A. P. Bochkov, State Hydrological Institute L. R. Struzer, M a i n Geophysical Observatory Leningrad, USSR
S U M M A R Y : The paper dcscribes methods for computation of average prccipitation ant1 siiowstorage in river basins und u system applied in thc USSR for the corrcction of measurccl precipitation which eliniinatcs systematic crrors of precipitation gauges. Techiiiqtie Ìor dcvelopment of such system in countries Lising precipitation gauges of otlicr designs is suggested.
E~~iitmtion of precipiruiion CIS ruuter halcinre element
E VALUA TION DES PRECIPITA TIONS C O M M E
ELEMENTS
D U BILAN D ’ E A U RESUME : Dans ce rapport, 011 expose les méthodes de calcul des précipitations moyennes et de l’enneigement dans Ics bassins versants et un système de correction des précipitations mesurées employé en URSS est prCsenté.L a méthode d’élaboration d’un tel Systeme pour les pays avec pluviomètres de différents types est proposée.
CALCULO D E LA PRECIPITACIdN C O M O ELEMENTO DEL BALANCE HIDR OL6 GICO
RESUMEN: E n el documento se describen los métodos para el cAlculo de la precipitación media y de Ia cantidad media d e agua proccdente de la nieve en las cuencas de los ríos, así c o m o u n sisteina utilizado en 1% URSS para la corrección de lac medidas de la precipitación que elimina los errores sistemáticos de los pluviómetros.
Se sugieren los medios técnicos para establecer este sistema en países q u e utilizan pluviómetros de diseño diferente.
In the USSR water balances are computed for monthly, seasonal and annual periods both for certain individual years (current water balances) and on the average for a long- term period (norinal water balances). Annual water balances are computed for hydro- logical and calendar years.
All the eleinents of water balances of river basins are computed for the a b o v e time intervals.
Pi,ecipitation is the main income element of water balance, on which the values of other components depend to a high degree. Therefore for water balance computation it is necessary to try to achieve the most accurate determination of average precipitation depth of a territory under study for a chosen time interval.
A. P. Bochkoo
DETERMINATION OF AVERAGE PRECJPITATTON FOR A DRAINAGE A R EA
While computing average precipitation depth for a river basin for a given time interval it is necessary to make use of precipitation data preferably of all stations and posts of the meteorological network situated within the basin and beyond its borders at a close reach from its water divide.
In the USSR computation of average precipitation for a river basin or any other area is performed by different methods depending on the tasks and physiological conditions: by plotting and planimetering isohyetes, by computation of mean weighed precipitation a n d by computation of mean arithmetic precipitation. The most accurate is considered to be the isohyete method.
Long-term aueruge monthly, seasonal and annual precipitation values are estimated for the same period of time for which all the rest water balance elements are computed.
According to these data maps of isolines of precipitation distribution over a territory are prepared and the average precipitation for a basin is deterniined by planimetering.
W h e n such maps are prepared for regions with scarce observational data, the use is made of the revealed regularities of precipitation variations related to the variations of other physiographical factors, for instance: the altitudinal gradient of precipitation on slopes of different orientation, the change of the amount of precipitation under the influence of forests and large water surfaces, etc.
For individual year.s average monthly, seasonal and annual average precipitation for river basins in plain regions are computed usually as mean arithmetic values and in case of uneven distribution of precipitation gauges, as mean weighed values.
Irrespective of the method of computation of the average precipitation for an area there should be performed the analysis of the influence of the type of the underlying surface (relief, forests, reservoirs, etc.) on the distribution of precipitation over the territory.
In moutainous regions graphical relations between the precipitation sum for each month a n d the altitude of precipitation gauges on the slopes of a given orientation are plotted.
From these relations the gradients of precipitation are determined. Provided the area situated at the given altitude (in parts from the total drainage area) and the amount of precipi talion over this area (with respect to the gradient) are known, the average precipita- tion value for a mountainous region might be computed by the method of mean weighed values.
In order to determine water equivalent of the snow cover in plain regions, snow surveys along snow courses are performed each month (in the field, in the forest, in ravines; the depth and the density of snow are measured along snow courses 3-2.5 kin long in the field, and 0.5 krn long in the forest and in 2-3 profiles across the ravines). O n the base of these data water equivalent of snow cover (snow storage) of a region is determined as m e a n weighed value from the percentage of areas occupied by the given landscapes.
T h e accuracy of estimation of snow storage in mountainous region is low and is used only as the first approxiination.
At present a method for determination of snowstorage from an airplane is being developed, i.e. a method which is based on the measuring of gamma-radiation of the earth with and without the snow cover.
T h e computation of average precipitation for small watersheds is performed according to no less than 3-5 precipitation gauges and the increase of the drainage area on each 1000-2000 kin2 requires 1-2 observational points more. Such density of observation points with the existing spatial variation of monthly precipitation sums makes il possi ble to determine mean monthly precipitation of a basin with an crror not exceeding 9-20%, Cof the average precipitation sum) on the greatest par1 of the USSR territory. In the southern (arid) zone the error inay reach 40-50%, [7, 81. These values characterize the
Estimation of precipituiion as wuier balance element
random error, but besides them in measuring precipitation there occur systematic errors;
method of eliminating them is described below. For all the above methods of determina tion of average precipitation on a watershed for water balance computation the corrected precipitation values are required, i.e. the values in which the systematic error was already eliminated.
SYSTEMATIC ERRORS IN MEASURING P R E C I P I T A T I O N
It is well known that precipitation gauges of all designs catch not all precipitation, especially solid precipitation [6, 9, IO]. This was proved also when water balances were computed, especially for the territories where the portion of solid precipitation is large [9].
However, in the fifties of our century meteorological and hydrological services of a number of countries (e.g. USSR, Great Britain) and W M O pointed out the necessity of the increasing of the accuracy of precipitation measurements.
After this in the USSR there were developed methods of quantitative estimation of measurement errors of precipitation gauges and beginning from I965 a system of correc- tion of measured values of long-term average monthly, seasonal and annual precipitation sums and of the same sums for individual years was put into action.
From January the Ist, 1970, corrections of the measured precipitation values for each observation terni will be carried out on all the station of the Hydrometeorological network of the USSR.
The description of the correction system and its application to water balance computa- tion makes a considerable part of the present report.
ERRORS OF P R E C T P I T A T I O N GAUGES
The errors of precipitation gauges were described in literature and were rather correctly determined already J O0 years ago in many well-known works.
According to these works precipitation gauges underestimate precipitation due to the wind infhence (“Yevons effect”), due to the losses of precipitation on the moistening of the raingauge bucket and on evaporation from it. There exist estimations of errors for various types of raingauges-American, English, German. However systematic ana- lysis of these errors was not carried out, and they were not taken into consideration in network measurements and in computation of precipitation.
Since I961 the Hydrometeorlogical Service of the USSR started an experimental study of the errors of the standard precipitation gauges (Tretyakov precipitation gauge, rain- gauge with Nipher shield).
Each physically different source of errors was studied separately [ 161. This is the main principal difference between these studies and those, performed earlier both in the USSR and other countries. Besides that it was decided that, while studying quantitatively the errors, it is necessary to seek for the relations between the errors and such factors or parameters, which are measured on ail meteorological stations and the data on which for previous years are available. This is the second peculiar feature of the studies perform- ed in the USSR. This provided possibility for developing methods for correction of not only the present observations but of the data for previous years.
Main results:
(u) The greatest errors ana tne most difficult to estimate appeared to be the errors caused by the zuind eflect. From the physical point of view it is clear that the value of the error depends on the wind speed at the height of the receiver and on the precipitation structure (the size of raindrops and snow-flakes). The estimation was performed by means of comparison of standard and reference precipitation gauges [3, 61.
A. P. Bochkov
The so-called pit raingauge which consists of a measuring bucket installed in it conic pit I30 cni in diameter scïved as a reference instrument for measuring liquid precipitation.
T h e walls of the pit are covered with turf. The receiving orifice of the bucket was at the level of the ground surface. Preliminary comparisons with raingauges of a more periect design installed in ground suggested by Bleasdale [i, 21 and special laboratory and theoretical investigations showed that systematic errors of such simple pit raingauge, including both the effect of splashing, do not exceed 1.5%.
Experimental observations for estimation of errors in measuring liquid precipitation due to wind effect were carried out since 1962 to 1965 on 180 stations, where wind speed at the height of two nietres and the intensity of precipitation according to pluviograph were recorded simultaneously.
Precipitation gauge, installed in a site protected from wind (e.g. on a forest glade), was assumed to be a reference instrument for ineasuring solid precipitation.
It was assumed that on forest glades the same amount of precipitation falls as on the open fields. However, there exist some publications and experimental data, though not quite proved, according to which solid precipitation on forest glades is 6-15% higher than in open fields. Therefore in some regions the assumed method m a y somewhat overesti- mate the errors of measurements of solid precipitation due to the wind effect. Investiga- tions were based on 8-10 year observational series from 32 pairs of neighbouring stations situated in open and in protected sites [3].
At data processing the error due to wind effect was characterized by the following relation:
is precipitation sum, measured by standard instrument;
precipitation s u m measured by reference instrument. In other words;
is the coefficient by which the measured precipitation value should be multiplied to obtain the value corrected for the wind effect error.
It appeared that the reduction coefficient
K
is almost completely determined by two parameters: wind speed U, at the height of the receiving orifice (2 in) and structural parameter L, which characterizes the fali velocity of rain drops (snow flakes):K = K(U,L).
(2)For liquid precipitation it is most convenient to determine structural parameter (L) as N value, which is the percentage of precipitation with the intensity Y
<
0.03 inin/min;for monthly sums of liquid precipitation it was possible to m a p out the value of N on the basis of pluviograph data. It is most expedient to take air temperature as the structural parameter of solid precipitation.
There were obtained parametrical families of curves of function (2) for liquid, solid and mixed precipitation measured by Tretyakov precipitation gauge and by raingauge with Nipher shield. Wind effect error of solid precipitation appeared to be very great and much geater than of liquid precipitation. O n the average correction on wind effect for annual sum of precipitation varies over the USSR territory from 8% to 40%".
(bj The losses of standard precipitation gauges of Lhe USSR Hydrometeorological Service on the i17itiu/ moistetiing were determined by simple tesls carried out with collectors with difTerent degree of amortization. A dry collector was fillcd with a known volume of water (or snow), and then the amount of water pourcd out of the collcctor was measured.
It was dctcrmined [i i], that at each nieasurcment thc loss of liquid precipitation equalled 0.2 nim (if the measured precipitation amount exceeded 0.05 min). Thc losses of solid