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Modelling of an Interzonal Multipurpose Water Management System in Morocco
CopYright © IFAC S\,stems Analysis Applied \Ianagement of\\'ater Resources. Rabat, \(oro((O, 1988
to
MODELLING OF AN INTERZONAL MULTIPURPOSE WATER MANAGEMENT SYSTEM IN MOROCCO M. Jellali*, T. Elafti*, G. Simeoni** and G. Mondada** *Dirf(lomlf
of
IralPr Rl'souras, .\fillislr),
0(
Pulilir Il'tll-k~, Ra/wl,
.\101'0((0
**Wall'l" Rfsourcl's Dl'pa 1'11111' 11 I, .Holor Co/ulII/ms COlI.\u/lillg ElIgillfl'l"s fll(', Hadi'll , 5 ll 'ilzl'l"/a lid
Abstract, Optimum schemes for conjunctive ground and surface water development have been defined for some river basins in the northwest of Morocco by means of a model capable of simulating the management repercussion of selected integrated water development systems at a given target year. The study area includes three major river basins (Sebou, Bouregreg and Oum Er Rbia) as well as some intern1ediate minor basins, totalling a surface of I 07000 km~, The population of the area in the year 2020 is expected to rise to more than twice as compared with the present, while the irrigated lands could expand from 500000 ha to some 800 000 ha within the same span of time, Accordingly, the overall yearly water demand is estimated to increase from 4000 Mio m 3 to some 9400 Mio m 3 to the project's target year 2020, thus prompting the local water authorities to undertake a master plan study for integrated water resources future development. The project included preliminary sectorial studies to assess surface and ground water resources, to make population and water requirement projections for water supply and irrigation and to define an optimum scheme of structures for water developement. The simulations led to the preselection of several altemative water resources developement schemes, In developing the alternatives. sensitivity analysis were applied to a basic solution defined in accordance with existing development plans and taken as a reference situation. Keywords: water management
INTRODUCTION The norhern basins of Atlantic Morocco benefit from relatively sizeable water resources as compared with the eastern and southem semi-arid to arid regions of the country.
agricultural development perimeters. Small scale agriculture areas are also disseminated along river flood plains and in other valleys and plateaux (Fig.2).
Yet, an increasing population and the need to expand agricultural areas in order to meet food self-production requirements, has motivated the local authorities to undertake a masterplan study to optimise the integrated water resources management of the most important river basins of the Atlantic Morocco and a series of minor coastal basins in the southwest.
The study'S final pupose is to define optimum schemes for conj unct i ve surface and water development in accordance with the socio-economic requirements and constrainls. To help the preliminary selection of a number of suitable alternative water development schemes, a mathematical model aimed at simulating multiple schemes water resource management has been used.
The population of the study area, which represented about 50 pecent of the country's total population in 1982, is expected to increase to more than twice the present level by the target year 2020. Still in 1982 the share of the rural population of the area was 43 percent of the total rural population, thus reflecting the general configuration of the country's economy, still greatly based on agriculture. This explains the presence in the area of some of the largest
DESCRIPTION OF THE STUDY AREA Geological and hydrogeological features The study area includes the catchments of the Sebou, Bouregreg and Oum Er Rbia rivers as well as a series of minor coastal basins, totalling a surface of 107 000 km 2 in all.
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Fig. 1 Location of the study area The primary basement is overlain by a thick sedimentary series, secondary, tertiary and quaternary in age. Distinguished in the lithological series are the limestones of the lower Lias in the Meknes-Fes region and of the middle Cretaceous (Turonian) in the Plateau des Phosphates - Tadla plain area, and constitute extensive and economically exploitable deep aquifers (Fig.2) . Plio-Miocene sediments of continental to marine-continental facies were deposited throughout the Pliocene and the Quaternary ages to constitute the Meknes-Fes plateau , the Tadla and Rharb plains as well as a series of superficial, detrital to biodetrital-licke variably thick deposits in the coastal plains. These Plio-quaternary sediments , distinguished by their generally fair to good hydrogeological characteri stics, const itute extensive aquifers, mostly in water table conditions, easily exploitable by means of wells. These aquifers constitute the major resource of water supply for rural areas. smal l towns and isolated industries as well as for sma ll scale irrigation system s. Hydroclimatological background The annua l average rainfall (period 1932-1984) exceeds 1000 mm in the summits of the Rif and Atlas mountains which bound the area northeast and eastward. Elsewhere, annual rainfall decrease progressively southwestward. Averages of 500 to 600 mm/year are registered in the northern plains (Rharb -Meknes-Fes). Minimum averarage rainfall occurs in the low Oum Er Rbia river basin with less than 300 mm/year.
The regional hydro-climatic events are quite well explainable but are rather unpredictable, al th ough they are characterised by 13 to 15 years of alternate rainy and dry series . The year 1986 marked the end of a deficitary period which started in 1972 and which assumed the characteristics of a serious drought between 1980 and 1985. WATER DEMAND AND RESOURCE DEVELOPMENT The demand The irrigated lands, which totalled some 500 000 ha in 1982 are expected to expand up to 800 000 in the year 2020 thus requiring 73 percent of this target year total forecasted water requirement for the study area. Present situation The developed groundwater resource in 1982 accounted for 57 percent of the water supply and for 16.5 percent of irrigation total water requirements, indicating a pronounced dependability of agriculture on surface water. At the reference year (1982) there were in the study area thirteen operationa l dam sites (Fig.2). Of these, seven are impounding mUltipurpose structures with a total live storage of 5650 Mio m 3 . The average annua l recharge of the thirteen most important aquifers existing in the area is of 1300 Mio m3, while the total net consumption of groundwater in 1984 is estimated to be of the order
l\loclelling of an lnterzonal Multipurpose ' ,V ater :'-Ianagement S\stem
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Fig. 2 Layout of the agriculture and water resources development plan of 660 Mio m j /year which represents some 77 percent of the ground water potential needs in 1982 (860 Mio m 3/year) and an overall return coefficient of 0.23. Developme nt program In Figure 2 are indicated the projected dams and other hydraulic structures which would have to be consrructed up to ne ar the year 2020 in order to insure a maximum development of surface and water resources. In se lecting this development scheme, which includes a total of 23 dams, and some transbasin canals , particular emphasi s has been given to the geographical distribution of the future water demand . Very pre liminary economical evaluations of each dam has been done by means of a simulation program described herea ft e r, in order to define in each sub basin, the most feas ible structures. Of the large-scale agricultural development areas in the course of imple mentation or being implemented, two (Doukkala and Haouz) are located outside the southwestern boundaries of the study area while two others (Tessaout amont and aval) lie acro ss the boundary, thus requiring transbasin canals. DESCRIPTION OF THE MODEL The program RIVER is conceived for simulating the simultaneous management of a rivers and reservoirs system with the purpose of developing water resources for water supply and irrigation needs, energy production, navigation , sanitary flow and flood control. The model allows for verification if a pre se lected reservoirs system with given characteristics and constraints can respond to pre requisites performances.
By adjusting in successive runs the data parameters , the optimum characteristics of the system are finally obtained following a trial and e rror prodecure. The model is composed of various interconnected elements (blocs), physically re presenting the simulated river ne twork. Basic elements in each system are the natural discharge and the blocs (Fig.3). The latte r consist of the following: the rese rvoir an hydropower station downstream (if any) a tributary three intakes two gauging stations locat e d upstream and downstre am of the res e rvoir. A groundwater aquife r is simulated as a reservoir of given characterist ics.
For each bloc , at the end of a given simulation run, the results are expressed as follows: the water level in the rese rvoir the discharge at the control points the di scharge out of the res e rvoir, through the powerplant and the spillway the fim1 and secondary energy production the modified discharge at the downstream control point the amounts of water alloted to irrigation and water supply. The model's calculation sequencies proceed from upstream to downstream unless different priorities are set by means of introducing data sequencies.
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Fig. 3 Stru cture of a simul at ion bl oc SIM ULATIO N C RITERIA Fundamental hvpothes is The whole area has bee n di vidcd int o 14 sub-bas ins (Fig.2) most of whi ch arc controll ed downstream by a dam . T wo supplementary sub-bas in s (E D and EM) re prese nt a reas o f wa te r de mand out of th e catchments. At each sub-ba sin outl ct point , monthl y di schargcs have bee n reconstitutcd for the 45 yea r pc ri od of available hydrol og ical data (1 939 -40 to 1983 -84 ). C o ns id e rin g th e exce pti o n a ll y de fi c it ary hydrolog ical conditi ons ha vin g prcv ail ed bctwee n 198 I to 1984. the whole se ri cs is co nsidc rcd as defi citary. Th e 43 ye ar long pe ri od 1939-40 to 1981-82 is taken to simul atc a normal hydrolog ical se rie. Bas ic pa rame te rs to take int o consid e ratio n for resou rces managemcnt are defincd as : the firm annu al rcg ul arize d vo lum c. whi ch
The latte r fac tor is the mos t imp ort ant co nstraint a nd is fund amc nt al for th e plann e rs dec isi on makin g. It is thu s important to defin e for eac h potentail wate r use r. reg ionall y appli cable defi cit criteria with respcc t to the normal (43 yea rs) and dcfi cit ary (45 yea rs), hydro log ica l sequencies . It is ass umcd th at th e ann ual de fi cit of the na tural reso urcc whi ch can be eas il y tolerat ed without any res tri cti on is o f 10 and 15 pe rcc nt fo r water suppl y a nd agri culturc res pec ti ve ly. On the othe r hand , cxperience shows that th c accep tabl e occ urence of defic it ary yea rs is one in ten for wat cr suppl y and onc in fi vc for irri gati on. Ma ximum to le rabl e defic it s for wa tcr suppl y in case of a drought within a givc n pc ri od have also bee n defin ed with respect to loc al ex pc ri ence (tow n of Marrakcc h) and se t to 20 % and 25 % for norm al and deficitary sequ encc respect ive ly.
The de finiti on of max imum acccptabl e de fi cits for ag ri culture call. fir st of all. fo r a di s tinc ti on bctwee n la rge scale and small sc al c irri gati on areas. r or large sca le agric ultural pc rimetc rs, whi ch are direc tl y o r indirec tl y und e r th c Go ve rnme nt a uthorit y, rulcs can cas ly be dictated. eve n on a yc ar-by-yea r bas is. Crop patt ern confi gurati on can bc mod ulat ed in such a way to enable the fl ex ibility o f a give n area to bc adap ted to a pa rti c ul ar hydro log ica l situ ati on. Relati ve ly limited reso urces ava il able in the Oum Er Rbia bas in , in duce planne rs
Table 1 Defic it criteria applied to the stud y area to detemline available resource Deficit crite ria Normal peri od
Deficit ary yea r
Water suppl y def. =I O%
Use r Irri ga ti on
Refe rence peri od
def.= 15 %
45 y.
Max imum occurrence of defi citary years
I in 10
2 in 10
45 y.
Max imum defi cit of supply
20%
32%
43 y.
Max imum deficit of supply
25 %
Rharb dev . area : 43% O.E.R .dev. area: 50%
45 y.
deficitary
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Fi g.4 Basic c rit e ria ~ for simulatin g basin wate r management with the program RIVER to minimi se permancnt and se llli-pcrm ~ lI1 e nt crops to 37 pe rcen t of the culti vate d land and in creasc fl ex ibilit y. with eve ntual suspel1Si on o f winter ClOp S. to ri se to 50 percent th e acceptan ce of the ma ximum deficit of supply in case of defi citary period. For small scale agricultural schemes. rural and di sse minat ed wa te r sup pl y. whi ch arc more frequently found in the upper ca tchm ent area~ of a given basin (F igA ) it is known that the use rs deri ve directl y the reso urces for the ir nee ds frol11 springs. temporary diversions. dug \\·e ll s. irrespec tive of any res triction rule. Limitati ons arc se t by nature it se lf in case of d ro ught eve nt s. With rega rd to the simulations. such short cut s of \vat er reso urce are re fl ec ted as a cut of an equi vale nt amount of th e "natural " monthl y re so urc es available do\\·nstrea m. Return coe ffi cient s In planning wa te r reso ur ces development. di stin cti on ha s to he made between tota l net demand (consumptive use) and the total reso urce (g ross demand ) mob ili ze d to sati sfy the forecas ted needs. Th e water non evapotran sp iratecl by crops as \\ell as dome stic and industrial sewe rs return s. treat ed or not. in the natural cycle within a generally limited span of time. to he reuse d.
APPLICATION EXAMPLE The chose n exa mple is that releva nt to th e Oum Er Rbia ba sin (Fig.5). whe re th e return s of surface irrigati on wate~r play a subs t:lIltial rol e in the hy (lraulics o f th e s urfa ce and grouJlc!water resources. The final possible development stage program of the Oum Er Rbia ba si n would co nt emplat e a towl of 22 dam~. say 12 in add iti on to the ten presently in ope rati o n. and a 1l1:l xill1l1l11 of three tran sfc rt s for irriga ti on Basica ll y. th e simulations consist of de fining the required withdrawals at each of the simulated bl ocs with an exce ption for the large Al Mas sira dam. \\'hi ch cont rol s the downstream part of the syste m. Discharges obtained at Al Mas sira. compared with th e mode l-g e ne rat e d flo ws. allow for the construction of the management curves of maximum deficit displayed in Fi g.6. For the multipurpose dam. th e primary concern is to res pect deficit criteria for water sup pl y. It appears that minimum annual water development can be granted with refe rence to th e deficitary pe riod (45 years), altough for thi s case the tolerance for deficit is ri sen to 25 percent.
In the non coastal basin s. the admitted re turn coefficients vary fr o m 0.28 for larg e sca le ag riculture de ve lopm e nt. to 0.8 for isolat ed indu stri es.
Rese rva ti on of wa te r to respect the pro du ction of firm po we r does not influ e nce s ignificantly the regulatin g capac ity of the system.
In order to preserve possibilities for wa te r use in the lowe r subcat chm ent s. minimum standard qualit y requirement s are attained by granting an adeq uate minimum sa nitary di sc har.!:'.e in the ri ve rbeds.
The appli ca tion of res tri ction criteria based on the all oted return period of one deficitary year out of te n (4 .5 years in 45) , shows co nsi ste nt results (Fig.6).
154
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Fig. 5 Water circulation and management scheme in the Qum Er Rbia basin
Resource development and demand comparison
AMENAGEMENT ,:
OUM ER RBIA
(AL MASSIRA)
DEFICIT MAXIMUM PENDANT 4~ ANS
The comparison between water resources a!1d needs is firstly conducted at the level of each sub-basin. The basis for comparison are: to define for each sub-basin the total gross demand, taking into account the historical evolution of the water needs and the necessary sanitary discharges
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to define the net water consumption, by taking into consideration the return coefficients and the possible water transfers. Finally, a water reso urces balance is established for each basin.
In a first instance, the above described procedure is applied with respect to what can be called a reference development scheme. Thi s results from taking into account the following major hypothesis. '---c~--«;;:-;;---
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For water supply: average population projections
and
water
demand
Fig. 6 Water resources management curves for the Qum Er Rbia river basin development scheme
For agricultural development areas : the maximum of potential development areas according to the govemment plans the water requirements estabilished by the relevant authorities or the water rights traditionally admitted.
In order to evaluate other possibilities offering eventually a better match between water demand and resource availability, alternative regional development schemes have been taken into consideration. With regard to agriculture, alternative developme nt schemes have been emphasized as follows :
\Iodcliing or an I ntcrzonal :\llIltipllrposc Water \ianagement S' "stcm
a) contemplating a cut of some components of the agriculture development including implementation slow - down b) giving priority to the development of small and medium schemes to the disadvantage of the largest, less flexible irrigation areas c) increasing the development areas located outside the basins d) introducing increasing ly efficient irrigation systems (sprinkler, trickle) , to reduce water losses. Other alternatives would contemplate: e) supplementary water transfers from northem to the southern catchments
the
f) to encourage preferential human settlements and consequent urban development in the northern basin where resources are relatively aboundant thus reducing the necessity for costly transfers structures. CONCLUSION Calibration and operational simulations to define optimum altemative schemes for conjunctive water resources development can be satsfactorily conducted on a monthly step simu lation base, over a reference period of 43 to 45 years.
155
Rules of management of water resources by means of large storage reservoirs are primarily set to satisfy most urgent present and projected needs for water supply and irrigation. However, hydropower schemes, when optimally located upstream of the large reservoirs, to benefit at best of suitab le topographic conditions, could satisfy a maximum of firn1 power demand yet allowing the fulfillment of nearly unaffected management program of the large regulation schemes. Experience gathered in conducting the simulatio ns shows that the choice of the reference period can result affected by the particular alternate wet and dry sequencies of years which characterize the climate prevailing in the study area and in other regions of Morocco. In this respect an equa l number of similarly representative wet and deficitary years should be se lected as a reference period for simulations under normal hydrological conditions. Conversely, a number of wet or dry years in excess cou ld significantly affect the overall water balances. Variations of the return coefficients can determine sign ificant changes in the evaluation of the water resources sti ll available. Therefore care has to be taken in selecting such coefficients with particular concern for the irrigation sector. A steady decrease of the returns is admitted in accordance with the forecasted progressive shift from traditional to more efficient irrigation methods and the introduction of other rules for water conservation.
to
MODELLING OF AN INTERZONAL MULTIPURPOSE WATER MANAGEMENT SYSTEM IN MOROCCO M. Jellali*, T. Elafti*, G. Simeoni** and G. Mondada** *Dirf(lomlf
of
IralPr Rl'souras, .\fillislr),
0(
Pulilir Il'tll-k~, Ra/wl,
.\101'0((0
**Wall'l" Rfsourcl's Dl'pa 1'11111' 11 I, .Holor Co/ulII/ms COlI.\u/lillg ElIgillfl'l"s fll(', Hadi'll , 5 ll 'ilzl'l"/a lid
Abstract, Optimum schemes for conjunctive ground and surface water development have been defined for some river basins in the northwest of Morocco by means of a model capable of simulating the management repercussion of selected integrated water development systems at a given target year. The study area includes three major river basins (Sebou, Bouregreg and Oum Er Rbia) as well as some intern1ediate minor basins, totalling a surface of I 07000 km~, The population of the area in the year 2020 is expected to rise to more than twice as compared with the present, while the irrigated lands could expand from 500000 ha to some 800 000 ha within the same span of time, Accordingly, the overall yearly water demand is estimated to increase from 4000 Mio m 3 to some 9400 Mio m 3 to the project's target year 2020, thus prompting the local water authorities to undertake a master plan study for integrated water resources future development. The project included preliminary sectorial studies to assess surface and ground water resources, to make population and water requirement projections for water supply and irrigation and to define an optimum scheme of structures for water developement. The simulations led to the preselection of several altemative water resources developement schemes, In developing the alternatives. sensitivity analysis were applied to a basic solution defined in accordance with existing development plans and taken as a reference situation. Keywords: water management
INTRODUCTION The norhern basins of Atlantic Morocco benefit from relatively sizeable water resources as compared with the eastern and southem semi-arid to arid regions of the country.
agricultural development perimeters. Small scale agriculture areas are also disseminated along river flood plains and in other valleys and plateaux (Fig.2).
Yet, an increasing population and the need to expand agricultural areas in order to meet food self-production requirements, has motivated the local authorities to undertake a masterplan study to optimise the integrated water resources management of the most important river basins of the Atlantic Morocco and a series of minor coastal basins in the southwest.
The study'S final pupose is to define optimum schemes for conj unct i ve surface and water development in accordance with the socio-economic requirements and constrainls. To help the preliminary selection of a number of suitable alternative water development schemes, a mathematical model aimed at simulating multiple schemes water resource management has been used.
The population of the study area, which represented about 50 pecent of the country's total population in 1982, is expected to increase to more than twice the present level by the target year 2020. Still in 1982 the share of the rural population of the area was 43 percent of the total rural population, thus reflecting the general configuration of the country's economy, still greatly based on agriculture. This explains the presence in the area of some of the largest
DESCRIPTION OF THE STUDY AREA Geological and hydrogeological features The study area includes the catchments of the Sebou, Bouregreg and Oum Er Rbia rivers as well as a series of minor coastal basins, totalling a surface of 107 000 km 2 in all.
SAMW- K
1-1')
150
1\1.
Jellali
et.
at.
~
SE"
1
R I VE l; ~ Ou N D t. R ¥
OF "..t.JOR 8 AS'"
MAJ OR AO L'IFE'" Ol SCHARG[ AT THE ~ O UiH
P8PJ L loTIO '" t rp! ,£Ob.:? ,
,.
; 'OU JDOIJ ~ C
r- ------- -- - - - - ·
.." ,
Fig. 1 Location of the study area The primary basement is overlain by a thick sedimentary series, secondary, tertiary and quaternary in age. Distinguished in the lithological series are the limestones of the lower Lias in the Meknes-Fes region and of the middle Cretaceous (Turonian) in the Plateau des Phosphates - Tadla plain area, and constitute extensive and economically exploitable deep aquifers (Fig.2) . Plio-Miocene sediments of continental to marine-continental facies were deposited throughout the Pliocene and the Quaternary ages to constitute the Meknes-Fes plateau , the Tadla and Rharb plains as well as a series of superficial, detrital to biodetrital-licke variably thick deposits in the coastal plains. These Plio-quaternary sediments , distinguished by their generally fair to good hydrogeological characteri stics, const itute extensive aquifers, mostly in water table conditions, easily exploitable by means of wells. These aquifers constitute the major resource of water supply for rural areas. smal l towns and isolated industries as well as for sma ll scale irrigation system s. Hydroclimatological background The annua l average rainfall (period 1932-1984) exceeds 1000 mm in the summits of the Rif and Atlas mountains which bound the area northeast and eastward. Elsewhere, annual rainfall decrease progressively southwestward. Averages of 500 to 600 mm/year are registered in the northern plains (Rharb -Meknes-Fes). Minimum averarage rainfall occurs in the low Oum Er Rbia river basin with less than 300 mm/year.
The regional hydro-climatic events are quite well explainable but are rather unpredictable, al th ough they are characterised by 13 to 15 years of alternate rainy and dry series . The year 1986 marked the end of a deficitary period which started in 1972 and which assumed the characteristics of a serious drought between 1980 and 1985. WATER DEMAND AND RESOURCE DEVELOPMENT The demand The irrigated lands, which totalled some 500 000 ha in 1982 are expected to expand up to 800 000 in the year 2020 thus requiring 73 percent of this target year total forecasted water requirement for the study area. Present situation The developed groundwater resource in 1982 accounted for 57 percent of the water supply and for 16.5 percent of irrigation total water requirements, indicating a pronounced dependability of agriculture on surface water. At the reference year (1982) there were in the study area thirteen operationa l dam sites (Fig.2). Of these, seven are impounding mUltipurpose structures with a total live storage of 5650 Mio m 3 . The average annua l recharge of the thirteen most important aquifers existing in the area is of 1300 Mio m3, while the total net consumption of groundwater in 1984 is estimated to be of the order
l\loclelling of an lnterzonal Multipurpose ' ,V ater :'-Ianagement S\stem
.. d
151
,_. ,. _ . _ -,-
=:J "'- ' _ . ::::J ::::J ,.- _- ,'- - ;::JI
...._ - -
Fig. 2 Layout of the agriculture and water resources development plan of 660 Mio m j /year which represents some 77 percent of the ground water potential needs in 1982 (860 Mio m 3/year) and an overall return coefficient of 0.23. Developme nt program In Figure 2 are indicated the projected dams and other hydraulic structures which would have to be consrructed up to ne ar the year 2020 in order to insure a maximum development of surface and water resources. In se lecting this development scheme, which includes a total of 23 dams, and some transbasin canals , particular emphasi s has been given to the geographical distribution of the future water demand . Very pre liminary economical evaluations of each dam has been done by means of a simulation program described herea ft e r, in order to define in each sub basin, the most feas ible structures. Of the large-scale agricultural development areas in the course of imple mentation or being implemented, two (Doukkala and Haouz) are located outside the southwestern boundaries of the study area while two others (Tessaout amont and aval) lie acro ss the boundary, thus requiring transbasin canals. DESCRIPTION OF THE MODEL The program RIVER is conceived for simulating the simultaneous management of a rivers and reservoirs system with the purpose of developing water resources for water supply and irrigation needs, energy production, navigation , sanitary flow and flood control. The model allows for verification if a pre se lected reservoirs system with given characteristics and constraints can respond to pre requisites performances.
By adjusting in successive runs the data parameters , the optimum characteristics of the system are finally obtained following a trial and e rror prodecure. The model is composed of various interconnected elements (blocs), physically re presenting the simulated river ne twork. Basic elements in each system are the natural discharge and the blocs (Fig.3). The latte r consist of the following: the rese rvoir an hydropower station downstream (if any) a tributary three intakes two gauging stations locat e d upstream and downstre am of the res e rvoir. A groundwater aquife r is simulated as a reservoir of given characterist ics.
For each bloc , at the end of a given simulation run, the results are expressed as follows: the water level in the rese rvoir the discharge at the control points the di scharge out of the res e rvoir, through the powerplant and the spillway the fim1 and secondary energy production the modified discharge at the downstream control point the amounts of water alloted to irrigation and water supply. The model's calculation sequencies proceed from upstream to downstream unless different priorities are set by means of introducing data sequencies.
152
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Fig. 3 Stru cture of a simul at ion bl oc SIM ULATIO N C RITERIA Fundamental hvpothes is The whole area has bee n di vidcd int o 14 sub-bas ins (Fig.2) most of whi ch arc controll ed downstream by a dam . T wo supplementary sub-bas in s (E D and EM) re prese nt a reas o f wa te r de mand out of th e catchments. At each sub-ba sin outl ct point , monthl y di schargcs have bee n reconstitutcd for the 45 yea r pc ri od of available hydrol og ical data (1 939 -40 to 1983 -84 ). C o ns id e rin g th e exce pti o n a ll y de fi c it ary hydrolog ical conditi ons ha vin g prcv ail ed bctwee n 198 I to 1984. the whole se ri cs is co nsidc rcd as defi citary. Th e 43 ye ar long pe ri od 1939-40 to 1981-82 is taken to simul atc a normal hydrolog ical se rie. Bas ic pa rame te rs to take int o consid e ratio n for resou rces managemcnt are defincd as : the firm annu al rcg ul arize d vo lum c. whi ch
The latte r fac tor is the mos t imp ort ant co nstraint a nd is fund amc nt al for th e plann e rs dec isi on makin g. It is thu s important to defin e for eac h potentail wate r use r. reg ionall y appli cable defi cit criteria with respcc t to the normal (43 yea rs) and dcfi cit ary (45 yea rs), hydro log ica l sequencies . It is ass umcd th at th e ann ual de fi cit of the na tural reso urcc whi ch can be eas il y tolerat ed without any res tri cti on is o f 10 and 15 pe rcc nt fo r water suppl y a nd agri culturc res pec ti ve ly. On the othe r hand , cxperience shows that th c accep tabl e occ urence of defic it ary yea rs is one in ten for wat cr suppl y and onc in fi vc for irri gati on. Ma ximum to le rabl e defic it s for wa tcr suppl y in case of a drought within a givc n pc ri od have also bee n defin ed with respect to loc al ex pc ri ence (tow n of Marrakcc h) and se t to 20 % and 25 % for norm al and deficitary sequ encc respect ive ly.
The de finiti on of max imum acccptabl e de fi cits for ag ri culture call. fir st of all. fo r a di s tinc ti on bctwee n la rge scale and small sc al c irri gati on areas. r or large sca le agric ultural pc rimetc rs, whi ch are direc tl y o r indirec tl y und e r th c Go ve rnme nt a uthorit y, rulcs can cas ly be dictated. eve n on a yc ar-by-yea r bas is. Crop patt ern confi gurati on can bc mod ulat ed in such a way to enable the fl ex ibility o f a give n area to bc adap ted to a pa rti c ul ar hydro log ica l situ ati on. Relati ve ly limited reso urces ava il able in the Oum Er Rbia bas in , in duce planne rs
Table 1 Defic it criteria applied to the stud y area to detemline available resource Deficit crite ria Normal peri od
Deficit ary yea r
Water suppl y def. =I O%
Use r Irri ga ti on
Refe rence peri od
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45 y.
Max imum occurrence of defi citary years
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32%
43 y.
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Fi g.4 Basic c rit e ria ~ for simulatin g basin wate r management with the program RIVER to minimi se permancnt and se llli-pcrm ~ lI1 e nt crops to 37 pe rcen t of the culti vate d land and in creasc fl ex ibilit y. with eve ntual suspel1Si on o f winter ClOp S. to ri se to 50 percent th e acceptan ce of the ma ximum deficit of supply in case of defi citary period. For small scale agricultural schemes. rural and di sse minat ed wa te r sup pl y. whi ch arc more frequently found in the upper ca tchm ent area~ of a given basin (F igA ) it is known that the use rs deri ve directl y the reso urces for the ir nee ds frol11 springs. temporary diversions. dug \\·e ll s. irrespec tive of any res triction rule. Limitati ons arc se t by nature it se lf in case of d ro ught eve nt s. With rega rd to the simulations. such short cut s of \vat er reso urce are re fl ec ted as a cut of an equi vale nt amount of th e "natural " monthl y re so urc es available do\\·nstrea m. Return coe ffi cient s In planning wa te r reso ur ces development. di stin cti on ha s to he made between tota l net demand (consumptive use) and the total reso urce (g ross demand ) mob ili ze d to sati sfy the forecas ted needs. Th e water non evapotran sp iratecl by crops as \\ell as dome stic and industrial sewe rs return s. treat ed or not. in the natural cycle within a generally limited span of time. to he reuse d.
APPLICATION EXAMPLE The chose n exa mple is that releva nt to th e Oum Er Rbia ba sin (Fig.5). whe re th e return s of surface irrigati on wate~r play a subs t:lIltial rol e in the hy (lraulics o f th e s urfa ce and grouJlc!water resources. The final possible development stage program of the Oum Er Rbia ba si n would co nt emplat e a towl of 22 dam~. say 12 in add iti on to the ten presently in ope rati o n. and a 1l1:l xill1l1l11 of three tran sfc rt s for irriga ti on Basica ll y. th e simulations consist of de fining the required withdrawals at each of the simulated bl ocs with an exce ption for the large Al Mas sira dam. \\'hi ch cont rol s the downstream part of the syste m. Discharges obtained at Al Mas sira. compared with th e mode l-g e ne rat e d flo ws. allow for the construction of the management curves of maximum deficit displayed in Fi g.6. For the multipurpose dam. th e primary concern is to res pect deficit criteria for water sup pl y. It appears that minimum annual water development can be granted with refe rence to th e deficitary pe riod (45 years), altough for thi s case the tolerance for deficit is ri sen to 25 percent.
In the non coastal basin s. the admitted re turn coefficients vary fr o m 0.28 for larg e sca le ag riculture de ve lopm e nt. to 0.8 for isolat ed indu stri es.
Rese rva ti on of wa te r to respect the pro du ction of firm po we r does not influ e nce s ignificantly the regulatin g capac ity of the system.
In order to preserve possibilities for wa te r use in the lowe r subcat chm ent s. minimum standard qualit y requirement s are attained by granting an adeq uate minimum sa nitary di sc har.!:'.e in the ri ve rbeds.
The appli ca tion of res tri ction criteria based on the all oted return period of one deficitary year out of te n (4 .5 years in 45) , shows co nsi ste nt results (Fig.6).
154
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Resource development and demand comparison
AMENAGEMENT ,:
OUM ER RBIA
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DEFICIT MAXIMUM PENDANT 4~ ANS
The comparison between water resources a!1d needs is firstly conducted at the level of each sub-basin. The basis for comparison are: to define for each sub-basin the total gross demand, taking into account the historical evolution of the water needs and the necessary sanitary discharges
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to define the net water consumption, by taking into consideration the return coefficients and the possible water transfers. Finally, a water reso urces balance is established for each basin.
In a first instance, the above described procedure is applied with respect to what can be called a reference development scheme. Thi s results from taking into account the following major hypothesis. '---c~--«;;:-;;---
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Fig. 6 Water resources management curves for the Qum Er Rbia river basin development scheme
For agricultural development areas : the maximum of potential development areas according to the govemment plans the water requirements estabilished by the relevant authorities or the water rights traditionally admitted.
In order to evaluate other possibilities offering eventually a better match between water demand and resource availability, alternative regional development schemes have been taken into consideration. With regard to agriculture, alternative developme nt schemes have been emphasized as follows :
\Iodcliing or an I ntcrzonal :\llIltipllrposc Water \ianagement S' "stcm
a) contemplating a cut of some components of the agriculture development including implementation slow - down b) giving priority to the development of small and medium schemes to the disadvantage of the largest, less flexible irrigation areas c) increasing the development areas located outside the basins d) introducing increasing ly efficient irrigation systems (sprinkler, trickle) , to reduce water losses. Other alternatives would contemplate: e) supplementary water transfers from northem to the southern catchments
the
f) to encourage preferential human settlements and consequent urban development in the northern basin where resources are relatively aboundant thus reducing the necessity for costly transfers structures. CONCLUSION Calibration and operational simulations to define optimum altemative schemes for conjunctive water resources development can be satsfactorily conducted on a monthly step simu lation base, over a reference period of 43 to 45 years.
155
Rules of management of water resources by means of large storage reservoirs are primarily set to satisfy most urgent present and projected needs for water supply and irrigation. However, hydropower schemes, when optimally located upstream of the large reservoirs, to benefit at best of suitab le topographic conditions, could satisfy a maximum of firn1 power demand yet allowing the fulfillment of nearly unaffected management program of the large regulation schemes. Experience gathered in conducting the simulatio ns shows that the choice of the reference period can result affected by the particular alternate wet and dry sequencies of years which characterize the climate prevailing in the study area and in other regions of Morocco. In this respect an equa l number of similarly representative wet and deficitary years should be se lected as a reference period for simulations under normal hydrological conditions. Conversely, a number of wet or dry years in excess cou ld significantly affect the overall water balances. Variations of the return coefficients can determine sign ificant changes in the evaluation of the water resources sti ll available. Therefore care has to be taken in selecting such coefficients with particular concern for the irrigation sector. A steady decrease of the returns is admitted in accordance with the forecasted progressive shift from traditional to more efficient irrigation methods and the introduction of other rules for water conservation.