ISSN 0097-8078, Water Resources, 2009, Vol. 36, No. 6, pp. 625–631. © Pleiades Publishing, Ltd., 2009.
WATER RESOURCES AND THE REGIME OF WATER BODIES
Groundwater Recharge: A Hydrogeologic Thought1 A. K. Taqveem Department of Geology, A.M.U.Aligarh-202002, India Received November 5, 2007
Abstract—The ensuing paper imparts vital information on an important component of the hydrologic cycle: recharge. Water flows through the porous media and forms a definite flow pattern that can be depicted in an elaborate manner through the micro level studies at the small watershed level. The estimation of recharge is indispensable for the groundwater budgeting studies. The advantages and disadvantages of some of the techniques have been reviewed. In the present paper an attempt is made to develop a fundamental understanding of the spatial and temporal distribution of the recharge component and to attract excellent research in the same field. DOI: 10.1134/S0097807809060025 1
Human population continues to grow. The demand placed on the groundwater resources is ever increasing. As an around 80% of the people of India rely on groundwater as their source of water supply. The insufficient recharge and unscrupulous withdrawal has induced declining trend in the water table of the country. For instance, the water table is reported to have fallen by over 10 meters in some localities in Delhi and in some other places such as Chennai and Chandigarh, it is reported they have fallen over 20 meters [56]. The situation is almost same in other parts of the country. The increasing demand has created a need to define accurately the spatial distribution of recharge across large areas. Understanding the recharge component and then its quantification is a prerequisite for effective water budgeting, liquid, solid and nuclear waste disposal. It has been suggested that the “dead cell” or “stagnation points” might be appropriate areas in which to inject waste fluid for permanent disposal [33]. The water moves through the porous media under the influence of fluid potential [12]. Recharge, can be defined as the water that crosses the vadose zone to join the water table. Before crossing the vadose zone and joining the water table it depends on a variety of factors. Winter [61] has proposed the three main factors in the hydrologic landscape that control water flow: climate, topography, and geologic framework. This makes the recharge component as one of the most complex and uncertain component of hydrologic cycle. The long term safe yield for the sustainable development is not related so much to the undisturbed recharge to the natural aquifer system, but rather to the recharge of the disturbed system and the proportion of the discharge that the ground water extraction centers are able to capture [4, 31]. The nonlinear relation interaction among recharge—discharge boundary conditions, and 1 The
article is published in the original.
changes in groundwater storage make solution to these problems difficult to resolve without careful accounting of the system parameters and their geographical distribution [48]. Hubbert’s Model Water level is an important component in delineating the recharge zones. Many workers [7, 19, 38, 43] have tried to delineate the water table often termed as “free surface”. Hubbert [22] presented the first descriptive model of regional steady state groundwater flow in an unconfined aquifer. In this model the relationship of water table, hydraulic head and equipotential lines are very well demonstrated. Figure 1 shows four piezometers A, B, C, and D installed with bottom end open. The hydraulic head is equal in the piezometers that end at the same equipotential lines, though their depth is different (A and B). However, the hydraulic head in the piezometer C and D varies as both ends at different equipotential lines. The hydraulic head in the piezometer is the level where equipotential lines cut the water table. At the piezometer “A” the hydraulic potential decreases with the depth. This is indicative of downward flow direction. Areas with this distribution of potential are recharge areas for a water table aquifer. In the deep piezometer D the hydraulic head is higher than of piezometer C. This indicates hydraulic potential increases with depth. This is typical of discharge area. This model depicts that the crest of the water table represent the groundwater recharge area while the valley bottom of the water table represent the discharge area [12]. Thus, knowing spatial distribution of recharge—discharge zones is imperative for a scientific approach towards solving the problem of unethical intervention in the natural system.
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TAQVEEM Recharge area
Discharge area
C
Equipotential lines Flow lines
D
Reccharege area B A
Water table A, B, C & D – Piezometers
Fig. 1. Water level in piezometers and the flow lines (Modified after Fetter [12]).
Water Table and the Recharge In a common reference it is considered that the water table follows the topography. It was the Toth [59] who offered a regional groundwater flow solution and assumed the position of the water table as the replica of the topography. In many settings the water table and surface topography seem poorly correlated or somewhat unrelated [3, 8, 37, 50]. Toth [59] in his conceptual model for local, intermediate, and regional flow pattern considered both the horizontal and vertical components in the flow regime. This view of nested flow components is in contrast to the Dupuit [10] and Forchhmeimer [13] approximation. These two ignored vertical component of flow in the regional scale setting and only considered the horizontal flow in their model. In the modern interpretation there is no room for the nested flow cells as proposed by Toth [59] but vertical flow is allowed with no resistance [26, 55]. Toth [59] developed the conceptual model while studying the hummocky terrain of Alberta, Canada. The region is known to contain areas of deep aquifers of low-permeability shales and sandstone overlain by a relatively thin layer of higher permeability sands [39]. The model he developed had a great depth and large distance between drainage divide and the stream. In order to the water table to rise to the highest point in this theoretical watershed, the ratio of recharge and hydraulic conductivity must be greater than 0.2 [36]. This recharge—hydraulic conductivity ratio is quite high and is only possible when the low permeability basin is overlain by a high permeability vadose zone, as the case was in Toth’s study. The thick permeable
vadose zone will store more precipitation rather than allow a high run-off and would give sustained recharge to the aquifer. Haitjema [16] has examined two situations (i) where the recharge—hydraulic conductivity ratio is relatively low and, (ii) where the recharge—hydraulic conductivity ratio is relatively high. In the first situation R/k = 0.08 and the head is specified (Figs. 2a, 2b). In Fig. 2a the water table is well below the land surface, the recharge area is more. The discharge is in sink, where the head meet the low-lying land surface. In Fig. 2b where the R/k is relatively more i.e. 0.4, the water table rises to intersect the land surface at two points, which are the discharging points. The flow pattern is divided into two units. A local flow cell has developed on the left hand side near the bottom of the watershed. On the right hand side the recharge area has shrunk due to the rise in the water table. In the Fig. 2b if the water table is measured it appears that it is the subdued replica of the topography. But when it is examined in the situation where water table is the replica of land surface (Fig. 3a) different flow system is obtained. Here numerous local flow cells are formed with zones of vertical flow. Numerous recharge and discharge cells are formed. This is the typical example that shows the heterogeneity of the aquifer system. If the head-specified upper boundary conditions are applied to a deeper aquifer as shown in Fig. 3b with an increase in basin depth ratio, there develops the threeflow systems local, intermediate and regional [16]. This intermediate flow systems have at least one local flow system between the recharge and discharge areas. Regional flow systems have the recharge area in the WATER RESOURCES
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(a)
305 m
Discharge zone
7620 m (b)
Discharge zone 305 m
Land Surface
Water Table
7620 m Fig. 2. Flow simulation of theoretical aquifer using recharge—specified boundary conditions, (a) recharge = 25.4 mm/year, hydraulic conductivity = 0.3 m/d; (b) recharge = 126 mm/year, hydraulic conductivity = 0.3 m/d (after Mitchell–Bruker [36]).
305 m
(a)
7620 m
305 m
(b)
7620 m Fig. 3. Flow simulation of theoretical aquifer using head—specified boundary conditions, (a) 1000 ft aquifer depth, (b) 10000 ft aquifer depth (after Mitchell-Bruker [36]).
basin divide and the discharge area at the valley bottom [12]. The model is the same as that of Toth [59]. This depicts that R/k ratio play a key role in forming the local cell units. More the R/k ratio, more the heterogeneous flow system is. Controlling Factors Hydrogeologist are working intensely on the recharge of groundwater, where unsaturated zone or vadose is quite thick [30, 35, 45, 57, 60]. Recharge depends on a wide variety of factors (e.g. vegetation, WATER RESOURCES
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precipitation, climate, topography and soil type) making it one of the most complex and uncertain hydrologic parameter to quantify [9]. If the climate and soil condition allows recharge to reach the water table at a rate that is greater than the saturated zone can transmit the recharge away, then the permeability of the geologic framework controls the recharge rate. This situation results in the condition of shallow water table because storage of water underground backup to the point that excess infiltration is diverted overland [61]. This situation is associated with relatively low permeable or anisotropic aquifers with
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TAQVEEM Evapotranspiration Precipitation Direction of Surface–water flow Surface Water
Water table Upland
Valley Side
Lowland Direction of groundwater flow
Geologic Framework Fig. 4. The dominant factors affecting recharge and groundwater flow (after Winter [61]).
high rainfall and flat terrain. Consequently there will be rise in the water table and mounds will form. Haitjema [16] offered a dimensionless relationship to estimate the mound formed due to the excessive recharge. Δh/d = 1RL /mkHd 2
( L ≥ k h /k v H ).
Where, Δh [m] is the mounding or rise in water table between streams, d [m] is the water level before the mounding commenced, L [m] is the distance between the streams/diameter in a radial flow, R [m/d] is the recharge rate, m is 8 or 16 depending on the flow problem being one dimensional or radial respectively, k [m/d] is the hydraulic conductivity of the aquifer, H [m] is the average thickness of the aquifer, kh is the horizontal hydraulic conductivity, and kv is the vertical hydraulic conductivity. In the climate control recharge, the saturated zone transmits more water than the climate and the vadose zone can provide. In this situation the water table shall be deep. In regions with relatively arid climate or high topographic relief, the climate controls the rate of recharge, whereas in regions of relatively humid climate or low topographic relief the geologic framework controls the rate of recharge. Variability of the topography (Fig. 4) or the geologic framework within the flow system causes different controls to operate in different regions [48]. Haitjema [16] offered a dimensionless criterion to asses water table is topography controlled or recharge controlled RL /mkHd > 1. 2
If the factor on the left hand side is more than one— the water table is controlled by the topography.
On the other hand if the water table is recharge controlled the following dimensionless condition will hold true. RL /mkHd < 1. 2
These two criteria are developed for Dupuit and Forchheimer flow conditions where distance between surface waters is large compared to the aquifer thickness i.e. the distance between two surface water bodies should be five times the thickness of the aquifer. Recharge Estimation Recharge estimation has always fancied the water scientist. In the estimation of the recharge components scale is considered an important factor. The scale of investigation influences the level of parameter detail needed to characterize the system, and the choice of an appropriate measurement technique is often evaluated according to the size of the study area [49]. The community scale extends upto several Sq. kms. and regional scale should encompass an area of 100 to 10000 sq kms. The scale to which recharge is defined has varied opinions, depending on the methods used for the purpose of quantification. Like, well hydrograph analysis [24], stream hydrograph separation [32] and geochemical tracers [49]. The Groundwater Resource Estimation Methodology [15] has given due consideration to the scale factor and suggested that instead of administrative boundaries, hydrogeological boundaries (watershed) be considered for the estimation of recharge. Historically, hydrograph analysis techniques have been developed for streams and spring discharge data [40]. Relatively few studies [1, 44, 51] have been conducted that yield quantitative data on aquifer parameWATER RESOURCES
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ters using well hydrographs. One motivation for examining the analysis of hydrographs for estimation of T and specific yield was that traditional aquifer testing methods require pumping large quantities of water from the aquifer [42]. The limitation with the hydrograph analysis method is that it is limited to a single well observation. For its expansion a strong network of observation stations and their regular monitoring is required. If this is done than the accuracy of the result will bear more credence. Stream hydrograph separation analysis is used to estimate recharge and discharge because daily stream—discharge measurements are the only data required [2, 11, 41]. This method can be used to an area, which has water divides as its boundaries. The automation of hydrograph separation has made the various techniques easier to implement [25, 34, 46, 47, 52]. The automated techniques estimate the recharge rate that is similar to manual estimates [32, 46, 47]. While an automated technique enforces a more consistent approach to identify groundwater discharge than the manual equivalent, the automated technique is still highly subjective and affected by the same underlying assumptions. If the basic assumptions are not met, automated hydrograph—separation—techniques become ideal tools for the preservation and spreading of hydrologic misconceptions [17, 27]. The assumptions made in the hydrograph—separation—analysis are there exist a direct correlation between groundwater recharge events and stream discharge peaks; evapotranspiration from the saturated zone, wetland, surface water bodies, and streams storage is negligible; bank storage is negligible; interaction between shallow aquifer and deep aquifer system is negligible; hydraulic characteristics of the contributing aquifer (recession index) can be estimated; the period of exclusively groundwater discharge can be estimated. Stream discharge peaks approximate the magnitude and timing of recharge events. The technique has its limitations too. Some of them are baseflow frequently is not equivalent to groundwater discharge because other hydrologic phenomenon can noticeably affect stream discharge; natural and human induced stresses such as climate and land management do affect the baseflow [14, 41]; it is assumed that streams have no slope and that groundwater discharges uniformly to all reaches, though which is otherwise; drainage from bank storage, wetlands, surface water bodies, and soils exceeds groundwater discharge; groundwater discharge during recession periods may be obscured by discharge from lakes, marshes, snow and ice, and bank storage [21]. Because of the violation of the main assumptions the technique is characterized as inconclusive [17, 18]. WATER RESOURCES
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Investigators wanting to use hydrograph separation technique to estimate groundwater discharge or recharge should also do with caution and with careful consideration of the degree of deviation from the model assumptions [17]. Groundwater models are used in water budgeting studies. In this, the hydraulic properties and boundary conditions are used as a major input for the estimation purposes. Recently, watershed models have been proposed as means to estimate groundwater recharge across scales ranging from local to regional watershed [6, 54]. Hydraulic conductivity varies with factors such as soils porosity, pore size, and water temperature. Many researchers [20, 28, 29, 53, 58] have attempted to clarify the relationships of hydraulic conductivity to these factors. The hydraulic conductivity shows a direct relation to the temperature and pore size [23]. The reliability of recharge in models depends on the accuracy of hydraulic conductivity inputs [49]. Thus, the models with uniform hydraulic conductivity or with limited values of the same give recharge results the accuracy of which is not known. So, more the hydraulic conductivity inputs better are the results. Cherkauer [6] has formulated an empirical relation between normalized recharge and readily available climatic, topographic, hydrogeologic, and land measures for small watershed in the glaciated terrain. A normalized annual recharge R/P (recharge per unit precipitation in cm/cm) was correlated with the multiple regression analysis. The formula is R/P = 0.0085 ( K v /SD ) – 4.18 { Dw/L f } 0.3
+ 0.0025 { N } + 0.0022. Where Kv is effective vertical soil conductivity (m/d); S is average hill slope in watershed (m/m); Dw is average depth to the water table (m); Lf is the length of flow to the main channel (drainage area/2 × channel length) (km); D is the portion of developed land in the watershed (as a %). This formula is best suited for wet land. The equation give best results with the following limits; K ≤ 2.7 (m/d); Natural land cover ≤30% average hill slope ≥0.03; Depht to the water table ≥9.1 m and development ≥5% [6]. SUMMARY Water table is an important parameter in delineating the recharge and discharge zones. For a pragmatic approach in the management of the groundwater resources a micro level study has to be carried out at the small watershed level. The study of groundwater should begin with a small watershed as a unit area of investigation. The crest of the water table is the area of recharge and the trough is the area of discharge. The recharge—
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hydraulic conductivity ratio is a key parameter in deciphering the heterogeneity of the aquifer system. The deep water table indicates the saturated zone transmit more water than the vadose zone can provide. The shallow water table depicts that the vadose zone provides more water than the saturated zone can transmit. The result of Recharge estimation has little credence without the proper consideration of the spatial and temporal distribution of the recharge component. Different techniques with a comprehensive approach are necessary for a credible estimation of the recharge. REFERENCES 1. Atkinson, T.C., Diffuse Flow and Conduit Flow in Limestone Terrain in the Mendip Hills, Somerset (Great Britain), J. Hydrology, 1977, vol. 35, pp. 93–110. 2. Bevans, H.E., Estimating Stream—Aquifer Interactions in Coal Areas of Eastern Kansas by Using Streamflow Records, in Selected Papers in the Hydrologic Sciences, Subitzky, S., Ed., USGS Water—Supply Paper, 1986, vol. 2290, pp. 51–64. 3. Blaskova, S., Beven, K., Tachecci, P., and Kulasova, A., Testing the Distributed Water Table Predictions of TOPMODEL (Allowing for Uncertainty in Model Calibration): The Death of TOPMODEL?, Water Resources Res., 2002, vol. 38, no. 11, pp. 1257–1268. 4. Bredehoeft, J.D., Papadopulos, S.S., and Cooper, H.H., Jr., Groundwater: the Water—Budget Myth, in U.S. Natural Research Council Studies in Geophysics: Scientific Basis of Water Resource Management, Washington DC: National Academy Press, 1982, pp. 51–57. 5. Cherkauer, D.D., Quantifying Ground Water Recharge at Multiple Scales Using PRMS and GIS, Ground Water, 2004, vol. 42, no. 1, pp. 97–110. 6. Cherkauer, D.S. and Ansari, Sajjad, A., Estimating Ground Water Recharge from Topography, Hydrogeology, and Land Cover, Ground Water, 2005, vol. 43, no. 1, pp. 101–112. 7. Davis, S.N. and De Wiest, Hydrogeology, New York: Wiley, 1966. 8. Desbarats, A.J., Logan, G.E., Hinton, M.J., and Sharpe, D.R., On the Kriging of Water Table Elevation Using Collateral Information from a Digital Elevation Model, J. Hydrogeology, 2002, vol. 255, no. 1, pp. 25– 39. 9. Dripps, W.R., Hunt, R.J., and Anderson, M.P., Estimating Recharge Rates with Analytic Element Models and Parameter Estimation, Ground Water, 2006, vol. 44, no. 1, pp. 47–55. 10. Dupuit, J., Estudes Theoriques at Practiques Sur le Mouvement des Eaux dans les Canaux Decouverts at a travers les Terrains Permeables (2nd ed.), France: Paris, Dumod, 1863. 11. Faye, R.E. and Mayer, G.C., Groundwater Flow and Stream-Aquifer Relations in the Northern Coastal Plain of Georgia and Adjacent Part of Alabama and South Carolina, WRI, USGS, 1990, pp. 88–4143. 12. Fetter, C.W., Applied Hydrogeology, Ohio: Merrill Publishing Company, 1988.
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