CHAPTER 4 DISCUSSION. Total Flow. For the study watershed, the water budget equation is defined as inflow equals

Transcription

1 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 inches. The following distribution of total input was based on the water budget calculated for the study watershed (fig. 23). Only inches (82.12% of total input) reached the watershed floor due to interception by the vegetative canopy. Approximately 80.51% (32.24 inches of 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,

2 75 INPUT = OUTPUT - LOSSES Input: Precipitation Loss I Interception I Ev apotranspirati on Overland Flow Contributions to Flow Zones /' I -, ~ Shallow Soil Flow <, ~ / Output: Tributary Flow Shallow - Fracture Flow Figure 43: Flow chart illustrating the water budget for the monitoring period. The input (precipitation) equals the output (tributary discharge) minus any losses. Losses are attributed to evapotranspiration, interception, and storage in the soil and shallow fracture flow zones. The amount lost can be inferred from precipitation and discharge measurements.

3 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 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

4 broken down into three periods (I, II, and ill) in order to explain how the system 77 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, 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

5 78 Figure 44: Comparison of (44B) through (44F) illustrates the gradual recharge of the system during the monitoring period. Based on the hydrograph analysis, the monitoring period was divided into three periods (I, II, and III). Period I lasted from 2/24/94 to 12/9/94~Period II lasted from 12/9/94 to 12/31/94~and Period III lasted from 12/31/94 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 the overland flow zone; lateral soil flow represents the shallow soil flow zone; and shallow fracture flow represents the shallow fracture flow zone. The tank model (After Barnes, 1940, fig. 44A) illustrates how each zone must recharge before it can contribute to output (tributary flow). Quick flow was the main contributor of flow during Period I. Most flow in the tributary during Period I occurred only during a storm (E). During Period I, the rainfall volume was lower than the historic averages (B) and evapotranspiration (C) reached its maximum. The soil moisture (0) also fluctuated between wilting point (0%) and field capacity ( <.). The threshold between quick flow and lateral soil flow is regulated by the antecedent soil moisture conditions. In mid-october, as the effects of evapotranspiration (C) diminished and rainfall volumes began to increase (B), the soil moisture (0) remained constant, near field capacity (100%). This indicates the soil zone is recharging. For lateral soil flow to occur and to contribute to tributary flow, the soils must remain saturated. Conditions are favorable for this to occur during the winter months when the effects of evapotranspiration are at their 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 tributary hydrograph (E). Lateral soil flow is the main contributor to tributary flow during Period II. The threshold between lateral soil flow and shallow fracture flow is regulated by the available storage capacity of both the soil zone and the shallow fracture flow zone. The available storage capacity of the soil zone must be depleted before the shallow fracture flow zone can begin to recharge and contribute to tributary flow. Conditions are most favorable for this to occur during the winter months when the effects of evapotranspiration are at their minimum. The shallow fracture flow zone began to contribute to tributary flow at the end of 12/94. The main contributors to flow during Period III were lateral soil flow as well as shallow fracture flow.

6 A 'II III RechlfCe to SFF tone Lalent now co""lb"ies 1oOu,,,,,, LAteral now conlrlbllles loou""'l Recha,ce 10SA' tone of Lalent flow rontn1mes looulplll Rain to Date: in Rain to Date: in Rain to Date: in M94 Ao M J J A S 0 N D J 95 F B 4" Precipitation Max c Eva potrans pi ra tion Min 100% Soil Moisture E.~.. Tributary Hydrograph 100% Quick Flow, Soil Flow, Shallow Fracture Flow F 0"1. QF SFF ~ ===========::=~====~---_:::::::::::::========~===tl==~4~l _

7 remained constant, near field capacity, from mid-october through February, At this point it is inferred that recharge to the shallow fracture flow zone commenced. 80 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, 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

8 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. whole. The inferences are intended to be representative of the watershed as a 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

9 striking the ground surface is greater than the available storage capacity of the soil to 82 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

10 rainfall. Runoff production will only occur during the high intensity episodes of 83 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 prestorm 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

11 ~~~_~:-T~n~'butary ' V Regional Water Table Figure 45: Comparison of runoff production along the hillslope. Gentle slopes at top of the basin (-200 feet from the tributary) and in flat lying areas along the slope (stations 1, 2, 3, and 5) generally produced smaller volumes of runoff than areas with steep slopes and at the banks of tributary (stations 4, 6, 7, and 8).

12 calculate available storage capacity of the soil (Hendrickx, 1990). Storms recorded 85 during the 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).

13 86 Figure 46: Photograph of water ponded at the soillbedrock interface after a Type A storm. Type A storms are characterized by soils at or near wilting point of the soil. Any water that infiltrates the soil zone will flow only vertically. The soil is too dry to induce lateral flow, which contributes flow to the tributary. The water is then lost to storage or re-evaporated back into the system through evapotranspiration.

14 When antecedent soil moisture conditions were at field capacity (Type B 87 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 reevaporated 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). flow in the soil zone. During Type B storms conditions are favorable to induce lateral 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.

15 88 Figure 47: Photograph of the inner tube of the mini-piezometer after a Type B Storm. The marking on the inner tube illustrates the preferential zones of flow within the soil zone recorded by the mini-piezometer. The base of the mini-pizoemeter is on the left side of the picture and represents the soillbedrock interface (at 0.0" on the tape). Flow is indicated where the pen mark is missing on the inner tube. From 0.0 to 4.5 inches and from 5.5 to 6.5 inches water was recorded in the minipiezometer. The area from 4.5 to 5.5 is a zone of no flow. The separation of two zones of flow only occurred during Type B storms. This implies that the storage capacity of the soil varies with depth. Soil flow will contribute flow to the tributary during Type B storms, when lateral flow dominates. -.

16 89 Figure 48: Photograph of the soil zone after a Type B storm. The photograph illustrates the preferential zones of flow in the soil column. A layer of soil was stripped away at different depths in a downslope direction. The darker zones positioned between the lighter zones indicate the preferential zones of flow.

17 Shallow Fracture Flow and Deep Flow 90 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 semipermeable boundary (figs. 16, 17). Water will flow horizontally along the tops of the

18 shale beds until enough head is built up to percolate deeper pass the shale beds. The 91 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 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 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.

19 CHAPTER 5 SUMMARY AND CONCLUSIONS 1. Forty-seven events accounted for the 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 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

20 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 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

21 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.

22 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 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-

23 February. Rainfall volumes must be great enough and frequent enough to 96 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.