¹Ming-Chee Wu, ¹Chih-shuo Chang, ²Nan-ko Chiu, ²Jun-kun Chen, ²Igor S. Zektser, ³Roald G. Dzhamalov

1 Department of Earth Sciences, National Cheng Kung University, TAIWAN

2 Tainan Hydraulics Laboratory, National Cheng Kung University, TAIWAN

3 Water Problems Institute, Russian Academy of Sciences, RUSSIA

 

Abstract:Three maps in the scale of 1:250,000 were compiled to parametrically characterize the groundwater resources of Taiwan; maps of Modulus of Groundwater Flow (M), Coefficient of Groundwater Flow (K₁), and Coefficient of River Recharge (K₂).  According to the results of the mapping process, the natural groundwater resource of a region was therefore estimated.  The total natural regional groundwater resources inTaiwan are 173.3×10⁸ m³, approximately; among this amount, 48.6×10⁸ m³ are for the plain regions, and 124.7×10⁸ m³ are for the mountain regions.

Keywords:Taiwan, Quantitative Assessment, Natural Groundwater Resources, Mapping

Introduction

Natural groundwater resources are characterized by recharges from infiltration of atmospheric precipitation, filtration from rivers, and leakage from adjacent aquifers.  It is therefore regarded as constantly recharged in the process of hydrological cycle; such that the regional natural groundwater resources are also considered as the natural productivity of regional aquifers that will be replenished within the process of a successive hydrological cycle.  In addition, the sustainable yield (or so called safe yield) of natural groundwater resources for a region indicates the maximum limit of regional groundwater exploitation for a long term period of time without diminishing the features and functions of the groundwater flow regime.  The relationship between sustainable yield of any regional natural groundwater resources can be described as following:

                     (1)

where, Q_s represents the sustainable yield of groundwater, Q_n is the regional natural groundwater resources (or regarded as groundwater recharge), W gives the aquifer groundwater storage,indicates the time of groundwater withdrawal, andis the additional groundwater resources.  For a long term period, if, withabsent, then the sustainable yield of groundwater regime is determined only by regional natural groundwater resources, Q_n.

According to the mathematical representation indicated above (Eq. 1), assessment of regional natural groundwater resources is practically performed based on two major concepts; one is the regional, the other is the natural.  The regional groundwater resources indicate the spatial average distribution of groundwater regime in space; namely the assessments are aimed to determine the groundwater resources on large areas.  Thenatural groundwater resources indicate the temporal average distribution of groundwater features which change in time; thus the assessments are to evaluate the occurrences of groundwater resources in a long period of time.  Namely, how to simply and accurately assess the regional natural groundwater resources reserve has always been an essential issue among the hydrologists and hydrogeologists; on purposes of groundwater resources development and management. 

There may have many theoretical or numerical methods been presented for evaluating the regional natural groundwater resources; however, among the previous studies, results of each approach can never reach a consentience; not only because of the variation of assessment techniques, but also due to the lack of hydrogeological database within the studied regions.  Nevertheless, in previous time, researches connected to the assessment of the regional natural groundwater resources were mostly performed with a view such as the groundwater resources can be easily accessed; namely, the assessment or estimation for groundwater resources was limited only to the plain area.  The mountain areas and the hilly areas were radically omitted. 

It was mentioned, in The Evaluation of Water Resources in China by the Hydrological Bureau of Hydraulic Power Ministry of China (1987):

“…the mountain area is counted in about 6.79×10⁶ km², and its water resources is estimated to approximately 676 billion cubic meters; yet, the plain area is counted nearly in 1.98×10⁶ km², and its water resources is estimated as high as to be 187 billion cubic meters…”

Obviously, the natural water resources in mountain areas are no less than those of plain area.  In above case, the water resources in the mountain area are approximately three and a half times more than that of the plain area.

Hwang (1994) also claimed, in The Evolution and Future Strategies of Groundwater Management, that:

“…from now on, we can’t pay our attentions only to the plain area, especially water resources in plain area have all been contaminated such that it is not suitable for drinking water supply any more.  As for mountain area, particularly the groundwater; such water resources have not yet been contaminated, it indeed should be protected as soon as possible in order to ensure that the water supply in the near future is secured…”  

Thus, advanced approaches established for current assessment of regional natural groundwater resources shall be conducted including the whole catchments of a river drainage basin; not just the downstream alluvial plain areas.  Namely, how to efficiently and reasonably evaluate the regional natural groundwater resources becomes part of the fundamental theme for regional water resources development and management.

This study has therefore revised the conventional approaches for regional natural groundwater resources assessment in Taiwan.  With quantitative characteristics of regional hydrogeology; namely, the Modulus of groundwater flow (M; liters/second*km²), the Coefficient of groundwater flow (K₁; %), and the Coefficient of river recharge (K₂; %) (Zektser and Dzhamalov 1988), a set of regional groundwater resources maps, in the scale of 1:250,000, had therefore accordingly been compiled.  In particular, the inclusion of the contribution from mountain area became the major issue along the compilation of the regional natural groundwater resources maps.  

In this study, the regional natural groundwater resources are quantified based on the hydrogeological conditions of the region, and the interactive relationship between surface water and groundwater within the hydrological cycle.  The natural groundwater resources of the region are therefore evaluated in accordance with two fundamental techniques; i.e., the stream base flow analysis and the modulus of groundwater flow.

Methodology

In this study, taking into account the scarce in measured database and uneven distribution of instrumented hydrological monitoring stations for Taiwan territory, the regional natural groundwater resources of whole Taiwan area were estimated via two fundamentals:  (1) stream base flow analysis for the mountainous region, and (2) Darcy’s groundwater flow theory for the plain area.All theses two fundamentals are based on the concept of water balance within a hydrological cycle.

In the base of the first method, the stream base flow analysis, the following two assumptions have been accepted:

(1)In the regions with homogeneous hydrogeological conditions, the river basin sections (with equal drainage squares and with equal distance from upstream source point) are characterized the same conditions of hydraulic connections and degree of interactions between groundwater and river water;

(2)The river sections with length less than 10 km from the upstream source point are characterized during monsoon season only by surface water or overshed runoff recharge; in other words, groundwater contribution to river runoff is negligible and may not be taken into account during such condition.  In that case, groundwater contributions to total river runoff have to be estimated only for river basin sections with length more than 10 km from the upstream source point. 

Stream base flow analysis

The so-called stream base flow has been regarded as the part of river runoff that interacts with groundwater flow system.  It can be either the net volume of the discharge from groundwater system to the river runoff or vice versa.  To estimate the groundwater flow, it is needed to select the hydrological monitoring stations (or called gauging stations) for river runoff with records under natural conditions.  The value of stream base flow may be expressed by adapting the lowest flow records over a long period of observation from a gauging station (Zektser and Dzhamalov, 1988).  Therefore, according to the long-term observation hydrograph of average daily runoff for a gauging station, the dry period in that gauging station during the observation period can be decided.  The average flow rate within that dry period is treated and translated into the stream base flow of that gauging station upon the representative river catchments.  The modulus of groundwater flow is therefore estimated in accordance with the stream base flow.

Modulus of groundwater flow ( M )

The modulus of groundwater flow is regarded as the capability of groundwater flow discharged from the river catchments area, with quantitative measures given in liters per second per 1 km².  The modulus of groundwater flow, characterizing the natural productivity of the aquifer been assessed, is defined by the following equation (Zektser and Dzhamalov, 1988):

                                                  (2)

where, Q_n represents the natural groundwater resources, is the area of the river catchments.

Coefficient of groundwater flow ( K₁ )

The coefficient of groundwater flow (K₁) is the percentage ratio of regional natural groundwater flow (Q_n) to quantity of annual precipitation (P).It demonstrates what percentage portion of groundwater flow is recharged by precipitation.  The coefficient of groundwater flow, characterizing the natural regional groundwater recharged by the precipitation, is defined by the following equation:

                                                   (3)

where,  indicates the natural groundwater recharges by precipitation .

Coefficient of river recharge ( K₂ )

The coefficient of river recharge (K₂) is the percentage ratio of regional natural groundwater flow (Q_n) to river runoff (Q_rf).  It shows what percentage portion of river runoff is from the discharge of groundwater.  The coefficient of river recharge, characterizing the portion of river runoff that is contributed by the groundwater, is defined as the following equation:

                                                   (4)

where,  gives the natural groundwater discharge into the river runoff .

Principle procedures of calculations

To estimate the modulus of groundwater flow, it is needed to select the monitoring stations for river runoff with records under natural conditions.  Under natural conditions indicates that the records of gauging stations haven’t been influenced by any human activities such as reservoirs, dams or any hydraulic constructions.  Besides the requirement for natural conditions; the length of observation period is also an essential factor in adapting the river runoff records to be used in computation of the stream base flow; and further more the modulus of groundwater flow, the coefficient of groundwater flow, and the coefficient of river recharge.

According to the length of the observation period, three groups of observation records are identified: one group of the observation period is more than 10 years, another group is between 5 to 10 years, and the other is less than 5 years.  However, all observation data selected are having complete records that include both dry and wet seasons.

To estimate the modulus numbers in plain area of the region, all available proceedings of Groundwater Monitoring Network Plan in Taiwan have been taken into account.  Besides that, the characteristics of subsurface geology and hydrogeology within the plain area were firstly been understood and analyzed, in accordance with the core and the drilling data provided by the Central Geological Survey of Taiwan. Conjunctively with all the available information, such as: transmissivity values, grain size distribution, lithology of sediments, watershed area, hydrological characteristics, and groundwater level distribution, the hydrogeological divisions within the plain area are constructed.  The groundwater flow for each hydrogeological subdivision is estimated in accordance with the regional groundwater flow regime via the concept of Darcy’s groundwater flow theory.  The modulus of groundwater flow in plain area can then be calculated.  The set of regional natural groundwater resources related maps for the region are therefore compiled.

For quantitative assessment of the groundwater resources in a region, areas for each subdivision of various modulus values are estimate.  Combining the modulus value with the corresponding subdivision area, computation for the amount of groundwater flow is then performed.

Results and Conclusions

According to the calculation results of each quantitative parameter, the corresponding maps were compiled.  The modulus of groundwater flow (Figure 1), the coefficient of groundwater flow (Figure 2), and the coefficient of river recharge (Figure 3).  Also, base on the fundamental process of the equation (2), reserves of regional natural groundwater resources equal to the product of modulus value and the area being evaluated. Namely, we are able to evaluate the reserves of regional natural groundwater resources from the calculations of the modulus value and the area under evaluation process.  The total reserves of natural groundwater resources in Taiwan region are 173.31×10⁸ m³/year (Table 1).  According to the similar processes , we can obtain the reserves of regional natural groundwater resources in plain area for approximately 48.60×10⁸m³/year, and about 124.71×10⁸ m³/year in mountain area (Table 1).

 

Acknowledgement

The appreciation is due to the Water Resources Agency, Ministry of Economic Affairs, TAIWAN, R.O.C., for the financial and informative support on this work.

 

References

[1]      Central Geological Survey, MOEA of Republic of China, 1997, The preliminary exploration for the hydrogeological framework in Pingtung plain. Annual Report of Central Geological Survey, pp 68-87 [In Chinese]

[2]      Environmental & Infrastructural Technologies, Inc., 2000, Estimation of groundwater recharge in Taiwan’s 10 regional aquifers, pp 5-94 [In Chinese]

[3]      Environmental & Infrastructural Technologies, Inc., 2000, Preliminary planning for conjunctive use of surface water and groundwater in Pingtung plain, Taiwan [In Chinese]

[4]      Geng, C. C., W. N. Chung, and W. L. Chang, 1994, Analysis of the river leakage and influence of reservoir on downstream groundwater resources. Proceedings, Conference for Protection of Groundwater Resources and Water Quality (I), pp 255-263 [In Chinese]

[5]      Hwang, J. S., 1994, The evolution of groundwater management and future policy measures. Proceedings, Conference for Protection of Groundwater Resources and Water Quality (I), pp 185-200 [In Chinese]

[6]      Hsu, T. L., 1961, The artesian water system beneath the Pingtung valley, Southern Taiwan. Proceedings, Geol Soc of China, 4:73-81

[7]      Hydrological Bureau, Ministry of Hydraulic Power, China, 1987, The evaluation of water resources in China [In Chinese]

[8]      Jean, J. S., 1993, Hydrogeologic framework of alluviums beneath the Pingtung by-product processing plant in the Pingtung valley, Taiwan. J. Geol. Soc. of China, 36(3): 311-329

[9]      Ting, C. S., Y. Zhou, J. J. Vries, and I. Simmers, 1998, Development of a preliminary groundwater flow model for water resources management in the Pingtung plain, Taiwan. Ground Water, 35(6): 20-36

[10]  Ting, C. S., 1994, The optimum model for groundwater management in Pingtung plain. Proceedings, Conference for Protection of Groundwater Resources and Water Quality (I), pp 265-284 [In Chinese]

[11]  Ting, C. S., 1997, Apply chloride ion balance method to estimate the groundwater recharge - A case study in Pingtung plain. Proceedings, Conference for Protection of Groundwater Resources and Water Quality (II), pp 703-714 [In Chinese]

[12]  Wu, M. C. and C. L. Cheng, 1996, Investigation regional hydrogeological characteristics and distribution of Artesian wells -A peculiar water resources in Pingtung Alluvial Pain (I). Report: NSC85-2111-M006-009, National Science Council, Taiwan, R.O.C. [In Chinese]

[13]  Wu, M. C. and C. L. Cheng, 1997, Investigation regional hydrogeological characteristics and distribution of Artesian wells -A peculiar water resources in Pingtung Alluvial Pain (II), Report: NSC86-2116-M006-010, National Science Council, Taiwan, R.O.C. [In Chinese]

[14]  Wu, M. C., C. L. Cheng, C. K. Chen, and C. K. Chen, 2001, Preliminary discussion for regional lateral recharge assessment of Pingtung plain. Proceedings, Conference for Protection of Groundwater Resources and Water Quality (VI), pp 217-223 [In Chinese]

[15]  Zektser, I. S. and R. G. Dzhamalov, 1988, Role of ground water in the hydrological cycle and in continental water balance. IHP III Project 2.3, UNESCO, Paris

 

Table 1 The regional natural groundwater resources assessment of Taiwan (x10⁶ m³).

 

  

Figure 2 Map for coefficient of groundwater flow.