chapter 4 discussion total flow flow (weir flow). losses...
TRANSCRIPT
CHAPTER 4
DISCUSSION
Total Flow
For the study watershed, the water budget equation is defined as inflow equals
outflow minus any losses. Inflow consists of precipitation. Outflow consists of stream
flow (weir flow). Losses are due to storage in the soil and shallow fracture flow zones,
interception, and evapotranspiration (fig. 43). Total input (precipitation outside the
vegetative canopy) was 40.05 inches. The following distribution of total input was based
on the water budget calculated for the study watershed (fig. 23). Only 32.89 inches
(82.12% of total input) reached the watershed floor due to interception by the vegetative
canopy. Approximately 80.51% (32.24 inches of 40.05 inches) of precipitation was lost to
evapotranspiration and interception. Thurow and Taylor (1995, p. 659) performed a study
on a juniper watershed in Texas and found that evapotranspiration accounted for 80 to
90% of water loss (Weltz, 1987, Carlson, and others, 1990). Preferential flow paths were
dependent upon the antecedent soil moisture conditions of the soil. Most of the recorded
stream flow in the study watershed was contributed from the soil zone (17.56% of total
input, 7.03
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INPUT = OUTPUT - LOSSES
Input: Precipitation
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Contributions toFlow Zones
Loss
I/' I -,
Overland ShallowShallow
Flow ~ Soil Flow - FractureFlow
<,~ /
Interception
I
Output:Tributary
Flow
Ev apotranspirati on
Figure 43: Flow chart illustrating the water budget for the 1994-95monitoring period. The input (precipitation) equals theoutput (tributary discharge) minus any losses. Losses areattributed to evapotranspiration, interception, andstorage in the soil and shallow fracture flow zones. Theamount lost can be inferred from precipitation anddischarge measurements.
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inches). Overland flow contributed 0.36 inches (0.9% of total input) to streamflow
(output) and shallow fracture flow contributed 0.23 inches (0.58% of total input) to
streamflow. (Due to rounding, the numbers do not total 100%.) These results are
consistent with other watersheds with similar physiographic characteristics. Burt and
Butcher (1885) found that the combination of steep slopes, permeable soils overlying
impermeable bedrock, and high intensity rainfall, will promote the production of large
volumes of soil flow. Troake and Walling (1973) found that surface runoff (overland
flow) comprised only about 1.0% of total annual runoff in such watersheds.
Hydrograph Analysis
The 94-95 monitoring year can be divided into two periods based on the
analysis of the tributary hydro graph (fig. 27): a deficit period and a surplus period.
The deficit period lasted from 2/24/94 until 12/9/94. During the deficit period, quick
flow was the main contributor to weir flow with only minor contributions from the
soil flow zone. During this period the watershed was characterized by unsaturated
soils and little percolation. The surplus period lasted from 12/9/94 until 2/24/95.
During this period, quick flow, soil flow, and shallow fracture flow contributed to
weir flow. The majority of flow came from the soil zone. The watershed was
characterized by saturated soils for most of this period and percolation to the shallow
fracture flow zone and deep zone was observed. The monitoring period was further
77broken down into three periods (I, II, and ill) in order to explain how the system
recharged (fig. 44).
Period I
Period I was the longest period of the three. It lasted from 2/24/94 until
12/9/94. During Period I, evapotranspiration reached its maximum, rainfall volumes
were low and rainfall events were infrequent. Soil moisture fluctuated greatly
between the wilting point and field capacity. Most of the tributary output was the
result of quick flow probably derived from water falling directly on the stream channel
and minor gullies in the watershed. During the summer months (the deficit period),
Burt and Butcher (1985) found only single peak hydrographs to occur. Figure 28a
illustrates the typical single peak response on the hydro graph after a storm that
occurred during the latter half of Period I.
In order for lateral soil flow to occur, the soil zone had to recharge to the
point that the vertical flow component of the soil was overwhelmed. During Period I,
26.68 inches of precipitation was recorded. Despite this volume of rainfall, the soil
zone did not recharge because of high evapotranspiration and the low frequency of
rainfall events. The storage capacity of the soils was at a maximum during Period I.
Because of the infrequent rainfall events, the soils dried out between events. As the
effects of evapotranspiration began to diminish in October and the volume and
frequency of rainfall began to increase, the soil zone began to recharge and the
moisture content
Figure 44:
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Comparison of (44B) through (44F) illustrates the gradual recharge of the systemduring the monitoring period. Based on the hydrograph analysis, the monitoringperiod was divided into three periods (I, II, and III). Period I lasted from 2/24/94to 12/9/94~Period II lasted from 12/9/94 to 12/31/94~and Period III lasted from12/31/94 to 2/24/95. Three types of flow contributed to output (tributary flow):quick flow, lateral soil flow, and shallow fracture flow. Quick flow represents theoverland flow zone; lateral soil flow represents the shallow soil flow zone; andshallow fracture flow represents the shallow fracture flow zone. The tank model(After Barnes, 1940, fig. 44A) illustrates how each zone must recharge before itcan contribute to output (tributary flow). Quick flow was the main contributor offlow during Period I. Most flow in the tributary during Period I occurred onlyduring a storm (E). During Period I, the rainfall volume was lower than thehistoric averages (B) and evapotranspiration (C) reached its maximum. The soilmoisture (0) also fluctuated between wilting point (0%) and field capacity(10001<.). The threshold between quick flow and lateral soil flow is regulated bythe antecedent soil moisture conditions. In mid-October, as the effects ofevapotranspiration (C) diminished and rainfall volumes began to increase (B), thesoil moisture (0) remained constant, near field capacity (100%). This indicatesthe soil zone is recharging. For lateral soil flow to occur and to contribute totributary flow, the soils must remain saturated. Conditions are favorable for thisto occur during the winter months when the effects of evapotranspiration are attheir minimum. By 12/9/94 (period II), the soil zone is contributing to output.Note the change in soil moisture content (0) and its relationship to the tributaryhydrograph (E). Lateral soil flow is the main contributor to tributary flow duringPeriod II. The threshold between lateral soil flow and shallow fracture flow isregulated by the available storage capacity of both the soil zone and the shallowfracture flow zone. The available storage capacity of the soil zone must bedepleted before the shallow fracture flow zone can begin to recharge andcontribute to tributary flow. Conditions are most favorable for this to occurduring the winter months when the effects of evapotranspiration are at theirminimum. The shallow fracture flow zone began to contribute to tributary flow atthe end of 12/94. The main contributors to flow during Period III were lateralsoil flow as well as shallow fracture flow.
RechlfCe Recha,ceto SFF tone Lalent LAteral 10SA' tone Lalent
now now flowrontn1Mesco""lb"Ies conlrlbllles
oflOOulplll1oOu,,,,,,
loOu""'l
Rain to Date: 28.68 in Rain to Date: 33.37 in Rain to Date: 40.05 in
M94 Ao M J J A S 0 N D J 95 F
A1
2-24-94 - 12-9-94'II12-94
III12-31-94 - 2-24-95
4" ·Precipitation
B
Eva potrans pi ra tionMax
cMin
100%Soil Moisture
E.~..Tributary Hydrograph
100% Quick Flow, Soil Flow, Shallow Fracture Flow
FQF
~SFF
0"1.
===========::=~====~---_:::::::::::::========~===tl==~4~L _
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remained constant, near field capacity, from mid-October through February, 1995. At
this point it is inferred that recharge to the shallow fracture flow zone commenced.
Periods II and III
As evapotranspiration diminished, the moisture content increased to field
capacity and remained high in the soil zone and lateral flow contributed to weir flow.
Period II lasted during December, 1994. A total of33.3? cumulative inches of
precipitation was recorded to this point. During Period II, soil flow was the main
contributor or weir flow with minor contributions from quick flow. As subsequent
rainfall events occurred, the soil remained at field capacity and water percolated pass
the soillbedrock boundary into the shallow fracture flow zone. The transition from
Period II to Period III was gradual. Subsequent rains continued to recharge the
shallow fracture flow zone. The effective porosity of the shallow fracture flow zone
is smaller than the soil zone (Bernhardt, 1991)~therefore, this zone required less
water and less time to recharge. The first evidence of shallow fracture flow based on
the tributary hydrograph occurred on 12/31/94. Period II is distinguished from
Period III by contributions to weir flow by the shallow fracture flow zone. Period III
lasted from 12/31/94 until 2/24/95. During Period ill, all three types offlow
contributed to weir flow. Soil flow was the dominant flow type. Shallow fracture
flow contributed more weir flow than quick flow did during this period. This is
consistent with work by Burt and Butcher (1985) who found that in winter months
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the majority of outflow is contributed from subsurface flow (soil and fracture flow).
His evidence was a secondary delayed discharge peak on tributary hydrographs which
occur several days after the rainfall event. This study found similar discharge peaks
during the surplus period. Figures 2Sb and 2Sc illustrate the typical signature of the
hydro graph response during which soil flow and shallow fracture flow respectively
contributed to streamflow. The mechanisms of each type of flow will now be
discussed.
Overland Flow and Shallow Soil Flow
It is impossible to discuss the overland flow and shallow soil flow zones
independently of one another because of their complex interrelationship. The runoff
volumes and soil flow volumes referred to in this section refer to the amount of runoff
and infiltration recorded in the catchment areas of the eight runoff plots. These data
were used to make inferences regarding runoff production and infiltration at the eight
stations. The inferences are intended to be representative of the watershed as a
whole. The mechanisms of surface and subsurface (soil and rock) flow are controlled
by the following: antecedent soil moisture conditions; the volume of the storm; the
intensity and duration of the storm; evapotranspiration; the slope of the site (station);
vegetation, and available storage in the soil and rock zone. The most influential of
these factors are the volume of rain produced during the storm and antecedent soil
moisture conditions. Runoff production implies that the rate and volume of water
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striking the ground surface is greater than the available storage capacity of the soil to
accommodate. This implies that with fluctuating soil moisture, runoff production will
vary.
The Incipient Runoff Curve defines a runoff producing storm as one in which
at least 3.0% of throughfall forms runoff (fig. 31). The curve indicates that a
combination of intensity and duration that approaches a volume between 0.6 and 1.0
inches of throughfall (precipitation) will produce runoff The clarification should be
made that the Incipient Runoff Curve applies to the deficit period. Therefore this
available storage capacity applies to dry soils, which would indicate a maximum
storage capacity. This suggests that a smaller volume of rain is required to produce
runoff during the surplus period. The variation in volume is required because of
antecedent soil moisture conditions. In addition, the effects of evapotranspiration are
at its minimum. If the soil is at field capacity (high antecedent soil moisture), then
only a small volume of precipitation is required to produce runoff When the soil is at
less than field capacity (low antecedent soil moisture), a greater volume of
precipitation is needed to produce runoff The storage capacity of the Aledo soil is
estimated to be less than 0.12 inches per inch when conditions are at or near the
wilting point.
The duration of the storm is significant for storms with durations greater than
two hours. Storms with this duration were typically characterized in this study by
bursts of high intensity rainfall preceded and followed by long periods of low intensity
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rainfall. Runoff production will only occur during the high intensity episodes of
rainfall. The overall intensity of the storm is low due to the long duration. Therefore,
using the intensity alone as an indicator of runoff production can be misleading. The
volume and duration of the storm should also be considered. Taylor (1990) drew the
following conclusions from hydrograph analysis of his data, which seem consistent
with those observed at this site. First, runoff production is affected by the intensity,
duration, and spatial variation of precipitation. In general, the hydro graph showed a
rainfall of high intensity and short duration would have quicker and higher peaks and
the system would return to pre-storm conditions faster than a storm with contrasting
characteristics. Moore (1992) and Taylor (1990) found the volume of runoff
produced during a storm varied spatially throughout a watershed. Spatial variation
was also observed during this study (fig. 33). This suggests that not only are pre-
storm characteristics of the soil important, but the physical characteristics such as
vegetation and slope are also controlling factors. Gentle slopes at the top of the basin
(approximately 200 feet from the tributary) generally produced smaller volumes of
runoff than areas with steep slopes and at the banks of tributary (fig. 45).
There are two types of flow in the soil zone: lateral and vertical. Only lateral
flow contributes to stream (weir) flow. The threshold between vertical and lateral soil
flow is highly dependent on antecedent soil moisture conditions. Antecedent soil
moisture conditions refer to the soil moisture content prior to a storm. Antecedent
soil moisture measurements are critical for the following reasons: to quantify
available soil water; to determine the hydraulic conductivity of the soil; and to
1 2 3
___ ~~~_~:-T~n~'butary __•••• -- 200'
V Regional Water Table-------------------------Figure 45: Comparison of runoff production along the hillslope.
Gentle slopes at top of the basin (-200 feet from thetributary) and in flat lying areas along the slope (stations1, 2, 3, and 5) generally produced smaller volumes ofrunoff than areas with steep slopes and at the banks oftributary (stations 4, 6, 7, and 8).
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calculate available storage capacity of the soil (Hendrickx, 1990). Storms recorded
during the 1994-1995 monitoring period are classified as either Type A or Type B
storms (fig. 37). Type A storms were dominated by vertical soil flow. Type B storms
were dominated by lateral soil flow as well as some vertical soil flow. The available
storage capacity of the soil during Type A storms is very high. Water infiltrating the
soil zone is absorbed in the pore spaces and held in storage. Any water not absorbed
by the pore spaces ponds up at the soillbedrock interface (figs. 38, 46). This vertical
movement may be primarily along macropores (fig. 29). Freeze (1971) states that the
hydraulic conductivity of the soil must be high enough for lateral soil flow to
contribute to weir flow. The hydraulic conductivity of the soil is at its maximum
when the soil is at field capacity (saturated) and will decrease with decreasing soil
moisture (Hendrickx, 1990). As the soil moisture and subsequently the hydraulic
conductivity of the soil zone increases, more lateral soil flow is produced and the
direction of flow is parallel to the slope. Burt and Butcher (1985) state that during
summer months (the deficit period) soil moisture is too low to induce lateral flow
through the soil zone. This implies that storms during the deficit period will not
contribute flow to the tributary. This is consistent with the results from the
hydro graph analysis as well as field observations. Once the vertical flow component
of the soil was recharged, lateral soil flow could occur. This happened when effects
of evapotranspiration were at their minimum and when the soil moisture remained
high (fig. 39).
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Figure 46: Photograph of water ponded at the soillbedrock interfaceafter a Type A storm. Type A storms are characterizedby soils at or near wilting point of the soil. Any waterthat infiltrates the soil zone will flow only vertically. Thesoil is too dry to induce lateral flow, which contributesflow to the tributary. The water is then lost to storage orre-evaporated back into the system throughevapotranspiration.
87
When antecedent soil moisture conditions were at field capacity (Type B
storms), the mini-piezometers showed evidence of preferential zones of flow through
the soil zone (fig. 47). This implies that storage in the soil zone changes with depth.
Moore (1992) suggests that frequent storm events produce a temporary, perched
water table in the soil zone. Evidence of a temporary perched water table during this
the study watershed was shown not only by the mini-piezometers but also by the
separation of two flow zones in the soil (fig. 48). The separation of two zones of
flow occurred only during Type B storms.
Recharge to the aquifer and contributions to weir flow (output) did not occur
during Type A storms. The volume of rainfall combined with the frequency of events
was too low. The soils dry out quickly during the deficit period (when the majority of
storms are of Type A) because the effects of evapotranspiration were at their
maximum. Any water that infiltrated the soil zone was lost to storage and re-
evaporated back into to the system through evapotranspiration. Recharge to the
aquifer and contributions to weir flow (output) will only occur during Type B storms
(the surplus period). During Type B storms conditions are favorable to induce lateral
flow in the soil zone. As soil moisture increases, the hydraulic conductivity of the soil
increases (Freeze, 1971). An increase in hydraulic conductivity induces lateral soil
flow. This was able to occur during the surplus period because soil moisture was
high, evapotranspiration was at its minimum, and rainfall was frequent enough.
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Figure 47: Photograph of the inner tube of the mini-piezometerafter a Type B Storm. The marking on the inner tubeillustrates the preferential zones of flow within the soilzone recorded by the mini-piezometer. The base of themini-pizoemeter is on the left side of the picture andrepresents the soillbedrock interface (at 0.0" on thetape). Flow is indicated where the pen mark is missingon the inner tube. From 0.0 to 4.5 inches and from 5.5to 6.5 inches water was recorded in the mini-piezometer. The area from 4.5 to 5.5 is a zone of noflow. The separation of two zones of flow onlyoccurred during Type B storms. This implies that thestorage capacity of the soil varies with depth. Soil flowwill contribute flow to the tributary during Type Bstorms, when lateral flow dominates.
-.
Figure 48: Photograph of the soil zone after a Type B storm. Thephotograph illustrates the preferential zones of flow in thesoil column. A layer of soil was stripped away at differentdepths in a downslope direction. The darker zonespositioned between the lighter zones indicate the preferentialzones of flow.
89
90Shallow Fracture Flow and Deep Flow
Prior to 12/30/94, no flow was observed in the piezometers that monitor the
shallow fracture flow zones. The volume of effective precipitation (32. 9 inches)
combined with the number and frequency of events until the beginning of 12/94 was
not enough to recharge either of these zones. Evapotranspiration was also reached its
maximum prior to this date. Recharge of the deeper zones will occur when the
storage capacity of the soil is at its maximum. Once the soil zone is at field capacity,
then water will percolate past the soillbedrock interface, infiltrating the shallow
fracture flow zone. During the deficit period, evapotranspiration is high and storms
did not occur with enough frequency to deplete the storage capacity of the soil. This
is why there was no recharge to the shallow fracture flow zone during the deficit
period. This could only happen during the surplus period when evapotranspiration
was at its minimum and rainfall volumes were high enough. The soils cannot have the
chance to dry out between rainfall events if recharge is to occur.
Based on the hydro graph analysis, the shallow fracture flow zone contributed
to weir flow before water was observed in the lower weir well for the first time on
12/31/94. This suggests three things. One, the shallow fracture flow zone will
contribute weir flow before it has reached its maximum storage capacity. Two, water
flows along preferential flow paths in this zone, primarily along horizontal bedding
planes. Three, the shale beds interbedded in the limestone beds act as a semi-
permeable boundary (figs. 16, 17). Water will flow horizontally along the tops of the
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shale beds until enough head is built up to percolate deeper pass the shale beds. The
preferential flow along the tops of the shale beds contributed to weir flow.
Prior to the first recorded rainfall event of the monitoring period (3/4/95, 0135
inches), there had been a period of greater than one month of no significant rainfall
(less than 0.10 inches). During the 1991 monitoring period, the minimum event which
generated a response in the piezometer monitoring the deep flow zone occurs after
events with volumes between 1.9 and 3.9 inches (Bernhardt, 1991). During the
period from 2/24/94 to 12/16/95, 64% of storms recorded had a volume of less than
one inch. Only 6% had a volume of greater than 1.9 inches, none greater than 2.17
inches. No response would be expected in the deep flow zone based on this data and
Bernhardt (1991). Water from the deep flow zone did not contribute flow to the
tributary (output) during the 1994-95 monitoring period. During the deficit period,
the deep flow zone contributes water to deeper down-dip flow systems found in the
Childress Creek Basin, of which the study watershed is a subbasin. The deep flow
zone is mainly a regional flow zone. During the deficit period, it is unlikely that the
deep flow zone contributes any subsurface flow to the regional system (Childress
Creek Basin) as well as the study watershed.
CHAPTER 5
SUMMARY AND CONCLUSIONS
1. Forty-seven events accounted for the 40.05 inches of precipitation recorded
outside the vegetative canopy. Based on historic rainfall records for Waco,
Texas, the period from February through September, 1994 experienced less
than average amounts of rainfall, while the period from October through
December, 1994 experienced greater than average amounts of rainfall.
2. The water budget for the study watershed is based on the data gathered during
the 1994-95 monitoring period. Approximately 80% of precipitation was lost
to evapotranspiration (interception and secondary evapotranspiration). The
majority of water not lost to evapotranspiration formed shallow soil flow
(17.56% of precipitation). Approximately 1.1% of throughfall formed
overland flow and less than 1.0% of throughfall formed shallow fracture flow
and deep flow.
92
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3. Precipitation, runoff production, and infiltration were found to vary spatially
throughout the watershed. This is due to varying physical characteristics,
such as vegetation, canopy interception, degree of slope, soil type, and soil
thickness. The results of the regression analyses performed on the data
suggest to accurately monitor precipitation, runoff: and infiltration, these
factors should be monitored at more than one station in the watershed on a
storm by storm basis.
4. The 1994-95 monitoring period was divided into two periods: a deficit period
and a surplus period. During the deficit period (2/24/94 through 12/94) only
quick flow contributed to stream (weir) flow. Quick flow is a combination a
overland flow and direct precipitation on the channel. The effects of
evapotranspiration, low rainfall volumes relative to historic averages, and
fluctuating soil moistures prevented recharge of the soil zone. The soil zone
must recharge completely before it will contribute to weir flow and before the
shallow fracture flow zone can recharge. As the effects of evapotranspiration
diminished and soil moisture remained at field capacity (saturated) due to
frequent rainfall events, the soil zone and subsequently the shallow fracture
flow zone contributed to weir flow. During the surplus period (12/94 through
2/24/95), soil flow (from the shallow soil flow zone) and shallow fracture flow
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(from the shallow fracture flow zone) contributed the majority of weir flow
with minor contributions from quick flow.
5. The mechanisms of preferential flow were defined by the following three
factors: antecedent soil moisture conditions, the volume of rainfall, and the
duration of the storm. One of three conditions prevailed, depending on the
combination of these factors. The physical characteristics of the watershed,
such as vegetation, slope length, and soils, also played a key role in defining
the preferential flow paths.
6. The incipient runoff curve defines a runoff producing storm as one in which at
least 3.1% forms runoff A volume of between 0.7 and 1.0 inches of
precipitation (throughfall) is required for a storm to produce runoff in the
study basin. The volume of rain required to produce runoff varies due to
antecedent soil moisture conditions.
7. The threshold between runoff production and infiltration into the soil zone
(vertical soil flow) was defined by the combination of antecedent soil moisture
conditions and the volume of rainfall. A wet soil favors runoff production
over infiltration. If the volume of rainfall surpasses the infiltration capacity of
the soil, runoff production occurred. Any water not lost to evapotranspiration
and that did not form runoff infiltrated the soil zone.
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8. There are two components offlow in the soil zone: vertical and lateral. Only
lateral flow contributes to stream (weir) flow. The threshold between vertical
soil flow and lateral soil flow is controlled primarily by the available storage
capacity of the soil. When the vertical flow component of the soil is
overwhelmed, the lateral flow component must compensate for the surplus of
water. The vertical flow component becomes overwhelmed when the soil is at
field capacity (no available storage).
9. The soil zone must recharge before the shallow fracture flow zone will begin
to recharge and contribute to stream (weir flow). The effective porosity of
the shallow fracture flow zone is smaller than the soil zone; therefore, the
shallow fracture flow zone requires less time and less water to recharge.
10. The deep flow zone did not contribute to weir (tributary) flow during the
1994-95 monitoring period. It is inferred that the deep flow zone is a regional
flow zone and contribute sot down-dip flow systems.
11. For the shallow aquifer system at the study site to recharge the following
conditions must occur. The effects of evapotranspiration must be minimal.
This occurs during the winter months, from mid-October through mid-
96February. Rainfall volumes must be great enough and frequent enough to
keep the soils at field capacity (saturated). The condition of the soil zone is
the driving force behind recharge. A dry soil will not allow for recharge to the
shallow aquifer system. Therefore, the conditions previously described are
essential to recharge.