A NEW METHODOLOGY TO ESTIMATE GROUNDWATER DISCHARGE, IN A LAKE.

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1 A NEW METHODOLOGY TO ESTIMATE GROUNDWATER DISCHARGE, IN A LAKE. I. Zacharias, E. Dimitriou, T. Koussouris National Centre for Marine Research, Institute for Inland Waters Agios Kosmas, Elliniko, GREECE thakis@ncmr.gr ABSTRACT In this study utilization of GIS techniques, simple hydrology models and remote sensing have been used to calculate a hydrogeological element of a specific catchment s water balance. Particularly, the underwater discharges in a great and deep freshwater body are difficult to quantify and demand high-tech and costly solutions. In this scientific effort a combination of the aforementioned techniques provides effectively, credible estimations of the underwater inflows in a large lake in Western Greece. Moreover, the formation of the Digital Elevation Model of the lake combined with the monthly water level fluctuation and the appropriate volumetric software produced the monthly water volume fluctuation in the lake. Then, all the hydrologic elements of the catchment, such as rainfall, evaporation, irrigation and additional water uses have been quantified by using simple but scientifically acceptable methods. Finally, the acquired and calculated data have been incorporated in the water balance model which provided its only unknown factor; the monthly underwater inflow. Thus, this simple and physically based approach illustrated an efficient and easy to apply scientific method that estimates a hydrologic element which is difficult to measure directly. Further, this method can be used in other catchments as well since the utilization of GIS techniques constitutes it widespread applicable. 1. Introduction Management of the water resources is one of the most important issues today, due to the relevant environmental problems that are caused, mainly because of the unsustainable use of the water storages that often occurs nowadays. Therefore, there is an apparent need for studying, comprehending and measuring the appropriate components of the hydrologic systems in order to facilitate the production of management plans for the water resources in a catchment scale. However, the hydrologic systems, sometimes, are difficult to be understood, parameterized and quantified. There are so many physical factors that operate and interact, often, in very complex ways, which cannot be analyzed adequately from the technologically advanced environmental models that are widespread adopted today. Thus, in such cases careful investigation of the site-specific characteristics in relation to the hydrological regime of the area should occur and possible alterations in the common used scientific approaches should be made (Shaw, 1994). In particular, it can be mentioned that measuring underground inflow into a water body is a difficult task that incorporates specialized equipment and personnel, as well as long-term measurements, which imply significantly high costs and timeconsuming efforts. Additionally, the possible underground water supplies of adjacent basins that may contribute in the amount of groundwater inflow into the water body

2 increase the uncertainty in the estimation of this inflow, especially when other indirect methods such as a simple water balance models or tracer-based methods are used. There are several methods for calculating underground inflows into water bodies such as lakes and rivers. Most commonly used ones incorporate water balance models that take into account meteorological data and land surface parameters like soil moisture and land cover types (Finch, 1998). Although, this is an effective approach for some specific environmental conditions, it cannot be used in areas with complex hydro-geologic regime where proportion of the underground water may be originate from adjacent catchments. Another approach is the mapping of hydraulichead surfaces, which combined with water level measurements in test boreholes, and GIS techniques can indicate good estimations of underground water supplies into water bodies (Salama, Ye Lin, Broun, 1995). Nevertheless, this method is difficult to implement under specific hydrologic conditions, especially where the aquifer depth is great and incorporates costly efforts such as the construction of boreholes. Finally, groundwater tracers such as 222 Rn and 226 Ra are used in order to investigate possible groundwater inflows and quantify them (Hussain, et al., 1998) but cannot illustrate very accurate results neither discriminate the possible different sources of the freshwater inflow. Consequently, a method that could produce reliable estimates of such a hydrologic parameter by taking into account all the possible inputs and outputs of water in the investigated catchment and without using high cost solutions would be extremely valuable for the scientists that conduct research in this field. Furthermore, estimating underground discharge in a large lake by using Digital Elevation model (DEM), aerial photographs, GIS technologies, water level measurements and a simple water balance equation is implemented in this project so as to acquire credible results in low costs and within short time period. All the aforementioned components are widespread applicable from many scientists today, can provide with credible estimations if appropriately used and do not impose significant problems that need great efforts to be overridden. 2. Site description The area under investigation concerns a deep and large lake catchment (Trichonis) which has an extent of 399 km 2 (Bertachas et. Al., 2000) and it is based on the western part of Greece in the prefecture of Aitoloakarnania. Trichonis catchment can be characterized as semi-mountainous since it is surrounded in the north by Panaitoliko Mountain and in south by Arakinthos Mountain while it incorporates low extent plains in the west and northwest part of the basin (map1). Lake Trichonis lies approximately in the middle of the catchment and it is a very important water body for this region since it has a potential water volume of about 3 billions m 3. However, the intense agricultural practices and the unsustainable use of water in this area have directed to the degradation of water quality and quantity during the last decade and therefore a need for applying a management plan to the water resources in the specific area has become apparent. For this purpose all the hydrological parameters of this catchment have been examined, analyzed and quantified and the results have been incorporated in a water balance equation. Nonetheless, the complex but well studied hydrogeologic conditions, the great amount of water storages in this area together with the relatively low extent of the basin indicate that this catchment s direct water inflows to the lake could not be able to sustain and re-circulate such an amount of

3 good quality water for so many decades. Further, by analyzing the water demand for irrigation and other uses in this area became apparent that the expected inflows into the lake would not be enough to cover all the needs and therefore an exponential reduction in the water storages should occur within a short time period which did not happen in this water body. Thus, a possible explanation for this fact could be the existence of underground inflows, supplied from an adjacent catchment that offers significant amount of water in Trichonis Lake. Indeed, after extensive research several high-yield springs have been found few meters under the lake s surface in its northeast coastline. However, measuring the discharge of such springs is a difficult task since special equipment has to be acquired and a long time measuring period has to be established. Consequently, an alternative method should be adopted that could provide easily and credibly the necessary measurements which will be analyzed in this study. Map 1. Study area with its hydrographic network 3. Methodology In order to estimate all the essential hydrologic parameters of the area several methods have been utilized. A simple water balance equation was initially adopted (Ward, 1990) : P + R + U input + V input = E + U output + V output (eq. 1) Where P: direct rainfall into the lake (m 3 ), R: overland flow that discharges into the lake (m 3 ), U input: underground water that discharges into the lake (m 3 ), V input : any additional water inflows to the lake (m 3 ), E: evaporation from the lake s surface (m 3 ),

4 U output : underground water that outflows from the lake (m 3 ) and V output : any further water outflows from the lake (m 3 ). Map 2. Trichonis catchment s Geology (Source: Institute of Geologic and Mineral Exploration, (1980)) Meteorological data and specifically, monthly rainfall, wind speed, solar radiation, vapor pressure and air temperature data have been used in order to estimate direct precipitation inflows to the lake and evaporation outflows which have been estimated by using the Penman method for an open surface water body (Hess, 1996): Eop / * a ( / ) 1 Where: Εοp : potential evaporation, Εa: aerodynamic equation based on humidity and actual vapor pressure ( Ea 0.35* (0.5 u2 /100) * ( ea ed )), u 2 Wind velocity 2m above the water bodies surface, ea: saturated vapor pressure in temperature Τa, ed: actual vapor pressure in average temperature, Η: available evaporation energy, ( H RI ( 1 r) R o ) R I : solar radiation corrected by taking into account geographic latitude and season, (R I (1-r)=0.95R a f a (n/n)), Ra: measured solar radiation, n: duration of sunlight, Ν: maximum potential duration of sunlight (hours), r: coefficient of reflection of solar radiation, Ro: reflected solar radiation,( R T 4 ( e ) * ( n / N ) Τa: mean air temperature, o a d

5 : rate of change of saturated vapor pressure in temperature t, ( e e ) /( T T )) and ( a d a d γ : the psychrometric constant, (γ = 0,27 mm Hg/ o F). Moreover, the characteristics of the irrigation pumping system of this area have been examined and the amount of abstracted water from the lake for agricultural uses has been calculated in a monthly basis. The necessary data concerning pumping capacity for each station and hours of daily operation have been acquired from the regional municipalities. Aerial photographs and GIS techniques have been used to validate this calculation by producing the land use map of the area and calculating the exact extent of each vegetation type that exists in the area (map 3). The necessary amount of irrigation water has been estimated by multiplying the extent of the agricultural fields with its respective daily water demand that has been retrieved from the bibliography compared and corrected with the information derived by the local farmers association. All the results concerning the irrigation demand from the different, aforementioned, approaches have been compared and averaged in order to eliminate potential errors. The outcome has been cross-examined with data that have been acquired from the local public authorities and with recent studies that have been conducted in the area (map 3). Map 3. Land uses of Trichonis catchment Further, the water demand for domestic and industrial uses are relatively low in this area due to the relatively small population and the limited number of industrial installations. Nevertheless, these demands have also been taken into consideration for the lake s water budget estimation, in order to achieve higher accuracy. These data are monthly and have been acquired by local authorities.

6 Additional seasonal inflows into Trichonis Lake from an adjacent basin (Acheloos downstream) through an irrigation canal (DXI) have been incorporated in the calculations by using monthly data recorded by Agricultural Organization for Land Reclamation of Agrinio. Another important element that has to be estimated is the significant controlled outflow towards lake Lysimachia that occurs through an artificial canal that connects these two lakes. Therefore, the monthly outflow from Trichonis lake to Lysimachia Lake, through the aforementioned canal s gates, is estimated by using Henderson s equation (1966): Q C d wb 2gy 1 Where: Q is the discharge from Lake Trichonis towards Lysimachia lake, C d is the discharge coefficient that depends on the geometry of the gate, w refers to the height of the gate opening, b represents the width of the bate opening, g is the gravitational force and y 1 is the difference in head acting on the gate (Mays W. L., 1996). C d C 2 c 1 C c w y 1 Where: C d is the discharge coefficient; C c is a coefficient that depends on the shape of the gate s lip and on the relative angle that this forms with the bottom level of the 2 canal (C c ,36, Θ = θ/90 ο, θ is the angle between the gate s lip and the horizon, figure 4). Moreover, the overland flow of this catchment s streams that discharge into the lake has been quantified by using monthly coefficients from recent hydrologic studies that have been conducted in the area and the necessary monthly rainfall data. It should be mentioned that these coefficients have been derived based on the area s hydrogeologic characteristics and on seasonal measurements of soil moisture (Kallergis et al, 1993). Finally, monthly water level measurements of the lake Trichonis have been acquired from the Technical Organization for Land Reclamation of Agrinio which is the responsible authority for the management of the water resources in the area, in order to be used for the calculation of the monthly remaining water volume in Trichonis Lake. The Digital Elevation Model (DEM) of the lake has been formed (figure 1) by using topographic maps of the area (1:50.000, designed by the Military Geographical Service) and with the contribution of ArcView 3.2 and 3D Analysis software (GIS software). Particularly, a Triangulated Irregular Network (TIN) polygon has initially been created by the program based on the relevant data of the above maps (contour lines of the area and z coordinates). TIN partitions a surface into a set of contiguous, non-overlapping, triangles. A height value is set for each triangle node and heights between nodes can be interpolated thus allowing for the definition of a continuous surface. TINs can accommodate irregularly distributed as well as selective data sets, which makes possible the representation of a complex and irregular surface with a relatively small data set. In this way the DEM of the lake was formed and combined

7 with the water level measurements provided, through a built-in utility of the 3D Analyst program, the monthly water volume fluctuations in high accuracy. Then these, remaining in the lake, water volumes have been placed in the analytical water balance equation which finally, indicated and quantified the only unknown component of the equation that is the monthly underground inflow to the lake Trichonis. Figure 1. Digital Elevation Model (DEM) of Trichonis Lake 4. Results Rainfall Evaporation. For the area s rainfall Thiessen polygons method has been used by implementing a network of 5 gauging stations that form the respective Thiessen polygons with the contribution of ArcView 3.2 GIS program. The acquired data concerns monthly values for the period and are taken from the National Meteorological Service of Greece. The calculated evaporation from the lake surface is estimated by using the aforementioned necessary input data in Penman s method and the outcome involves monthly evaporation values (figure 2). Evaporation Rainfall mm Months Figure 2. Rainfall and Evaporation in Trichonis catchment

8 Monthly overland flow The overland flow from the area s streams towards Trichonis lake has been estimated in a recent study of the specific catchment s hydrologic regime (Kalergis et al., 1993) and monthly overland flow coefficients have been produced which have been used, in this study, together with the relevant rainfall data, in order to provide the surface water that discharges in the lake (figure 3). Overland flow 50 Millions m Months Figure 3. Overland flow in Trichonis catchment Irrigation scheme and additional water uses The water demand for irrigation purposes is covered, mainly, by direct pumping from the lake and then the acquired water is distributed through open canals to the agricultural fields. The irrigation period extends from April to September each year and the area that is served by the lake s water is approximately 97 Km 2 within Trichonis basin and additional 175 Km 2 outside this particular catchment. The amount of water that flows towards this external area moves through a controlled-flow canal that connects Trichonis lake with the adjacent lake of Lysimachia and with the hydrologic system of Acheloos river (map 4). This canal incorporates three gates placed against the water flow, which possess mechanisms that adjust the amount of water that moves through them (figure 4). Figure 4. The controlled-flow canal with sluice gate that connects Trichonis with Lysimachia lake

9 The monthly consumption of lake s water for irrigation is calculated from the land use map (map 3) by taking into account the extend of agricultural land estimated by Image analysis software and an annual average of 650 m 3 of water per 1000 m 2 of land (Hydretme, 1998). Further, the total area of agricultural land in this specific catchment is 97 Km 2 and therefore the annual irrigation need for this area is approximately 62x10 6 m 3. Part of this amount is covered by direct pumping from the lake (figure 5) while the rest (about 33x10 6 m 3 ) is taken by a canal that carries water from Acheloos river. Moreover, another 175 km 2 of agricultural land, which is outside Trichonis catchment, uses irrigation water from Trichonis and Lysimachia system. Therefore, approximately 115x10 6 m 3 is taken from Trichonis Lake for this purpose by adjusting the opening of the canal s gates that connects these two lakes. However, because the outflow from this canal is significantly higher than 115x10 6 m 3 since it also discharges the surplus of Trichonis water during the wet periods, the exact amount of this outflow will be estimated by using data relating to the lake s water level as well as to the gates characteristics Total irrigation needs covered by Trichonis lake 10 6 m Months Figure 5. Irrigation demand covered by Trichonis lake Additionally, it has to be stated that even though the water supply for domestic and industrial use in the area is limited in relation to the rest of the water uses, is included in our calculations by using monthly data that public authorities have supplied, in order to achieve higher precision.

10 Map 4. Pumping stations of Trichonis catchment Monthly outflow from Trichonis lake towards Lysimachia lake Daily data concerning each gate s opening and Trichonis lake s water level have been used, (acquired by the responsible public authority for the management of this canal), with the aforementioned Henderson (1966) equation to calculate the daily and then monthly outflow from Trihonis towards Lysimachia lake (figure 6). m 3 (milions) Outflow towards Lysimachia Lake Months Figure 6. Water outflow towards Lysimachia lake Quantification of the water level fluctuations with the contribution of GIS software. The fluctuations of Lake Trichonis water level were significant in the past and it has reached approximately a maximum annual range of 3 meters. However, today,

11 due to the construction of the canal that discharges the water surplus to Lysimachia lake, and the decrease of the annual rainfalls caused the reduction of the water level fluctuations which fall to approximately, 1.50 meters, within a year s time period. The use of ArcView GIS program together with the 3D Analyst component facilitated the fast and accurate transformation of the water level fluctuation to monthly values of remaining water in the lake, which represents the difference between the total monthly inflows and the respective outflows. Therefore, the DEM of the lake has been formed using the 3D Analyst application (figure 1), and the exact water volume remaining in the lake every month has been calculated with this application by utilizing the monthly water level values that have been acquired from the local public authorities. Then each month s total water volume has been subtracted from the previous month s respective value and this provides the monthly surplus or deficiency of water due to the lake s monthly water balance (figure 7). Finally the estimated values and the acquired data are applied to the simple monthly water balance equation where the only unknown quantity is the underground discharge in Trichonis Lake. Therefore, by varying the monthly underground discharge in an properly programmed spreadsheet, the remaining water volume in Trichonis Lake (Total monthly inflows Total monthly outflows) produced by the water balance equation reaches the respective values calculated by the 3D Analyst application and this provides the real monthly underground inflow into the lake (figure 8). (x10 6 m 3 ) Water volume fluctuation Water level fluctuation 16, ,5 14,5 13, , (meters) Figure 7. Remaining water volume in Trichonis Lake 5. Discussion After analyzing the hydrologic components of Trichonis catchment, several interesting characteristics become apparent. As concerns the rainfall in the area it can be stated that the wet period extends from October to April (582 mm, 69% of the annual rainfall) and specifically the wettest months are December (217 mm, 24% of the annul rainfall) and November (211 mm, 23% of the annul rainfall). The dry period that extends from May to September falls only 150 mm of rainfall (17% of the annual value) and the driest month is June (6 mm of rainfall, 0.7% of the annual value). Nonetheless, evaporation from the lake is significantly increased due to the relatively high temperatures, especially during the summer season, and the great availability of water from Trichonis Lake. Further, the annual evaporation reaches 1096 mm and

12 according to figure 2, there is a significant water deficiency during the period between May and September when evaporation is high and rainfall low, while there is water surplus encountered from October to April when evaporation decreases and the weather becomes wet. The specific area incorporates approximately 20 seasonal streams that discharge significant amount of water during the wet period into lake Trichonis. Particularly, November and December are the months that present the greatest overland flow (19% and 29% of the annual value, respectively) while the lowest value has been recorded in June (0.4% of the annual overland flow). The total amount of the annual overland flow reaches 133x10 6 m 3, which is relatively high considering the limited extent of the continental part of this basin (approx. 300 Km 2 ) and the small number of streams that drain the area. Thus, the responsible factor for the increased overland flow is the characteristics of the geologic formations which most of them are impermeable as well as the high gradients dominating in a large part of the basin that facilitate the development of overland flow. Irrigation demand from Trichonis Lake is significantly increased since agriculture is the most common fiscal activity in the broader area. A proportion of the water needed for irrigation is pumped directly from the lake (approx. 33x10 6 m 3 or 19%) and distributed to the farming land that surrounds the lake, another proportion is provided from the irrigation canal which brings water from Achelloos river (aprox. 30x10 6 m 3 or 17%) and finally the rest of the water demand for agricultural purposes for areas outside this basin is discharged through the controlled flow canal that connects Trichonis with Lysimachia lake (114x10 6 m 3 or 64%). Therefore, the total irrigation needs covered by Trichonis Lake reach 176x10 6 m 3 and are abstracted from the lake during the irrigation period (April September) which cause serious environmental impacts since during this time period the inflows are the lowest of the year while the outflows are extremely high. Thus, there is a great seasonal water level fluctuation that disturbs the existed ecosystems in the area and degrades the land around the coastline. Another significant outflow from Trichonis Lake concerns the discharge of the controlled flow canal that connects Trichonis and Lysimachia lake. This canal not only provides the aforementioned necessary irrigation water for some regions outside the specific catchment but yields the surplus of water during the wet period to Lysimachia lake to avoid floods. Estimation of this water quantity is given by the Henderson equation (1966), which takes into account the constructional characteristics of the canal, the gates openings and the respective water level. This canal is managed according to the downstream areas water needs and therefore provides high discharges mainly during the dry period while it maintains approximately low discharge values during the wet period except from November and December when extremely high rainfall records are observed. Thus, from January to May only 22x10 6 m 3 of water (10% of the annual value) are directed towards Lysimachia lake through the controlled flow canal while 112 x10 6 m 3 (50% of the annual value) end up in Lysimachia lake from June to September and 90x10 6 m 3 (40% of the annual value) discharges into Lysimachia from November to January. The monthly remaining water volume in Trichonis lake has been calculated by using the water level records from the local authorities and the 3D Analyst software as mentioned before. Thus, the estimated values indicate a significant surplus of water in February (37x10 6 m 3 ) which is mainly due to the very low outflows of this month

13 while there is a great deficit of water in the lake in June (-32x10 6 m 3 ) which is expected since the inflows during this month are limited and the outflows are high, mainly because of the irrigation. More, there is significant water deficit observed during the whole irrigation period (figure 8) while great water excess is recorded from January to May. These monthly remaining water volumes represent the difference between the monthly inflows and the respective outflows from Trichonis lake and therefore they can be used, with the contribution of the water balance equation, to estimate the monthly inflows and particularly the monthly underground discharges which is the only unknown parameter of the water balance equation. The annual underground inflow in Trichonis lake is approximately 123x10 6 m 3 and the highest monthly value is measured in July (24x10 6 m 3 or 20% of the annual value). Further, high values have been observed in August (20x10 6 m 3 or 16% of the annual value) and in February (14x10 6 m 3 or 11% of the annual value) while very low underground inflows have been recorded in November and December. The pattern that these results present, incorporates a physical explanation of the system since there is a significant lag time between rainfall, infiltration and consequently underground inflows that probably come from an adjacent basin. A statistical elaboration process that occurred for the purposes of this study confirms this notion. Monthly, Rainfall and water level fluctuations data for a period of 13 years ( ) have been correlated and the relative coefficient has been estimated for different lag times. The highest correlation has achieved for a lag time of 2 months (r=0.47) which indicates that on average the lake s inflows needs approximately two months to end up in the lake, following all the different routes described by the hydrologic cycle. The most time consuming process in this cycle is the underground flow and therefore it should be expected that the lag time for this part of the inflows could be larger than 2 months. Thus, the rainfalls cause the great underground discharges observed during the summer in winter while the very low respective discharges recorded in November and December are due to the low height of rainfalls during the summer. The total annual amount of underground inflow (123x10 6 m 3 ) cannot represent the infiltrated water produced within Trichonis catchment because the hydro-geologic properties of the area, and the annual rainfall cannot provide such an amount of underground water. Furthermore, the geologic formations of the area have been studied and the relevant infiltration coefficients have been calculated for each formation. Finally, the proportion of the rainfall that moves towards the underground water table is estimated by multiplying the aforementioned coefficients with the annual rainfall in mm and this reaches 43x10 6 m 3 which is considerably lower than the total underground inflow observed in the area (123x10 6 m 3 ). This supports the concept of additional underground supplies from an adjacent basin, which is difficult to be profoundly examined since the geomorphology of the area does not facilitate a hydraulic gradient survey. Nevertheless, the already mentioned estimations strongly indicate the existence of significant underground inflows from nearby catchment(s) of the broader area (map 2). The annual water balance of Trichonis lake is positive, which is in accordance with previous hydrologic studies in the area (Tzimopoulos et al., 1993), since the total annual inflow is 390x10 6 m 3 while the total annual outflow is 382x10 6 m 3 (approx. 8x10 6 m 3 water excess). However, the monthly water balances indicate great variation of the inflows and outflows seasonally. The greatest water excess has been recorded in February (34.2x10 6 m 3 remaining in the lake) while the higher water deficit has been encountered in June (-34.7x10 6 m 3 remaining in the lake). The most important

14 factor for the significant variations that occur in this area is the human activities and particularly the agricultural practices. From April to September 176x10 6 m 3 of water are abstracted from the lake for irrigation (46% of the total annual outflows) and this imposes great stresses in the hydrologic system and its biotic environment since at the same period the inflows are limited. Table 1. Water balance components in Trichonis lake and the estimated underground Inflows Water volume (x10 6 m 3 ) J F Μ Α Μ June July Aug. S Ο Ν D Ann. Rainfall directly in Trichonis lake Overland flow Water inflow from DXI irrigation canal Underground inflows Total inflows Evaporation from the lake Domestic and industrial water use Irrigation needs within Trichonis basin Irrigation needs outside Trichonis basin Total irrigation needs from the lake Outflow towards Lusimachia lake Total outflows Remaining water volume in the lake Cummulative water vol. In the lake Monthly inflow s Monthly outflow s 80 m3 (millions) Months Figure 8. Total inflows and outflows in Trichonis lake Thus, each specific hydrologic component that affects Trichonis lake has been studied and quantified using simple but accurate methods and the latest possible data. The geometric and geomorphologic characteristics of the area have been illustrated with the contribution of GIS technology and the existence of underground inflows from adjacent basin have been revealed after combining all the acquired data in a simple water balance equation. The estimation of these underground inflows by direct measurements is costly and time-consuming and therefore an indirect method has been applied incorporating all the available hydrologic data of the area combined with

15 the lake s DEM and 3D Analyst program. The monthly remaining water volumes in Trichonis lake have been, initially, calculated by using the aforementioned method and finally all the estimated hydrologic components have been placed in the area s water balance equation in order to calculate the monthly underground discharge in the lake which is the only unknown component of the equation. Comparing them with respective results from recent hydrologic studies of the area has validated the results of the estimated values and they presented significant similarity. Additionally, the simple, well-known and comprehensive methods that have been used increase the reliability of the estimations and present a widespread, easy to adopt method for calculating such hydrologic elements. Conclusively, it can be stated that measuring underground inflows in a large and deep lake is a difficult scientific task, especially when a proportion of them comes from an adjacent hydrologic catchments. Therefore, indirect measurements incorporating GIS technology, a simple water balance equation and profound understanding of the hydrologic regime of the area together with the necessary relevant meteorological and water level data can comprise an easily applicable technique that can provide credible and accurate results. Acknowledgments This study has been conducted under the purposes of a EU-funded, LIFE-Nature 99 project, in the area of Trichonis Lake, with title: Actions for the protection of the Calcareous Bogs/Fens in Trichonis Lake. References 1. Bertachas I., Zacharias I., Koussouris Th., 2000, Evaluation of the anthropogenic impact on Trichonis Lake catchment, final report, pp.66. In: Zacharias I. and Koussouris Th. (Editors). Actions for the protection of calcareous bogs/fens in Trichonis Lake. Technical Report, NCMR / IIW 2. Finch W. J., (1997), Estimating direct groundwater recharge using a simple water balance model sensitivity to land surface parameters, Journal of Hydrology, Vol.: 211, p.: Henderson (1966) in Psilovikos A., et al. (1995), Appraisal and management study of water resources in Acheloos downstream basin for the development and environmental reclamation of the area s estuary, Ministry of Environment, Town Planning and Public Works, Technical Report, University of Thessaloniki, Department of Geology and Physical geography, pp.: Hess T. (1996), Evapotranspiration estimates for water balance scheduling in the UK, Cranfield University 5. Hussain, N., Church, T.M. and Kim, G., Use of 222-Rn and 226-Ra to trace groundwater discharge into the Chesapeake Bay. Marine Chemistry, 65: Hydretme Ltd., (1998), Study for the reclamation of the irrigation networks at the area of Paravolas and Pamfias, Prefecture of Aitoloakarnania, Ministry of Agriculture 7. Institute of Geologic and Mineral Exploration, (1980): Geologic maps of Trichonis area, scale 1:50000, 4 sheets

16 8. Kallergis G. (1974), Basic principles of groundwater hydrology, Bulletin of Greek Geologic Association, Volume IX, Issue 1, pp.: , Athens 9. Mays L. W., (1996), Water Resources Handbook, Mc Graw Hill. 10. Salama B. R., Ye L., Broun J., (1995), Comparative study of methods of preparing hydraulic-head surfaces and the introduction of automated hydrogeological GIS techniques, Journal of Hydrology, Vol.: 185, p.: , Shaw M. E., (1994), Hydrology in Practice, Chapman & Hill 12. Tzimopoulos Ch., Spiridis A., (1993), The water balance of Trichonis lake, Appraisal study of the hydrology and management of the water resources of Acheloos estuary, Ministry of National Economy. 13. Ward R. C. and Robinson M., (1990), Principles of Hydrology, Mc Graw Hill.