* Rainfall simulations had pointed out the high infiltration capacity of
the soils. Under such conditions a HORTON-type overland flow cannot oc-
cur on large parts of the basin (COSANDEY, BOUDJEMLINE, ROOSE et LELONG,
1990).
*
Floods a r e always l i n k e d w i t h h i g h r a i n f a l l events, though amounts o f p r e c i p i t a t i o n may v a r y a l o t , ( f r o m 44 mm, o c t o b e r 10, 1987, f i g . 8 t o 241 mm, o c t o b e r 4, 1987, f i g . 7 ) , i n r e l a t i o n w i t h t h e water s t o r a g e i n t h e watershed.*
F l o o d r i s e i s always sudden and t i m e t o p e a k f l o w i s s h o r t , g i v i n g e v i - dence o f sudden changes i n t h e w a t e r pathways and o f occurrence o f a gene r a l i z e d o v e r l a n d f l o w . Discharge i n c r e a s e s i n a few hours 0.3 m3/s/km2 t o one, and perhaps two m3/s/km2 when t h e r a i n does n o t s t o p . The storm r u n o f f c o e f f i c i e n t exceeds sometimes 50 p e r c e n t (COSANDEY,1990).These s t u d i e s , combined w i t h f i e l d o b s e r v a t i o n s , suggested t h a t t h e storm f l o w s a r e generated b y sudden and s i g n i f i c a n t over1 and f l o w which can o c c u r when wide p a r t s o f t h e b a s i n a r e s a t u r a t e d , a c c o r d i n g t o t h e
" c o n t r i b u t i n g areas" concept (DUNNE and BLACK, 1970). When a t h r e s h o l d c o r r e s p o n d i n g t o a maximum v a l u e f o r t h e groundwater s t o r a g e i s reached, t h e w a t e r t a b l e r i s e s t o t h e s u r f a c e o f t h e s o i l and " o v e r f l o w s " , i n d u - c i n g a h i g h volume of s t o r m r u n o f f . The aim o f t h i s paper i s t o show how t h i s t h r e s h o l d was determined.
4 D e t e r m i n i n g t h e t h r e s h o l d f o r f l o o d r i s e
run o f f
I
tiole ( d q s )5 10
Fig. 2 Evolution of discharge during recessing periods
The f i g u r e 2 shows a break i n t h e r e c e s s i o n l i m b s a t t h e v a l u e o f 63 l / s . T h i s break c l e a r l y i n d i c a t e s an i m p o r t a n t m o d i f i c a t i o n i n t h e w a t e r p a t h - way p a t t e r n w i t h i n t h e basin, when a r a p i d s u r f a c e r u n o f f decreases t o a s l o w e r s u b s u r f ace r u n o f f . A c c o r d i n g t o Mai 1 l e t ' s Law (CASTANY, 1963),
63 l / s corresponds, f o r t h e groundwater storage, t o a v a l u e o f about 170 mm. I t seems t h e r e f o r e p o s s i b l e t o admit t h a t t h i s v a l u e r e p r e s e n t s t h e maximum s t o r a g e c a p a c i t y o f t h e s o i l , and t h a t , above t h i s threshold, t h e w a t e r t a b l e " o v e r f l o w s " ; a l l t h e r a i n f a l l i n g on t h e s e s a t u r a t e d s u r f a c e s p a r t i c i p a t e s t o t h e s t o r m f l o w e i t h e r as an o v e r l a n d f l o w r u n - n i n g o v e r t h e s o i l s u r f a c e , o r as a " p i s t o n - f l o w " .
Consequently, t h e 63 l / s v a l u e f o r d i s c h a r g e i s assessed t o be t h e t h r e - s h o l d f o r t h e b e g i n n i n g o f t h e f l o o d .
Under such c o n d i t i o n s , s i g n i f i c a n t s t o r m f l o w s c a n n o t o c c u r b e f o r e he groundwater s t o r a g e reaches t h e t h r e s h o l d o f 170 mm. However, t h e r a i n w a t e r can i n f i l t r a t e t h r o u g h t h e s o i l and r e s t o r e t h e w a t e r t a b l e o n l y a f t e r t h e s o i 1 has r e a c h e d h i s f i e l d c a p a c i t y . P r e v i o u s s t u d i e s , based on f i e l d o b s e r v a t i o n (DURAND, 1989) and model 1 in g approach (GUERIN 1987) showed t h a t t h i s f i e l d c a p a c i t y i s h e r e o f labout 100 mm.
So, t h e v a l u e f o r t h e t o t a l w a t e r s t o r a g e o f t h e b a s i n b e f o r e f l o o d r i s e may be assumed t o be : 170 + 100 = 270 mm.
comparisons between computed and m o n i t o r e d v a l u e s f o r t h e amount o f r a i n n e c e s s a r y t o i n d u c e a f l o o d have been r e a l i z e d i n o r d e r t o a s s e r t a i n t h i s h y p o t h e s i s .
5 R e s u l t s
The amount o f r a i n n e c e s s a r y t o r e a c h t h e 270 mm t h r e s h o l d
-
t h u s t o g e n e r a t e a f l o o d-
i s computed as t h e s i m p l e d i f f e r e n c e between 270 mm and t h e v a l u e o f t h e t o t a l w a t e r s t o r a g e i n t h e b a s i n a t t h e b e g i n n i n g o f t h e r a i n e v e n t . The t o t a l w a t e r s t o r a g e i s t h e sum o f groundwater s t o r a g e ( a c c o r d i n g t o M a i l l e t ' s Law, i n r e l a t i o n w i t h t h e b a s e f l o w ) and s o i l w a t e r s t o r a g e (computed by t h e w a t e r b a l a n c e ) .Seven s t o r m e v e n t s ( r a i n >I50 mm) o c c u r r e d d u r i n g t h e 8 y e a r s o f t h e f i e l d s t u d i e s . S i x o f them have been r e c o r d e d and a r e a n a l y z e d below.
F i v e o f them produced f l o o d s :
5.1 November 7-9, 1982 ( F i g . 3 )
5.3 October 13-15, 1986 ( F i g . 5 )
5.5 October 4-6, 1986 ( F i g . 7 )
6 D i s c u s s i o n
*
t h e second remark a p p l i e s t o h y d r o l o g i c a l processes : anThe f i n a n c i a l s u p p o r t was g r a n t e d w i t h t h e h e l p o f t h e EEC (Recherche e t Developpernent dans l e dornaine de 1 'Environnernent).
References
Castany, G. 1963. T r a i t 6 P r a t i q u e des Eaux S o u t e r r a i n e s . Dunond, P a r i s . Cosandey, C. 1986. De l ' o r i g i n e de 1'6coulernent r a p i d e de crue, dans un
p e t i t b a s s i n - v e r s a n t f o r e s t i e r b r e t o n . Z. Geomorph., s u p p l . 60 Cosandey, C., D. Boudjernl i n e , E. Roose, and F. L e l o n g 1989. Etude exp6- r i m e n t a l e du r u i s s e l l e m e n t s u r des s o l s 5 v 6 g 6 t a t i o n c o n t r a s t 6 e du Mont-LozGre. Accept6 pour pub1 i c a t i o n dans Z. f u r Geornorph.
Cosandey, C. and J.-F. Didon-Lescot 1990. Etude des c r u e s c6venoles : c o n d i t i o n s d ' a p p a r i t i o n dans un p e t i t b a s s i n f o r e s t i e r sur l e v e r - s a n t sud du Mont-Lozkre ( F r a n c e ) . (Proceedings o f t h e L j u b l j a n a Symposium, A p r i l 1990). IAHS Publ., n0191.
Didon, J.-F. 1985. C o n t r i b u t i o n 5 1 1 6 t u d e de l a v a r i a b i l i t 6 s p a t i o - t e m - p o r e l l e des p l u i e s s u r l e Mont-LozGre. DEA, U n i v . de M o n t p e l l i e r . Dunne, T. and R . D. B l a c k 1970. An e x p e r i m e n t a l i n v e s t i g a t i o n o f r u n o f f p r o d u c t i o n impermeable s o i l s . Water r e s o u r c e s Res. n02, pp.478-490 Guerin, F. 1987. Etude du fonctionnement de t r o i s p e t i t s b a s s i n - v e r s a n t s du Mont-Lozkre 5 l ' a i d e d ' u n rnod5le h y d r o l o g i q u e g l o b a l e t des analyses c o r r 6 l a t o i r e s e t s p e c t r a l e s . Memoire de DEA soutenu 5 1 1 U n i v e r s i t 6 d 1 0 r 1 6 a n s l e 13 o c t . 1987.
Lelong, F. P . Durand and J.-F. Didon 1988. Cornparaison des b i l a n s h y d r o - chirniques, des t a u x d l a l t & r a t i o n e t d ' a c i d i f i c a t i o n dans t r o i s p e t i t s b a s s i n s - v e r s a n t s g r a n i t i q u e s s i r n i l a i r e s 5 v 6 g 6 t a t i o n con- t r a s t 6 e (Mont-Lozere, France). S c i . G6ol. B u l l . 41 (3-4), pp.263- 278, Strasbourg.
USE OF EXPERIMENTAL BASINS DATA FOR
Six main categories of soils have been defined on the territory of the forest zone of the European part of the U.S.S.R. as a result of the detailed lithological description of the soil with the following generalization over the degree quadrants of the 1 : 1 500 000 scale map. Soil-physical characteristics have been obtained for 186 degree quadrants using the long-term field data of the GGI Valdai Branch, the Vyatsk expedition and the poor observation data from an agrometeoro- logical network. The Valdai Branch data are noted to be representative for the prevailing part of the territory under study.
Hydrological processes in any physiographic zone significantly depend on general humidification of a territory and accumulating capacity of the soil as a medium, transforming precipitation into river runoff.
Accumulating capacity and water regime of soils depend on soil-physical parameters and, in the first turn, on porosity, density and water retaining capacity.
Disturbance of natural soil structure due to man's activity (forest cutting, reclamation, etc.) causes changes of soil-physical properties and of the water regime of soils which results in changes of the runoff
.
In general, there is a lack of soil-physical parameter data in hydrometeorological observations. So, when carrying out vast areal generalizations concerning runoff formation and water balance investigations, natural research data obtained from experimental basins
may be used for ungauged territories. Such generalization has been performed for the forest zone of the U.S.S.R. European part, which is characterized by the great variety of soil lithology. The territory under study covers 17 regions and 5 autonomic republics (about 12 x 10,5 km2). It occupies the large part of the Russian plain and the Upper Volga landscape area. Its north-western and central parts are related to the mean- and south-taiga subzone, its south-eastern part to the forest-steppe zone.
The north-western and central parts of the territory were subjected to glaciation which resulted in an extreme variety of soils in terms of texture, as well as in lack of soil structureness. The south-eastern part of the forest zone is characterized by the relatively homogeneous texture and by a rather high degree of its structure.
Soils have been mainly formed by quaternary and present deposits represented by moraine and topsoil loams, sandy loams and sands, carbonate and loessial loams, glacial lake sands and sandy loams, underlied by loams. Sands are mainly placed nearby sea beaches, river and lake valleys. Loamy and sandy-loamy deposits are mostly spread. The soil cover is characterized by podzol, sod-podzol and grey forest types of soil.
The territory covers 186 degree quadrants of a soil map with the scale 1 : 1 500 000. When analyzing, each degree quadrant was subdivided into 100 quadrants and description of ground was made, taking into account all information available. Then the generalization was performed within each degree quadrant which resulted in marking out 6 soil types (Table 1).
Using the experimental data of the Valdai Branch and the IHP experimental basins network as well as the GGI expedition episodic data, the values of the main soil-physical parameters such as volume weight, full, capillary and field capacities, and wilting point were determined. Herein, the poor information of the agrometeorological network was used as comparative. Preliminary comparison showed small differences of these parameter values (within the limits of each soil type) for the regions of the north-western and central parts of the forest zone. In general they are close to the values obtained at the experimental basins of the Valdai Branch (Table 1). As to the south- eastern part of the zone there have been noted some peculiarities due
to more homogeneous soil textures (Table 2).
Table 1. Soil-physical parameters* for different soil types of the north-western and central parts of the forest zone
VW FC CC FiC WP VW FC CC FiC WP
Table 2. Soil-physical parameters* for different soil types of the south-eastern part of the forest zone
w w m m r 1 0
m m N d N m
rl rl rl d
-wm mm 00 d S 2 Z
d d rl
- Q W W O N u l
m m o ~ d d
d r l d d
- W W N r n r - m
m m o d o o
d d d d
- m m o o . t m
m b ~ ~ m o
4 d d
W I .
a rl
IIIIIllllllllllllll Illlllil
260 270 217 270 241 241 268 282 235 240 268 268 250 250 250 278 278 271 268 268 268 268- -
270 270 282 267 262 262 276 280 257 268 282 282 278 278 278 271 246 235 210 242 257 253 - 'Leningrad-
242 267 282 294 282 282 282 228 205 282 285 286 275 282 282 282 277 269 300 282 282 300 300 300 239 239 274 287-
"Vologda "Kirov-
248 235 282 273 274 282 263 267 267 244 294 300 289 285 288 284 264 221 178 169 256 256 248 291 288 292 290 291-
"Pskov "Valdai-
269 242 282 270 282 282 282 282 282 289 272 282 277 287 283 275 239 178 248 292 248 291 221 204 221 265 266 264-
- 242 240 274 241 282 282 282 282 261 276 262 283 267 247 256 260 246 182 186 195 212 230 239 230 220 230 210 300-
"Moscow "Vladimir "Gorki-
249 266 275 287 298 305 305 294 220 163 189 172 248 291 300-
'Smolensk-
I 1 1 2701 27q 2621 2551 2701 2761 305, 3031 3031 189 289 193, 2311 2661 2261 1 I I I I I I I I Figure 2. Field capacity distribution over the forest zone territory (0-
100 cm layer)Tables 1 and 2 show volume weight for two distinctive layers: 40-50 cm as being a transitional one from humus horizon to parent rock; and 90- 100 cm where any incidental factors (e.g. plant root system, organics, etc.) practically have no influence on the special variability of the volume weight and other parameters. Data on full, capillary and field capacities and wilting point refer to the 0-100 cm layer. The characteristics given in Tables 1 and 2 were used when calculating their averages for the degree quadrants of the forest zone map. In case of the great lithological variety within a quadrant the characteristic value was determined as a weighted average.
Taking into account, that in some regions of the forest zone the greater part of the territory is covered by fields, the ratio of soil- physical properties of the field and forest soils is of interest. The main distinctions are known to be dependant on differences in porosity, which is greater in the forest compared with the field due to the loosing effect of the forest vegetation root system (Kapotov, 1980).
This fact explains the smaller volume weight and the larger full capacity values for forest soils (Table 1). Relative discrepancies for soil-physical characteristics are 10-15%. The least difference (2-4%) is for field capacity values.
The results of the study are presented in the form of map-schemes given as an example in Figures 1 and 2, and also in summary tables (Kapotova, 1988), containing the full data for 186 degree quadrants. The results show that the distribution of soil-physical parameters is in accordance with the soil type variation. For example, the volume weight in the north-western and central parts changes from 1.56 to 1.80 in the 40-50 cm layer and from 1.61 to 1.88 in the 90-100 cm layer. However, it is possible to distinct rather vast regions in which the volume weight fluctuations are negligible (the Valdai Hills, Leningrad, Smolensk and Moscow regions). More uniform distribution of volume weight is noted in the south-eastern part of the forest zone. The volume weight of structure loams in these regions is 0.2-0.3 g/cm3 less compared with that in the north-western and central parts.
The character of the water content changes is also subjected to the soil distribution regularity. The wilting point and the field capacity have the greatest changes over the area (2-2.5 times and 1.5-2.0 times, respectively) while the full and capillary capacity fluctuations do not
exceed 10%.
The present investigation is mainly based on experimental basins data and resulted in soil-physical parameters for vast poor gauged territory. A conclusion can be drawn about the representativity of the Valdai Branch experimental data to the larger area of the forest zone of the U.S.S.R. (European part).
References
Kapotov A.A. and Kapotova, N.I., 1980. Sravnitelni analiz vodno
-
phizicheskikh svoistv pochvenno-gruntovoi tolshchi polevogo i lesnogo vodozvorov. (The comparative analysis of water-physical properties of soils of field and forest small basins).
-
"Trudi GGI", v.266, s.29-49.Kapotova N.I. and Kapotov A.A., 1988. Issledovanie vodno-phizicheskikh kharakteristiks pochvogruntov territorii Nechernozemnoi zoni R.S.F.S.R. (Investigation of water-physical characteristics of soils of Non-Chernozem R.S.F.S.R. zone). - "Trudi GGI", v.331, s.57-80.