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