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Small basin response in a Mediterranean mountainous abandoned farming area: Research design and preliminary results
CATENA
vol. 19, p. 309 320
Cremlingen 1992 !
Small Basin Response in a Mediterranean Mountainous Abandoned Farming Area: Research Design and Preliminary Results P. LIorens & E Gallart
Summary A 36 ha basin named Cal Parisa has been studied since early 1988 in the eastern Pyrenees (Spain). This basin was selected as a representative of extensive areas on clayey bedrock formerly used for farming, nowadays almost abandoned, where badlands are of great importance as far as sediment production is concerned. As we are interested in the hydrological behaviour of terraced areas, and badland areas could strongly modify the hydrological and sediment response, the basin is devoid of badlands but has some bedrock scars and small gullies. Severe geoecological disturbances were produced in it by agricultural land use in the past: the basin was deforested, topography was terraced, and the drainage net was increased by ditching. Monitoring design is based on two measuring stations, one at the main outlet and the other in a 16 ha sub-basin, and two sampling stations at the outlets of the two main sub-basins; a weather station and four rain recorders. Preliminary data show a hydrological response fully controlled by antecedent moisture conditions, suggesting that soils ISSN 0341-8162 ~c)1992 by CATENA VERLAG, W-3302 Cremlingen-Destedt, Germany 0341 8162/92/5011851/US$ 2.00 + 0.25
(:\][NA
A n I n l e r d i s c i p l i n a r y .)ournal of S O I L S ( ' I E N d K
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have high infiltration capacities and that runoff is generated by saturation overland flow; delayed flow being less significant. Stream water samples show relatively high dissolved solids concentrations, dominated by calcium bicarbonate, and suspended sediments concentrations about ten times less than dissolved ones, as a consequence of the strong sediment conveyance discontinuities observed in the field.
1
Introduction
The study on the small basin of Cal Parisa forms part of the CSICL U C D E M E project no. 6: Small basin responses in Mediterranean ecosystems: monitoring, experiments and modelling. The main purpose of this Project is to coordinate the work of several research teams concerned with the study of 14 small drainage basins along the mediterranean part of Spain from two main points of view: biogeochemical and geomorphological (fig. 1). This small basin was selected as a representative of mountain areas on clayey bedrock prone to mass movements and gullying, strongly modified by human activity. Research on this basin performs therefore an "unplanned experiment" according to Rohdenburg (1989) in the sense that modifications of natural conditions are much
(]EOMORPII()LI~)(;Y
310
[
r~
Llorens & Gallart
] Neogene depressions and
troughs
Tertiarydepression Pyrenees
~--~ Betic Ranges [ ~ Weackly deformedMesozoicrocks ]
Hercynianbasement
(~)
Filabres
(~
Prades
~) Montseny (~) Vallcebre (~) Izas
older than the beginning of the monitoring period. The immediate purposes of the study of the Cal Parisa basin are: First, to assess the hydrological behaviour of the basin, and its response to different rainfall events• The agricultural terraces are expected to increase the infiltration capacity of the basin, whereas the ditches which connect the saturated areas of the inner part of the terraces to the drainage net are expected to shorten and sharpen the response to rainfall when the system becomes saturated• After field evidence, Horton overland flow is produced on the bare mudrock areas. One of the main questions is whether high discharges are related to saturation conditions (continuous weak rainfalls), or to excess precipitation (intense rainstorms).
Fig. 1: Location o / the drainage basins studied in the research project "'Small basin responses in Mediterranean Ecosystems: monitoring, experiments and modelling". The basin labelled V is the Cal Parisa basin, presented in this paper.
Second, to assess the behaviour of erosion and transport processes in the basin. There are small but very active sediment source areas on hillslopes, but only few of them are connected to the drainage net. Furthermore, there are sediment conveyance discontinuities on hillslopes and on the drainage system which produce sediment stores. Under these conditions the sediment exported out of the basin, that is only a small part of the total volume of sediment produced, seems to come mainly from the channels. In addition to the,' monitoring of sediment yielded by the basin, the aim of the study is to describe the pattern of sediment sources, sinks and pathways, with an assessment of the volumes of sediment transferred (Dietrich & Dunne 1978). The relationships between the sed-
CAIENA An InterdisciplinaryJournal o[SO[I S(IEN(E HYDROLO(JY
(iI:OMORPIIOLOGY
Monitoring, oM farmed mountain basin response
iment collected at the basin outlet and the sediment sources are to be studied by fingerprinting techniques (Walling & Kane 1984). Less immediate purposes concern the role of human impact on hydrology and gully initiation, and the long term geomorphic evolution of these areas, especially after land abandonment.
2
The study area
311
the lowest part of the basin. Two areas of bare clayey bedrock of 390 and 540 m 2, located at the foot of the cliff, were produced as scars of debris flows during the extreme rainfall of November 1982 (see Gallart & Clotet 1988). These and other small bare areas which occur on gullies, are active sediment sources usually disconnected from the drainage net (fig. 2). Severe geoecological disturbances were produced by agricultural land use in the past; the basin was deforested, terraced, and a net of ditches was made to convey the water out of the saturated areas. About 45% of the basin is nowadays covered by grassland and used for cattle and sheep stockbreeding; 38% of the basin, corresponding to the marginal areas for agricultural use that were early abandoned, is covered by bushes (Buxus
The elementary basin of Cal Parisa is located in the high Llobregat basin in the Eastern Pyrenees. Continental late Cretaceous mudrocks (Garumnian facies) are the main sources of sediment (Clotet & Gallart 1986), specifically from relatively small badland areas with erosion rates as high as 9 mm/year (Clotet et al. 1988). The area of the whole basin is 36 ha between 1400 and 1700 m a.s.1, on sunny sempervirens, Genista scorpio, Juniperus aspect. The climate is Mediterranean communis) and some small wood armountainous with a mean annual pre- eas and dispersed trees mostly of Pinus cipitation of nearly 900 mm, showing sylvestris. These bushes and trees replace a bimodal distribution with two rainfall the old wood formed mainly by Quercus peaks one in spring and the other in pubescens. Topography is terraced in most of the autumn; frequently this distribution has another less important peak in August basin, but the best terraces occur in the due to convective rainstorms. The mean central area. Terraces are usually about annual temperature is about 9°C, with 3 m wide and 1.5 m high, and have a great annual and daily thermic ampli- man made containing wall of limestone. tudes. Freezing occurs about 100 days Most of them are flat or gently dip into per year, and snowfalls are frequent dur- the inner part, where soil is thinner, the ing winter but snow usually melts during water table frequently outcrops and is subsequent days. collected by shallow ditches. The bedrock is mainly a smectite rich The drainage network is poorly orgamudstone of Garumnian facies (nonma- nized because of mass movements and rine upper Cretaceous). At the upper and human disturbances. The drainage dennortheastern boundary the basin has a sity is about 20 k m / k m 2, 24% of which cliff of tilted continental limestones, and is provided by man made ditches. at the N W boundary an old landslide The nested sub-basin (left) has an area left a characteristic formation of clay of 16 ha. In general it is more disturbed with limestone boulders which reaches than the mean; 48% of the length of
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F._~Woods , -~7 Bare mudstone
~
aare limestone Irregular poorly drained area
~
Terraced abandonned
~ S ~
~
Bushes and scattered trees
~ ~
Debris flow 1982
. . . .
0 I
Saturated areas
""
200 m I
Fig. 2: Main geoecological units of the Cal Parisa basin.
[]
Rain recorder
Flume
[]
Weather station
--**- Pipe culvert
[]
Water level sensor
(~ Automatic sampler
[]
Conductimeter
(~ Stage sampler
[]
Water temp. sensor
V
200m
0
Fig. 3: Monitoring instrumentation design.
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Monitoring old Farmed mountain basin response
the drainage net is formed by man made ditches, and 60% of the area is terraced.
3
Instrumental design
The instrumental design, set up on May 1989 consists of (fig. 3):
3.1
Pluviometric and meteorological stations
Three perimetral rain recorders with tipping bucket devices are connected to small data loggers especially designed, which record the time when every movement of the bucket (0.1 mm) is produced instead a reading of the number of movements every time period; this system allows us a better computation of instantaneous rainfall intensities without recording null values. A meteorological station, connected to a more conventional data logger, records: precipitation, air temperature, relative humidity, wind speed and direction, and global radiation; the sensors are read every 10 seconds, and values are recorded every 5 minutes.
3.2
Hydrological stations
The nested (left) subcatchment station has a type H steel flume (76 cm) able to measure a peak discharge of 450 1/s. It is provided with a capacitive water level sensor connected to a data logger allowing continuous recording of water levels, which are converted to water discharges with the help of rating tables (Brakensiek et al. 1979). The discharge at the main outlet is controlled by a pre existing concrete pipe with a diameter of 1 m, provided with a capacitive sensor which is connected to
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a data logger. Because of the lack of calibration data, water levels are converted to discharges with the help of an equation (Brakensiek et al. 1979). The highest measurable discharge is about 4.5 m3/s" accuracy is low for small discharges but the slope of the pipe is enough to allow self cleaning. The water discharge for the right subcatchment is calculated by substraction. Both measuring stations have been designed to obtain good estimates of high discharges, which are assumed to be the more important for sediment transport. Unfortunately, low discharges cannot be accurately measured neither at the main outlet because of the control design, nor at the left outlet because of intiltration of low discharges in the stream bed before reaching the station.
3.3
Sampling stations
There are three points of sampling and measuring of temperature and electrical conductivity of water: the two discharge stations and a third in the main channel (right subcatchment) before the confluence with the nested channel (left subcatchmentl. The three points are provided with electrical conductivity and temperature sensors connected to data loggers. The temperature of water is recorded continuously, but the conductimeter is connected to a mechanism that switches on the electric power when the flow reaches a preselected level. The two discharge stations are provided with stage samplers which have 12 sampling bottles that allow taking samples of water and transported sediment during the water rising. The stage sampler at the flume is built to take a sample every increment of 45 1/s of the flow.
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Date
precipitation (mm)
5 min. max. rain intensity(mm/h)
antecedent conditions
37.3 33.8 41.5
6 12 85
wet wet dry
9-10/04/89 25/04/89 28/10/89
runoff coefficients wholebasin left basin 0.66 '~ 0.01
0.33 0.31 0.02
Tab. 1 : Summary of the recorded.flood events.
Automatic sampling devices are also installed in the two subcatchments; these consist o f an electronical p r o g r a m m i n g system, a peristaltic p u m p and 24 sampling bottles. The instrument takes the first sample when the level of water rises, because it is connected to the same mechanism as the conductimeters; subsequent samples can be taken sequentially (uniform or non uniform time). These instruments are connected to the data logger that records the time o f each sample.
3.4
Data loggers
The data loggers on the hydrological stations are o f the same type as that on the meteorological station. Sensors and automatic samplers are read every 15 seconds, means being c o m p u t e d and recorded every 5 minutes. D a t a are weekly transferred to a portable computer to store and analyze the information.
4 4.1
Preliminary results Hydrological results
1989 was a dry year in the study area, because o f that we have only few registered events, the more significant data being shown on tab. 1. These data demonstrate that the basin response has significant differences depending on the antecedent moisture conditions. In dry periods, the basin is able to absorb noticeable storms
CAIENA
without producing any runoff at the outlet; it is suggested therefore that runoff at the outlet is generated by saturation overland flow, and that Horton overland flow produced on bare areas during rainstorms is infiltrated before reaching the drainage network. During the event on 9th to 1 l th April 1989, which occurred during a rainy week, the total rainfall collected in the northernmost rain recorder (the event took place before the end o f the instrumenting works) was 37.3 ram, with a 5 minutes m a x i m u m rainfall intensity o f 6 m m / h r and a rainfall duration of 18h40' (fig. 4). The volume o f quickflow at the basin outlet was about 8835 m 3 with a runoff coefficient o f 66% and a discharge peak o f 227 1/s. The contribution o f the left subcatchment to the total outlet was 22%, the runoff coefficient being 33%. In the two hydrological stations only 1% o f the runoff had been produced by the time that nearly 45% o f the rain had fallen. The lag time between the centroids of rainfall and discharges was 10 h 55 min at the main outlet and 9 h 40 min at the outlet o f the left subcatchment (fig. 5). These lag times are long for such small catchments, suggesting that, even in wet conditions, about the first third o f the rainfall was needed to reach a sufficient saturation level. A more detailed analysis o f the hydrological response o f the basin during this
A n I n t e r d i s c i p l i n a r y J o u r n a l ¢~f S O I l S C I E N C E
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o E 2 E E 4 &-6
2O0
160
Total
~" 120 v
o 8o
40 0
I
0
210
40
60
"80
t (hours)
Fig. 4: Hyetograph and hydrographs of the event on 9 to l l t h April, 1989. Basin areas are 36 ha (total), and 16 ha (left).
100
90 80 ¢J
70 Ra
"5 60
5 50 v lI
40
/
/
/
Run°tT/
//Total
30 20 10 0 2O
30
40 t (hours)
50
60
Fig. 5" Cumulative rainfall and discharge for the same event shown on .fig. 4.
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event has been performed following an idea from Dunne & Leopold (1978). In order to obtain net discharges produced by rainfall, a simple recession equation has been applied to every five minutes discharge readings of the central part of the hydrograph; net discharges are the results of the differences between every discharge reading and the recession discharge obtained from the previous reading. The formula used is:
4.2
Sediment yield results
The chemical analysis of the water samples show a chemical composition dominated by calcium bicarbonate: according to average values 64% of the Total Dissolved Solids corresponds to HCO 3 and 25% to Ca 2~, representing mean concentrations respectively of about 200 and 80 ppm. In these conditions precipitation of carbonates often occurs. This hydrochemical characteristic has an important role in the mechanisms of clay aggregaQ,, = Q, - Qt 110~at lion, which is being studied beside parwhere ticle size, mineralogy and chemical comQ,, is the net discharge at time t position of the suspended sediment. Q~ is the discharge reading at time t The Total Dissolved Solids response is the recession coefficient dt is the time interval for the 9th to l lth April event is shown The correlation coefficients for differ- on fig. 8. Both sub-basins show dissolved ent time spans between rainfall readings sediment concentrations negatively corand the net runoff obtained with this related with discharge, due to dilution procedure are shown on fig. 6; there is effect. The lack of high concentration no significant correlation for the main values shown by the left subcatchment outlet, but a lag time of 30 35 min is can be explained by the fact that low disdemonstrated for the left subcatchment; charges do not reach the station because this lag is usual for dry areas (Dunne & of infiltration in the stream bed. HysLeopold 1978). teresis loops are not well defined (probIn spite of the lack of correlation be- ably somewhat faded by the low resotween rainfall and net discharge readings lution of the sampling), but clockwise for the central part of the hydrograph at and counterclockwise trends are shown the main outlet, there is a good correla- respectively by the right and left subtion between actual discharges at the two basins. measuring sites for the whole event, flow The total yield of dissolved solids durat the main outlet being about l0 rain ing the whole event was about 1700 kg later (fig. 7). These results show that for all the basin. the hydrological response of the left subThe Total Suspended Sediments recatchment, when it is wet enough to prosponse for this event is shown on fig. 9. duce runoff, is a direct function of rainIt is interesting to highlight the low sedfall, while the main basin has a somewhat iment concentrations in both subcatchmore intricate response, well correlated ments and their different behaviour. In to this from the sub-basin, but indepenthe right subcatchment a positive corredent of small variations of rainfall. lation between TSS and discharge, with a very small slope and without definite hysteresis loop at the resolution level of the sampling, can be observed: this sug-
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Monitoring old farmed mountain basin response
0.8 u Left subcatchment
0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 -0.1 -0.2
r
0
I
L
i
20
I
6i0
40 /X t (minutes)
Fig. 6: Correlation coefficients between rainfall and net discharges o/" the central part qf the hydrographs at both gauging stations Jbr different time spans (see text).
0.9l
0.9,
Y
0.92
0.90 i 30
i -10
t
~0
i
310
L
L 50
I
J 70
t (minutes)
Fig. 7" Correlation coefficients between discharges at the two gauging stations./or different time spans, taking the whole event; positive values represent that discharges at the left sub-basin are compared with discharges measured at the main outlet At minutes later.
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210'
200
E 03
Q 190 p-
180 [] Left s u b c a t c h m e n t ,x Right s u b c a t c h m e n t 170
I
0
I
I
20
I
I
40
I
I
60
I
I
I
80
100
I
L
I
120
140
Q (I/s)
Fig. 8: Total Dissolved Solids response of the two sub-basins during the same event shown on former figures.
200
\ \ k
160
00
a Left s u b c a t c h m e n t t, Right s u b c a t c h m e n t
~
20
40
60 Q (I/s)
80
100
120
Fig. 9: Total Suspended Sediments response o f the two sub-basins during the same event shown on former figures.
(?ATENA
An Interdisciplinary Journal of SOIL S(IENCE
HYDROLOGY
GEOMORPttOLO(JY
Monitoring oM farmed mountain basin response
gests a poor but maintained suspended sediment supply. On the contrary, the left subcatchment shows a negative correlation between TSS and discharge, and a main clockwise hysteresis loop, probably due respectively to a dilution effect and to the exhaustion of the sediments available in the stream bed. The total yield of suspended sediments for all the basin was about 130 kg, an order of magnitude smaller than the dissolved one. According to field evidence, the basin shows a great difference in sediment production and transport between the bare areas, with high runoff and sediment production, and the areas with dense vegetation cover (grassland or bushes), agricultural terraces and thick soils, with low runoff and very low sediment production. The difference between these two kinds of areas are related to the different infiltration rates, vegetation protection, and especially to the behaviour of the materials against erosive processes; samples taken from mudrock outcrops slake in water, while adjacent soil samples in spite of the same lithologic composition, are more resistant to erosive processes (Soi~ et al. this volume).
5
Conclusions
During the eight-month record, the Cal Parisa basin has shown a hydrological response influenced by antecedent moisture conditions. Although the bedrock is impermeable, it has thick soils that have a high infiltration capacity and water storage. When the basin reaches saturation it has a fast response, with high runoff coefficients. On the other hand, Hortonian overland flow frequently occurs in bare areas, where the bedrock is covered by a thin re-
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golith. Nevertheless, these areas are usually disconnected from the drainage net, and overland flow coming from them infiltrates when it reaches areas with thick soils. The samples of stream water collected at the two sampling points exhibit relatively high dissolved solids concentrations, dominated by calcium bicarbonate close to saturation levels, and low suspended sediment concentrations, about ten times lower than dissolved ones. This is the basin response to weak rainfalls during wet periods. In these conditions, runoff is generated by saturation overland flow, and suspended sediments seem to be produced by the stream beds and banks. The high intensity rainstorms recorded occurred during dry spells, and did not produce streamflow at the gauging stations, nor sediment yield, in spite of the significant volumes of sediments moved on the slopes within the basin.
Acknowledgements This work is being financed by an agreement between the Consejo Superior de Investigaciones Cientificas and the Instituto Nacional para la Conservaci6n de la Naturaleza ( L U C D E M E project). We are very grateful for technical support from M. Fernfindez and field support from M. Soler, E. Vfizquez and M. Olaya. We are also very grateful to the anonymous referees from CATENA for their critical comments which significantly improved the manuscript.
References BRAKENSIEK, D.L., OSBORN, H.B. & RAWLS, W.J. (Coordinators) (1979): Field manual for research in agricultural hydrology. Agriculture Handbook 224, USDA, 550 pp.
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CLOTET, N. & GALLART, F. (1986): Sediment yield in a mountainous basin under high Mediterranean climate. Zeitschr. fiir Geomorphologie Suppl. Bd. 60, 205-216. CLOTET, N., GALLART, F. & BALASCH, C. (1988): Medium-term erosion rates in a small scarcely vegetated catchment in the Pyrenees. CATENA SUPPLEMENT 13, 37 47. DIETRICH, W.E. & DUNNE, T. (1978): Sediment budget for a small catchment in mountainous terrain. Zeitschr. ffir Geomorphologie Suppl. Bd. 29, 191-206. DUNNE, T. & LEOPOLD, L.B. (1978): Water in environmental planning. Freeman & Co., New York, 818 pp. GALLART, F. & CLOTET, N. (1988): Some aspects of the geomorphic processes triggered by an extreme rainfall event: the November t982 flood in the Eastern Pyrenees. CATENA SUPPLEMENT 13, 79-95. R O H D E N B U R G , H. (1989): Methods for the analysis of agroecosystems in Central Europe, with emphasis on geoecological aspects. CATENA 16, 1-57. SOLE, A., JOSA, R., PARDINI, G., ARINGHIERI, R., PLANA, F. & GALLART, F. (this volume): How mudrock and soil physical properties influence badland tbrmation at Vallcebre (Pre-Pyrenees, N.E. Spain). CATENA 19, 287-300. WALLING, D.E. & KANE, P. (1984): Suspended sediment properties and their geomorphological significance. In: Catchment experiments in fluvial geomorphology, 311-334. Geobooks, Norwich.
Address of authors: Dr. Pilar Llorens Dr. Francesc Gallart Institute of Earth Sciences Jaume Almera (C.S.I.C.) Ap. 30102 08028 Barcelona Spain
('AIENA AnlnterdisciplinaryJournalofSOILSCILNCt
IIYDROLOGY (}fOMORPIIOLO¢P~
vol. 19, p. 309 320
Cremlingen 1992 !
Small Basin Response in a Mediterranean Mountainous Abandoned Farming Area: Research Design and Preliminary Results P. LIorens & E Gallart
Summary A 36 ha basin named Cal Parisa has been studied since early 1988 in the eastern Pyrenees (Spain). This basin was selected as a representative of extensive areas on clayey bedrock formerly used for farming, nowadays almost abandoned, where badlands are of great importance as far as sediment production is concerned. As we are interested in the hydrological behaviour of terraced areas, and badland areas could strongly modify the hydrological and sediment response, the basin is devoid of badlands but has some bedrock scars and small gullies. Severe geoecological disturbances were produced in it by agricultural land use in the past: the basin was deforested, topography was terraced, and the drainage net was increased by ditching. Monitoring design is based on two measuring stations, one at the main outlet and the other in a 16 ha sub-basin, and two sampling stations at the outlets of the two main sub-basins; a weather station and four rain recorders. Preliminary data show a hydrological response fully controlled by antecedent moisture conditions, suggesting that soils ISSN 0341-8162 ~c)1992 by CATENA VERLAG, W-3302 Cremlingen-Destedt, Germany 0341 8162/92/5011851/US$ 2.00 + 0.25
(:\][NA
A n I n l e r d i s c i p l i n a r y .)ournal of S O I L S ( ' I E N d K
IIYDROIO(]Y
have high infiltration capacities and that runoff is generated by saturation overland flow; delayed flow being less significant. Stream water samples show relatively high dissolved solids concentrations, dominated by calcium bicarbonate, and suspended sediments concentrations about ten times less than dissolved ones, as a consequence of the strong sediment conveyance discontinuities observed in the field.
1
Introduction
The study on the small basin of Cal Parisa forms part of the CSICL U C D E M E project no. 6: Small basin responses in Mediterranean ecosystems: monitoring, experiments and modelling. The main purpose of this Project is to coordinate the work of several research teams concerned with the study of 14 small drainage basins along the mediterranean part of Spain from two main points of view: biogeochemical and geomorphological (fig. 1). This small basin was selected as a representative of mountain areas on clayey bedrock prone to mass movements and gullying, strongly modified by human activity. Research on this basin performs therefore an "unplanned experiment" according to Rohdenburg (1989) in the sense that modifications of natural conditions are much
(]EOMORPII()LI~)(;Y
310
[
r~
Llorens & Gallart
] Neogene depressions and
troughs
Tertiarydepression Pyrenees
~--~ Betic Ranges [ ~ Weackly deformedMesozoicrocks ]
Hercynianbasement
(~)
Filabres
(~
Prades
~) Montseny (~) Vallcebre (~) Izas
older than the beginning of the monitoring period. The immediate purposes of the study of the Cal Parisa basin are: First, to assess the hydrological behaviour of the basin, and its response to different rainfall events• The agricultural terraces are expected to increase the infiltration capacity of the basin, whereas the ditches which connect the saturated areas of the inner part of the terraces to the drainage net are expected to shorten and sharpen the response to rainfall when the system becomes saturated• After field evidence, Horton overland flow is produced on the bare mudrock areas. One of the main questions is whether high discharges are related to saturation conditions (continuous weak rainfalls), or to excess precipitation (intense rainstorms).
Fig. 1: Location o / the drainage basins studied in the research project "'Small basin responses in Mediterranean Ecosystems: monitoring, experiments and modelling". The basin labelled V is the Cal Parisa basin, presented in this paper.
Second, to assess the behaviour of erosion and transport processes in the basin. There are small but very active sediment source areas on hillslopes, but only few of them are connected to the drainage net. Furthermore, there are sediment conveyance discontinuities on hillslopes and on the drainage system which produce sediment stores. Under these conditions the sediment exported out of the basin, that is only a small part of the total volume of sediment produced, seems to come mainly from the channels. In addition to the,' monitoring of sediment yielded by the basin, the aim of the study is to describe the pattern of sediment sources, sinks and pathways, with an assessment of the volumes of sediment transferred (Dietrich & Dunne 1978). The relationships between the sed-
CAIENA An InterdisciplinaryJournal o[SO[I S(IEN(E HYDROLO(JY
(iI:OMORPIIOLOGY
Monitoring, oM farmed mountain basin response
iment collected at the basin outlet and the sediment sources are to be studied by fingerprinting techniques (Walling & Kane 1984). Less immediate purposes concern the role of human impact on hydrology and gully initiation, and the long term geomorphic evolution of these areas, especially after land abandonment.
2
The study area
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the lowest part of the basin. Two areas of bare clayey bedrock of 390 and 540 m 2, located at the foot of the cliff, were produced as scars of debris flows during the extreme rainfall of November 1982 (see Gallart & Clotet 1988). These and other small bare areas which occur on gullies, are active sediment sources usually disconnected from the drainage net (fig. 2). Severe geoecological disturbances were produced by agricultural land use in the past; the basin was deforested, terraced, and a net of ditches was made to convey the water out of the saturated areas. About 45% of the basin is nowadays covered by grassland and used for cattle and sheep stockbreeding; 38% of the basin, corresponding to the marginal areas for agricultural use that were early abandoned, is covered by bushes (Buxus
The elementary basin of Cal Parisa is located in the high Llobregat basin in the Eastern Pyrenees. Continental late Cretaceous mudrocks (Garumnian facies) are the main sources of sediment (Clotet & Gallart 1986), specifically from relatively small badland areas with erosion rates as high as 9 mm/year (Clotet et al. 1988). The area of the whole basin is 36 ha between 1400 and 1700 m a.s.1, on sunny sempervirens, Genista scorpio, Juniperus aspect. The climate is Mediterranean communis) and some small wood armountainous with a mean annual pre- eas and dispersed trees mostly of Pinus cipitation of nearly 900 mm, showing sylvestris. These bushes and trees replace a bimodal distribution with two rainfall the old wood formed mainly by Quercus peaks one in spring and the other in pubescens. Topography is terraced in most of the autumn; frequently this distribution has another less important peak in August basin, but the best terraces occur in the due to convective rainstorms. The mean central area. Terraces are usually about annual temperature is about 9°C, with 3 m wide and 1.5 m high, and have a great annual and daily thermic ampli- man made containing wall of limestone. tudes. Freezing occurs about 100 days Most of them are flat or gently dip into per year, and snowfalls are frequent dur- the inner part, where soil is thinner, the ing winter but snow usually melts during water table frequently outcrops and is subsequent days. collected by shallow ditches. The bedrock is mainly a smectite rich The drainage network is poorly orgamudstone of Garumnian facies (nonma- nized because of mass movements and rine upper Cretaceous). At the upper and human disturbances. The drainage dennortheastern boundary the basin has a sity is about 20 k m / k m 2, 24% of which cliff of tilted continental limestones, and is provided by man made ditches. at the N W boundary an old landslide The nested sub-basin (left) has an area left a characteristic formation of clay of 16 ha. In general it is more disturbed with limestone boulders which reaches than the mean; 48% of the length of
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F._~Woods , -~7 Bare mudstone
~
aare limestone Irregular poorly drained area
~
Terraced abandonned
~ S ~
~
Bushes and scattered trees
~ ~
Debris flow 1982
. . . .
0 I
Saturated areas
""
200 m I
Fig. 2: Main geoecological units of the Cal Parisa basin.
[]
Rain recorder
Flume
[]
Weather station
--**- Pipe culvert
[]
Water level sensor
(~ Automatic sampler
[]
Conductimeter
(~ Stage sampler
[]
Water temp. sensor
V
200m
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Fig. 3: Monitoring instrumentation design.
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the drainage net is formed by man made ditches, and 60% of the area is terraced.
3
Instrumental design
The instrumental design, set up on May 1989 consists of (fig. 3):
3.1
Pluviometric and meteorological stations
Three perimetral rain recorders with tipping bucket devices are connected to small data loggers especially designed, which record the time when every movement of the bucket (0.1 mm) is produced instead a reading of the number of movements every time period; this system allows us a better computation of instantaneous rainfall intensities without recording null values. A meteorological station, connected to a more conventional data logger, records: precipitation, air temperature, relative humidity, wind speed and direction, and global radiation; the sensors are read every 10 seconds, and values are recorded every 5 minutes.
3.2
Hydrological stations
The nested (left) subcatchment station has a type H steel flume (76 cm) able to measure a peak discharge of 450 1/s. It is provided with a capacitive water level sensor connected to a data logger allowing continuous recording of water levels, which are converted to water discharges with the help of rating tables (Brakensiek et al. 1979). The discharge at the main outlet is controlled by a pre existing concrete pipe with a diameter of 1 m, provided with a capacitive sensor which is connected to
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a data logger. Because of the lack of calibration data, water levels are converted to discharges with the help of an equation (Brakensiek et al. 1979). The highest measurable discharge is about 4.5 m3/s" accuracy is low for small discharges but the slope of the pipe is enough to allow self cleaning. The water discharge for the right subcatchment is calculated by substraction. Both measuring stations have been designed to obtain good estimates of high discharges, which are assumed to be the more important for sediment transport. Unfortunately, low discharges cannot be accurately measured neither at the main outlet because of the control design, nor at the left outlet because of intiltration of low discharges in the stream bed before reaching the station.
3.3
Sampling stations
There are three points of sampling and measuring of temperature and electrical conductivity of water: the two discharge stations and a third in the main channel (right subcatchment) before the confluence with the nested channel (left subcatchmentl. The three points are provided with electrical conductivity and temperature sensors connected to data loggers. The temperature of water is recorded continuously, but the conductimeter is connected to a mechanism that switches on the electric power when the flow reaches a preselected level. The two discharge stations are provided with stage samplers which have 12 sampling bottles that allow taking samples of water and transported sediment during the water rising. The stage sampler at the flume is built to take a sample every increment of 45 1/s of the flow.
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Date
precipitation (mm)
5 min. max. rain intensity(mm/h)
antecedent conditions
37.3 33.8 41.5
6 12 85
wet wet dry
9-10/04/89 25/04/89 28/10/89
runoff coefficients wholebasin left basin 0.66 '~ 0.01
0.33 0.31 0.02
Tab. 1 : Summary of the recorded.flood events.
Automatic sampling devices are also installed in the two subcatchments; these consist o f an electronical p r o g r a m m i n g system, a peristaltic p u m p and 24 sampling bottles. The instrument takes the first sample when the level of water rises, because it is connected to the same mechanism as the conductimeters; subsequent samples can be taken sequentially (uniform or non uniform time). These instruments are connected to the data logger that records the time o f each sample.
3.4
Data loggers
The data loggers on the hydrological stations are o f the same type as that on the meteorological station. Sensors and automatic samplers are read every 15 seconds, means being c o m p u t e d and recorded every 5 minutes. D a t a are weekly transferred to a portable computer to store and analyze the information.
4 4.1
Preliminary results Hydrological results
1989 was a dry year in the study area, because o f that we have only few registered events, the more significant data being shown on tab. 1. These data demonstrate that the basin response has significant differences depending on the antecedent moisture conditions. In dry periods, the basin is able to absorb noticeable storms
CAIENA
without producing any runoff at the outlet; it is suggested therefore that runoff at the outlet is generated by saturation overland flow, and that Horton overland flow produced on bare areas during rainstorms is infiltrated before reaching the drainage network. During the event on 9th to 1 l th April 1989, which occurred during a rainy week, the total rainfall collected in the northernmost rain recorder (the event took place before the end o f the instrumenting works) was 37.3 ram, with a 5 minutes m a x i m u m rainfall intensity o f 6 m m / h r and a rainfall duration of 18h40' (fig. 4). The volume o f quickflow at the basin outlet was about 8835 m 3 with a runoff coefficient o f 66% and a discharge peak o f 227 1/s. The contribution o f the left subcatchment to the total outlet was 22%, the runoff coefficient being 33%. In the two hydrological stations only 1% o f the runoff had been produced by the time that nearly 45% o f the rain had fallen. The lag time between the centroids of rainfall and discharges was 10 h 55 min at the main outlet and 9 h 40 min at the outlet o f the left subcatchment (fig. 5). These lag times are long for such small catchments, suggesting that, even in wet conditions, about the first third o f the rainfall was needed to reach a sufficient saturation level. A more detailed analysis o f the hydrological response o f the basin during this
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o E 2 E E 4 &-6
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Total
~" 120 v
o 8o
40 0
I
0
210
40
60
"80
t (hours)
Fig. 4: Hyetograph and hydrographs of the event on 9 to l l t h April, 1989. Basin areas are 36 ha (total), and 16 ha (left).
100
90 80 ¢J
70 Ra
"5 60
5 50 v lI
40
/
/
/
Run°tT/
//Total
30 20 10 0 2O
30
40 t (hours)
50
60
Fig. 5" Cumulative rainfall and discharge for the same event shown on .fig. 4.
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event has been performed following an idea from Dunne & Leopold (1978). In order to obtain net discharges produced by rainfall, a simple recession equation has been applied to every five minutes discharge readings of the central part of the hydrograph; net discharges are the results of the differences between every discharge reading and the recession discharge obtained from the previous reading. The formula used is:
4.2
Sediment yield results
The chemical analysis of the water samples show a chemical composition dominated by calcium bicarbonate: according to average values 64% of the Total Dissolved Solids corresponds to HCO 3 and 25% to Ca 2~, representing mean concentrations respectively of about 200 and 80 ppm. In these conditions precipitation of carbonates often occurs. This hydrochemical characteristic has an important role in the mechanisms of clay aggregaQ,, = Q, - Qt 110~at lion, which is being studied beside parwhere ticle size, mineralogy and chemical comQ,, is the net discharge at time t position of the suspended sediment. Q~ is the discharge reading at time t The Total Dissolved Solids response is the recession coefficient dt is the time interval for the 9th to l lth April event is shown The correlation coefficients for differ- on fig. 8. Both sub-basins show dissolved ent time spans between rainfall readings sediment concentrations negatively corand the net runoff obtained with this related with discharge, due to dilution procedure are shown on fig. 6; there is effect. The lack of high concentration no significant correlation for the main values shown by the left subcatchment outlet, but a lag time of 30 35 min is can be explained by the fact that low disdemonstrated for the left subcatchment; charges do not reach the station because this lag is usual for dry areas (Dunne & of infiltration in the stream bed. HysLeopold 1978). teresis loops are not well defined (probIn spite of the lack of correlation be- ably somewhat faded by the low resotween rainfall and net discharge readings lution of the sampling), but clockwise for the central part of the hydrograph at and counterclockwise trends are shown the main outlet, there is a good correla- respectively by the right and left subtion between actual discharges at the two basins. measuring sites for the whole event, flow The total yield of dissolved solids durat the main outlet being about l0 rain ing the whole event was about 1700 kg later (fig. 7). These results show that for all the basin. the hydrological response of the left subThe Total Suspended Sediments recatchment, when it is wet enough to prosponse for this event is shown on fig. 9. duce runoff, is a direct function of rainIt is interesting to highlight the low sedfall, while the main basin has a somewhat iment concentrations in both subcatchmore intricate response, well correlated ments and their different behaviour. In to this from the sub-basin, but indepenthe right subcatchment a positive corredent of small variations of rainfall. lation between TSS and discharge, with a very small slope and without definite hysteresis loop at the resolution level of the sampling, can be observed: this sug-
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0.8 u Left subcatchment
0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 -0.1 -0.2
r
0
I
L
i
20
I
6i0
40 /X t (minutes)
Fig. 6: Correlation coefficients between rainfall and net discharges o/" the central part qf the hydrographs at both gauging stations Jbr different time spans (see text).
0.9l
0.9,
Y
0.92
0.90 i 30
i -10
t
~0
i
310
L
L 50
I
J 70
t (minutes)
Fig. 7" Correlation coefficients between discharges at the two gauging stations./or different time spans, taking the whole event; positive values represent that discharges at the left sub-basin are compared with discharges measured at the main outlet At minutes later.
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210'
200
E 03
Q 190 p-
180 [] Left s u b c a t c h m e n t ,x Right s u b c a t c h m e n t 170
I
0
I
I
20
I
I
40
I
I
60
I
I
I
80
100
I
L
I
120
140
Q (I/s)
Fig. 8: Total Dissolved Solids response of the two sub-basins during the same event shown on former figures.
200
\ \ k
160
00
a Left s u b c a t c h m e n t t, Right s u b c a t c h m e n t
~
20
40
60 Q (I/s)
80
100
120
Fig. 9: Total Suspended Sediments response o f the two sub-basins during the same event shown on former figures.
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gests a poor but maintained suspended sediment supply. On the contrary, the left subcatchment shows a negative correlation between TSS and discharge, and a main clockwise hysteresis loop, probably due respectively to a dilution effect and to the exhaustion of the sediments available in the stream bed. The total yield of suspended sediments for all the basin was about 130 kg, an order of magnitude smaller than the dissolved one. According to field evidence, the basin shows a great difference in sediment production and transport between the bare areas, with high runoff and sediment production, and the areas with dense vegetation cover (grassland or bushes), agricultural terraces and thick soils, with low runoff and very low sediment production. The difference between these two kinds of areas are related to the different infiltration rates, vegetation protection, and especially to the behaviour of the materials against erosive processes; samples taken from mudrock outcrops slake in water, while adjacent soil samples in spite of the same lithologic composition, are more resistant to erosive processes (Soi~ et al. this volume).
5
Conclusions
During the eight-month record, the Cal Parisa basin has shown a hydrological response influenced by antecedent moisture conditions. Although the bedrock is impermeable, it has thick soils that have a high infiltration capacity and water storage. When the basin reaches saturation it has a fast response, with high runoff coefficients. On the other hand, Hortonian overland flow frequently occurs in bare areas, where the bedrock is covered by a thin re-
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golith. Nevertheless, these areas are usually disconnected from the drainage net, and overland flow coming from them infiltrates when it reaches areas with thick soils. The samples of stream water collected at the two sampling points exhibit relatively high dissolved solids concentrations, dominated by calcium bicarbonate close to saturation levels, and low suspended sediment concentrations, about ten times lower than dissolved ones. This is the basin response to weak rainfalls during wet periods. In these conditions, runoff is generated by saturation overland flow, and suspended sediments seem to be produced by the stream beds and banks. The high intensity rainstorms recorded occurred during dry spells, and did not produce streamflow at the gauging stations, nor sediment yield, in spite of the significant volumes of sediments moved on the slopes within the basin.
Acknowledgements This work is being financed by an agreement between the Consejo Superior de Investigaciones Cientificas and the Instituto Nacional para la Conservaci6n de la Naturaleza ( L U C D E M E project). We are very grateful for technical support from M. Fernfindez and field support from M. Soler, E. Vfizquez and M. Olaya. We are also very grateful to the anonymous referees from CATENA for their critical comments which significantly improved the manuscript.
References BRAKENSIEK, D.L., OSBORN, H.B. & RAWLS, W.J. (Coordinators) (1979): Field manual for research in agricultural hydrology. Agriculture Handbook 224, USDA, 550 pp.
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CLOTET, N. & GALLART, F. (1986): Sediment yield in a mountainous basin under high Mediterranean climate. Zeitschr. fiir Geomorphologie Suppl. Bd. 60, 205-216. CLOTET, N., GALLART, F. & BALASCH, C. (1988): Medium-term erosion rates in a small scarcely vegetated catchment in the Pyrenees. CATENA SUPPLEMENT 13, 37 47. DIETRICH, W.E. & DUNNE, T. (1978): Sediment budget for a small catchment in mountainous terrain. Zeitschr. ffir Geomorphologie Suppl. Bd. 29, 191-206. DUNNE, T. & LEOPOLD, L.B. (1978): Water in environmental planning. Freeman & Co., New York, 818 pp. GALLART, F. & CLOTET, N. (1988): Some aspects of the geomorphic processes triggered by an extreme rainfall event: the November t982 flood in the Eastern Pyrenees. CATENA SUPPLEMENT 13, 79-95. R O H D E N B U R G , H. (1989): Methods for the analysis of agroecosystems in Central Europe, with emphasis on geoecological aspects. CATENA 16, 1-57. SOLE, A., JOSA, R., PARDINI, G., ARINGHIERI, R., PLANA, F. & GALLART, F. (this volume): How mudrock and soil physical properties influence badland tbrmation at Vallcebre (Pre-Pyrenees, N.E. Spain). CATENA 19, 287-300. WALLING, D.E. & KANE, P. (1984): Suspended sediment properties and their geomorphological significance. In: Catchment experiments in fluvial geomorphology, 311-334. Geobooks, Norwich.
Address of authors: Dr. Pilar Llorens Dr. Francesc Gallart Institute of Earth Sciences Jaume Almera (C.S.I.C.) Ap. 30102 08028 Barcelona Spain
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