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A.M.J. Meijerink
mechanisms and difficulties in estimation are described in the good overview by
Lerner et al. (1990).
First, qualitative estimation of the recharge is discussed, followed by semiempirical approaches and quantitative methods.
Qualitative approach. It must be admitted that in hydrogeological practice often
an empirical estimate of the recharge is uniformly distributed over the aquifer.
Although a qualitative approach for estimating the recharge remains qualitative, at
least an attempt can be made to differentiate spatial patterns of recharge. By visual
interpretation of remotely sensed imagery, especially stereo-aerial photographs,
the geomorphology can be used for delineation of the various soil/overburden
units. Textures and depths of overburden require field observations. Segmentation
of terrain in physiographic units each having a set of geomorphological and soil
properties, possibly with addition of vegetation, have been described by;
Goosen,1967; Verstappen,1977; Way,1978; Townshend, 1981; Meijerink,1988.
Once the units and their attributes have been mapped, transfer functions are
needed to convert meteorologic and terrain data into quantities of recharge, and
this is where difficulties are met.
One way of conversion is using data from the literature describing similar areas
or by using appropriate field methods. Specific terrain features may be added, such
as micro-drainage patterns, which may influence the recharge.
The approach can be best explained by continuing the qualitative example of
Figure 14.1. The relative recharge can be assessed to be negligible on the slate
outcrops. The depth of the weathered zone and the clayey soil varies from 40 to
100 cm. After dry periods, initial rainfall enters into the cracks of the soil which
close rapidly while the subsoil is impermeable. The gravel deposits can store
temporarily some infiltrated water, but they are too thin to have an effect, because
they rest on the impermeable slates. The soil depth on the sandstones is negligible,
as can be deducted from the fact that bedding and fractures can be seen on the
photo. Consequently, the direct runoff is high. The recharge on the quartzites is
judged to be only a little better than that of the area with the slates, except where
the surface drainage is impeded by obstructions. Quite effective for recharge is the
capturing of surface runoff by the blocked valleys. The recharge is a function of
the sizes of the micro-catchments draining into each field and occasional overflow
from one walled field to another. During field work it was assessed that the yield
of open wells in or near such blocked valleys was more then elsewhere, in terms of
irrigated acreage and frequency and duration of water application.
In general, bias is minimized when the mapping is limited to three or four
relative classes only, such as units with either "high" or "low" recharge and leave
the remainder as a "medium" category until further information is available. Units
such as sand and gravel deposits, highly fractured rock outcrops, limestones with
surface karst and so on, can be identified on imagery. In such units in dry climates
the recharge can be substantial proportions of the annual rainfall, see the overview
and discussion in Lerner et al. (1990). Units with no or little recharge are the
exfiltration areas (see below), or sloping units with shallow soils and outcrops.
A.M.J. Meijerink
mechanisms and difficulties in estimation are described in the good overview by
Lerner et al. (1990).
First, qualitative estimation of the recharge is discussed, followed by semiempirical approaches and quantitative methods.
Qualitative approach. It must be admitted that in hydrogeological practice often
an empirical estimate of the recharge is uniformly distributed over the aquifer.
Although a qualitative approach for estimating the recharge remains qualitative, at
least an attempt can be made to differentiate spatial patterns of recharge. By visual
interpretation of remotely sensed imagery, especially stereo-aerial photographs,
the geomorphology can be used for delineation of the various soil/overburden
units. Textures and depths of overburden require field observations. Segmentation
of terrain in physiographic units each having a set of geomorphological and soil
properties, possibly with addition of vegetation, have been described by;
Goosen,1967; Verstappen,1977; Way,1978; Townshend, 1981; Meijerink,1988.
Once the units and their attributes have been mapped, transfer functions are
needed to convert meteorologic and terrain data into quantities of recharge, and
this is where difficulties are met.
One way of conversion is using data from the literature describing similar areas
or by using appropriate field methods. Specific terrain features may be added, such
as micro-drainage patterns, which may influence the recharge.
The approach can be best explained by continuing the qualitative example of
Figure 14.1. The relative recharge can be assessed to be negligible on the slate
outcrops. The depth of the weathered zone and the clayey soil varies from 40 to
100 cm. After dry periods, initial rainfall enters into the cracks of the soil which
close rapidly while the subsoil is impermeable. The gravel deposits can store
temporarily some infiltrated water, but they are too thin to have an effect, because
they rest on the impermeable slates. The soil depth on the sandstones is negligible,
as can be deducted from the fact that bedding and fractures can be seen on the
photo. Consequently, the direct runoff is high. The recharge on the quartzites is
judged to be only a little better than that of the area with the slates, except where
the surface drainage is impeded by obstructions. Quite effective for recharge is the
capturing of surface runoff by the blocked valleys. The recharge is a function of
the sizes of the micro-catchments draining into each field and occasional overflow
from one walled field to another. During field work it was assessed that the yield
of open wells in or near such blocked valleys was more then elsewhere, in terms of
irrigated acreage and frequency and duration of water application.
In general, bias is minimized when the mapping is limited to three or four
relative classes only, such as units with either "high" or "low" recharge and leave
the remainder as a "medium" category until further information is available. Units
such as sand and gravel deposits, highly fractured rock outcrops, limestones with
surface karst and so on, can be identified on imagery. In such units in dry climates
the recharge can be substantial proportions of the annual rainfall, see the overview
and discussion in Lerner et al. (1990). Units with no or little recharge are the
exfiltration areas (see below), or sloping units with shallow soils and outcrops.