David Archer

Conflicting Signals of Climatic Change in the Upper Indus Basin

Abstract Temperature data for seven instrumental records in the Karakoram and Hindu Kush Mountains of the Upper Indus Basin (UIB) have been analyzed for seasonal and annual trends over the period 1961–2000 and compared with neighboring mountain regions and the Indian subcontinent. Strong contrasts are found between the behavior of winter and summer temperatures and between maximum and minimum temperatures. Winter mean and maximum temperature show significant increases while mean and minimum summer temperatures show consistent decline. Increase in diurnal temperature range (DTR) is consistently observed in all seasons and the annual dataset, a pattern shared by much of the Indian subcontinent but in direct contrast to both GCM projections and the narrowing of DTR seen worldwide. This divergence commenced around the middle of the twentieth century and is thought to result from changes in large-scale circulation patterns and feedback processes associated with the Indian monsoon. The impact of observed season...

Contrasting hydrological regimes in the upper Indus Basin

Since much of the flow abstracted from the River Indus for irrigation originates in the Himalayas, Karakoram and Hindu Kush Mountains, an understanding of hydrological regimes of mountain rivers is essential for water resources management in Pakistan. Broad characteristics of hydrological regimes are investigated using streamflow data from nineteen long-period stations in terms of annual and seasonal runoff. Regression between climatic variables and streamflow for three key basins, the River Hunza, River Astore and Khan Khwar have first been carried out followed by regional analysis of twelve further basins. Analysis shows distinct hydrological regimes with summer volume governed by: melt of glaciers and permanent snow (thermal control in the current summer), melt of seasonal snow (control by preceding winter and spring precipitation), or winter and monsoon rainfall (precipitation control in current season). Satisfactory levels of correlation were achieved between streamflow and measurements of temperature and precipitation at valley sites, which offer promise as a basis for assessing seasonal flow volumes. They also suggest the possibility of extending the flow record back on the basis of historical climatic records, which commence early in the twentieth century.

The use of historical flood information in the English Midlands to improve risk assessment

Abstract The Easter 1998 flood was the largest flood event in the gauged record of many basins of the English Midlands. Flood frequency analysis, using such gauged records only, placed the 1998 event at a return period of over 100 years on several basins. However a review of historical (pre-gauged) flooding on some rivers gives a different perspective. Examples are given of the use of historical flood information on the River Leam, the River Wreake at Melton Mowbray, the River Sence (tributary to the River Soar) and the River Frome at Stroud. The cost of acquiring such historical flood data is trivial in comparison to gauged data, but the benefits are demonstrated as significant. In particular, historical flood data provide a better basis for risk assessment and planning on flood plains through revised estimates of flood discharge and depth.

Assessment of Runoff Sensitivity in the Upper Indus Basin to Interannual Climate Variability and Potential Change Using MODIS Satellite Data Products

Abstract The Upper Indus Basin (UIB) covers an area of more than 200,000 km2 and has an elevation range from below 1000 to over 8000 m above sea level. Its water resources underpin Pakistans food security and energy supply. Vertical and horizontal variations in key climate variables govern the runoff contributions of the UIBs various elevation zones and subcatchments. Remote sensing climatic data products from NASAs Moderate Resolution Imaging Spectrometer (MODIS) instrument platform provide an opportunity to develop a spatial characterization of the climatology of remote and rugged regions such as the UIB. Specifically, snow-covered area (SCA) and land surface temperature (LST) have been shown to provide good analogues, respectively, for precipitation and air temperature. As such, SCA and LST quantify regional variations in mass and energy inputs to runoff generation processes. Although the 10-year (2000–2010) MODIS observational record is not adequate to evaluate long-term trends, it does provide a consistent depiction of annual cycles and a preliminary assessment of interannual variability. This study presents a summary of the period means and interannual variability found in remotely sensed SCA and LST products for the UIB. It then provides an update of locally observed recent climate trends for the 1962 to 2007 period. Nonparametric trend tests are applied both to the local observations and to remote sensing records to assess patterns in recent variability. The climatic noise (intense variability) of the past decade, however, renders conclusions on nascent trends in SCA and LST premature. Finally, runoff sensitivity to temperature change—spatially applied as summer (JJA) nighttime 0°C LST isotherm migration—is assessed for a range of potential scenarios. Results indicate that changes in mean summer (JJA) runoff could range from −30 to +35% or more, depending on whether recent locally observed changes continue or scenarios derived from current regional climate model (RCM) simulations unfold.

Assessment of severity of the extreme River Tyne flood in January 2005 using gauged and historical information

Abstract The extreme Tyne (Northumbria, UK) flood in January 2005 provided the opportunity to reassess flood risk and to link peak discharge and flooded area to probability of occurrence. However, in spite of the UK guidance on flood risk assessment given in the Flood Estimation Handbook (FEH), there is still considerable subjectivity in deriving risk estimates. A particular problem for the Tyne arises from the effects of river bed gravel extraction both on the reliability of gauged discharges and in the interpretation of historical level data. In addition, attenuation and drawdown of Kielder Water has reduced downstream flood risk since 1982. Estimates from single-site, pooled estimates and historical information are compared. It is concluded that the return period of the observed flood was around 71 years on the lower Tyne and was probably the largest flood since 1815.

Characterising flash flood response to intense rainfall and impacts using historical information and gauged data in Britain

We analyse chronologies of historical flash floods derived from searches of newspaper archives and other sources commencing before 1800 and recent gauged rainfall and stream flow data. Five key examples are chosen to illustrate specific features of flash floods. Pluvial flash floods arise from rainfall before it reaches a watercourse and may cause severe flooding of land and properties far from rivers. River flash floods, like pluvial floods, have the characteristic of rapid speed of response, a principal source of risk to life. Intense rainfall can generate ‘walls of water’ in river courses which can propagate long distances downstream and steepen, without upstream structural failure. Steeply rising wavefronts more commonly occur on steep upland catchments but, where intensities of extreme short period rainfall are sufficient, such wavefronts can also occur on lowland catchments. A definition of flash floods from intense rainfall, relevant to British landscape and climate, is proposed.

A Detailed Cloud Fraction Climatology of the Upper Indus Basin and Its Implications for Near-Surface Air Temperature*

AbstractClouds play a key role in hydroclimatological variability by modulating the surface energy balance and air temperature. This study utilizes MODIS cloud cover data, with corroboration from global meteorological reanalysis (ERA-Interim) cloud estimates, to describe a cloud climatology for the upper Indus River basin. It has specific focus on tributary catchments in the northwest of the region, which contribute a large fraction of basin annual runoff, including 65% of flow originating above Besham, Pakistan or 50 km3 yr−1 in absolute terms. In this region there is substantial cloud cover throughout the year, with spatial means of 50%–80% depending on the season. The annual cycles of catchment spatial mean daytime and nighttime cloud cover fraction are very similar. This regional diurnal homogeneity belies substantial spatial variability, particularly along seasonally varying vertical profiles (based on surface elevation).Correlations between local near-surface air temperature observations and MODIS c...

CURRENT RESEARCH IN BRITISH SNOWMELT RIVER FLOODING

ABSTRACT An adaptation of the degree-day method has been used to analyse a number of snowmelt events on two catchments as a first step in a programme of research on snowmelt river flooding in Britain. The analysis indicates that the degree-day factor varies during events and between events on the same catchment. A snowmelt event is seen as consisting of three phases, an initial lag phase, a phase of nearly constant degree-day factor and a recession phase. The degree-day factor in the constant part of each event has significant correlation with the total flood volume on both catchments and with liquid precipitation during the snowmelt on one catchment only. Separate procedures are considered necessary for forecasting the initial lag phase and runoff during the recession.