Page 176 - Hydrogeology Principles and Practice

HYDC05 12/5/05 5:35 PM Page 159

Groundwater investigation techniques 159

The available methods for calculating direct
recharge, three of which are discussed in the follow-
ing sections, can be classified as: direct measurement
2
using lysimeters over areas up to 100 m (for further
details, see Kitching et al. 1980; Kitchen & Shearer
1982); Darcian approaches to calculate flow in the
unsaturated zone above the water table (eq. 5.12);
borehole and stream hydrograph analysis; empirical
methods that simplify recharge as a function of rain-
fall amount; soil water budget methods either at a
field (Section 5.4.2) or catchment scale; and applica-
tion of environmental or applied tracers to follow the
saturated movement of water in the unsaturated
zone (see Fig. 4.10a).
Fig. 5.17 Borehole hydrograph record representing the change in
aquifer storage during a single recharge period. The net recharge
5.5.1 Borehole hydrograph method is equal to the product of the amplitude of groundwater level rise
(∆h) and the aquifer storage coefficient, S.
The borehole hydrograph method, in conjunction with
stream hydrograph separation, provides a convenient is zero and surplus precipitation (the hydrological
means of calculating the partitioning of effective rain- excess, HXS) is routed to surface water and ground-
fall between surface water runoff and groundwater water as recharge. The most difficult aspect is to cal-
discharge during a recharge season. Fluctuations in culate actual evapotranspiration (AE) and, in general,
borehole hydrographs represent changes in aquifer a quantity known as potential evapotranspiration
storage and, as shown in Fig. 5.17, multiplication (PE) is first defined as the maximum rate of evapo-
of the amplitude of water level change, ∆h, by the transpiration under prevailing meteorological con-
aquifer storage coefficient provides a value for the net ditions over short-rooted vegetation with a limitless
recharge. The total recharge is equal to the addition water supply. A budgeting procedure is used to con-
of net recharge and groundwater outflows (baseflow, vert PE to AE with the degree to which potential and
found by hydrograph separation (Section 5.7.1) and actual evapotranspiration rates diverge being con-
spring flow). The method is useful in the preparation trolled by a root constant (RC), a function of soil and
of a preliminary catchment water balance or in sup- vegetation characteristics and a measure of readily
port of regional groundwater flow modelling, but is available water within the root range. Representative
limited by the need for a good distribution of observa- values of RC expressed as an equivalent depth of
tion boreholes in the catchment of interest. water are given in Table 5.4.
The extent to which PE and AE diverge is a matter
of debate, with various models having been proposed
5.5.2 Soil water budget method to represent the reduction in plant transpiration with
decreasing soil moisture content. An example of a
The conventional method of estimating recharge drying curve is shown in Fig. 5.18, illustrating the
using a soil water budgeting approach is based on the decline in AE as plant wilting occurs until finally die-
studies of Penman and Grindley (Penman 1948, 1949; off is reached. The complexity of the wilting process
Grindley 1967, 1969). The method is conceptually is simply represented as a single step function in
simple. Water is held in a soil moisture store, pre- Fig. 5.18 with the AE equal to the full PE until a SMD
cipitation adds to the store and evapotranspiration equal to RC + 0.33RC is reached, at which point the
depletes it. When full, the conceptual quantity of soil AE decreases drastically to one-tenth of the PE rate.
moisture deficit (SMD), a measure of the amount No further decline in AE is shown until die-off occurs.
of water required to return the soil to field capacity, By adopting a daily, weekly or monthly budgeting