Showing papers in "Hydrological Sciences Journal-journal Des Sciences Hydrologiques in 1991"

Selection of calibration objective fonctions in the context of rainfall-ronoff modelling in a Sudanese savannah area

Abstract: In the context of rainfall-runoff modelling carried out on the Sudanese savannah area in the northwest of the Ivory Coast, attempts are being made to reconstitute the flow at the outlets of catchments in 10 day time steps. By using algorithms with automatic fitting procedures for the parameters, it appeared necessary to make a choice concerning the calibration objective functions to be used. The paper presents the algorithms, data and objective functions that have been used. The results obtained from the calibrations made have been analysed. That analysis was done principally with the help of a comparative evaluation modulus which takes into account elements other than the value of the objective function alone and which enables the quality of the results to be picked out from a hydrological point of view. At the conclusion of the analysis, the objective function defined by Nash seems to stand out quite clearly in relation to the other formulae examined.

Extreme rainfall estimation by combining data from several sites

Abstract: Nowadays it is well recognized that inference about the form of the upper tail of the distribution of precipitation amounts has to be based on a joint evaluation of several records in a region. Three methods of regional estimation are reviewed: (a) the station-year method; (b) regional averaging of statistics of the data; and (c) maximization of a joint likelihood function. Methods (b) and (c) are illustrated with the regional estimation of parameters of the generalized extreme value (GEV) distribution for 1 day annual maximum precipitation amounts in The Netherlands. The adequacy of regional models can be tested in several ways. Special attention is paid to the number of exceedances of regional estimates of certain large quantiles (return periods 250, 500 and 1000 years) in a catalogue of all daily precipitation amounts ≥80 mm at official KNMI stations during the period 1866–1989. The consistency of frequency curves for different durations is also considered. Other methods to evaluate the suitab...

Isoerosivity and erosion risk map for Sicily

Abstract: This paper reviews simplified methods for evaluating the rainfall erosivity index and proposes two relationships for estimating the annual value of Wischmeier's rainfall erosivity index at sites equipped with recording and non-recording raingauges. For the Sicilian region, the FAO index was also found to represent the erosion risk. A regional relationship to estimate the standard deviation of the annual erosivity index is proposed. The isoerosivity map is plotted by using 41 values of the rainfall erosivity index calculated by Wischmeier's procedure and 128 values estimated according to a relationship proposed by the authors. Finally, for each of 169 sites, an erosion risk index is calculated and an erosion risk map is plotted.

Hydrological balance of two Mediterranean forested catchments (Prades, northeast Spain)

Abstract: Precipitation and discharge have been measured for several years in two small forested catchments located in the Mediterranean area of Spain. Actual evapotranspiration has been calculated as the difference between annual precipitation and discharge. Results show that: (a) most of the precipitation is evaporated rather than lost by streamflow, even in the most humid years; (b) there is a high inter-annual variability both in discharge and evapotranspiration; and (c) annual evapotranspiration correlates significantly with annual precipitation, in contrast to the constancy of annual evaporation in catchments of wet, colder climates. Finally, a simple expression is proposed in order to calculate annual actual evaporation from the ratio of precipitation to potential evapotranspiration. This expression uses a derived exponent, k, which takes into account the characteristics of individual catchments.

The effects of climate change on discharges and snow cover in Finland

Abstract: The effects of climate change obtained through model simulations on runoff and snow cover in Finland are presented in this study. The results are based on hydrological model simulations using the information from one climatic general circulation model (GISS) scenario with doubled atmospheric carbon dioxide as input. According to the GISS model scenario, the temperature increase in Finland will be 2–6®C, the precipitation increase 10–30 mm per month and the evaporation increase 5–30 mm per month. The effects of these climatic changes on runoff and snow cover have been evaluated in twelve different watersheds by hydrological watershed models. According to the results the mean discharge (MQ) increases by 20–50%, the mean minimum discharges (MNQ) increase considerably in winter due to a two to three months shorter snow cover period and the mean maximum discharges (MHQ) decrease due to diminished maximum snow water equivalents. In southern Finland persistent winter snow cover will vanish.

A comparison of the hydrology of moorland under natural conditions, agricultural use and forestry

Abstract: Long term research has been conducted into the hydrological effects of different land usage of a wetland mire in southern Germany. Drainage for agriculture lowered the water table and reduced evaporation from about 110% of open water losses to just under the Penman short grass potential rate. The runoff regime was altered and peak flows increased. Afforestation of agricultural land increased evaporation losses to much higher levels than open water evaporation, and annual runoff was nearly halved. Forest growth reduced soil water and baseflows. Peak flows became smaller; the rate of reduction was particularly rapid in the early years of tree growth.

Rainfall network design through comparative kriging methods

Abstract: The methods of simple and disjunctive kriging are applied and compared in the estimation of optimum locations of recording raingauges as part of a network for the determination of storm characteristics to be used in forecasting and design. Some advantages are shown but problems arise when there are large differences in storm structures and movements. Another source of uncertainty is in the modelling of the semi-variogram. Application is made to the management of an area of the Severn-Trent water basin, UK, with 13 autographic raingauges.

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.

Hydrological response of a medium-sized mountainous catchment to climate changes

Abstract: The long term hydrological response of a medium-sized mountainous catchment to climate changes has been examined, The climate changes were represented by a set of hypothetical scenarios of temperature increases coupled with precipitation and potential evapotranspiration changes. Snow accumulation and ablation, plus runoff from the study catchment (the Mesochora catchment in central Greece) were simulated under present (historical) and altered climate conditions using the US National Weather Service snowmelt and soil moisture accounting models. The results of this research obtained through alternative scenarios suggest strongly that all the hypothetical climate change scenarios would cause major decreases in winter snow accumulation and hence increases in winter runoff, as well as decreases in spring and summer runoff. The simulated changes in annual runoff were minor compared with the changes in the monthly distribution of runoff. Attendant changes in the monthly distribution of soil moisture and...

Statistical modelling of annual maximum flows in Turkish rivers

Abstract: The lognormal, Gumbel, smemax, log-Boughton, log-logistic, Pearson-3 and log-Pearson-3 distributions are applied to the annual flood peaks series longer than 21 observations of 112 unregulated natural streams in 23 major basins in Turkey. The parameters of most of those distributions are estimated by the methods of: (a) moments; (b) maximum likelihood; and (c) probability weighted moments. A detailed chi-squared goodness-of-fit test is performed both with equal-length sliced histograms and with equal-probability-area sliced histograms, each one being repeated three times for three different but consecutively increasing slice numbers, such as 5, 6, 7 or 6, 7, 8 etc. A Kolmogorov-Smirnov goodness-of-fit test is also included. According to the evaluations of those three tests, a single definite distribution did not appear to be the best consistently for all the sample series taken, but the log-logistic and log-Pearson-3 models scored a larger number of better fits.

Intercomparison of evapotranspiration estimates

Abstract: Five methods of computing reference evapotranspiration from a reference crop (Penman, radiation, Blaney-Criddle, Hargreaves and pan evaporation) have been studied for their applicability under different climatic conditions. The Penman method was taken as the standard and the other four methods were compared against this method. Good correlation was obtained between the values estimated by the four methods and the Penman method although differences in magnitude were found. Regression equations were developed to correct those differences in magnitude. The method suitable for the estimation of reference evapotranspiration for each climatic condition is also suggested.

Evaluation of the response of a water table to a variable recharge rate

Abstract: A new method of evaluation of the response of a water table to artificial recharge is presented. The solution is formulated in the form of a simple numerical model. The model has several advantages over the traditional methods of mounding prediction. The effect of the unsaturated zone, which modifies the recharge rate as compared with the infiltration rate, is considered. Mounding is calculated for a variable recharge rate induced by a variable infiltration rate. Also, the effect of in-transit water in reducing the tillable pore space above a rising water table is considered. The validity of the model results is illustrated using several sets of field data collected in the Ghazvin Plain (Iran). Sample calculations proved that the model predicts mounding more accurately than the traditional methods and, therefore, more realistic recommendations for the design and operation of artificial recharge schemes are possible using the model.

Spatially varied soil erosion under different climates

Abstract: The spatial variability of the factors of the universal soil loss equation is examined on the mediterranean basin of Conca de Tremp covering 43.1 km2 in Spain. The evaluation of the rainfall erosivity R and the soil erodibility K is relatively straightforward and spatially-averaged values of these parameters can be applied to the entire basin. Conversely, the spatial variability of annual soil erosion losses on large basins depends primarily on the factors L, S and C describing topographic, vegetation and land use parameters. A grid size analysis of soil erosion losses from the Conca de Tremp basin under mediterranean climatic conditions in Spain shows excellent agreement with the earlier results on the Chaudiere basin in Canada. It is concluded for both basins that unbiassed estimates of soil erosion losses are obtained for grid sizes less than about 0.125 km2. The analysis of the Conca de Tremp basin validates the use of the grid size factor proposed by Julien & Frenette (1987). It is also foun...

Regional climate change impacts: I. Impacts on water resources

Abstract: Regional effects of greenhouse warming on water resources, and more specifically on surface runoff, are assessed for a mountainous region of central Greece comprising four drainage basins by using a conceptual model and plausible hypothetical scenarios of temperature and precipitation change Results show considerable sensitivity of runoff characteristics to climate change and indicate certain basin morphoclimatic characteristics such as snow cover, basin aridity and morphology, runoff coefficients etc, which modify considerably basin response Research work is complemented by assessing the climate change impacts on water management works of the area This second part of the work is published separately in the following paper in this issue

Reliable yield of reservoirs and possible effects of climatic change

Abstract: Statistics of seasonal and daily rainfall in nine regions covering the United Kingdom are presented, together with the best currently available predictions of changes in rainfall and evaporation, based on various general circulation model (GCM) assessments of the climate 40 years ahead. On that basis, runoff sequences are generated for the region of northwest England with north Wales (NWE & NW, humid temperate) and for southeast England (SEE, temperate but less humid), both for the current climate and for the year 2030 estimate, the latter incorporating higher evaporation losses. The future annual runoff is reckoned as 8% less than the current average in the SEE region and 4% less than that in the NWE & NW region. Direct supply reservoir yield versus storage graphs are then derived in each case for a range of duration-based probability of emptiness, their yields (i.e. sustained draw-off rates) and storage capacities being expressed as proportions of current annual runoff. The year 2030 graphs fall systematically below those for the current climate, representing 8% to 15% loss of yield from existing storage in the SEE region and 4% to 25% loss of yield in the NWE & NW region.

Regional climate change impacts: II. Impacts on water management works

Abstract: The sensitivity of some critical water management issues, such as reservoir storage and hydroelectric production, to climatic variability and change on a regional basis is analysed and presented. This is the second of two papers presented in this issue dealing with the regional hydrological effects of climatic change. By using a set of plausible hypothetical climatic scenarios to represent both greenhouse warming and precipitation changes, the sensitivity of the guaranteed annual fresh water and energy supply levels has been evaluated under conditions of altered runoff for four connected multi-purpose reservoirs in central Greece. It is shown that reservoirs designed and operated under current climatic conditions are, in general, affected by the climatic changes examined. A dramatic increase of the risk associated with the annual guaranteed quantity of water supply and energy production would occur if greenhouse warming were accompanied by a reduction in annual precipitation. Significant increase...

Rhythmic karst springs

Abstract: Rhythmic springs (ebb and flow springs, intermittent springs, potajnice) belong to the group of springs which appear exclusively in karstified terrains. The paper describes various types of rhythmic springs and gives their classification. It also develops a mathematical model for the functioning of this type of springs based on the principle of recharge and emptying of the underground reservoir through siphon action. Applying this model, according to the observed hydrographs of some rythmic springs in Yugoslavia, the paper explains in detail the structure of the underground reservoir located in the karst.

A modified method for drought identification

Abstract: Water management under drought conditions is a challenging task to irrigation and agricultural engineers. The parameters of interest in identifying drought include onset, termination and severity. In this paper, a methodology to identify those parameters from the available historic data on streamflow and rainfall having seasonal pattern is proposed. The methodology follows and modifies a procedure by Herbst et al. (1966) suggested for the analysis of drought. This modified methodology is applied to the streamflow series of the Bhadra river and the mean areal rainfall series for the catchment of the Bhadra reservoir in Karnataka State, India. The droughts identified by the proposed methodology are concurrent with the historically realized droughts, thus proving the viability and applicability of the methodology in the identification of drought conditions.

Rain-induced outflow from deep snowpacks in the central Sierra Nevada, California

Abstract: In many mountainous areas of the Pacific coast of North America, rainfall onto snowpacks causes massive floods, probably the single greatest cause of changes in channel morphology and lotie habitat...

Water in the Universe

Abstract: Water appears to be one of the most abundant molecules in the Universe. It dominates the environment of the Earth and is a main constituent of numerous planets, moons and comets. On a far greater scale it possibly contributes to the so called “missing mass” of the Universe and may initiate the birth of stars inside the giant molecular clouds. This paper gives a brief description of water and ice environments with an emphasis on their possible origin and subsequent development in the Solar System. Expanding the scope of hydrology to cover phenomena encountered on other celestial bodies is postulated and discussed.

Sensitivity of evapotranspiration and soil moisture in wheat fields to changes in climate and direct effects of carbon dioxide

Abstract: A physically based model of moisture transfer in the soil-canopy-atmosphere system, fitted to measured data in wheat fields, was applied to calculate the sensitivity of evapotranspiration to the combined effect of possible CO2-induced climate changes and the direct impact of doubling CO2 content in the atmosphere. The impact of carbon dioxide was taken into account by changing the stomatal resistance and the leaf area index. Several climate change scenarios were used. Simulation studies showed that when the changes of a number of climatic and plant factors are considered, evapotranspiration responses can differ greatly from those that consider only temperature change. The temperature effect of the increase in CO2 content in the atmosphere can be compensated by changes in the physiological parameters of vegetation. Thus, under certain conditions, one can expect a decrease in evapotranspiration instead of an increase such as is predicted by a number of models which neglect the direct effects of CO2...

Changes in flood regime by use of the modified curve number method

Abstract: This paper describes the use of a simple two stage rainfall-runoff model in which a curve number (CN) principle is used to calculate the soil water content and, subsequently, the rainfall contribution to direct runoff and groundwater flow. The maximum soil water retention, S, is used to express various characteristics of a catchment (infiltration rate, soil cover and land use, as in the CN method) relevant to flood formation. Using historical flood events, the model is calibrated, and the statistical distribution parameters of peak flows determined. With the same historical input data scenarios (rainfall), sets of flood hydrographs are simulated for various values of the parameter S, and corresponding distribution parameters of peak flows are determined. This procedure is used to demonstrate possible changes in flood regime to be expected due to changes of the catchment soil properties and its vegetation cover. A case study is presented for the River Hron catchment, area 582 km2, in the mountaino...

The variation in rainfall catch from standard UK Meteorological Office raingauges: a twelve year case study

Abstract: Rainfall catches from three standard MKII Meteorological Office gauges are examined. These gauges differ in that one is exposed, one is surrounded by a turf wall and the third is mounted at ground level. Rainfall records between 1976 and 1988 are analysed to assess the variations in rainfall catch between the three types of gauge. During the early period of study, differences between gauges exceeded those in later years and this has been attributed to the settling-in of the new turf wall and ground level collector surfaces. After this initial period the annual differences in catch relative to the standard exposed gauge are 2% and 5% for the turf wall and ground level gauges respectively. Regression analysis has produced correction equations which can convert standard exposed gauge data to either turf wall or ground level equivalents. The equation for ground level/exposed gauges is found to be very similar to one developed in Nigeria and has been found to be applicable to water balance studies els...

Practical computation of gamma frequency factors

Abstract: A numerical algorithm for computation of gamma frequency factors is given and verified in the ranges of −9.0 ≤ skewness coefficient ≤ 9.0 and 10−8 ≤ non-exceedance probability ≤ 1—10−8, which are wide enough for all hydrological applications.

Dynamic-stochastic models of rainfall and snowmelt runoff formation

Abstract: Possibilities for the development of dynamic-stoch astic models of runoff formation with random inputs are discussed. Two models are described: the first allows the calculation of the statistical distribution of the maximum discharges of rainfall floods, and the second the statistical distribution of snowmelt flood volumes. Meteorological inputs are generated by the Monte- Carlo method. Physically-based models are used for the transfor­ mation of input data into runoff. The various models are applied to observation data from two watersheds. Modeles dynamico-stochastiques de formation de l'ecoulement pluvial et de l'ecoulement de fonte des nieges Resume On considere les possibilites d'elaboration des modeles dynamico-stochastiques avec des entrees au hasard. Deux modeles sont decrits: l'un permet de calculer la distribution statistique des debits maximaux des crues pluviales, l'autre, la distribution statistique des volumes de l'ecoulement des eaux de fonte de neige. Des valeurs des facteurs meteorologiques de l'ecoulement sont generees par la methode de Monte-Carlo. On emploie des modeles dynamiques partant d'une base physique pour la trans­ formation des valeurs d'entree. L'essai des modeles est effectue en partant des donnees des observations sur deux bassins versants.

The Vigil Network : a means of observing landscape change in drainage basins

Abstract: Long-term monitoring of geomorphic, hydrological, and biological characteristics of landscapes provides an effective means of relating observed change to possible causes of the change. Identification of changes in basin characteristics, especially in arid areas where the response to altered climate or land use is generally rapid and readily apparent, might provide the initial direct indications that factors such as global warming and cultural impacts have affected the environment. The Vigil Network provides an opportunity for earth and life scientists to participate in a systematic monitoring effort to detect landscape changes over time, and to relate such changes to possible causes. The Vigil Network is an ever-increasing group of sites and basins used to monitor landscape features with as much as 50 years of documented geomorphic and related observations.

Dynamic simulation model of vertical infiltration of water in soil

Abstract: One of the most important problems of hydrological forecasting is to obtain a reliable estimation of effective rain. Infiltration is one of the variables which greatly influences the partitioning of rainfall into surface runoff and subsurface flow. This paper presents an infiltration model which describes the unsaturated zone as a multi-layer system. For this purpose a relationship developed by Denisov (1978) for total hydraulic potential versus soil moisture content has been used. The model contains a system of ordinary differential equations for describing soil moisture movement and it can be interpreted as an aggregated simulation model with lumped parameters. Some basic equations and results of simulation runs are presented.

A maximum entropy view of probability-distributed catchment models

Abstract: The probability-distributed catchment model, as originally proposed by Moore & Clarke (1981), is re-examined from a maximum statistical entropy viewpoint. The distribution of water within a catchment is treated as a problem of statistical inference and resolved using an entropy maximization technique. A simple runoff generating mechanism is employed, which, together with the catchment mass balance equation, yields a catchment model involving just one dynamic parameter, y, and two constants, k and λ. The parameter y determines the temporal variation of catchment storage V and runoff q. The latter is nonlinearly related to V through q = k(1—λyV), where y provides the nonlinear departure from the simple linear reservoir q = kV.

Climate and water—a call for international action

Abstract: The work undertaken by climatologists and atmospheric physicists during the 1980s has led to far more consistent and precise predictions of future climate change being made at the close of the decade than had been made at its start. Governments have responded by calling for more information and, in some cases, have taken steps to mitigate the effects of such a change. The hydrological community has not been idle and many projects have been launched to study the interaction between climate change, the hydrological regime and water resources. However, the move towards larger scale international projects with major hydrological components will make new and extensive demands on the hydrological community, to which it must respond.

Earth's surface temperatures and the global water cycle

Abstract: At a global scale the troposphere is a closed water vapour reservoir with interlinked cycles of heat and water. The global (macroscale) water vapour dynamics are analysed in three stages: (a) a semi-empirical simulation procedure quantifying the internal relationships among the vapour pressure gradients at the Earth's surface, within the overheated and unsaturated nearsurface layer, and the precipitation formation zone; (b) an analytical description of the effect of changing the Earth's surface temperature on the vapour pressure gradient at cloud base, giving insight into the feedback mechanism between the Earth's surface evaporation and precipitation formation; and (c) a combination of the previous two stages leading to the equilibrium criteria for vertical vapour flux. Needs and opportunities for applications combined with other approaches in climate research are outlined. Fuller details are given elsewhere (Szesztay, 1991).