I. HYDROGEOLOGY OF ARID REGIONS
9
All items except changes in groundwater storage can be measured or estimated. The difference between the two sides of the equation for a given
budget period is, therefore, ascribed to changes in groundwater storage.
If observation wells are available, changes in water levels in the wells can
be related to changes in groundwater storage, and the accuracy of the hydrologie budget can be checked. Unfortunately, the groundwater increment
of a given budget within a desert region is so small in comparison to other
items that it is masked by errors of measurement of precipitation as well
as the other budget items.
Another more direct hydrologie budget can commonly be used in local,
well-defined groundwater basins where man has not upset the natural balance and where all groundwater eventually comes to the surface in springs
or in areas of dense vegetation. Water discharge and évapotranspiration
are estimated for these areas of groundwater discharge and are assumed
to be equal to the long-term groundwater recharge. Such calculations must,
of course, assume steady-state conditions for the basin.
In the budget methods, usually the only unknown value in Eq. (6) is
T which can then be estimated by Eqs. (4) and (5).
A second method for solving Eq. (6) requires test drilling to measure m
and laboratory measurements of K from samples of the aquifer recovered
during drilling. Thus, Km (or T) is known and Q T can be calculated. The
number of laboratory determinations of K needed is excessive, so this
method is expensive and tedious.
The third general method, that of aquifer tests, is most reliable. By injecting water into wells or by pumping water from the wells, artificial
changes of hydraulic head are produced in the aquifer. These changes can
be analyzed (Davis and De Wiest, 1966; Hantush, 1966; Walton, 1970)
to determine values of T.
In practice, it is common to use both hydrologie budget studies and
aquifer tests as independent methods to analyze desert basins.
Figure 6 presents a simple application of Eq. (6). An estimate of the
total amount of water flowing in a desert valley is of interest. Aquifer tests
indicate a value of T of 1200 m
2
/day. By construction of the flow net,
n = 3 and Ah = 0.4 meter. The total flow Q r is therefore
Q T = nTAh = (3)(1200)(0.4) m
3
/day
Q T = 1440 m
3
/day or 381,000 gallons/day
Although a scale showing horizontal distance is given in Fig. 6, it is
important to note that this information is not needed for the calculation.
Flow nets can be used to assess the amount of recharge or discharge
from a stream. The example in Fig. 7 shows two stream channels crossing
9
All items except changes in groundwater storage can be measured or estimated. The difference between the two sides of the equation for a given
budget period is, therefore, ascribed to changes in groundwater storage.
If observation wells are available, changes in water levels in the wells can
be related to changes in groundwater storage, and the accuracy of the hydrologie budget can be checked. Unfortunately, the groundwater increment
of a given budget within a desert region is so small in comparison to other
items that it is masked by errors of measurement of precipitation as well
as the other budget items.
Another more direct hydrologie budget can commonly be used in local,
well-defined groundwater basins where man has not upset the natural balance and where all groundwater eventually comes to the surface in springs
or in areas of dense vegetation. Water discharge and évapotranspiration
are estimated for these areas of groundwater discharge and are assumed
to be equal to the long-term groundwater recharge. Such calculations must,
of course, assume steady-state conditions for the basin.
In the budget methods, usually the only unknown value in Eq. (6) is
T which can then be estimated by Eqs. (4) and (5).
A second method for solving Eq. (6) requires test drilling to measure m
and laboratory measurements of K from samples of the aquifer recovered
during drilling. Thus, Km (or T) is known and Q T can be calculated. The
number of laboratory determinations of K needed is excessive, so this
method is expensive and tedious.
The third general method, that of aquifer tests, is most reliable. By injecting water into wells or by pumping water from the wells, artificial
changes of hydraulic head are produced in the aquifer. These changes can
be analyzed (Davis and De Wiest, 1966; Hantush, 1966; Walton, 1970)
to determine values of T.
In practice, it is common to use both hydrologie budget studies and
aquifer tests as independent methods to analyze desert basins.
Figure 6 presents a simple application of Eq. (6). An estimate of the
total amount of water flowing in a desert valley is of interest. Aquifer tests
indicate a value of T of 1200 m
2
/day. By construction of the flow net,
n = 3 and Ah = 0.4 meter. The total flow Q r is therefore
Q T = nTAh = (3)(1200)(0.4) m
3
/day
Q T = 1440 m
3
/day or 381,000 gallons/day
Although a scale showing horizontal distance is given in Fig. 6, it is
important to note that this information is not needed for the calculation.
Flow nets can be used to assess the amount of recharge or discharge
from a stream. The example in Fig. 7 shows two stream channels crossing