MOSAH, An Agro-hydrological Simulation Model

Copyright © IFAC Systems Analysis Applied to Water a nd Related Land Reso urces. Lisbon. Portugal. 1985

MOSAH, AN AGRO-HYDROLOGICAL SIMULATION MODEL G. Galleguillos, G. Mendez and A. Lucchini Departamelllo r/I' Ob m; Cil'iles, L'lIil'l'rsir/ai/ Terllim Fl'Ill'I'iw Sall la ,\faria. \'alpar(liso. Chill'

Abstract . A simulation model considering the relevant factors affecting agricultural development in a system conformed by four important Chilean valleys is discussed . WOSAH , the simulation model , uses on a monthly basis , information about surface and groundwater hydrology, climate and land resources, irrigat ion water works and , also, current and projected crop patterns and water distribution legal regulations. The economic assessment of farmers net income of an irrigation district is based on the determination of agricultural production and sowed and harvested areas . A water distribution strategy is used to supply water irrigation demand , under a scarcity situation, upon the results of a parameterized moisture balance in the irrigated land, applying a criterion that represents a trade-off between crops profitability differences and local land property distribution . A MOSAH's application example is put forward to demonstrate its wide use characteristics and its effective potential as a valuable tool for planning and decision making processes . Keywords. Agriculture ; computational methods simulation ; system analysis; water resources .

large-scale

systems

models

B&P- HTS-cNR , 1983) . the The criteria for modelling of the physical phenomena in the irrigated areas have been adopted from a Stanford- type simulation mobel , adapted by L. Stowhas (1979) .

I NTROCX.X::TI ON

The purpose of increasing crop production in a valley , implies the necessity of analyzing a system conformed by social , economic and physical element , characterised by a great degree of interdepe ndence .

CONCEPTUAL FRAMEWORK The information flow on a monthly basis , by shown in Fig . 1. Due most relevant issues are

Starting from a current situation of utilization of natural resources of land, water and climate by farmers and related organizations, the projects that might be conceived must study the potentiality of alternative hydraulic works to support more attractive crop patterns development, to increase irrigated areas and to suplement adequate water supply safety .

feeded and produced , MOSAH is schematically to space reasons , only described next .

MOSAH is designed to handle the required information to determine the annual net incomes of farmers in each irrigation district . This determination is obtained by calculating the economic value of agricultural products minus the corresponding operational costs, and also subtracting the failure costs that might take place due to sowed but not harvested areas . These computations are done for every crop . A harvested area is considered to be the minimum irrigated area of a crop after the application of the monthly water distribution strategy , as discussed further . The model makes an estimation of sowed areas as a function of harvested areas; the relationship has been obtained from the studies performed about farmers' sowing adjustment, considering their hydrological expectations after the rain and snow accumulation period . The sowed area of each crop is determined as the harvested area, if this is greater or equal than half the total area assigned to that crop in an

This implies also to consider problems of encouraging the conceivable agronomic development by providing technological, entrepreunerial and financial facilities that are needed as factors allowing to reach productivity levels and production goals high enough to justify the investments, normally high , that must be done , mainly on water works systems . One extended tool of analyzing the economic impact and other issues of such complex development projects is the methodology of mathematical simulation . WOSAH, the simulation model presented in this paper, has been built using the information a vailable for the project area from a study made by a Chilean- British partnership (CICA-

21

22

G. Galleguillos, G. I\Iendez and A. Lucchini

irrigation sector ; and as a hal f of the total area if the harvested area , in a year , is these concepts less than that . Obviously , are not applicable neither to pernanent plantations nor to natural meadows . Concerning WOSAH ' s input information, as shown in Fig . 1 . some data sets are directly feeded into it (hydrometeorological data, water works characteristics, water distribution factors , etc . ) . The rest of the information needed is produced by previous analysis and treatment of different relevant factors. Crops water demand . Monthly consumptive use by crops is determined multiplying the potential evapotranspiration by the crop coefficient . The first term has been computed in agreement with a climatic zonification by means of Blanney & Criddle formula. Crops pattern determination. This is a very fundamental informat ion ; its current and projected configuration is the resultant of the confluence of various factors . These are those indicated in Fig. 1 . The possibility of reaching a more attractive crop pattern, increasing for example the areas of frui t trees or high gross margin crops , in general , is conditioned by safe water availability due to new water works and by the knowledge of land and climate conditions. In addition to these physical characteristics, those associated with socio-economical local conditions must also be considered . It is clear that technological levels, entrepreunerial capacity , technical assistance, credit facilities , farmer organization levels and marketing circumstances are items that have contributed to the occurrence of current crop patterns and that must be considered in an agricultural project . Main water works development projects . Preliminary studies of different dam sites , aquifer exploitation by pumping and waterways as project alternatives for providing necessary water supply demanded by agricultural developments , are the basis for the defini tion of the fundamental design variables of the schemes whose performance can be tested by the simulation model. WOSAH WODEL ' S DESCRIPTION The Study Area The WOSAH model was built looking for adequately represent the agrohydrological reality of some Chilean valleys . The study area is limited by 32° - 33° S . Lat.; and 70° - 71° 30' W. Long . (Fig . 2) . The study area covers 12.470 Km' , including the basins of rivers Aconcagua : 7 . 183 Km', Putaendo: 1 . 343 Km', Ligua : 1 . g97 Km' and Petorca: 1.947 Km' . The agricultural land covers a surface of 81 . 861 Ha ..

General Model's Basis The WOSAH model has been conceived as a multiple set of fundamental interconnected elements . Each element is governed by well defined equations and relations . are water distribution nodes, The elements river reaches between nodes, interbasins canals , irrigation districts , dams and aquifers . Each element represents one or more specific hydrological processes which take place inside the system . Therefore , each of them is governed by the water balance equation (continuity equation) , in a time interval. The planning purpose justifies choosing a time interval of one month , and also, spatially concentrated parameters; this implies the assumption that the hydrological processes, occurring in an element, are concentrated at one point . Elements Conceptual Simulation Basis Node . It concentrates surface water contribu tions and distribute them among different downstream node users . Legal aspects and intake capacities must be considered . Water surplus runs downstream through rivers and creeks . Interbasin canal . It simulates water transport from a point to another , into a valley , or among different valleys . The flow at the downstream point is calculated as the intake flow , minus conveyance losses . This last figure is obtained through a factor less or equal to unity , that multiplies the intake flow . Dam . This element regulates water flowing into a reservoir . Input and output var iables are governed by the continuity equation . Aquifer . It acts like a groundwater reservoir that regulates groundwater flows. It is ru l ed by the continuity equation under the assumption of linear behaviour . This means that it exists a direct proportionality between groundwater exit flow and storage water volume in an aquifer . The linearity constant has a parametric value . This element also considers the depletion efect upon exit flows due to upstream pumping in the aquifer . This computation is done by means of a depletion unitary function that gi ves month ly depletion along time , due to a unitary constant pumping during one month. The function has been obtained from groundwater models of main system aquifers . By superposition , the effect of a given pumping sequence can be obtained . Irrigation district. This is the main element of the system . The area called irrigation sector is divided , in fact, into an area of non irrigated land and an area of irrigation agriculture . Non saturated zones in both areas have been modelled as water tan\s with a maximum water depth , a parameter called

\IO S.-\H HW\X .. '. ;sed in .'Ia te r'

Fig . 3a SOws the tr,e i '-r-igated a r ea .

t::>alance

equa t ion

for"

conceptual scheme The correspond ing this tank can be

:"ri tte n as : (11

:. Ile ,-e HF IS tre soil O1ois ture at the end t"le olontl' ; 1-'1 is the initial 'Tloisture; HLL 1S the infilu-ation f r oro precipitation ; HR is the irrigation infiltration (HR= O i n tile unirrigated a'-eal ; and HeR is the actual eVdPot ranspiraLon . The eve nt ua l excess of HF upon H'1AX 1S inco q ,o ,-ated into the aquifer . Fig . 30 s"o~s the i nout and exit resource s tlH t pal'ti'2ipate in the global wat e r balance of the i"r' igation dis tn c t. In that figure QDBB is the maximun intake flow for irrigation , QBCM 1S th e pumped flow for irrigatio n, PREC i s the p,-e ei l' ! tation ; CR 1S the total actual ev ar,o transr,irati o n; QRET is the total surface r'e turn flow , composed by drai nage flow from precipi tat1 0 n a nd irrigation returns ; QPP is the percolation t o aqui fer, composed by chan ne l s percolati o n , deep pe r colation fr om p r ecipi tati o n a nd by the ove,-flow from modelation tank s prev iously described ; AT is the total area o f the sec t or and H i s the moisture of non saturated so il l a ye r s .

saturation a d i rec t evapo ration is assumed t o from the soil , whi c h is lesser than the water potent ial evapo trans pi ra t ion . Nevertheless , as tile uppe r soi l layer- s become drier , it has been stated that the evaporation decreases rapidly and lineally w1t h humidity until ze,-o . ~' athemat ically, EDR =K ' ETP ' (HNULl /( HMAX-HMUL II , if H>HNUL

,~ r

The precipitation balance equatio n is : HLL=PRE C- EI - PPLL

(2)

where PREC is the total rainfall ; El is surface runoff and PPLL is the dee p percolation from rainfall . El and PPLL are determined as functions of related parametric thresholds and soil moisture l evels . The water comsumpt ion mode 1 used cons iders c rops consumpt ive - use and direct evaporation from the ground . As long as the existi ng moisture level in t he no n saturated layer is greater than a c ritical mcisture value - somewhat greater than the c rop permanent vlilti ng point crops evapo ra te at maximum rate . In the oppos i te c a se , the evapotran sp i ration rat e stro ngly declines as the mcisture decreases approaching the permanent wilting point , where crop consump tion 1S assumed to be ze r o . Jathematica lly, (31

UCR=UC TR , if H> HCRI T

EOR=O , if

H ~ HNUL

(4 )

where EDR : actual di rect evapotranspiration ; ><:-1' 1 is the pote nti al evapo tran spira tlOn ( ETPI reduction factor and HNUL : moisture va lue that makes evaporation null (pa rameter l . Real consumptive use is defined as the greatest value from both effects . The comsumption model is graphically shown in F1g . 3c . Finally , the irrigati o n water balance equations are put forward . The water volume needed to be artificially applied In the irrigation area is called irrigation effec ti ve demand (DERI . It is determined as f o llows : DER= (UCTR- HLL)/EFS

(51

where EFS is the parameter representing irr igat io n ef fi cie ncy at an irrigation sector . The available water flow to be used i n a district is the corresponding intake flow minus the conveyance losse a nd the non used water by unavailability of e nough night regulation facilities . He nce , QDS~QDBB ' EFC ' ALFAR

(6)

where QDS : available flow ; efficiency (parameter) ; ALFAR : efficiency (pa ramete r ) .

EFC : conveyance night regulation

Eventual water defic i ts are d ete rmined through the difference between the effective demand
deficit

(71

at

the

irrigation

UCR= UCTR ( 1-E XP( H/( HCRIT - HIII , if H(H(RIT QU=AMAX1( O. , (HI - HCRIT ) ART/EFSI whe re UCR : a ctual evapotranspiratlO n ; UCTR: potential cons';l'1ption , this value 1S equal to the or od~ ct C C the crop coefficient and the Gotential evapotranspir ation; H' humidity ana HCR IT : h UI'1 ~ d ~ ty that restrains the COnSJ'np tive L;se (::;ara"le ter) . In the non ~ rr igatea area a const ant crop coeff ic ient C><:NT , has been used a.s another model parameter . Otherwise ,

if

soi I

is

saturated

Or

(8)

where QU : initial utilizable moisture; ART : total area demanding water during the month . Determ ination

of

monthly

irriga ted

area

of a

crop . As it has been stated previously , this is a very fundamental issue leading to the determi nati on of the year l y agricultural production .

near If

mon thly water

deficit

DEFS

is

zero ,

then

24

G. Galleguillos, G. Mend ez and A. Lucc hini

all crops are irrigated. On the contrary , if DEFS is greater than zero a scarcity situation emerges and a water use strategy must be applied within an irrigation district. Briefly, the strategy consists of introducing a reduction in the irrigation area of some crops to fit with water availability . The area reduction is performed sacrifying crop areas in an order established by their relative profitability at the moment of the deficit. First, the reduction affects natural meadows which is the less profitable crop. I f deficit still persists, half annual crop areas are sequentially reduced, and if necessary the other half, looking for the adjustment to remanent deficit. If this has not still been enough , permanent plantations are reduced following the same criterion. MODEL APPLICATION EXAMPLE The simulation model MOSAH is applicable to the study of different configurations of dams , canals and pumping station systems in the valleys under study. As an example of model application, the one performed in the Putaendo Valley, where the profi tabili ty of agrohydrological developments is relatively high, is described . Putaendo valley has been modelled with 3 nodes, 2 river reaches , 2 irrigation sectors and one aquifer. The water currently available for irrigation is taken from the Putaendo river. Upper farmers have rights on a 5~1o of natural river flow, lower sector farmers may use all the remanent flow . new crop The project considers to reach a pattern, using the best land resources to extend permanent plantations (apricots , peaches , vineyards, etc.) and increasing agricultural productivi ty by intervening on raising up the technological level , thus also reaching higher economic gross margins .

MOSAH allows the determination of the economic and hydrological valley response derived from different combinat ions of supposed ly independent actions on the Putaendo valley . The combinations studied are put forward in TABLE 2. TABLE 2

Current Situation and Project Alternatives Combinations

Symbol CCP/CTL- NP-CIW CCP/CTL- P-CIW CCP/CTL-NP-FIW CCP/CTL- P- FIW FCP/FTL- NP-CIW FCP/FTL- P-CIW FCP/FTL-NP-FIW ECP/FTL- P- FIW

Crop pattern Pumping Irrigation & Tech . level works current no current current current yes current no future future current yes future no current future current yes no future future future future yes

Figure 4 shows the present value of agronomic income obtained from the updating of a constant annui ty which is equal to the expected value of the agronomic income; the discount rate used is 1~1o . The figures correspond to the 8 combinations described above. Figures 5a and 5b represent the corresponding hydrological responses associated with the economic impact calculated on the basis previously mentioned. The economic figures exhibited do not comprise investments nor operational costs derived from civil works, highlighting the importance of projected changes in crop patterns and farmers gross margins, which are less costly than water works, being the complementarity of such issues really not so high as it is sometimes stated . Central agencies can find the way to supply economic safety , instead of water safety, being this a matter whose benefits must be studied. COMPUTATIONAL FEATURES

A pumping station is projected to serve the lower sector; consequent ly, upper sector water rights might be increased to 10~1o. Canals network is improved by means of a projected increase of canals conveyance efficiency (EFC) and capacity (CCR). The project also considers an improvement of irrigation efficiency (EFS) and of night regulations facilities of the water (ALFAR) . The changes adopted are shown in TABLE 1 . TABLE 1

EFS EFC ALFAR CCR

(Par . ) (Par . ) (Par . ) (Var.)

Current and Projected Parameters and Variables Current 0.48 0 .8 0.9 8 . 2 m'/s

Projected 0 . 55 0.9 1.0 9.2 m'/s

Computer code. MOSAH is conformed by 22 subroutines which are operated by a main program that controls execution . The computer code is wr i tten in FORTRAN 77 and the mode 1 has been run in a Digital computer DEC-2020. Model parameters. The model uses 21 parameters whose values are obtained from a calibration process that allows its adaptation to observed information available. 17 parameters are used in the irrigation district subroutine . An 11 years period has been used in the calibration process, 4 years data have been used for validation purposes and the simulation applications have been run over a period of 35 years input information . CONCLUSIONS The

methodological

and

computational

efforts

25

M OSA H carried on to represent the relevant variables that characterize an agro- hydrological system have been briefly described . The amount of information that must be handled and the crytical analysis of model ' s performance - calibration and application results- seem to justify these efforts.

for a better understanding of this complex system and for the visualization of development alternatives, and its effects. ACKNOWLEDGEMENTS This paper is originated in a research performed under the financial support of the University Federico Santa Maria . The Chilean Irrigation Comission has contributed with the information needed . The authors are also grateful to Pablo Isensee and Ludwlg Stowhas, who contributed wi th their experience and knowledge to better attain the research work goals .

The relationship between agricultural products quality versus the amount of irrigation water applied and irrigation techniques used has not been considered . The accuracy level pursued , in relation with the information currently available, explains this decision . In the future , if information affords it, this and some other issues, e . g. aquifer linearity , stream-aquifer interaction , could be treated differently with some advantage .

REFERENCES CICA , Binnie & Partners, Hunting Technical Services, Comision Nacional de Riego de Chile (1983) . cstudio integral de riego de los valles Aconcagua, Putaendo, Ligua y Petorca . Vols. I, ll , Ill , l V, V and VI . Republica de Chile . Crawford , N. H. , and R.K . Linsley (1966 ) . Digital simulacion in hydrology: Stanford Watershed Mode 1 IV . -,-T:;e"c::.h::.n::i:.::c:.::a::l~-.:R~e:!p:::o:::r~t=---,-N-,-u_~3",9,,-,-,--,D",e=p::.t.:.. of Clvil Engineering, Stanford University. Stowhas, L. (19191 . Retormulacion de un modelo de simulacion hidrologica para la hoya del rio Aconcagua. Instltuto Nacional de Investigacion de Recursos Naturales de Chile .

The paper contribution is thought to be in the description of the decision rules leading to the output information that has been looked for , as well as , in eliciting the application possibilities of MOSAH to -the analysis of various scenarios that can be conceived for an agro- hydrological system . The applications results obtained until now suggest the needing of a careful selection of the model parameters and input variable information to make a valuable use of this tool for planning and decision making processes . I n any case , MOSAH' s building and use by the authors have shown to be an effective means

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fl ows In

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1 - - - - - - - --------'

WCR KS STuDI ES

Fig . 1 . MOSAH ' s main information flow.

Agrn:ulturOI

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Q1C1:l~c! l on

So wcf ar . as Wat er d . flclts :montl'll.,1

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26

G. Galleg uillos. G. Me ndez and A. Lucc hini

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Fig . 2 . Study area map .

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FIW·NP

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Fig . 4. Total net income of alternatives .

1.5 . - - - - - - -_ _ _ _ _ _ _ _ _ _ _ _ _--,

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Fig . 5 . Duration curves of total net income .

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