Detecting flood drivers through large-sample geomorphology

<p>Many fluvial processes have long been treated as stationary, fluctuating within an unchanging envelope of variability. However, a large body of evidence has revealed that shifts in climate, land cover and river basin management may manifest locally along river networks through hydrological and geomorphic change. Measuring the effect of these changes on the local flood risk requires a large sample approach. Large sample geomorphology has existed for many decades but is currently undergoing a step-change characterised by computational techniques, scalability, and growing interdisciplinarity. This step-change has been assisted by the availability of remotely sensed datasets describing the land surface (including satellite, airborne and ground-based acquisitions), alongside other datasets more conventionally employed in hydro-climatology (including weather and climate observations, reanalysis, and projections). Within this context, data science and AI approaches facilitate pattern detection and the testing of both long-standing and emerging theories, to derive insights about processes and mechanisms at play. Here, we will discuss the value of large-sample geomorphology for understanding nonstationary landscapes and the associated flood risk. We will provide insights into the promise and pitfalls of large-sample approaches within an evolving discipline, and discuss ways forward, with more systematic hypothesis testing and developing projections of future change.    </p>

All dried up: An interdisciplinary analysis of drought risk in Ladismith, South Africa

<p>Droughts have always been part of Earth climate, yet today these phenomena are becoming more alarming due to their increasing severity and their disastrous socio-ecological impacts. Different scientific definitions or diverse understanding of drought risk have been proposed also because of the simultaneously social and ecological complexity which characterizes droughts relative to other hazards and/or vulnerabilities. This work sets out to confront the distinctive complexity of drought risk throughout a novel approach which combines political ecology perspectives with hydro-climatological insights. Our engagement with political ecologies of land, water, and vulnerability helps to explain the socio-political processes that intersect with the production of droughts and their consequences. Concurrently, hydro-climatology unravels the physical or material processes that both constitute and transform drought phenomena into socio-ecological disasters. The drought-stricken Ladismith in Western Cape, South Africa, is the point of departure of our empirical analysis which portrays the socio-ecological disruption reached by this rural community after five years of below-average rainfall (meteorological drought). We show that Ladismith socio-ecological crisis was mostly engendered by a distinct mechanism of capital accumulation through land and water dispossession, which emerged locally in the form of white commercial agriculture. Our interdisciplinary approach examines these socio-political processes in relation to the drought physical transformations over time and across space. By relating societal and physical processes we advance a novel understanding of drought that sheds light on the crucial interactions between social power, climate, land use, and hydrology, which all too often transform a meteorological event into a soil moisture drought, a hydrological drought, and eventually into a major socio-ecological crisis. Secondly, combining hydro-climatology with political ecology reveals that social power not only influences the vulnerability of the systems affected by droughts, but also shapes the occurrence and manifestation of the hazard itself. This novel conceptualization of drought risk as socially produced is key to intercept the material spaces and physical dynamics through which social power plays out in more extreme and disruptive drought events. A similar approach, by identifying unjust and unsustainable socio-ecological changes, can make drought management policies and strategies more proactive rather than constrain them to relief or adaptation measures.</p>

Impacts of Climate Change on the Hydro-Climatology and Performances of Bin El Ouidane Reservoir: Morocco, Africa

In arid and humid contexts, dams’ reservoirs play a crucial role in water regulation and flood control. Under the projected climate change (CC) effects, even a preoptimized management approach (MA) of a reservoir needs to be assessed in this projected climate. This chapter aims to assess the impacts of CC on the Hydroclimatic (HC) variables of the basin upstream the reservoir of Bin El Ouidane (Morocco), and the effects on the performances of its preoptimized MA. The applied Top-Down assessment procedure included CORDEX climate projections, hydrological, siltation, evaporation, and management models. Concerning the HC variables, the results obtained concord with those reported in the literature in terms of trend, but not always in terms of intensity of change. On the other hand, the projections expected a decrease in the performances of the reservoir, except for criterion allocations’ standard deviation, calibrated during the optimization. Also, interesting conclusions have been found like: the change in precipitation dominant form, the accentuation of the pluvial hydrological regime, the advanced snow melting due to the temperature increase. This chapter presents a typical case study on how to use climate projections for reservoir MA adaptation, without being highly and negatively influenced by the climate model uncertainties.

A 50-year analysis of hydrological trends and processes in a Mediterranean catchment

The Réal Collobrier hydrological observatory in south-eastern France, managed by Irstea since 1966, constitutes a benchmark site for regional hydro-climatology. Because of the dense network of stream gauges and rain gauges available, this site provides a unique opportunity to evaluate long-term hydro-meteorological Mediterranean trends. The main catchment (70 km2) and its sub-catchments are located in the Massif des Maures of south-eastern France, close to the Mediterranean coast. The vegetation is composed of forest mainly calcified on crystalline soils (maquis of heath, cork-oak, maritime pine and chestnut). Direct human influence has been negligible over the past 50 years. The land use and land cover has remained almost unchanged, with the notable exception of a wildfire in 1990 that impacted a small sub-catchment. Therefore changes in the hydrological response of the catchments are caused by changes in climate and/or physical conditions. This study investigates changes in observational data using up to 50-year daily series of precipitation and streamflow. The analysis used several climate indices describing distinct modes of variability, at inter-annual and seasonal timescales. Trends were assessed by the Mann–Kendall method. The analysis also used hydrological indices describing drought events based on daily data for a description of low flows, in particular in terms of timing and severity. The analysis shows that there is a marked tendency towards a decrease in the water resources of the Réal Collobrier catchment in response to climate trends, with a consistent increase in drought severity and duration. But the changes are variable among the sub-catchments.