REPORT ON THE ACTIVITIES OF IAHS FOR THE PERIOD 1971–1975

ABSTRACT

Much of New Hampshire and Vermont (combined area = 50 000 km²) has hilly to mountainous topography. Elevations range from 0 to 1900 m a.s.l. (average = 360 m), and many peaks exceed 1200 m. Mean annual precipitation increases strongly with elevation (adjusted for additional orographic effects and distance from moisture sources), as do mean monthly precipitation, snow depth, and snow water equivalents. Mean monthly temperatures decrease with elevation, largely masking latitudinal effects, and can be used with other information to show how potential evapotranspiration changes with elevation. These effects combine to produce strong elevational increases in mean annual streamflow and, more surprisingly, cause streamflow variability, both short term and annual, to decrease with mean drainage basin elevation. Low flows for a given exceedance probability increase markedly as mean basin elevation increases above 340 m. Flood peaks for a given return period also increase with mean basin elevation. Slope and aspect affect the timing of snowmelt runoff, but otherwise appear to have only second order effects on hydrology. The effect of elevation is so dominant in the region that it can be used as the single independent variable in predicting many streamflow parameters.     

MEAN STREAMFLOW FROM DISCHARGE MEASUREMENTS

Abstract

Mean flow of a stream is usually computed from a continuous record of flow ai a gaging station. A less costly method consists of (1) estimating 12 individual monthly flows from one discharge measurement per month and a concurrent gaging station record on a nearby stream, using a different relation for each month, (2) computing the annual mean from the estimated monthly means, and (3) using a relation based on gaging station records to estimate the long-term mean from the one annual mean. An annual mean can be estimated within about 10 percent of its measured value even though both the gaged and ungaged streams are affected by diversions and have different runoff characteristics. The method has greatest utility in regions where mean runoff is not closely related to drainage area.     

An intercomparison of hydrological network-design technologies

Abstract

Two network-design technologies are compared by random sub-sampling of actual streamflow data. The technologies, Network Analysis for Regional Information (NARI) and Network Analysis Using Generalized Least Squares (NAUGLS), have a common objective, viz. to maximize regional information within a limited budget and time horizon. The data used for intercomparison are from a network of 146 streamgauges in the central part of the United States. In general, the results for the illustrative example indicate that the NAUGLS method conveys more information than the NARI method to the network designer interested in maximizing regional information about mean annual flows with a limited budget.     

Trend detection in river flow series: 1. Annual maximum flow / Détection de tendance dans des séries de débit fluvial: 1. Débit maximum annuel

Abstract

Results of a study on change detection in hydrological time series of annual maximum river flow are presented. Out of more than a thousand long time series made available by the Global Runoff Data Centre (GRDC) in Koblenz, Germany, a worldwide data set consisting of 195 long series of daily mean flow records was selected, based on such criteria as length of series, currency, lack of gaps and missing values, adequate geographical distribution, and priority to smaller catchments. The analysis of annual maximum flows does not support the hypothesis of ubiquitous growth of high flows. Although 27 cases of strong, statistically significant increase were identified by the Mann-Kendall test, there are 31 decreases as well, and most (137) time series do not show any significant changes (at the 10% level). Caution is advised in interpreting these results as flooding is a complex phenomenon, caused by a number of factors that can be associated with local, regional, and hemispheric climatic processes. Moreover, river flow has strong natural variability and exhibits long-term persistence which can confound the results of trend and significance tests.     

Trends in low flows in Spain in the period 1949–2009

ABSTRACT

This study analyses trends in low flows in Spain in the period 1949–2009, based on daily flow data collected at 60 gauging stations located in near-natural catchments. Two low-flow indicators were considered: (i) the seven-day annual minimum streamflow and (ii) the 10th percentile of the yearly flow duration curve. Catchments were clustered into three regions in terms of monthly mean flows. The Mann-Kendall test was used considering four periods between 1949 and 2009. A multi-temporal trend analysis was also applied to the longest series to identify wet and dry periods that could influence the results. Lastly, a field significance test provided a regional assessment of the at-site detected trends at each region. The results for each indicator reveal a clearly decreasing trend in low flows throughout the northern half of Spain that was found to be field-significant over the (Atlantic and Mediterranean) regions.

Editor Z.W. Kundzewicz; Associate editor not assigned     

THE GENERATION OF SPRING PEAK FLOWS BY SHORT-TERM METEOROLOGICAL EVENTS

Abstract

Spring peak flows recorded over a 25-year period in Benton Creek, a small forested watershed in northern Idaho, were studied in their relation to meteorological events. More peak flows were generated by rain-on-snow than by clear-weather snowmelt; the two types of peaks differ in magnitude and in other characteristics. Two rather simple techniques were used to calculate the generative capacity of a rainfall-temperature event and the hypothetical outflow of water from a snowpack during rainy and rainfree periods. Similar data on spring peaks on two regional subbasins were considered also.     

Analysis of the temporal variability of runoff in Ivory Coast: statistical approach and phenomena characterization

Abstract

The runoffs at four Ivory Coast hydrometric stations (monitoring flows from an area covering between 5930 and 66500 km²) were analysed with a set of statistical methods for the detection of breaks in the time series. The variables studied were the annual mean discharge and some characteristic discharges. From a quantitative standpoint, the existence of a clear break in the series of annual mean discharges at the beginning of the decade from 1970, the date from which the runoffs decrease significantly, was noted. A more qualitative study of the results showed that low flows were more affected than high flows by this variability of the regime. This fluctuation appears to be in accord with the drought phenomena observed during the same period in the Sahel, to the north of Ivory Coast.     

A variable-length block bootstrap method for multi-site synthetic streamflow generation

Abstract

The complexity of stochastic streamflow generators limits their practical use, highlighting the need for effective but simpler approaches. An attempt to meet this objective is presented using variable-length bootstrapping (VLB) of annual flows, and a weighted method of fragments for disaggregation and perturbing the bootstrapped annual flows. The perturbations enable generation of annual flows different from those present in the historical record, thereby overcoming one of the main limitations of the classical bootstrap method. The VLB replicates adequately nine historical annual statistics of a five-site problem, and reproduces the annual serial and cross-correlations better than STOMSA—a state-of-the-art parametric generator. The VLB achieves reasonable validation using the sum of minimum flows and the reservoir storage size test. Because of the modification of the monthly flow distribution caused by the weighted averaging of fragments, the VLB cannot be safely used for within-year analysis, but is a potentially robust annual streamflow generator.

Citation Ndiritu, J. (2011) A variable length block bootstrap for multi-site synthetic streamflow generation. Hydrol. Sci. J. 56(3), 362–379.     

Regional analysis of low flow using L-moments for Dongjiang basin,South China

Abstract

Dongjiang water has been the key source of water supplies for Hong Kong and its neighbouring cities in the Pearl River Delta in South China since the mid-1960s. Rapid economic development and population growth in this region have caused serious concerns over the adequacy of the quantity and quality of water withdrawn from the Dongjiang River in the future. Information on the magnitude and frequency of low flows in the basin is needed for planning of water resources at present and in the near future. The L-moment method is used to analyse the regional frequency of low flows, since recent studies have shown that it is superior to other methods that have been used previously, and is now being adopted by many organizations worldwide. In this study, basin-wide analysis of low flows is conducted for Dongjiang basin using five distributions: generalized logistic, generalized extreme value, lognormal, Pearson type III and generalized Pareto. Each of these has three parameters estimated by the L-moment method. The discordancy index and homogeneity testing show that 14 out of the 16 study sites belong to a homogenous region; these are used for further analysis. Based on the L-moment ratios diagram, the Hosking and Wallis goodness-of-fit statistical criterion and the L-kurtosis criterion, the three-parameter lognormal distribution is identified as the most appropriate distribution for the homogeneous study region. The regional low-flow estimates for each return period are obtained using the index flood procedure. Examination of the observed and simulated low flows by regional frequency analysis shows a good agreement in general, and the results may satisfy practical application. Furthermore, the regional low-flow relationship between mean annual 7-day low flows and basin area is developed using linear regression, providing a simple and effective method for estimation of low flows of desired return periods for ungauged catchments.     

THE UTILIZATION OF RIVER WATER RESOURCES IN ARID ZONES

ABSTRACT

Flood data were assembled for 168 Scottish basins containing 3071 station-years of record. Multiple regression techniques were used to produce equations for predicting mean annual flood from physiographic and climatological basin characteristics. Mean annual rainfall (SAAR) gave better results than measures of extreme rainfall (M52D and RSMD). Percentage area of lake storage (LOCH) was found to be a better predictor than fraction of the basin draining through a lake (LAKE). Apart from SAAR and LOCH, the recommended equation requires the basin area (AREA), stream frequency (STMFRQ) and an index of the soil type (SOIL). Measures of average basin slope (AVES) and main stream slope (S1085) were not statistically significant. The standard error of the estimate of the predicted mean annual flood is 0.147. The equation has a coefficient of determination, R², of 0.914 and appears robust over a wide range of basin types without requiring regionally derived multipliers or coefficients. No significant improvement in fit was acheived by ridge regression, Stein estimation or use of the influence function.     

Prediction of base flows from basin characteristics: a case study from Zimbabwe / Prévision de débits de base à partir de caractéristiques du bassin: une étude de cas au Zimbabwe

Abstract

Abstract Base flows make up the flows of most rivers in Zimbabwe during the dry season. Prediction of base flows from basin characteristics is necessary for water resources planning of ungauged basins. Linear regression and artificial neural networks were used to predict the base flow index (BFI) from basin characteristics for 52 basins in Zimbabwe. Base flow index was positively related to mean annual precipitation (r = 0.71), basin slope (r = 0.76), and drainage density (r = 0.29), and negatively related to mean annual evapotranspiration (r = –0.74), and proportion of a basin with grasslands and wooded grasslands (r = –0.53). Differences in lithology did not significantly affect BFI. Linear regression and artificial neural networks were both suitable for predicting BFI values. The predicted BFI was used in turn to derive flow duration curves of the 52 basins and with R ² being 0.89–0.99.     

Evaluation of temperature trends over India / Evaluation de tendances de température en Inde

Abstract

Abstract The impact of climate change is projected to have different effects within and between countries. Information about such change is required at global, regional and basin scales for a variety of purposes. An investigation was carried out to identify trends in temperature time series of 125 stations distributed over the whole of India. The non-parametric Mann-Kendall test was applied to detect monotonic trends in annual average and seasonal temperatures. Three variables related to temperature, viz. mean, mean maximum and mean minimum, were considered for analysis on both an annual and a seasonal basis. Each year was divided into four principal seasons, viz. winter, pre-monsoon, monsoon and post-monsoon. The percentages of significant trends obtained for each parameter in the different seasons are presented. Temperature anomalies are plotted, and it is observed that annual mean temperature, mean maximum temperature and mean minimum temperature have increased at the rate of 0.42, 0.92 and 0.09°C (100 year)^-1, respectively. On a regional basis, stations of southern and western India show a rising trend of 1.06 and 0.36°C (100 year)^-1, respectively, while stations of the north Indian plains show a falling trend of –0.38°C (100 year)^-1. The seasonal mean temperature has increased by 0.94°C (100 year)^-1 for the post-monsoon season and by 1.1°C (100 year)^-1 for the winter season.     

LONG-TERM FLUCTUATIONS AND TRENDS IN WATER LEVEL CHANGES IN THE OUTFLOW LAKES IN NORTHERN POLAND

ABSTRACT

The occurrence of cyclic fluctuations and trends in water levels has been tested by examining a long-term series of mean annual water levels in 11 outflow lakes, mostly groove lakes, located in Northern Poland between 14° E and 23° E and situated at different altitudes up to 120 m above sea level.

Analysis of the occurrence of cyclic fluctuations of water levels has been carried out by means of mass curves. The regression method together with the moving average method applied to 10-year values of mean annual water levels were used to establish the trend in water levels.

On the basis of the research completed, it has been stated that long-term fluctuations of mean annual water levels of the lakes under investigation, in spite of the absence of a distinct periodicity, are characterized by a cycle with a 23-year period. The studies of the trends of mean annual water levels revealed a negative tendency ranging from—0·059 to—0·402 cm per annum.     

Riparian responses to reduced flood flows: comparing and contrasting narrowleaf and broadleaf cottonwoods

Abstract

To enable assessment of risks of water management to riparian ecosystems at a regional scale, we developed a quantile-regression model of abundance of broadleaf cottonwoods (Populus deltoides and P. fremontii) as a function of flood flow attenuation. To test whether this model was transferrable to narrowleaf cottonwood (Populus angustifolia), we measured narrowleaf abundance along 39 river reaches in northwestern Colorado, USA. The model performed well for narrowleaf in all 32 reaches where reservoir storage was <75% of mean annual flow. Field data did not fit the model at four of seven reaches where reservoir storage was >90% of mean annual flow. In these four reaches, narrowleaf was abundant despite peak flow attenuation of 45–61%. Poor model performance in these four reaches may be explained in part by a pulse of narrowleaf cottonwood expansion as a response to channel narrowing and in part by differences between narrowleaf and broadleaf cottonwood response to floods and drought.

Editor Z.W. Kundzewicz; Guest editor M. Acreman

Citation Wilding, T.K., Sanderson, J.S., Merritt, D.M., Rood, S.B., and Poff, N.L., 2014. Riparian responses to reduced flood flows: comparing and contrasting narrowleaf and broadleaf cottonwoods. Hydrological Sciences Journal, 59 (3–4), 605–617.     

Response of runoff to climate change in the Wei River basin,China

Abstract

Climate change will likely have severe effects on water shortages, flood disasters and the deterioration of aquatic systems. In this study, the hydrological response to climate change was assessed in the Wei River basin (WRB), China. The statistical downscaling method (SDSM) was used to downscale regional climate change scenarios on the basis of the outputs of three general circulation models (GCMs) and two emissions scenarios. Driven by these scenarios, the Soil and Water Assessment Tool (SWAT) was set up, calibrated and validated to assess the impact of climate change on hydrological processes of the WRB. The results showed that the average annual runoff in the periods 2046–2065 and 2081–2100 would increase by 12.4% and 45%, respectively, relative to the baseline period 1961–2008. Low flows would be much lower, while high flows would be much higher, which means there would be more extreme events of droughts and floods. The results exhibited consistency in the spatial distribution of runoff change under most scenarios, with decreased runoff in the upstream regions, and increases in the mid- and lower reaches of the WRB.

Editor Z.W. Kundzewicz; Associate editor D. Yang     

Climate-driven trends in mean and high flows from a network of reference stations in Ireland

Abstract

This paper introduces a reference hydrometric network for Ireland and examines the derived flow archive for evidence of climate-driven trends in mean and high river flows. The Mann-Kendall and Theil-Sen tests are applied to eight hydroclimatic indicators for fixed and variable (start and end date) records. Spatial coherence and similarities of trends with rainfall suggest they are climate driven; however, large temporal variability makes it difficult to discern widely-expected anthropogenic climate change signals at this point in time. Trends in summer mean flows and recent winter means are at odds with those expected for anthropogenic climate change. High-flow indicators show strong and persistent positive trends, are less affected by variability and may provide earlier climate change signals than mean flows. The results highlight the caution required in using fixed periods of record for trend analysis, recognizing the trade-off between record length, network density and geographic coverage.

Editor Z.W. Kundzewicz; Associate editor H. Lins

Citation Murphy, C., Harrigan, S., Hall, J., and Wilby, R.L., 2013. Climate-driven trends in mean and high flows from a network of reference stations in Ireland. Hydrological Sciences Journal, 58 (4), 755–772.     

Modelling the impacts of land-cover change on streamflow dynamics of a tropical rainforest headwater catchment

Abstract

A modelling experiment is used to examine different land-use scenarios ranging from extreme deforestation (31% forest cover) to pristine (95% forest cover) conditions and related Payment for Ecosystem Services (PES) schemes to assess whether a change in streamflow dynamics, discharge extremes and mean annual water balance of a 73.4-km² tropical headwater catchment in Costa Rica could be detected. A semi-distributed, conceptual rainfall–runoff model was adapted to conceptualize the empirically-based, dominant hydrological processes of the study area and was multi-criteria calibrated using different objective functions and empirical constraints on model simulations in a Monte Carlo framework to account for parameter uncertainty. The results suggest that land-use change had relatively little effect on the overall mean annual water yield (<3%). However, streamflow dynamics proved to be sensitive in terms of frequency, timing and magnitude of discharge extremes. For low flows and peak discharges of return periods greater than one year, land use had a minor influence on the runoff response. Below these thresholds (<1-year return period), forest cover potentially decreased runoff peaks and low flows by as much as 10%, and non-forest cover increased runoff peaks and low flows by up to 15%. The study demonstrated the potential for using hydrological modelling to help identify the impact of protection and reforestation efforts on ecosystem services.

Editor Z.W. Kundzewicz

Citation Birkel, C., Soulsby, C., and Tetzlaff, D., 2012. Modelling the impacts of land-cover change on streamflow dynamics of a tropical rainforest headwater catchment. Hydrological Sciences Journal, 57 (8), 1543–1561.     

BANKFULL DISCHARGE: AN EXAMPLE OF ITS STATISTICAL RELATIONSHIPS

Abstract

Analysis of data for the White and Wabash Rivers suggests means of determining discharge at the natural bankfull stage, despite the effects of artificial embanking and clearance of channels. Bankfull discharge, and discharge at mean annual flood, undergo an orderly downstream increase in percentage duration of flow.     

A BRIEF ACCOUNT OF THE GEOGRAPHY OF THE CENTRAL OETZTAL ALPS WITH SPECIAL REFERENCE TO ITS GLACIOLOGY

Abstract

Beginning with an energy loss relation of the form S = aVⁿ, it is possible to describe a number of basic analytical types of flows. The derived general equation may be shown to give solutions for simple cases such as radial and vortex flows as well as reducing to Laplace's equation. Laminar flow is a special case with n = 1, and further solutions range from laminar to fully turbulent flow.