Groundwater recharge. MIEA & Erasmus Mundus GroundwatCH Groundwater Pollution and Protection (M. Teresa Condesso de Melo)

Transcription

1 Groundwater recharge Precipitation, Runoff, Infiltration, Evaporation, Transpiration, Aquifer Recharge

2 Groundwater recharge of an aquifer system may be defined as the volume of water that reaches the saturated zone of the aquifer contributing for replenishing the groundwater reservoir. Groundwater recharge is the quantity which, in the long term, is available for both abstraction and supporting the baseflow component of rivers (Rushton & Ward 1979).

3 Groundwater recharge is an important component of most groundwater flow models and it may be either specified or estimated during model calibration (Sanford, 2002). However, the idea that knowing the groundwater recharge is important in determining the size of a sustainable groundwater development (long-term safe yield) is a myth because it is an oversimplification of the information that is needed to understand the effects of developing a groundwater system (Alley et al., 1999; Bredehoeft, 2002).

4 Groundwater recharge can be either a naturalor an artificialprocess. The natural recharge occurs when it stems from the direct infiltration of rainfall or from the water percolation of adjacent water bodies, and the artificial recharge when it is induced by human activity such as irrigation, urbanisation, construction of injection boreholes or river spreading. Depending on the route followed by percolating water towards the water table, recharge can be classified as direct recharge when talking about diffuse infiltration of recharge water towards groundwater, or as indirect recharge when alongriverandother mainchannels. Some authors, such as Lerner (1997), also refer to localised/ focussed recharge to describe the recharge that suffered some horizontal movement before recharging groundwater, such asstreamsand lakes. Rushton (1997) also refers to actual and potential recharge to distinguish between the infiltrated water that reaches indeed the water table from that recharge, estimated from surface-water and unsaturated zone studies, which may or may not reach the water table.

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6 Nowadays there are several methods available to estimate natural groundwater recharge, from the well-known: physical methods (using either direct techniques based on the use of lysimeters or on the water table fluctuation, or indirect techniques based on the estimate of soil physical parameters); geochemical methods (using chemical - chloride and isotope techniques tritium, for example); inverse methods (using numerical models to solve the groundwater flow equation for recharge instead of groundwater heads).

7 Penman-Grindley method The Penman-Grindley (Penman, 1950; Grindley, 1967) is the conventional and most widely used method of estimating recharge. The method was initially developed to calculate soil moisture deficit and actual evaporation, but allows recharge to be calculated as a function of effective rainfall. It conceptualises that water is held in store as soil moisture, which can be increased by rainfall and depleted by evapotranspiration. When the field capacity is attained, excess rainfall is routed to surficial runoff and to groundwater asrecharge.

8 Penman-Grindley method In this method, the soil water balance equation is defined for a uniform zone and certain interval of time as follows: P = E A ΔS = P -E + R + ΔS A R where P is rainfall [L], E A is the actual evapotranspiration [L], R is the runoff [L L -2 ] and S is the change in soil water storage [L]

9 Penman-Grindley method (limitations) The accuracy of this method relies on the accuracy of the initial data. Actual evapotranspiration is one of the critical parameters. It can either be determined directly from open water evaporation pans or irrigated lysimeters, or calculated from meteorological data using for instance the Penman (1948) or the Thornthwaite (1948) formulae, just to mention two of the most used ones. The method works best for humid climates with seasonal patterns of recharge, widespread and relatively uniform rainfall, in areas with well-developed soils that never drycompletely (Lerner, 1997). And, it is unlikely to be successful in semi-arid areas because of the long periods of less than potential evaporation, when errors in estimating the actual evapotranspiration are greatest, and the precipitation and the actual evapotranspiration are nearly equal (Allison, 1981).

10 Water table fluctuation method The water table fluctuation method for estimating groundwater recharge is based on the premise that rises in groundwater levels in unconfined aquifers are due to recharge water arriving at the water table and going immediately into storage (Healy & Cook, 2002). The recharge rate is calculated as follows: q w = Sy dh dt h = Sy t where Sy is the specific yield, h is the water table height and t is time. How to calculate h? Hydrograph of average groundwater level and bar graph of weekly average precipitation for Beaverdam Creek Basin, Maryland USA (after Rasmussen and Andreasen, 1959). Dashed lines represent the expected level to which the water table would have receded in the absence of precipitation. How to calculate h?

11 Sy is the specific yield

12 Water table fluctuation method (limitations) This method is very easy to use and has been applied in numerous studies as early as the 1920s. However, the method is not able to account for steady rate of recharge (e.g. when rate of recharge is equal to rate of drainage) and works best in areas with shallow water tables that display sharp water level rises and falls. Another very important constraint is the difficulty and uncertainty on the determination of the specific yield, despite the fact that there are several laboratory, field or geophysical methods availableto calculate it. The analysis of the water table fluctuations provides spatially averaged recharge rates as long as the monitoring wells are located such that the water levels are representative of the catchment as a whole. The time period represented by the recharge estimates will depend on the length of the hydrographic record.

13 Chloride mass balance in the saturated zone The groundwater chloride mass-balance method is frequently used for estimating groundwater recharge, specially related to the fact that it is a inexpensive method, does not require sophisticated instrumentation and is independent of whether recharge is focused or diffuse. The method yields recharge estimates that are integrated in the space and in time, and was originally applied in the late sixties to estimate recharge rates in the coastal plain of Israel (Eriksson & Khunakasem, 1969). The use of the chloride mass balance approach in groundwater requires the knowledge of three environmental variables, which are: (1) the mean annual rainfall for the study region; (2) the average annual total chloride fallout; and, (3) the average groundwater chloride concentrations inthe studyarea.

14 Chloride mass balance in the saturated zone According to the chloride mass-balance method, the mean annual recharge flux (R) is calculated as stated by the basic equation (e.g. Allison & Hughes,1978): R P = ( C + C ) p C gw D where P is the long-term mean annual precipitation [L T -1 ], C P is the weighted mean concentration of chloride in rainfall [M L -3 ], C D is the amount of chloride in the dry deposition [M L -2 T -1 ] and C gw [M L -3 ] is the average chloride concentration ingroundwater within the rechargearea.

15 Chloride mass balance in the saturated zone (limitations) The method assumes that chloride ion behaves as a conservative, nonadsorbed environmental tracer under steady-state conditions, and the validity of its application is restricted by several assumptions: the only origin of groundwater chloride is either from rainfall or from dry deposition, and it doesn t occur any recycling of chloride within the aquifer; rainfall and atmospheric input of chloride (wet and dry fallout) is considered to be constant with time over long periods of time; rainfall is evaporated and/ or recharged to groundwater without any significant surface runoff; no groundwater evaporation occurs upgradient from the groundwater sampling points.

16 Chloride mass balance in the unsaturated zone Schematic depth profiles of the chloride concentration of soil water (Allison, 1988). (a) Piston flow with abstraction of soil moisture by roots; (b) Abstraction of soil moisture by roots with either preferred flow of water to beneath the root zone or diffusive loss of chloride to the water table; (c) A profile which may reflect different recharge rates and conditions for one site.

17 Chloride mass balance in the unsaturated zone Assuming that water and solute (chloride ion) behave conservatively and that their downward movement is by piston flow below the surface mixing layer, the total chloride content C Tz [M L -2 ] down to a depth z can be calculated according to (e.g. Allison et al. 1985; Allison et al. 1992; Selaolo, 1998): C T z θ ρ Z Z G i = 10 hici = 10 i= 1 ρw i= 1 h C θ i i w where h i is the depth interval [L]; C i is the chloride content in soil moisture [M L -1 ]; θ G the gravimetric moisture content and is expressed as grams of moisture per 100 g of dry sediment and ρ i the bulk density [M L -3 ]. The term θ G ρ i /ρ w =θ w where θ w is the volumetric moisture content. ρ w [M L -3 ] is the water density and is considered ~1. Similarly, the total moisture content θ Tz [M L -2 ] is given by: θ T z = 10 Z i= 1 h i θ G ρ ρ w i

18 Chloride mass balance in the unsaturated zone Furthermore, if the average annual total chloride fallout C fallout [M L -2 T -1 ] at the surface is assumed to be constant over the long term, then the vertical soil moisture flux q w [L T -1 ] can be expressed as: Cfallout θtz qw = C And the actual soil moisture velocity ν w is given by: q υw = θ To study changing environmental conditions with time, cumulative chloride content (C T ) is plotted against cumulative soil moisture content (θ T ), and the moisture flux q w becomes: θt qw = Cfallout C T z If the total chloride deposition is assumed to be constant in time, the soil moisture age t wz [T] at the point of interest within a profile can be calculated as follows: C t w Z = C Tz w w Tz fallout

19 Chloride mass balance in the unsaturated zone Chloride profiles in the unsaturated zone determined by analysis of pore waters extracted from drilled cores. (a) Typical smooth profile encountered where only direct recharge is operative. (b) Typical noisy profile encountered in situations where bypass recharge routes introduce indirect recharge to the unsaturated zone at various depths in the sub-soil profile. (Developed after Lerner et al )

20 Comparison between groundwater recharge estimation methods (space scales, costs & complexity)

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22 Problems

23 Problems

24 PROBLEM The Aveiro Quaternary groundwater body is an unconfined sandy aquifer (specific yield, S y =0.05) and occupies a total area of approximately 931 km 2. Climatological data indicate that: Average rainfall: mm/year Average potential evapotranspiration (ETP, Penman method): mm/year Surfacerunoff: 300 mm/year Average chloride concentration in rainfall: 5.3 mg/l Averagechloride concentration in groundwater: 30 mg/l Averageannual fluctuation ofwater level: 5.6 m Estimate the groundwater recharge for the Aveiro Quaternary aquifer using the: Penman-Grindley method Water fluctuation method Chloride mass balance method WATER LEVEL IN WELLS (m) Oct Nov Dec Jan h=5.60 m Feb Mar Apr May Jun Jul Aug WELL #23 Sep

25 PROBLEM 1. Resolution of the problem using the Penman-Grindley method. MIEA & Erasmus Mundus GroundwatCH OCT NOV DEC JAN FEB MAR APR MAY JUN JUL AUG SEP TOTAL P (mm) Rainfall ETP (mm) P-ETP (mm) ETR (mm) Potential Evapotranspiration (Penman) Actual Evapotranspiration, Ea (Penman) R w (mm) Reserve of water for plants R w (mm) Variation in water reserve for plants EXC (mm) Runoff + Stored water, S DEF (mm) Water deficit Verification (P = ETR + EXC + R w ) ANSWER: The total excess water (EXC) is mm/year corresponding to surficial runoff + groundwater recharge. Assuming that surficial runoff is 300 mm/year (initial data of the problem), the groundwater recharge (R) is approximately 243 mm/year. We have groundwater recharge from October to June and deficits (requires irrigation for plants to survive) from July to September.

26 PROBLEM 2. Resolution of the problem using the chloride mass balance method. R P = ( C + C ) p C gw D mm / year 5.3mg / l = 30.0mg / l = 190mm / year ANSWER: The groundwater recharge (R) is approximately 190 mm/year.

27 PROBLEM 3. Resolution of the problem using the water fluctuation method. RR = SSSS h tt = mm 1yyyyyyyy = mmmm 1yyyyyyyy = 280mmmm/yyyyyyyy 20 WELL #23 WATER LEVEL IN WELLS (m) h=5.60 m 15.6 Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep ANSWER: The groundwater recharge (R) is approximately 280 mm/year.

28 Estimates of the groundwater recharge for the Aveiro Quaternary aquifer using the: Penman-Grindley method = 243 mm/year Chloride mass balance method = 190 mm/year Water fluctuation method = 280 mm/year The estimates of groundwater recharge vary between 190 and 280 mm/year depending on the method used to calculate. On average we can consider a value of 238 mm/year (= 238 litres/m 2 /year= 238 dm 3 /m 2 /year) for the groundwater recharge ofthe region. If we consider the total area of the region (931 km 2 ) we can calculate the total volume of groundwater recharge for the studied region: VV RR = mm 3 mm mm 2 = mm 3 /yyyyyyyy = 221hmm 3 /yyyyyyyy