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Groundwater investigation techniques 161

Table 5.5 Example of the Penman–Grindley soil water budget method to calculate values of actual evapotranspiration (AE), soil moisture
deficit (SMD) and recharge (hydrological excess, HXS) using precipitation (P) and potential evapotranspiration (PE) data. The calculation
assumes that the permeable soil is covered by a short-rooted grass with a root constant (RC) of 75 mm and that the soil is at field capacity
on 1 April (SMD = 0).
Month P (mm) PE (mm) P − PE (mm) AE (mm) DSMD (mm) SMD (mm) HXS (mm)

Apr. 20 48 −28 48 28 28 0
May 12 56 −44 56 44 72 0
Jun. 24 72 −48 (24 + [100 − 72] + 0.1[20]) = 54 (28 + 2) = 30 102 0
Jul. 9 68 −59 (9 + 0.1[59]) = 15 6 108 0
Aug. 31 42 −11 (31 + 0.1[11]) = 32 1 109 0
Sep. 60 28 32 28 −32 77 0
Oct. 75 20 55 20 −55 22 0
Nov. 106 10 96 10 −22 0 74
Dec. 94 5 89 5 0 0 89
Jan. 69 5 64 5 0 0 64
Feb. 40 18 22 18 0 0 22
Mar. 72 30 42 30 0 0 42
Apr. 18 50 −32 50 32 32 0
Total recharge 291

correct daily recharge. On a monthly basis, and as the balance of a number of measurements including
input to a regional groundwater flow model, the baseflow, which is assessed directly from streamflow
monthly distribution of recharge calculated this way gauging. Flow data are also necessary for the deriva-
is regarded as acceptable. tion and application of operating rules for surface
reservoirs, groundwater resources and river regula-
tion (Section 8.2.3). Historic flow data are used in
5.5.3 Chloride budget method setting the minimum residual flow in a river in order
to support the aquatic ecology or provide sufficient
Soil water budgeting methods as described above dilution to achieve water quality standards. A num-
were developed for temperate climates and therefore ber of simple and advanced techniques are employed
have less validity in semi-arid and arid zones where to measure or estimate river flows (discharge), with
these methods normally underestimate recharge, the main techniques described in the following sec-
often giving zero values. An alternative, geochemical tions. General guidelines for the selection of methods
method is to use a conservative tracer species such of discharge measurement (velocity–area, slope–area,
as chloride to estimate the amount of recharge and, dilution, ultrasonic, electromagnetic, weirs and flumes)
in favourable circumstances, the recharge history. An are included in BSI (1998).
example of the application of the chloride budget
method is given in Box 5.1.
5.6.1 Velocity–area methods

Surface floats
5.6 Stream gauging techniques
This velocity–area method is particularly useful when
The recording of streamflow data is fundamental to conditions, for example during a flood, make it dan-
water resources studies and management, flood stud- gerous to deploy other discharge measurement pro-
ies and water quality management. In hydrogeology, cedures. The method requires the choice of a length
the importance of river flow data extends to ground- of river reach sufficient to allow accurate timing of a
water resources, with aquifer recharge deduced from float released in the middle of the channel and far