and the flow regime are under discussion. The upper Aare River catchment is therefore an appropriate case study for analysis of the interactions between climatic, hydrological, hydraulic, economic as well as ecological parameters. The simulation of runoff in Alpine catchment areas is essential for optimal hydropower exploitation under normal flow conditions, but also for the analysis of flood events. The semi-distributed conceptual modeling approach Routing System contains a reservoir-based precipitation-runoff transformation model (GSM-SOCONT), extended by dynamic glacier simulation tool. Spatial precipitation and temperature distributions are taken into account for simulating the relevant hydrological processes, such as glacier melt, snowpack constitution and melt, soil infiltration and runoff. The model development, calibration and validation are illustrated for the 2005 flood event, where the flood reduction capacity of the HPP is discussed, as well as future long-term runoff estimations. Climate change scenarios, based on a reference climate period, take into account intra-annual temperature and precipitation variations as well as their long-term tendencies. Runoff series of daily resolution are produced by hourly updating of the meteorological, glaciological and hydrological parameters. An almost complete deglacierization of the upper Aare River basin is simulated for the late 21st century. The resulting reduction of glacier melt in summer and earlier snowmelt in spring change the runoff regime from glacio-nival to nival. The implemented heuristic hydropower modeling tool in Routing System allows simulation of the operating mode of complex HPP. Within the case study of the upper Aare River catchment and despite the complexity of the HPP network, the influence of climate change, electricity market issues, plant enhancements as well as hydropeaking constraints is simulated and assessed. Despite the reduction of future runoff, increased flexibility due to new turbine and pumped-storage capacities allows compensation, especially in the case of volatile electricity prices, and could even partially restore the natural flow regime. Several operational and construction measures to reduce hydropeaking are implemented in the model. Resulting flow regimes as well as the related costs are defined. Operational constraints, such as limitation of turbine discharge, increase of residual flow or limited drawdown range, generate relatively high costs compared to their environmental effectiveness. Better ecological and economic response is achieved by construction measures, such as flow deviation systems or compensation basins installed downstream of the power house outflow where the water is temporarily stored and then released to the river by a guided system. The simulated flow regimes are rated by a river specific habitat model for representative morphologies and three life stages of the target species brown trout (Salmo trutta fario). This is based on results from a 2D hydrodynamic model and in situ investigations undertaken in the framework of a joint project of EAWAG. Steady and dynamic indicators quantify fish habitat suitability and allow comparison through economic indices of the implemented mitigation measures. For the Hasliaare River, investments for mitigation of hydropeaking are only justified by morphological improvements. The developed approach is useful for the enhancement of complex storage hydropower schemes regarding mitigation of altered flow regimes. Despite several uncertainties, it allows operators, authorities and researchers to define and rate the impact of HPP operation on the river network, to ecologically and economically assess mitigation measures and thus to address hydropeaking in a straightforward manner.

EPFL2012