THE BASIC PRINCIPLES OF THE METHODS AND THE PROGRAMME OF THE FIELD WATER BALANCE RESEARCH IN THE USSR

Transcription

1 THE BASIC PRINCIPLES OF THE METHODS AND THE PROGRAMME OF THE FIELD WATER BALANCE RESEARCH IN THE USSR Dr. URYVAEV V. A. State Hydrological Institute of the Head Office of the Hydrometeorological Service under the Council of Ministers of the USSR, Leningrad ABSTRACT The report deals with the methods used in the water balance research, which is carried out in representative and experimental basins under various physiographical conditions. This research aims not only to study the hydrologie regime in river basins, but also the ways to influence hydrologie processes in order to get a proper ratio between the elements of the water balance. The necessity is stressed to obtain quantitative values of the terms of the water balance equation by means of direct measurements by independent accurate methods. The paper also deals with the different ways of measurement of the water budget elements at different points of the river basin and with the evaluation of different ways for averaging the data of a drainage system as a whole, depending on variability of a given water blance element in time and space. In connection with the statement mentioned above and the problems connected with the evaluation and increase of accuracy of water balance calculations some considerations are given about the sizes of experimental and representative river basins in different natural zones. Some statements of the report are illustrated with examples. RÉSUMÉ Le rapport touche des questions de la méthode des recherches du bilan d'eau, effectué sur des bassins représentatifs et expérimentaux qui se trouvent dans les conditions physiographiques différentes. Ces recherches prévoient les études de régularités dans la formation du régime hydrologique des bassins fluviaux, ainsi que celles des procédés de l'influence effective sur les aspects différents du processus hydrologiques pour obtenir un résultat favorable dans la corrélation entre les éléments du bilan d'eau. Le rapport souligne la nécessité de recevoir l'estimation quantitative de tous les termes de l'équation du bilan d'eau pendant les recherches de cette sorte en les mesurant directement par les méthodes indépendantes et assurant, en outre, leur dénomination assez élevée. On examine dans ce rapport les différents moyens de mesure des éléments du bilan d'eau dans certains points du bassin et on apprécie la légitimité de l'emploi de différentes méthodes pour trouver les moyennes de ces données pour tout le bassin fluvial dans l'ensemble par rapport aux modifications de cet élément du bilan d'eau en temps et en territoire. Compte tenu de ce qui prédède et ayant en vue la question d'appréciation exacte et la mesure plus exacte dans la calculation du bilan d'eau, on exprime quelques idées sur la dimension des bassins versants représentatifs et expérimentaux dans différentes zones climatiques. Certains passages du rapport sont illustrés par des exemples. It is very difficult to find general forms of connexion between water budget elements and the geographical environment because of a great number of various interconnected factors. The availability of voluminous data about water budget elements gives the opportunity to carry out some investigations concerning the laws of areal and temporal variations of water budget components. These investigations in their turn, must be the basis for a general theory, which will be able to give a quantitative estimate of dynamics of water budget elements. Besides we must pay peculiar attention to find out physical natur of phenomena 700

2 and quantitative evaluation of complex processes taking place over a large territory. The transition principle from observation at a given point to areal observations is connected with evaluation of the variation of the studied element over the drainage area, dependent on microlandscape; their integral values are obtained by means of geographical-statistical generalizations. Active experiments and such investigations, which help to reveal the nature of the processes, determining the composition of water budget are of great significance to solve this problem. In our country with hundred of thousands rivers and lakes it is quite impossible to install observation stations on every river or lake. Combined use of the results of experimental investigations and base network observations gives the opportunity to analyse hydrological processes and their areal generalization. In the USSR, experimental investigations of water budget elements are based on the following principles: 1. Simultaneous (synchronous) study of all water budget elements (or the majority ofthem). 2. Direct measurement of all water budget elements (if possible) by independent methods. 3. Study of elementary processes of hydrological cycle (evaporation, precipitation, runoff, infiltration, snow melt etc.) in connexion with water budget elements investigations. 4. Study of processes of runoff formation under various artificially changed conditions: during different soil cultivation, in different types of vegetative associations, different soil moistening etc. The transition of experiments on a small area to wide experiments on vast regions appears to be possible. 5. Carrying out experimental investigations in different climatic zones. It helps to study the effect of different factors under various climatic conditions; there is also an opportunity to study their influence upon the trend of hydrological processes transformation with different climatic zones peculiarities, taken into account. 6. Study of the influence of physiographical factors and human activities upon hydrological regime as a whole or upon water budget elements is carried out by means of the comparison method of two or more basins. These basins must be of similar natural conditions and differ from each other by the factor, which influence upon runoff is under study on a given water body (the influence of forest, swamps, lakes, the difference in relief, soils, and the influence of cultivation, draining, irrigation etc. as well). 7. Simultaneous study of water and heat budgets of water bodies, providing the most complete characteristics of hydrological regime. Many soviet specialists got positive results of some schemes of joint solution of water and heat budget equations. 8. Appliance of new principles and instruments for water budget and experimental investigations, which enable to take into account pulsatory character of the variation of many meteorological and hydrological elements and their areal changeability as well. PROGRAMME AND METHODS OF INVESTIGATIONS Water budget investigations in the USSR are carried out at special stations and field laboratories where all the water budget components are being studied and measured by independent methods, and at base state network, where some water budget elements are being observed. Special stations carry out investigations on experimental and representative basins; they are field laboratories, equipped with modern measuring devices and 701

3 located in different geographical zones. Investigations are carried out on experimental basins and plots, equipped with special measuring instruments. Fig. 1 and 2 give an example of measuring devices distribution at two representative basins of the Valdai scientific-research hydrological laboratory: at "Tajezhny Fig. 1 Forest catchment "Tajezhny Log" (0.45 km 2 ). 1 - watershed divide, 2-measuring weir with limnigraph, 3 - runoff plot, 4 - Tretiakov's precipitation gauge, 5 - self recording rain gauge, 6 - snow survey line and points of snow density measurements, 7 - observation post of water yield from snow cover, 8 - ground water level observation well, 9 - swamp water level gauge, 10 - soil water storage observation post, 11 - Danilin's frozen ground gauge, 12 - meteorological instruments for temperature, humidity, wind velocity and other observations, 13 - evaporation from snow cover observation post (during snow melt only), 14 - a set of gravimetric GGI soil evaporimeters (2 evaporimeters and 1 rain gauge), 15 - a set of gravimetric 0.1 m 2 GGI-B-1000 swamp evaporimeters (2 evaporimeters and 1 rain gauge), 16 - hydraulic soil evaporimeter (small model), 17 - hydraulic soil evaporimeter (large model), 18 - lysimeter, 19 - device for rainfall running down the tree trunks measurements, 20 - "Yelovy Log" catchment, 21 - soil evaporation plot, 22 - water budget plot N3, 23 - water budget plot N4, 24 - lysimeter plot, 25 - runoff plots, 26 - meteorological gradient towers (height 45.0 m). 702

4 Fig. 2 Woodless catchment "Usadjevsky Log" (0.36 km 2 ). The symbols are the same as for fig. 1. Log" forest basin with the area of 0.45 km 2 and woodless "Usadjevsky Log" basin with the area of 0.36 km 2. The programme of these stations includes observations of the following water budget elements: precipitation, snow cover, channel runoff from small basins, surface runoff from experimental plots, evaporation from soil, evaporation from vegetative cover, evaporation from water surface and snow cover, water content in soil, ground water regime. This programme also includes the observation of principal meteorological elements, connected with water budget study, infiltration and soil freezing. Besides, these stations carry out agrometeorological and phenological observations, topographical, geological and hydrogeological surveys; some stations perform actinometric observations, necessary for heat budget study. The programme of the stations also includes special observations, connected with the solution of some specific scientific problems, for example: study of the influence of the following factors upon runoff: forest and field-protecting forest-belts, cultivation, swamp draining; besides the programme includes investigations of peculiarities of mud- 703

5 stream floods formation, erosion processes etc. At every station, within its borders, there is a set of catchment basins of different dimensions and of runoff plots, their number varies from units up to several dozens, depending on natural conditions and tasks set to the station. Study of precipitation - one of the principal and most important water budget elements - consists of two separate tasks: rainfall measurements and water equivalent of snow cover measurements. The quantity of precipitation is measured by means of standard rain gauges and self-recording rain gauges. The distribution of rain gauge stations over the experimental or representative basins area depends on the basin relief, its dimensions and vegetative cover. The number of rain gauge stations depends on the basin area and on the set task, averaging intervals and the desired detailization in rainfall distribution over the drainage basin. On experimental basins precipitation is studied for investigation of storm runoff and for checking the flood runoff formulae parameters. For this purpose special dense network of standard and self-recording raingauges is established at some stations, covering areas from 100 up to km 2 ; the density of network reaches 125 stations on 100 km 2. For example, on a mountainous experimental basin of the Malaja Almaatinka river on the area of 120 km 2, where mud-flows often occur, at the altitude of m above sea-level, 130 rain gauges are installed, with 80 self-recording rain gauges among them. The results of observations at many special rain gauge networks allow to investigate the laws of rainfall distribution over territory and to determine relationship between the precipitation quantity at the observation point and the precipitation height of the same frequency over the territory of rain distribution. These generalizations are performed for the north and the south of the European part of the USSR. In particular, a stable coefficient of skew, characterizing the precipitation variations from the centre of the storm to the periphery for areas up to km 2 was determined. Experimental investigations and analysis of long-term data allowed to define the error of rain gauge observations under different conditions of gauges installation in different geographical zones and calculate differential corrections to mean annual precipitation. When the height of rain gauge is 2 m above ground the diminishing error of precipitation measurement due to wind influence, evaporation, moistening of the receiver reaches 30 per cent of mean annual value, depending on the type of gauge installation and geographical region. The subsequent efforts to determine the true quantities of precipitation are directed towards the establishment of correction coefficients to precipitation for given months and years and towards the search of a new type of rain gauge, which will record the precipitation most exactly. With this aim the investigations are carried out at special proving grounds where different types of rain gauges are located under different conditions (different height above ground, different wind protection etc.). Soviet specialists also carry out investigations of precipitation measurement by means of radar. Snow cover investigations at experimental basins include: definition of water equivalent of snow pack; study of snow accumulation and snow melt in woodland and woodless areas depending on the weather conditions ; study of the heat budget and water budget of snow cover; Measurements of evaporation and condensation on snow surface; water yield during snow melt and the velocity of snow melt surface runoff. The definition of water esuivalent of snow cover at experimental basins is carried 704

6 out by "route" snow surveys and by "landscape" snow surveys. "Route" snow surveys are described in the report of W.D.KOMAROV. "Landscape" snow survey routes are laid on landscape elements most typical of the given region. The weighed average value of water equivalent of snow cover for the given basin is calculated then with respect of the area of several landscape elements. The area covered by with snow during snowmelt is sometimes determined exactly by means of air reconnaissance. Some means of the application of aerial methods for the definition of water equivalent of snow pack have been found. The investigations of processes of runoff formation are carried out in accordance with the principal phases of runoff: overland flow, runoff from small drainage basins and streamflow. Runoff-plots are used for overland flow investigations. A runoff-plot is a homogenous part of a slope (elementary drainage area), isolated from the surrounding locality by small dividing ridge. The quantity of water, flowing from the runoff-plot into a flume, situated at the lower part of the plot is measured by means of volumetric method (if the volume of water is small) and by means of a sharp-crested weir with self-recorder (when the volume of water is great). The dimensions of the runoff-plots, according to local conditions, may be different: from 10 to 30 meters wide and from to and more meters long. In dry steppe zones of the South the dimensions of runoff-plots sometimes are increased to km 2. This increase is necessary to exclude or reduce the accidental influences of the dividing ridges and of the microrelief, which can be great enough during small water yield from the slopes; besides the data received on great runoff-plots more correspond to the conditions of runoff from great plain areas typical of these zones. Sometimes there are used runoff-plots with dividing ridges and screens deepened into ground to isolate ground and surface waters of a given plot. The influence of the following factors on overland flow is studied by means of runoff-plots: (a) soil cultivation methods; (b) forest age and composition; (c) different artificially created states of soil: freezing or thawing, drying or wetting of soil, ice crust on soil; (d) infiltration during sprinkler irrigation. With the increase of the dimensions of slopes and the transition to the primary drainage network the conditions of the surface runoff are changed. The "verkhovodka " (temporary perched ground water) can appear on the surface (especially at foothills), the ground water (baseflow) part in total stream flow increases with the increase of the depth of the river valley and the homogeneity of runoff formation is more or less disturbed. The studying of the runoff formation under such conditions is carried out at small drainage areas, where the runoff shows the integral influence of all physiographical factors. For studying it is necessary to select such drainage areas and slopes, which enable to study the influence of human activities and natural factors differentially. The selection of such experimental drainage areas is a complicated task, because it is difficult to carry out the condition of the runoff factors homogeneity all over the drainage area and therefore the experimental drainage areas are chosen, as a rule, of small dimensions. The dimensions of these drainage areas range from some hectares up to dozens of km 2 ; the drainage areas in the plain steppe zones of the South can reach some hundreds km 2. Every experimental station disposes from 5-6 drainage areas and 1-2 runoff-plots up to drainage areas and runoff-plots. It is necessary to pay special attention to the distinctness of the watershed divides, the coincidense of surface and underground watershed divides, the homogeneity of the 705

7 drainage areas in respect of the conditions of runoff formation while choosing slopes and small watercourses for investigations. Runoff from drainage areas is measured by means of hydrometrical weirs and flumes and in some cases, when drainage areas are great, using hydrometric "areavelocity" method. Selfrecording instruments are installed for the continuous registration of runoff. As it is necessary to take into account the total amount of runoff, the base State network of gauging stations, situated at small and middle rivers and great streams of the USSR is the next link of the organization of runoff investigation. As an example of the influence of different factors on runoff let us examine the influence of cultivation and forest amelioration. The results of the investigations carried out at special stations showed that in comparison with layland the cultivation of soil lying fallow and of automn ploughland considerably reduced (at average 20-40%) surface runoff especially spring runoff; winter-crops and stubble field slightly increase (within 10-20%) spring runoff. In wet years these fluctuations and difference of runoff are smoothed; in dry years they increase, especially in arid zones. Surface runoff is reduced by the influence of forest and of field-protecting forest-belts. At small drainage basins the influence of cultivation is less than at slopes; and at large rivers it is less than at small basins or is quite negligible. The investigations at experimental basins together with the use of basic river stations network data enabled to state quantitative criteria of the influence of agricultural practices and reclamation upon runoff to use it in practical purposes in water management in the USSR. The study of runoff at small catchments of the steppe zone enabled also to get interesting quantitative criteria of evaluation of catchment area dimensions influence upon runoff. It has been noticed that the runoff amount changes distinctly with catchment dimensions change, especially in case of small watershed. In steppe zones the runoff amount decreases distinctly with the increasing of area dimensions, while in forest regions these changes are of inverse character. Study of évapotranspiration play an important role for water budget investigation. Evaporation is directly measured by the use of soil gravimetric and hydraulic evaporimeters; it is also calculated by means of formulae containing meteorological data or determined using heat and water budget methods. Standard gravimetric 500 cm 2 evaporimeters GGI-500, 50 and 100 cm deep, with a removable bottom are widely used on the base network of the USSR. By means of these instruments evaporation from soil covered with vegetation or without it is determined. Gravimetric evaporimeters and lysimeters of m 2 and more are used for measurement of evaporation from intertilled and technical crops. In order to obtain reliable observation data, heat and water regime of evaporimeter soil monolith is necessary to be almost the same of that of natural soil (regime). The error of the applied systems of evaporimeters, as the investigations showed, is not more than 10% of measured values. In order to reach higher accuracy in evaporation measurement a new type of evaporimeter based on hydrostatic weighing principle was designed in the USSR since These evaporimeters make possible to use large soil monoliths and carry out continuous measurements of higher accuracy to observe daily evaporation fluctuations. As a result of this work in the USSR at the Valdai scientific research hydrological laboratory a big hydraulic evaporimeter (BHE) was installed, BHE is a reference instrument with a surface area of 5 m 2 and 2 m deep. Registration of change of weight is accurate within mm of water layer. The total weight of the floating system with soil monolith is 40 tons. Changeable forest soil evaporimeters, with a surface area of 3 m 2 and 1.5 m deep weighed on hydraulic balance were installed to study daily variations of evapotrans- 706

8 piration in monoliths with different kinds of trees in them. Small models of hydraulic soil evaporimeters with area of 0.2 m 2 and 1.5 m deep are also wide spread. In cases when it is necessary to apply calculation methods for evaporation evaluating (turbulent diffusion method and heat budget method) different installations for gradient measurements of meteorological elements and installations for heat budget elements are used. Lysimetric installations with natural or artificially controlled ground water level are also used for evaporation and soil moisture dynamics study. The dimensions of lysimeters are as follows: depth m, surface area m 2. The majority of investigators come to the conclusion that a lysimeter area must be no less that m 2 and its depth depends on aeration zone depth. Aeration zone models is created by means of taking out soil monoliths usually in undisturbed state. Besides percolation and moisture losses by evaporation, precipitation, ground water level, soil moisture change, evaporation from soil, runoff are observed at the lysimeters site, and thus all the elements of water budget are measured directly. As it is proved by lysimeters use water budget estimated by means of lysimeters has a minimum discrepancy. Another advantage of this methods lies in the possibility to carry out active experiments. Its main drawback is that the water budget elements measured with lysimeters characterize only a point on Earth's surface. Joint lysimetric observations and observations on water budget plots will be wide spread for the study of water budget elements. Evaporation values measured by lysimeters are in good accordance with the data obtained by heat budget method and water budget plots. For example, the difference in evaporation values measured by the BHE hydraulic evaporimeter at the Valdai hydrological laboratory and calculated by the method of heat budget made only 8% at the average during (for vegetative period only). According to measurements at the catchment "Tajezhny Log" mean annual évapotranspiration of a fir-wood makes 390 mm in average for the period May- September; 44% of this value falls to forest transpiration, 26% to evaporation of rainfall, intercepted by crowns of trees and 30%-to évapotranspiration under crowncover. The data of simultaneous observations by evaporation pans, placed on fields with different crops indicate that minimum evaporation occurs during the period of the most intensive crops growth which exceeds mean evaporation rate during the whole of vegetative period by 40%. For example, the ratio of monthly évapotranspiration from summer-wheat to evaporation from virgin soil in the steppe region makes 1.39 in June, 1.53 in July, and 1.09 for the period May-September. While studying river basins water budget, evaporation from water surface is of great interest from the point of view of evaluation of water losses from reservoirs, losses of moisture during irrigation on arid territories etc. For observations of evaporation from water surface sunken or floating evaporimeters GGI-3000 and evaporation tanks with a surface area of 20 m 2 and 100 m 2 are used. In the USSR simultaneous observations by means of the evaporimeters GGI-3 000, Class A (USA) evaporimeters and the 20 m 2 evaporation tanks as reference devices took place. The results of the analysis showed that the evaporimeter GGJ distorts heat and water regimes, daily and seasonal evaporation variations significantly less than the Class A (USA) evaporimeter. Transitional coefficients of this evaporimeter are stable and are approximate to 1. All over the territory of the USSR they variate only within the limits of up to 1.0. The GGI evaporimeter records are close to these of the 20 m 2 evaporation tank. Observations of evaporation from water surface are performed at all large water budget stations of the USSR. On the basis of the observation data characteristics for drainage area water budget calculations are obtained in particular such as evapo- 707

9 ration from snow surface, evaporation during different levels of ground water, evaporation in the presence of semisubmerged aquatic plants etc. Mean moisture storage in soil, which is necessary to know for water budget calculations is obtained by taking samples of soil by means of boring; the determination of soil moisture is carried out by the weighing method. Sufficient accuracy of mean water storage determination by this method can be obtained only when there is a very great number of observation points. Recently the number of representative observation points and their location is determined by means of the statistic methods. The system of observations of soil humidity dynamics provides for measurements of soil humidity in different layers up to 1.0 m deep, and in special cases up to 3 m and more as a minimum programme. The frequency of measurements cannot be great up to date, because of the difficulty of their carrying out. Meanwhile, as a minimum, these determinations should be performed at the end or at the beginning of seasonal change of soil moisture. In some cases simultaneously with soil humidity determination experiments on soil nitration capacity using special filtration devices and sprinkler irrigation are carried out. In the USSR in order to increase the accuracy of measurements of water content in soil and to facilitate field work new methods are being worked out such as neutron method, gamma-ray method, method using measurements of natural earth radioactivity and others. Ground water regime observations include observations of following elements: ground water level, thermal regime, ground water chemical composition, physical and hydrological qualities of soil. The greatest interest presents the observations of ground water immediately connected with river runoff, i.e. the top layer of ground water. Because of difficulties of ground water study field investigations are often carried out only at selected parts of drainage basins or at selected basins and the results are extrapolated over similar drainage areas. At special stations and at the stations of the state basic network all the observations are carried out in accordance with instructions unified for all the USSR and under methodical directions which are worked out by the State Hydrological Institute and other scientific research institutions. The results of the observations are published in periodical publications of Hydrometeorological service of the USSR. In the USSR special experimental water budget stations network began to appear about 50 years ago and by the present time it accumulated great experience of investogation, developed significantly and spreaded all over the main physio graphical zones of our country. This experience and the investigation results will be useful for realization of the International Hydrological Decade programme. In the USSR the development of this experience is aimed at large scale field expeditionary investigations of water budget. This work is carried out on vast territories (up to many hundreds of thousands of square kilometres) in connection with solving problems of irrigation, water supply, swamp drainage, assimilation of virgin lands, water transfer from one river basin to another and other water management projects. 708