Page 295 - Hydrogeology Principles and Practice
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HYDC08  12/5/05  5:31 PM  Page 278






                 278    Chapter Eight





































                 Fig. 8.1 Diagram showing the inputs, flowpaths and outputs within a river basin that comprise a catchment water balance. The total
                 groundwater discharge, G , is the sum of Q + Q + Q .
                                 R         G  A   U

                 P = ET + S + G ± ∆S                 eq. 8.1  tion (Q ) will be at the expense of river baseflow (Q )
                         R   R                                    A                                 G
                                                             and so potentially causing environmental impacts.
                 On a short timescale of weeks to months, equation  As a general rule, a useful measure of the quantity
                 8.1 is balanced by changes in the water held in soil and  of water required to maintain acceptable minimum
                 groundwater storage (∆S) as represented by changes  river flows is the Q95 low flow statistic. This means
                 in soil moisture content and groundwater levels.  that during the baseflow recession period, when river
                 Over longer timescales of several years, changes in  flows are dominated by the groundwater input (Q ),
                                                                                                    G
                 storage balance out to zero and, expressing G = Q +  it is undesirable for the river flow to fall below the
                                                   R   G
                 Q , equation 8.1 becomes:                   long-term average of flow that is equalled or exceeded
                  A
                                                             for 95% of the time.
                 P − ET = S + Q + Q                  eq. 8.2   For sustainable groundwater development, and in
                         R   G   A
                                                             order to meet conflicting environmental (Q ) and
                                                                                                 G
                 In other words, the difference between precipitation  socioeconomic (Q ) demands, it is highly desirable
                                                                           A
                 and evapotranspiration, or effective precipitation,  that these demands do not exceed the rate of ground-
                 supports surface runoff, groundwater discharge as  water recharge (equated to G ). Three conditions can
                                                                                   R
                 river baseflow and borehole abstractions. The total  be defined in which: (i)  Q < G − Q and further
                                                                                   A   R   G
                 flow in a river is calculated as the sum of S and Q .  groundwater resources are available for exploitation;
                                                  R     G
                 Methods for calculating the amount of effective   (ii)  Q = G − Q , in which case the safe yield has
                                                                 A   R    G
                 precipitation (hydrological excess) are discussed in  been achieved; and (iii) Q > G − Q when the ground-
                                                                               A   R   G
                 Section 5.5. Clearly, if equation 8.2 is to be balanced,  water resources are over-exploited or mined. In the
                 any increase in the amount of groundwater abstrac-  case of (iii), groundwater in support of abstractions
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