Space - time analysis of water deficit

Transcription

1 European Water 3/4: 3-8, E.W. Publcatons Space - tme analyss of water defct G. Mendcno and P. Versace Department of Sol Defence, Unversty of Calabra, Ponte Petro Bucc Arcavacata d Rende (CS), Italy Abstract: Keywords: The paper descrbes the man characterstcs of an ntegrated system amed to predct space-tme varatons of water defct occurrng n southern Italy. The system has been developed wthn the European INTERREG IIC Programme. It, usng GIS technologes ntegrated wth tele-meterng network, hydrologcal databases and dstrbuted water balance models, allows to analyse dfferent aspects of the wder drought phenomenon. The ntegrated system heren proposed represents both an orgnal methodologcal approach and a sutable tool to determne the crtcal areas charactersed by water defct that for the analysed regon they appear more frequent and wder. Drought, GIS, Water Balance 1. INTRODUCTION In the last decades water defct problems rse n dfferent areas of the world, and many of these have become by now extremely ntense, reachng stuatons of dramatc crss. Ths scenery seems to be worsened by the progressve ncrease of drought observed n varous southern Europe areas. In ths context, numerous ntatves have been promoted by the European Communty to face the effects nduced by drought on the member States. Among these, the man ntatve s the INTERREG IIC Programme, whch has permtted the actuaton of several transnatonal plannng programmes, and ncludes among ts objectves the development of a strategy for the preventon and co-operaton n facng the reducton of water resources due to drought. Specfcally, wthn the sub-programme Analyss of the hydrologcal cycle the southern Italan regons have been analysed n detal because of ther meta-stable clmatc regme charactersed by strong ntermttences of the water balance. For these regons, an ntegrated montorng system for the analyss and the forecast of the effects produced by prolonged water defct perods has been realsed. The system s made up of a commercal GIS (Arc-Vew) connected wth a Data Acquston System whch stores real tme data recorded by the Natonal tele-meterng hydrometeorologcal network managed by the Servzo Idrografco Mareografco Nazonale (SIMN). The system s runnng snce the begnnng of the year 2000 and the tele-meterng data are constantly updated and returned accordng to the nput requred by the smulaton models. These models allow the estmate of spatally dstrbuted hydrologcal quanttes, such as solar radaton, potental evapotranspraton, water defct. The paper, after a bref analyss of the man crtera used for developng the ntegrated montorng system, descrbes the theoretcal structure of the spatally dstrbuted water balance models, presents the analyses carred out for parameter estmaton, and dscusses some results obtaned through the smulaton of water balance n southern Italy durng the last fve years. 2. INTEGRATED MONITORING SYSTEM The ntegrated system developed n southern Italy for the analyss of the prolonged water defct effects s based on a GIS-embedded water balance modelng, drectly lnked wth a Data Acquston System amed to store real tme data recorded by the Natonal tele-meterng hydro-

2 4 G. Mendcno and P. Versace meteorologcal network managed by the SIMN (Mendcno and Versace, 1999; Mendcno, 2001). Ths network has been strengthened wth a great number of statons, most of whch are amed to measure specfc quanttes to be drectly used for the estmate of water balance of the southern Italy regons. For the whole southern Italy, all the geographc nformaton has been stored wthn the Arc-Vew GIS accordng to two data structures, respectvely vector and raster. All the spatally dstrbuted hydrologcal quanttes are elaborated, managed and stored usng a raster data structure charactersed by a 250 m square grd spatal resoluton equal to the one utlsed for the southern Italy Dgtal Terran Model. Hydrologcal coverages are drectly obtaned by lnkng the GIS wth the hydrologcal databases. Specfcally, hydro-meteorologcal nformaton s managed consderng two dfferent data types. The frst s based on daly hydro-meteorologcal values recorded by all gauge statons (hstorc DB) startng from The latter s based on real tme data recorded by tele-meterng statons wth a 20-mnute tme step (real tme DB). The hydrologcal databases have been developed usng Mcrosoft SQL Server software wth the am of makng easy the data management and updatng. The nteracton wth databases has been mproved realsng some Wndows-based software applcatons capable of guaranteeng a more smple and flexble data query, and allowng the user to extract synthetc graphs and reports. These nterfaces user-db can work both autonomously and drectly nsde the GIS by smple graphcal selectons on the gauge statons. These addtonal functonaltes are drectly embedded wthn the GIS through a set of Avenue scrpts. Specfcally, they allow to dynamcally query the dfferent staton types avalable n the databases, showng the results obtaned nsde the GIS. 3. SIMULATION MODELS On the examned regon, water defct s evaluated on a monthly bass usng a spatally dstrbuted water balance model. Ths model follows the orgnal approach suggested by Thornthwate and Mather (1955) and smulates sol mosture varatons, evapotranspraton and runoff on sngle 5 km regular grd cells usng data sets that nclude clmatc drvers, vegetaton and sol propertes (Fg. 1). Ths model does not consder horzontal moton of water on the land surface, or n the sol. The governng equaton s based on a smplfed mass balance: P = S + E + ΔW (1) where P s precptaton, E s evapotranspraton, S s water surplus, and ΔW s the change n sol mosture storage. All the quanttes are evaluated n mllmeters per month. Equaton (1) does not dfferentate between surface runoff and groundwater runoff, t allows to determne water surplus S as the water whch does not evaporate or reman n sol storage and s avalable to generate surface and subsurface runoff. The procedure schematses the sol column through a reservor whose maxmum capacty s gven by the sol-water holdng capacty WHC. The state varable representng the sol mosture at the end of the month s defned W and depends on the dfference between precptaton P and potental evapotranspraton PE values. If P PE then: W [ W + ( P PE ) WHC] mn 1, = (2) and sol mosture s recharged up to the maxmum value WHC. When W s equal to WHC, further postve values (P - PE ) are consdered as surplus S. Negatve values of (P - PE ) ndcate the amount by whch precptaton fals to supply PE requrements. In ths case water wll be wthdrawn from the sol mosture, resultng n an exponental sol mosture depleton, and the actual evapotranspraton E s less than PE. Actual evapotranspraton E equals PE when P > PE,

3 European Water 3/4 (2003) 5 otherwse t s equal to the precptaton P plus the change n sol mosture storage ΔW durng the month. When E < PE, then the dfference (PE - E ) represents the water defct D or the amount of water that would be suppled by rrgaton to the sol durng the month. Surplus S s hypothessed to be subdvded nto two quanttes, μ S and (1-μ )S respectvely. The former quantty μ S represents the monthly surface runoff, whereas the latter (1-μ )S descrbes the groundwater recharge. A monthly runoff coeffcent μ s locally determned startng from the Curve Number method suggested by the Sol Conservaton Servce (1968), and modfyng t to account for antecedent sol mosture condtons (Heatwole et al., 1987). Groundwater recharge (1-μ )S s used to determne the runoff delay caused by water transport through groundwater before t enters rver channels. If a lnear groundwater reservor s assumed, then ts monthly runoff detenton Dr (mm) can be expressed as follows: Dr Dr Δt S Dr β 1 1 ( 1 μ ) = (3) where β s the lnear reservor constant and Dr -1/β s the groundwater runoff. Groundwater runoff plus surface runoff μ S determne the monthly rver runoff Q. Potental evapotranspraton PE n the sol-water balance procedure s estmated consderng the Penman equaton (1948), modfed by Monteth (1965): 1 Δ PE = λ ( Rn G) ρ cp ( ea ed )/ ra Δ + γ ( 1+ r r ) c a (4) where PE s the evapotranspraton rate (mm d -1 ), 86.4 s a converson factor, γ s the psychrometrc constant (kpa C -1 ), λ s the latent heat of vaporsaton (MJ kg -1 ), Δ s the vapour pressure gradent wth temperature (kpa C -1 ), Rn s the net radaton (MJ m -2 d -1 ), G s the sol heat flux (MJ m -2 d -1 ), ρ s the atmospherc densty (kg/m -3 ), e a s the saturated vapour pressure at ar temperature (kpa), and e d s the actual vapour pressure (kpa), c p s the specfc heat most ar (kj kg -1 C -1 ), r a s the aerodynamc resstance (s m -1 ) and, r c s the crop canopy resstance (s m -1 ). Spatal dstrbutons of ths last resstance have been obtaned consderng Leaf Area Index (LAI) coverages derved from monthly Normalsed Dfference Vegetaton Index mages. Penman-Monteth equaton requres a great amount of observed data that s not always avalable on the whole regon and for long tme perods. A smplfed equaton for the estmate of potental evapotranspraton n southern Italy has been obtaned as follows (Mendcno et al., 2002): PE = a' PERAD + b' T + c' LAI + d' (5) where PERAD (mm d -1 ) s the radaton term of equaton (4), T s the monthly ar temperature ( C), a (-) s a constant equal to 0.63, b (mm d -1 C -1 ), c (mm d -1 ) and d (mm d -1 ) are seasonal parameters shown n Table 1. Usng observed data, equaton (5) has shown good performances equal to those obtaned through Penman-Monteth equaton. Furthermore, the smplfed equaton has allowed to extend the smulaton perod also n the past years where only precptaton and temperature data was avalable. Table 1. Monthly values of parameters b, c and d Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec b (mm d -1 C -1 ) c (mm d -1 ) d (mm d -1 )

4 6 G. Mendcno and P. Versace (a) (250 m) Precptaton Evapotransp. Water Holdng Capacty (5 km) Sol mosture (b) January Rver runoff August (c) Fgure 1. a) Southern Italy grd (5 x 5 km) used for water balance smulatons; b) Example of user-selected areas where monthly GIS-water balance procedure s actvated; c) Output data mapped by GIS 4. WATER DEFICIT Model performance has been verfed on few expermental basns n southern Italy before proceedng to the analyss of the regonal water balance. Further analyses have been carred out varyng some characterstc parameters (β, WHC, Dr 0 ) and observng model senstvty. Comparson between model-smulated and observed monthly rver flows ponted out that groundwater ntal detenton Dr0 dd not apprecably wegh on smulatons; then, t was hypothessed as unformly dstrbuted on the whole southern Italy and fxed equal to 100 mm. Solreservor constant β was determned consderng dfferent classes of values. Results based on the mnmsaton of root mean square errors and sum of mass dscrepances nvolved the use of three optmum values (5, 10 and 20 months) dependng on sol permeablty. These values mean that groundwater runoff n the present month s assumed to be equal to 20% of groundwater detenton observed n the prevous month for moderate rate of water transmsson, equal to 10% for slow rate of water transmsson and equal to 5% for very slow rate of water transmsson. Changes n solwater holdng capacty dd not produce apprecable dfferences n smulated rver runoff wth the excepton of months charactersed by very extreme precptaton events. Results obtaned at basn scale have been extended to the whole southern Italy and the correspondng water balance has been developed by consderng hstorc monthly mean data frst (for the perod ), and then monthly values observed durng the years Smulatons carred out on actual data, offer useful nformaton about hydrologcal characterstcs of

5 European Water 3/4 (2003) 7 the examned regon, n absolute terms, but can be subjected to crtcsms because of the smplfcatons ntroduced nto the water balance procedure. Dfferently, ther comparson wth hstorc mean quanttes unvocally defnes the crtcal areas monthly nterested by water defct. Fgures 2a, b and c show the comparson between actual and hstorc monthly values obtaned averagng the spatally dstrbuted water balance quanttes on the whole southern Italy. The same fgures pont out a net reducton n the actual precptaton data that nvolves a more evdent reducton n rver runoff. Crtcal water defct values are manly observed durng the summer months, wth a maxmum n August For the observed perod, October 2001 shows the hghest crtcal water defct charactersed by a 80 mm dfference from the correspondng hstorc mean value (Fg. 2c). For these months GIS-embedded modelng allow to automatcally determne crtcal areas whose water defct values appear to be substantally dfferent from the correspondng hstorc mean (Fg. 2d). (mm/month) (a) RAINFALL HISTORIC DATA (mm/month) (b) RIVER RUNOFF HISTORIC DATA 0 0 1/97 7/97 1/98 7/98 1/99 1/97 (mm/month) 7/99 1/00 7/00 1/01 7/01 7/97 1/98 7/98 1/99 7/99 1/00 7/00 1/01 7/ (c) WATER DEFICIT (PE-E) HISTORIC DATA DIFFERENCES August 2000 _ = /97 7/97 1/98 7/98 1/99 7/99 1/00 7/00 1/01 7/01 (d) Fgure 2. Comparson between actual and hstorc monthly water balance quanttes n southern Italy. a) Spatallyaveraged ranfall values; b) Spatally-averaged rver runoff values; c) Spatally-averaged water defct values; d) Example of spatally dstrbuted water defct dfferences for the crtcal August CONCLUSION An ntegrated GIS-Web montorng system amed to analyse, control and map water defct perods occurrng n southern Italy has been descrbed n ths paper. Real tme hydro-meteorologcal data are stored n databases, whch are drectly lnked to a GISembedded water balance modelng. GIS s able to spatally determne crtcal areas whose water defct values appear to be dfferent from the correspondng hstorc means. Ths s realsed on a monthly bass comparng hstorc mean quanttes wth actual observed values. Analyses carred out durng the years have shown a dffuse drought phenomenon characterzed by an ncreasng water defct trend. Specfcally, ths trend s more evdent along the Ionan zones. The nnovatve aspect of the proposed system s ts GIS-Web archtecture, through whch all tele-meterng recorded data and spatally dstrbuted water balance analyses are avalable, both as tables and maps, va World Wde Web at Future developments wll be addressed to two man drectons: the frst amed to mprove the water balance procedure, especally for what concerns sol propertes and the correct defnton of groundwater volume; the other regardng the ntroducton nto the water balance of quanttes related to rrgaton and water supples.

6 8 G. Mendcno and P. Versace REFERENCES Mendcno G., Versace P., Water balance model based on GIS procedures, Workshop INTERREG IIC Assetto del Terrtoro e Lotta Contro la Scctà, Taormna, Italy. Mendcno G., Spatally dstrbuted predcton of water defct perods, Proceedngs of the NATO Advanced Research Workshop on Integrated technologes for envronmental montorng and nformaton producton, Marmars, Turkey. Mendcno G., Senatore A., Versace, P., Stma dell evapotrasprazone potenzale n regon merdonal attraverso l anals spazale de dat altmetrc, d temperatura e radazone solare, XXVIII Convegno d Idraulca e Costruzon Idraulche, Potenza, Italy. Monteth J. L., Evaporaton and the envronment. In: The state and movement of water n lvng organsms, XIXth Symp. Soc. Exp. Bol., Swansea, Cambrdge Unversty Press, Penman H. L., Natural evaporaton from open water bare sol and grass, Proc. R. Soc. London; Ser. A 193: Thornthwate C. W., Mather J. R., The water balance, Clmatology, Drexel Inst. of Technology, Centeron, New Jersey. Sol Conservaton Servce, Hydrology, Supplement A to Secton 4, Natonal Engneerng Handbook. Washngton, D.C.: U.S. Department of Agrculture. Heatwole C.D., Campbell K.L., Bottcher, A.B., Modfed CREAMS hydrology model for Coastal Plan flatwoods, Trans. ASAE; 30(4):