chapter 4 discussion total flow flow (weir flow). losses...

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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 74

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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

74

INPUT = OUTPUT - LOSSES

Input: Precipitation

75

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.

76

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:

78

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

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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

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===========::=~====~---_:::::::::::::========~===tl==~4~L _

80

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

81

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

82

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).

84

85

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).

86

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.

88

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

91

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

93

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

94

(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.

95

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.