Large-scale variations in hydrological characteristics

Given heightened concerns about climate change and human impacts upon water resources, it is critical to provide information about current and future variations in hydrological characteristics. By elucidating patterns and drivers of hydrological response, it is possible to assess those regions and time-periods most susceptible to climate change/variability and anthropogenic influences and thus inform decision-makers so that water hazards and stress (e.g. floods and droughts) may be mitigated. NE-FRIEND Project 3 aims to identify and understand variations in hydrological behaviour at a range of spatial (within basin to global) and temporal (event to multi-decadal) scales, so much of the research effort is targeted toward regional hydrology. Research themes include:

1) methods for detection of hydrological variability and change: regionalisation/

interpolation, mapping hydrological characteristics, time-series analysis, classification, assessing climatic sensitivity of flows, hydrostochastics, etc.

2) multi-scale hydrological systems/scale issues

3) large scale climate-hydrology interactions, including teleconnections 4) modelling hydrological fluxes/budgets and water use

5) prediction and forecasting of hydrological variability, including floods and droughts.

The research philosophy of the group is holistic. Research spans the spectrum of hydrological descriptors (average, minimum (drought), maximum (floods), annual regimes, duration curves, moments, etc.) rather than focusing upon certain aspects of the hydrological regime.

Both inductive (data mining) and deductive (hypothesis-driven) approaches are adopted.

Many of the group’s innovative techniques for hydrological analysis have been developed to be adaptable to a range of hydrological descriptors to increase applicability. Moreover, several participants work on the atmosphere-surface water (including snow and ice)-groundwater process cascade and, thus, bridge traditional (sub-)discipline boundaries.

Although a range of research themes and approaches exist within the group, participants are united by a common goal: to share expertise, methods, findings and data. The benefit and unique aspect of NE-FRIEND Project 3 is the cross-cutting nature of research activities, which allow the group and participants to collaborate with AMIGO (Latin America and Caribbean), AMHY (Alpine and Mediterranean) and HKH (Hindu Kush-Himalayan) FRIEND programmes, as well as other NE-FRIEND groups (e.g. Project 2: Low Flow).

The ability to ‘think beyond the drainage basin’ and identify, understand and predict drivers of hydrological variability are fundamental to assessment of the impact of global environmental change (both climate and human-induced) upon water resources. In addition, the regionalisation and interpolation of hydrological characteristics coupled with the investigation of multi-scale hydrological systems are important in two contexts: (1) addressing the ‘ungauged basin problem’ (e.g. the IAHS-PUB initiative) and (2) upscaling finer-scale process-based understanding to the large scale (and vice versa downscaling).

These scientifically and practically important issues are being addressed by NE-FRIEND Project 3, as demonstrated by the following key achievements. To organise information,

projects are grouped under the above research themes; however, it should be noted that these research strands are interwoven rather than mutually exclusive.

Methodological advances and scale issues

Several participants have worked on advancing regionalisation/interpolation and time-series analysis methods (theme 1 and theme 2). This research includes:

• quantification and explanation of regional hydrological variations and evaluation of competing time-series analysis methodologies for studying hydrological records in Wales and the English Midlands

• regional frequency analysis to examine changes in extreme rainfall causing flooding in the UK (1961-2000) and assessing the ability of regional climate models to reproduce extreme rainfall patterns and assessing future changes

• new statistical methods for interpolation of runoff at different space and time scales, termed ‘hydrostochastics’; these techniques allow consistent regionalisation of all river flow characteristics (from mean runoff and coefficient of variation to low flow and floods) and they respect ‘contextual rules’, that is, all parts of the basin and river network are interrelated.

• evaluation of flood risk in Slovakia, including computation of design discharges in small mountain basins, regional flood frequency analysis, flood risk analysis, rainfall-runoff modelling and assessing climate change impacts on flooding

• development of a national low flow regionalisation method for Austria, including seasonality indices classification, comparison of various approaches for grouping hydrologically similar basins and mapping schemes, investigation of the value of short records for regionalisation and the Consensus Modelling Approach

• analyses of the spatial and temporal distribution of floods within Britain over the last 1000 years, including methods for the incorporation of historical hydrological information into flood frequency analysis

• regionalisation, using basin properties, of hydrological parameters for lumped models and river flow prediction at ungauged sites

• explicit consideration of scale issues (within a geostatistical framework) to interpolate runoff, regionalise catchment parameters and flood frequency, and model regional water availability

• identification of a regional variability in hydrological/basin characteristics and flood frequency for small basins in Slovakia as a basis for assessing flood risk (Figure 2.5).

Climate-hydrology interactions (theme 3)

Both empirically based and modelling (also see theme 4 below) approaches have been adopted to investigate large-scale hydroclimatological patterns and interactions. At the Western European scale, empirical research has focused upon identifying and understanding spatial and temporal variability in climate and river flow regimes, and the impact of climate variability/change upon river flow regimes. This work has included:

Figure 2.5 Flood risk for small basins in Slovakia, based upon evaluation of rock-soil complex permeability (source: Solín) [Note: I = basins with no permeable and very poorly permeable soils having high variability of mean daily discharge values (Cv = 100-160%) and low percentage of baseflow (30-50%); II = basins with permeable soil having medium variability of mean daily discharge values (Cv = 50-100%) and medium percentage of baseflow (50-75%); III = basins with highly permeable soils with very low variability of mean daily discharge values (Cv < 50%) and very high percentage of baseflow (over 75%)]

• developing a methodology for classification of river flow regimes (i.e. annual hydrographs) at the continental scale to aid characterisation of spatial and temporal variability in Western European river flows (Figure 2.6)

• deriving a new air-mass climatology for Western Europe

• developing a transferable Sensitivity Index to assess the impact of climatic variability upon flow regimes

• investigating linkages between Western European air-mass types and their frequencies, and river flow regimes

• developing methodologies for detecting changes in river flow regimes in space and time, e.g. for Scandinavia and France.

In addition, these methodologies (or derivatives) have been applied to characterise large-scale precipitation and runoff patterns and to assess the potential impact of climate variability/change upon river flows for Himalayan river basins of Nepal. Similar techniques have also been used to examine climate-hydrology links in the Upper Indus Basin (Hindu Kush Himalaya) to assess recent climatic trends and associations between atmospheric circulation patterns and hydrology response.

For the northern North Atlantic region, ocean-atmosphere (including the North Atlantic Oscillation) controls upon river flow have been investigated to identify the key climatological drivers of both high and low flows. Scenario-based modelling has been undertaken across

Figure 2.6 Variability in inter-annual river flow regimes across Western Europe (source:

Bower; Hannah; McGregor) [Note: (1) This sample illustrates that in individual years certain regimes are prevalent, e.g. 1975=1, 1980=4 and 1990=1; (2) Lower magnitude regimes span larger geographical regions than higher magnitude regimes; this may reflect (hydro-)climatological conditions leading to their development – dry, high-pressure, anti-cyclonic systems tend to persist over a larger area than low-pressure cyclonic systems; and (3) Nordic countries tend to follow a different pattern of variability to that of the rest of Europe.]

Europe to assess the hydrological impact of climate change; this research is novel as it employs a probabilistic framework to yield a range of possible futures, rather than single central estimates.

At the basin scale, detailed modelling has been conducted to downscale climate change scenarios and analyse impacts on high and low flow indicators for UK rivers. Methods were developed to assess and compare the main uncertainties inherent to climate change impact studies and models run for current and future time-horizons. Stochastic rainfall generators have been used for broad-scale modelling of runoff, which shows potential for estimation at ungauged sites and simulation of future river flow conditions.

Modelling hydrological fluxes and water use (theme 4)

At the global scale, the WaterGAP model has been applied to investigate hydrological (water resources) and water use patterns. This work entails model development and application of models; it is geared towards assessing the current situation and deriving scenarios of global

change. Groundwater recharge is the major limiting factor for the sustainable use of water resources. Using a global hydrological model, specifically modified to obtain good estimates of groundwater recharge in the semi-arid and arid regions of the globe (WaterGAP Global Hydrological Model WGHM), the impact of climate change on groundwater recharge was assessed (Figure 2.7; Döll and Flörke, 2005). In some semi-arid areas (northeast Brazil, southwest Africa and the southern Mediterranean rim), groundwater recharge will decrease markedly by the 2050s according to the four climate scenarios applied.

Figure 2.7 Average annual groundwater recharge: baseline (1961-90) and scenarios for the 2050s showing the impact of climate change (source: Döll and Flörke, 2005;

http://www.geo.uni-frankfurt.de/ipg/ag/dl/publikationen/index.html)